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WO2007147419A1 - Multihead weigher and method of operating such multihead weigher - Google Patents

Multihead weigher and method of operating such multihead weigher Download PDF

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Publication number
WO2007147419A1
WO2007147419A1 PCT/DK2007/050074 DK2007050074W WO2007147419A1 WO 2007147419 A1 WO2007147419 A1 WO 2007147419A1 DK 2007050074 W DK2007050074 W DK 2007050074W WO 2007147419 A1 WO2007147419 A1 WO 2007147419A1
Authority
WO
WIPO (PCT)
Prior art keywords
multihead weigher
vibration
operating
frequency
feeder
Prior art date
Application number
PCT/DK2007/050074
Other languages
French (fr)
Inventor
Henrik Skyum
Henning SKRÆDDERDAL
Søren PEDERSEN
Original Assignee
Scanvaegt International A/S
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Scanvaegt International A/S filed Critical Scanvaegt International A/S
Priority to EP07764494A priority Critical patent/EP2038621A1/en
Publication of WO2007147419A1 publication Critical patent/WO2007147419A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G13/00—Weighing apparatus with automatic feed or discharge for weighing-out batches of material
    • G01G13/003—Details; specially adapted accessories
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/0207—Driving circuits
    • B06B1/0223—Driving circuits for generating signals continuous in time
    • B06B1/0238—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave
    • B06B1/0246—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave with a feedback signal
    • B06B1/0253—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave with a feedback signal taken directly from the generator circuit
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G19/00—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups
    • G01G19/387—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for combinatorial weighing, i.e. selecting a combination of articles whose total weight or number is closest to a desired value
    • G01G19/393—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for combinatorial weighing, i.e. selecting a combination of articles whose total weight or number is closest to a desired value using two or more weighing units

Definitions

  • the present invention relates to the field of multihead weighers, in particular multihead weighers comprising vibration driven feeders.
  • Multihead weighers also sometimes referred to as combination weighers or associative weighers, provide an advantageous way of dividing a particulate product, typically food products such as peas, shrimps, meat balls, etc., or hardware products such as nuts, washers, rivets, etc., into portions according to predetermined weight criteria.
  • the particulate products are delivered to a central part of the multihead weigher, wherefrom it is randomly divided by a, typically vibrating, dispersion feeder and distributed to several weighing hoppers or other weighing means by means of, typically vibrating, linear feeders and possibly storage hoppers.
  • the weighing hoppers are typically emptied into a collection chute and the particulate products delivered from there to post processing, e.g. packaging.
  • a central control unit monitors each weighing hopper, and possibly further weighing and measuring units, in order to control for each portion which hoppers' content should contribute and thereby control the establishment of portions meeting the criteria as accurately as possible.
  • the cost effectiveness of a multihead weigher highly depends on the speed by which the particulate product is distributed to the weighing hoppers, i.e. the speed by which any previously emptied weighing hopper is ready to contribute to an additional product portion, and/or the amount that it is filled with in a certain amount of time.
  • Fast filling of empty weighing hoppers gives the central control unit more weighing hoppers to choose from to combine accurately sized portions, it facilitates using fewer weighing hoppers for each combination as they get filled by a larger amount in the same time, it facilitates fewer weighing hoppers overall, i.e. fewer "heads", and it facilitates a fast system.
  • Fast distribution of material to the weighing hoppers thus facilitates cheaper, faster and more accurate multihead weighers.
  • the distribution speed highly depends on the various feeders, especially the linear feeders, but also the dispersion feeders and any other links in the distribution chain.
  • a typical 14-head multihead weigher operating at medium speed, e.g. 40 - 80 drops per minute, will normally be set to use 4 weighing hoppers per drop.
  • the 4 weighing hoppers used at the first drop will not be ready for use at the next drop, i.e. there will normally be 10 hoppers to choose from.
  • the number of different possible combinations with 4 hoppers with a total of 10 available hoppers is 210. If the product distribution become weaker and the machine in average will have to use, e.g., 5 hoppers in stead of 4 because the hoppers are less filled, there will only be, e.g., 9 hoppers to choose from.
  • the number of different possible combinations with 5 hoppers with a total of 9 available hoppers is only 126, which is a 40% reduction in the number possible combinations. This will at first inflict on the accuracy of the machine of reaching the target weight and thereby the customer's "giveaway". If the product distribution is weakened further the number of different possible combinations will decrease further and the multihead weigher will not be able to find a portion within the allowed limits and may no longer be able to keep the speed, as it has to wait for more hoppers to be filled.
  • the accuracy of the hopper portions fluctuates reasonably, e.g. as a result of an uneven product layer, the accuracy of the multihead weigher will at first be reduced and secondly, the speed will be reduced.
  • the feeders in a multihead weigher are typically utilizing vibrators to move the particulate products.
  • the possible product distribution speed is important in multihead weighers, and vibration driven feeders provide the fastest product movement only at a certain vibration frequency, their resonance frequency.
  • either the resonance frequency or the vibration frequency is changed until they substantially match.
  • the resonance frequency of a vibrator may fluctuate over time, and thereby destroy the optimization.
  • Short-term reasons comprise, e.g., moisture, temperature and load variations, etc.
  • long-term reasons comprise, e.g., permanent change of material characteristics, especially spring characteristics, due to wear, moisture, friction, dirt accumulation etc.
  • a few prior art multihead weighers make use of more advanced vibrators with adjustable vibration frequency, e.g. by changing the frequency at which the current alternates. If the vibration of such vibrators becomes nonoptimal due to mismatch between the resonance and vibration frequencies, a regulation may be performed by changing the resonance frequency as described above by changing mechanical characteristics, or by changing the adjustable vibration frequency, or both. The optimization is thus much easier for these vibrators, as it may be done by changing electrical parameters, such as the pulsed current frequency, instead of mechanical characteristics. Finding the resonance frequency, however, still requires a technician with both experience and skills. Whereas it, thus, theoretically should be possible to optimize regarding short-term changes as well as long-term changes as electrical parameters may actually in most systems be changed at runtime, it, however, appear still not possible.
  • adjustable frequency vibrators highly facilitates the optimization process, as the technician can quickly try different vibration frequencies until the optimal frequency is determined, and this is a great improvement in multihead weighers which often comprises 14, 20, 24 or even more heads, each with an independently changed resonance frequency. But they still do not provide a method of dealing with the short-term changes.
  • the present invention relates to a method of operating a multihead weigher 10 comprising at least one vibration driven feeder 11, 12 comprising at least one vibrator 110, 120, said method comprising measuring electrical properties of said at least one vibrator, and on the basis thereof determining at least one control parameter of said multihead weigher.
  • an advantageous method for operating a multihead weigher whereby efficiency and precision is improved and maintained due to adaptive, automatic or semi-automatic, control based on measurements performed within the system.
  • the present invention may be used with any multihead weigher or similar equipment that utilizes vibration driven feeders for distributing products that have been or are to be processed, in order to make the distribution process more effective, or, alternatively, to better control the distribution process.
  • said at least one control parameter comprises at least one control parameter of said at least one vibration driven feeder 11, 12, an advantageous embodiment of the present invention is obtained.
  • the method controls the vibration driven feeders in order to achieve improved feeding efficiency and/or better control of the product distribution.
  • said at least one control parameter comprises the vibration frequency of at least one of said vibrators 110, 120, an advantageous embodiment of the present invention is obtained.
  • the method involves controlling the vibration frequency of the vibrators.
  • the efficiency of a vibration driven feeder is related the vibration frequency and its difference to the resonance frequency of the feeder, the efficiency may be controlled by controlling the vibration frequency.
  • said determining at least one control parameter comprises determining an optimal vibration frequency for at least one of said vibrators 110, 120 on the basis of said electrical properties
  • an optimal vibration frequency is determined from the measured electrical properties.
  • said determining an optimal vibration frequency for at least one of said vibrators 110, 120 comprises measuring said electrical properties of said at least one vibrator at different vibration frequencies, an advantageous embodiment of the present invention is obtained.
  • the vibrators are operated at different frequencies and electrical properties measured, before an optimal frequency is determined.
  • the optimal vibration frequency is the resonance frequency of the feeder, at which the maximum product movement may be obtained.
  • the optimal vibration frequency is determined as the resonance frequency when the feeder is loaded with an average amount of products.
  • the resonance frequency and the average amount of products may be determined at long or short intervals, e.g. once a week or several times a minute, according to the operating conditions and variation of load.
  • the vibration driven feeders controlled by the method are linear feeders, preferably each associated with a storage hopper and a weighing hopper.
  • the present invention may be used with great advantage for controlling linear feeders, both in order to optimize efficiency of product movement by optimizing the vibration frequency, and in order to precisely control product movement by controlling the vibration frequency, e.g., according to current product load, the current portioning requirements, etc.
  • At least one of said vibration driven feeders 11, 12 comprises a dispersion feeder 11, an advantageous embodiment of the present invention is obtained.
  • one of the vibration driven feeders is a dispersion feeder distributing the products to the linear feeders.
  • a dispersion feeder distributing the products to the linear feeders.
  • the vibrators are operated by means of a coil, and preferably a core and an armature.
  • Other kinds of vibrators facilitating determining a relationship between resonance frequency and electric properties are, however, also within the scope of the present invention.
  • said measuring electrical properties comprises measuring the current in said coil 40, 50, an advantageous embodiment of the present invention is obtained.
  • the coil current is measured as it relates to the efficiency and vibration frequency of the vibrator. It is noted that any method or means for measuring the coil current is within the scope of the present invention, and such current measuring techniques are widely known within the art, including measuring voltages over resistors coupled in serial with the coil, using current measuring transformers, using dedicated current measuring circuits or detectors, or any other suitable, current measuring means.
  • said measuring at different vibration frequencies comprises sweeping through an interval of vibration frequencies, an advantageous embodiment of the present invention is obtained.
  • the current is measured at several different vibration frequencies during a sweep, whereby the optimal vibration frequency may be determined.
  • any scheme of frequency sweeping is within the scope of the present invention, including linear sweeps at regular or irregular intervals, logarithmic sweeps, or non-linear sweeps where, e.g., the frequency alternates between high and low frequencies, etc.
  • said interval of vibration frequencies comprises a predetermined interval, an advantageous embodiment of the present invention is obtained.
  • a predetermined interval of frequencies forms basis for the sweep and determination of the optimal frequency. This is advantageous when the optimal frequency is relatively fixed and may be relatively well estimated before the sweep, e.g. on the basis of data sheets or tests performed during development.
  • the sweep interval is determined adaptively on the basis of previous measurements, or on the basis of a few trial measurements performed prior to the sweep.
  • said measuring at different vibration frequencies comprises measuring at different frequencies within a dynamic interval comprising an estimated optimal vibration frequency
  • an advantageous embodiment of the present invention is obtained.
  • an estimated optimal vibration frequency forms basis for determining which frequencies should be tried in order to determine the true optimal vibration frequency.
  • the estimated optimal frequency may be determined from the manufacturer of the vibrator or the multihead weigher, or may, preferably, be determined on the basis of previous measurements.
  • the control parameter e.g. the optimal vibration frequency or the product load
  • the control parameter is determined repeatedly in order to detect changes thereof. It can be shown that such changes comprise long- term changes over months due to, e.g., wear of springs, etc., and short-term changes over minutes or hours due to, e.g., temperature and moisture changes, load variations, etc.
  • the optimal vibration frequency is tracked by determining it repeatedly.
  • the method is carried out continuously as it in a preferred embodiment does not reduce the efficiency of the feeders and weigher.
  • the optimal control parameters during production use of the weigher realistic and optimal parameters are determined.
  • control parameters according to the present invention are determined at intervals, e.g. once an hour, once a day, once a week, etc.
  • control parameters according to the present invention are determined at predetermined operational states, e.g. during maintenance checks, cleaning, certain states of normal use, etc.
  • control parameters according to the present invention are determined at predetermined operational states, e.g. immediately after a hopper has been emptied, etc.
  • said estimated optimal vibration frequency comprises a recently determined optimal vibration frequency
  • an advantageous embodiment of the present invention is obtained.
  • the estimated optimal frequency is estimated on the basis of one or more recently determined optimal frequencies.
  • said estimated optimal vibration frequency comprises a predetermined frequency
  • an advantageous embodiment of the present invention is obtained.
  • the estimated optimal frequency is predetermined, e.g. on the basis of tests performed by the manufacturer.
  • said measuring at different vibration frequencies comprises coarsely sweeping through a first interval of vibration frequencies and determining the most optimal vibration frequency of those, then finely sweeping through a second interval of vibration frequencies, whereby said second interval is narrower that said first interval, and said first interval at least partly overlaps said second interval
  • a first, coarse frequency sweep is made in order to determine an interval in which to find the optimal vibration frequency, and subsequently a second, finer frequency sweep is through the most likely interval determined by the coarse sweep.
  • finer and finer frequency sweeps are made within narrower and narrower intervals.
  • the optimal vibration frequency i.e. the resonance frequency
  • the resonance frequency may be determined as the vibration frequency that causes the highest coil current.
  • a user may initiate the procedure of determining the optimal control parameters.
  • the procedure is carried out automatically during normal use, and no user-interaction is normally required. It may however be advantageous to allow a user to initiate the process, e.g. for initial installation, testing, etc. In embodiments where the process is only carried out at certain intervals or operational stages, a user initiation may be the preferred embodiment.
  • Measuring the optimal control parameters while the feeders are empty may provide information about the multihead weigher, the feeders and vibrators and their state of wear, etc., and may form basis for determining an estimated resonance frequency for the feeders for use as basis for the run-time measurements. Hence, by running an empty test at certain intervals, the general maintenance state of the multihead weigher may be monitored.
  • the method is carried out during normal use, i.e. with load on the feeders. Thereby production need not to be halted, and optimal, realistic control parameters are determined.
  • said multihead weigher comprises at least one dispersion feeder 11 and at least 10, preferably at least 14, and even more preferably at least 24 linear feeders 12, an advantageous embodiment of the present invention is obtained.
  • the method may be carried out on loaded feeders at a power level too low to operate the feeder. Thereby the optimal vibration frequency may be determined without moving any products.
  • said driven by low power comprises driving said at least one vibrator 110, 120 at a power low enough to substantially not move any particulate products comprised by said vibration driven feeder 11, 12, an advantageous embodiment of the present invention is obtained.
  • the optimal control parameters are initially determined and tracked by controlling the vibration according to a gradient control algorithm that is preferably carried out continuously during normal use. Thereby the most optimal parameters, and thereby the most efficient and precise control of the weigher, are continuously determined, without reducing the efficiency of the weigher at any time.
  • the gradient control algorithm comprises rules for initially determining the optimal vibration frequency, as well as rules for tracking it during use.
  • the gradient control algorithm causes the vibration frequency to vary between the optimal frequency, a frequency a little lower and a frequency a little higher, according to a predetermined or adaptively determined scheme.
  • the new optimal frequency be used as primary, centre frequency, and a large-scale sweep or other means for finding the optimal frequency will not be necessary until the tracking has been paused, e.g. due to maintenance, etc.
  • said determining at least one control parameter comprises determining the current product load on at least one of said vibration driven feeders 11, 12 on the basis of said electrical properties
  • the product load currently located on each feeder may be determined from the measurements.
  • an advantageous tool for improving the control of multihead weighers is provided, as knowledge about the product load may be derived into information about the amount of incoming products, the distribution of products, the efficiency of the feeders, the risk of overloading the storage hoppers, etc.
  • the product load is used as a regulation parameter for controlling the feeder efficiency by controlling the vibration frequency.
  • detection of a large amount of products on a certain feeder may cause the efficiency of that feeder to be reduced to a less-optimal degree by operating it at a non-optimal vibration frequency, in order to reduce the risk of overloading the associated storage hopper.
  • detection of a small amount of products possibly in combination with a parameter indicating the associated storage hopper and/or weighing hopper is more or less empty, may cause the feeder to be operated at increased efficiency, e.g. at the optimal vibration frequency, in order to speed up distribution of products on that feeder.
  • the product amount change detected when a piece drops from the feeder or is placed on the feeder may be so significant that it can be used for determining the number of products added to or removed from the feeder.
  • the method of the present invention may be used for determining the number of pieces delivered to the hoppers, and, in turn, for facilitating portioning the products on the basis of numbers as well as, or alternative to, weight.
  • determining the current product load on at least one of said vibration driven feeders 11, 12 on the basis of said electrical properties comprises measuring the current in at least one coil 40, 50 comprised by said at least one vibrator 110, 120, an advantageous embodiment of the present invention is obtained.
  • the amount of products may preferably be determined from measuring the current through the coil of the vibrator, as, at a fixed vibration frequency within a period short enough to assume that the resonance frequency depends primarily on the product layer, the coil current will decrease with increasing product load.
  • the current amount of product on a feeder may be used for controlling its feeding speed, e.g. by operating it at a non-optimal vibration frequency, in order to prevent overload of the associated storage hopper.
  • the knowledge about the number of products distributed by the feeders determined on the basis of the coil current is used for facilitating the multihead weigher to establish portions with a predetermined number of product pieces, as well as, or alternative to, a predetermined weight.
  • the present invention further relates to a multihead weigher 10 comprising at least one vibration driven feeder 11, 12, each vibration driven feeder comprising at least one vibrator 110, 120, characterised in that said multihead weigher further comprises at least one measuring means 73 for determining electrical properties of said at least one vibrator 110, 120.
  • an improved and advantageous multihead weigher which facilitates improved efficiency and precision due to means for operating the vibration driven feeders in the most optimal and/or precise mode.
  • efficiency and precision of a multihead weigher depend on the feeders' ability to distribute products efficiently and precisely to the hoppers, it is important that the optimal vibration conditions for the feeders are known.
  • the present invention provides means for determining optimal vibration conditions, as well as knowledge about the amount of product being distributed by the feeders.
  • said at least one vibrator 110, 120 comprises at least one coil 40, 50
  • an advantageous embodiment of the present invention is obtained.
  • the vibrators are operated by means of a coil, and preferably a core and an armature.
  • Other kinds of vibrators facilitating determining a relationship between resonance frequency and electric properties are, however, also within the scope of the present invention.
  • said measuring means 73 comprises at least one current detector 73 for measuring the current in at least one of said at least one coil 40, 50, an advantageous embodiment of the present invention is obtained.
  • the coil current is measured as it relates to the efficiency and vibration frequency of the vibrator.
  • any kind of current detector suitable for measuring the coil current is within the scope of the present invention, and such current detectors are widely known within the art, including measuring voltages over resistors coupled in serial with the coil, using current measuring transformers, using dedicated current measuring circuits or detectors, or any other suitable, current measuring means.
  • said multihead weigher comprises at least one processing unit 74 for determining an optimal vibration frequency of said at least one vibrator 110, 120 on the basis of said electrical properties, an advantageous embodiment of the present invention is obtained.
  • a processing unit for processing the measurements and determining the optimal vibration frequency. Any kind of processing means are within the scope of the present invention.
  • the multihead weigher may comprise a processing unit for each feeder or monitored feeder, or it may comprise one or more processing units capable of monitoring and controlling more than one feeder each.
  • the processing unit may form part of, or simply be, the processing means also used by the multihead weigher for monitoring and controlling the hoppers and establishing portions of predetermined number or weight.
  • said multihead weigher comprises at least one processing unit 74 for determining the current product load on said at least one vibration driven feeder 11 , 12, an advantageous embodiment of the present invention is obtained.
  • a processing unit for processing the measurements and determining the current product load of the feeder. Any kind of processing means are within the scope of the present invention.
  • the multihead weigher may comprise a processing unit for each feeder or monitored feeder, or it may comprise one or more processing units capable of monitoring and controlling more than one feeder each.
  • the processing unit may form part of, or simply be, the processing means also used by the multihead weigher for monitoring and controlling the hoppers and establishing portions of predetermined number or weight.
  • vibration driven feeders 11, 12 is a dispersion feeder 11 and at least 10, preferably at least 14, and even more preferably at least 24 of said vibration driven feeders 11, 12 are linear feeders 12, an advantageous embodiment of the present invention is obtained.
  • said multihead weigher comprises control means for operating said multihead weigher according the above-described method, an advantageous embodiment of the present invention is obtained.
  • the multihead weigher comprises means, e.g. its major processing means, or the processing units mentioned above, for carrying out the operation method described above.
  • fig. 1 illustrates in general the concept of a multihead weigher
  • fig. 2 illustrates an embodiment of a multihead weigher
  • fig. 3 illustrates in more detail an embodiment of a feeding mechanism for one head of a multihead weigher
  • fig. 4 illustrates an embodiment of a vibration-driven linear feeder of a multihead weigher
  • fig. 5 illustrates an embodiment of a vibration-driven dispersion feeder of a multihead weigher
  • fig. 6 illustrates a vibrator controller for a vibration-driven feeder
  • fig. 7 illustrates the how the coil current correspond to the vibration frequency
  • fig. 8 illustrates an adaptive vibrator controller according to a preferred embodiment of the present invention, fig.
  • fig. 9 illustrates a gradient control algorithm according to a preferred embodiment of the present invention
  • fig. 10 illustrates an example of a correlation between the product amount and the coil current
  • fig. 11 illustrates a traditional vibrator intensity regulation
  • fig. 12 illustrates a vibrator intensity control according to a preferred embodiment of the present invention.
  • Figure 1 illustrates the general concept of a multihead weigher 10.
  • the particulate product to be divided into portions of desired sizes is delivered by an in-feeder 17, which may be any means of delivering the product, e.g. a conveyor band, a vibration feeder, a delivery hopper or chute, manual delivery by shovel or bag, etc.
  • the particulate product is delivered to a, typically vibrating, dispersion feeder 11, wherefrom it is divided by random dispersion to a number of, typically vibrating, linear feeders 12, typically corresponding to the number of heads of the multihead weigher 10.
  • each linear feeder 12 the particulate product is typically collected in a storage hopper 13, also occasionally referred to as a feeding hopper, and when the corresponding weighing hopper 14 is empty, delivered as a portion to the weighing hopper 14, which determines the weight of the portion.
  • a central processing unit monitors the different weights determined by the different weighing hoppers 14, and when a number of weighing hoppers contain portions which in combination amount to an acceptable weight, it triggers those weighing hoppers to substantially simultaneously deliver their portions into a collection chute 15, 16, wherefrom the combined portion of acceptable weight may delivered to postprocessing means 18, e.g. packaging means, sorting means, quality control, etc., possibly directly subsequently to leaving the chute 15, 16.
  • the storage hoppers 13 are omitted, and the particulate product delivered to the weighing hoppers 14 directly from the linear feeders 12.
  • the storage hoppers 13 as intermediate collectors is however facilitated that full portions may be delivered to the weighing hoppers immediately after they are emptied, except in the few cases where a particular weighing hopper is emptied at intervals too short for the corresponding storage hopper 13 to collect a significant portion of particulate product.
  • even further storage hoppers, or other intermediate feeders, hoppers, etc. may be provided to further increase the effect and swiftness of the multihead weigher.
  • a linear feeder 12 may, e.g. deliver to two or more storage hoppers 13.
  • more than one level of weighing hoppers and division or combination of portions may be provided, in order to achieve increased precision or faster flow.
  • Figure 2 is an example of an embodiment of a multihead weigher 10. It comprises a central, vibrating dispersion feeder 11, from where a particulate product may be distributed to a number of vibrating linear feeders 12. At the end of each linear feeder is a storage hopper 13 for temporarily collecting portions of the product. Below each storage hopper 13 is a weighing hopper for determining the weight of a portion of the product. When the weighing hoppers 14 are emptied, the products are catched by a chute 15, 16 or combination of skids and a chute, and delivered to packaging means or transportation means located below the chute. As the multihead weigher 10 of figure 2 comprises 24 weighing hoppers 14, it is said to have 24 heads.
  • Figure 3 illustrates in more detail an example of an embodiment of a feeding mechanism for one head of a multihead weigher 10, seen from the side. It comprises a vibrating dispersion feeder 11 , which is common to all the heads of the multihead weigher.
  • the dispersion feeder 11 is preferably conical, so particulate products placed on the feeder is transported to its outer rim by a combination of vibration and gravity, but any means for distributing the particulate product to the feeders of the different heads are within the scope of the present invention.
  • Figure 3 further comprises a vibrating linear feeder 12 for receiving a part of the particulate product delivered to the dispersion feeder.
  • the linear feeder 12 is typically mounted so that a small part of it extends under the dispersion feeder 11 to ensure that the entire particulate product is catched, and with its bottom surface being slightly inclined downwards away from the dispersion feeder. Thereby vibration and gravity cause the particulate product to travel away from the dispersion feeder.
  • the linear feeder 12 is furthermore typically provided with feeder sidewalls 128 to ensure that the particulate product does not fall off the sides of the feeder. This is more clearly seen in figure 2. It is noted, that any type and design of a feeder suitable for delivering particulate products from a dispersion feeder to a storage- or weighing hopper, is within the scope of the present invention.
  • the particulate product leaves the far end of the linear feeder 12, it is typically collected in storage hopper 13, before being delivered to a weighing hopper 14.
  • the hoppers are arranged with suitable means for fastening, weighing, emptying, etc., according to a typical multihead weigher embodiment, and any hopper means and hopper supporting and control means are within the scope of the present invention.
  • Figure 3 further comprises a dispersion feeder vibrator 110 and a linear feeder vibrator 120, which in a preferred embodiment are both mounted to a mounting plate 30 of the multihead weigher 10.
  • Both vibrators 110, 120 comprise an electromagnet 111, 121 provided with an iron core, for establishing an alternating magnetic field when supplied with alternating, or preferably pulsed, current, and an armature 112, 122 arranged at a small distance from the electromagnet, and substantially perpendicular to the magnetic force established by the electromagnet.
  • the armature is made of a material subject to reaction to a magnetic force, typically a ferromagnetic material.
  • the electromagnets are preferably driven by pulse width modulated vibrator controls, or similar types of controls, in which it is possible to alter the pulsed current frequency by which the vibrators are driven.
  • the electromagnets 111, 121 are preferably each securely fixed to a base 117, 127, mounted on the mounting plate 30.
  • a number of flexible springs 113, 114, 115, 123, 124, are provided for keeping the armature in position.
  • To the same flexible springs are also fixed feeder mounts 116, 126, which thus move together with the armatures. At the feeder mounts are mounted the dispersion feeder 11 and linear feeders 12, respectively, so that they also move in correspondence with the armatures.
  • FIG 4 illustrates in more detail an embodiment of a linear feeder vibrator 120 of a multihead weigher. It comprises a base 127 on which an electromagnet 121 is mounted.
  • the electromagnet preferably comprises an iron core 41 and a coil 40 of insulated, electrically conducting wire. When a pulsed current is provided to the coil, a magnetic field which reciprocates as illustrated by the arrow 42 is established.
  • To the base is also fastened the ends of one or more flexible springs 123, 124, preferably two leaf springs, preferably made of laminated epoxy and glass sheets, e.g. as obtainable from 3M under the trade name Scotchply or from Red Sheet Electric Company under the trade name Cyply, but alternatively made of any suitable material.
  • a feeder mount 126 and an armature 122 At the other end of the springs are fastened a feeder mount 126 and an armature 122.
  • the armature is made of a material which reacts to magnetic force, and because of the flexible mounting, it will reciprocate in accordance with the reciprocation of the magnetic field established by the electromagnet, and thereby cause the feeder mount 126 to reciprocate or vibrate substantially as indicated by the arrow 43.
  • the reciprocation frequency is directly proportional to the frequency of the alternating or pulsed current, whereas the power of the vibrating movement is determined by several parameters, such as how well the armature engages with the magnetic field, the type, condition and mounting of the springs, the weight of the parts mounted to the springs, e.g. the linear feeder, the spring constant of the system, etc.
  • the entire vibrating part of the system i.e. the springs, armature, feeder mount, feeder and possible contents placed thereon, determine a resonance frequency at which the highest vibrating power is established from a certain amount of electrical power. At other frequencies the efficacy of the system is less optimal.
  • FIG. 5 illustrates in more detail an embodiment of a dispersion feeder vibrator 110 of a multihead weigher. It comprises a base 117 on which an electromagnet 111 is mounted.
  • the electromagnet preferably comprises an iron core 51 and a coil 50 of insulated, electrically conducting wire. When a pulsed current is provided to the coil, a magnetic field which reciprocates as illustrated by the arrow 52 is established.
  • To the base is also fastened the ends of one or more flexible springs 113, 114, 115, preferably three leaf springs, preferably made of laminated epoxy and glass sheets, e.g. as obtainable from 3M under the trade name Scotchply or from Red Sheet Electric Company under the trade name Cyply, but alternatively made of any suitable material.
  • the armature is made of a material which reacts to magnetic force, and because of the flexible mounting, it will reciprocate in accordance with the reciprocation of the magnetic field established by the electromagnet, and thereby cause the feeder mount 116 to reciprocate or vibrate substantially as indicated by the arrow 53, i.e. by a combination of vertical and rotational movement.
  • the reciprocation frequency is directly proportional with the frequency of the pulsed or alternating current, whereas the power of the vibrating movement is determined by several parameters, such as how well the armature engages with the magnetic field, the type, condition and mounting of the springs, the weight of the parts mounted to the springs, e.g.
  • the entire vibrating part of the system i.e. the springs, armature, feeder mount, feeder and possible contents placed thereon, determine a resonance frequency at which the highest vibrating power is established from a certain amount of electrical power. At other frequencies the efficacy of the system is less optimal.
  • the dispersion feeder and the linear feeders of a multihead weigher each determine an individual resonance frequency at which they work most efficiently, and as described above, the resonance frequency for a certain feeder depends on several parameters. Because those parameters change over time, the resonance frequency fluctuates. Over relatively long time, typically months, the resonance frequency changes because of permanent changes such as, e.g. spring characteristics changing due to wear, moisture, friction, dirt accumulation, etc. Over short time, e.g. hours or even minutes, the resonance frequency may change because of momentary changes such as, e.g., moisture, temperature and load variations, etc.
  • Figure 6 illustrates a common driving concept for vibrators. It comprises an electromagnet 111, 121 to be driven by an alternating or preferably pusled current in order to establish vibration of an armature, and a vibrator controller 60 for establishing the driving current.
  • the controller 60 comprises a driver signal generator 61, e.g. an AC generator, a pulse width modulator, etc., and an amplifier 62, e.g. a transistor, a switch-mode amplifier, etc.
  • the driver signal generator 61 establishes a small-signal representation of a desired driving signal, and the amplifier 62 steps up the signal to an appropriate level.
  • the driver signal generator 61 is preferably adjustable as regards the frequency of the driver signal. Thereby the vibration frequency of the corresponding feeder may at any time be changed to the currently actual resonance frequency of that feeder, preferably independent of any of the other feeders, provided the actual resonance frequency is known or can be determined, e.g. by trial and error.
  • the determination of the actual resonance frequency and corresponding regulation of the vibration frequency should, however, preferably be performed as often as possible, preferably several times an hour, or even continuously.
  • the resonance frequency of the vibrator can be determined as the frequency at which the coil current is strongest and thus, where the system is provided with the most effect.
  • the coil current is a manifestation of the job performed by the vibrator; the higher the amplitude on the vibrator, the higher the current through the vibrator coil.
  • Figure 7 illustrates a typical correspondence between the driver signal frequency fdnver and the current through the coil of the electromagnet l co ⁇ ⁇ .
  • the resonance frequency f reson ance of the system is the driver signal frequency at which the coil current is strongest.
  • the resonance frequency may, e.g., be 44.2Hz, and the coil current at the resonance frequency be, e.g., 1.96A.
  • a significant change in the coil current may typically be observed within a change of IHz of vibration frequency.
  • the coil current may have dropped to, e.g., 1.73 A at a vibration frequency of 43.6Hz, and 1.78A at 45.0Hz. It is noted, however, that any kinds and embodiments of vibrators with any resonance frequency and current/frequency-relationships are within the scope of the present invention.
  • FIG. 8 illustrates a preferred embodiment of the present invention. It comprises an electromagnet 111, 121 which drives a vibrating feeder. It further comprises an adaptive vibrator controller 70 for establishing the driving pulsed current for the electromagnet.
  • the adaptive controller 70 comprises an adjustable frequency driver signal generator 71, e.g.
  • the adjustable frequency driver signal generator 71 establishes a small-signal representation of a desired driving signal, and the amplifier 62 steps up the signal to an appropriate level. In an alternative embodiment the generator may establish a large-signal representation, in which case no amplification is needed.
  • the shape of the driving signal is preferably a pulse width modulated signal (PWM signal), but may be any kind of alternating or pulsed signal such as, e.g., square, triangle, sawtooth or sine waves, pulse density modulated signals, etc., and it may be analog or digital.
  • PWM signal pulse width modulated signal
  • the type of amplifier is insignificant as long as it is suitable for amplifying the kind of driver signal established by the generator to a level suitable for driving the vibrator.
  • the adjustable frequency driver signal generator 71 is adjustable as regards the frequency of the driver signal. Thereby the vibration frequency of the corresponding feeder may at any time be changed to the currently actual resonance frequency of that feeder.
  • the adaptive vibration controller 70 further comprises a current detector 73 for measuring the current through the coil. A representation of the current, e.g. a voltage level or a digital value proportional to the measured current value, is received by a processing unit 74, which processes the measured current and accordingly adjusts the driver signal frequency by controlling the adjustable frequency driver signal generator 71.
  • the processing unit 74 may be any kind of means suitable for receiving a measured value or signal and on the basis thereof establishing a control value or signal, e.g. a microprocessor, a digital signal processor, a programmable gate array, etc.
  • the processing unit merely maps measured current values into appropriate frequency control values.
  • the processing unit comprises means for performing different control algorithms, e.g. one or more algorithms for finding the resonance frequency for initialization or at certain intervals, one or more algorithms for tracking the resonance frequency during normal use, and possibly further algorithms for test purposes, for certain uses of the multihead weigher, for considering further parameters, e.g. amount of contents, etc.
  • the processing unit further comprises user input means for enabling a user to select a desired control algorithm or setting control parameters, and communication means for enabling the processing unit to receive commands, e.g. algorithm selection or parameter settings, or transmit values or commands, from/to a central processing unit of the multihead weigher, a central control computer, the other adaptive vibrator controllers, etc.
  • commands e.g. algorithm selection or parameter settings, or transmit values or commands, from/to a central processing unit of the multihead weigher, a central control computer, the other adaptive vibrator controllers, etc.
  • an adaptive vibrator controller 70 which comprises feedback in order to automatically optimise the driver signal frequency, preferably according to the currently actual resonance frequency of the vibrator, and thereby optimise the overall system efficacy.
  • Different algorithms may be provided for determining the currently actual resonance frequency of a vibration feeder according to an embodiment of the present invention.
  • One type of algorithm comprises sweeping through the frequency range in which the resonance frequency is expected to be, and simultaneously monitor the coil current.
  • the sweeping may be performed by one thorough sweep, or by performing several sweeps with frequency values of increasing resolution but in frequency bands of decreasing width.
  • the sweep method is most useful for initialization purposes, for testing once in a while, or when other, more optimal algorithms of the processing unit have lost track of the resonance frequency. Because of the relatively long time needed by sweep algorithms, and because of the very ineffective vibration produced at many of the tested frequencies, and hence for relatively long time, the use of a sweep algorithm during normal productive use of a multihead weigher is in many embodiments unacceptable.
  • the sweep algorithms may be available for manual selection by a technician, operator or cleaning crew, or for automatic performance at certain intervals, e.g. during cleaning or maintenance breaks, etc. Therefore the sweep algorithms are in most circumstances not optimal for counteracting short-term variations of the resonance frequency, whereas they are excellent for automatically dealing with long-term variations over weeks or months.
  • the adaptive vibrator control comprises modified sweep algorithms that are better suited for more frequent use and use during productive operation of the multihead weigher.
  • the amplification by the amplifier 62 may, e.g., be decreased or turned off during sweep, so that the magnetic field established is not powerful enough to cause a significant vibration, but still enabling the current detector to measure a current through the coil.
  • the sweep may be performed faster as the mechanical reciprocation is far less significant and the current measurement thus faster responding to adjustments.
  • a sweep algorithm is performed at the different feeders at different times during normal productive use.
  • the sweeps for the resonance frequencies may be performed sequentially for the different feeders, and the productivity is thus only reduced by 1/24 during sweeping.
  • Such sequential sweeping by different vibrator controllers may, e.g., be controlled by a central processing unit of the multihead weigher, or by the vibrator controllers communicating with each other.
  • a further modified sweep algorithm optimised for use during normal productive operation of the multihead weigher comprises performing sweeps for feeders whose corresponding weighing hoppers are being reset or not being relevant for combination in the current packing cycle, and therefore not requiring optimal product movement at first.
  • An alternative embodiment of this algorithm causes a sweep to be started for each feeder for which the corresponding weighing hopper complies with the above, and just halt the sweep prematurely if it turns out that the weighing hopper is to be used and thus have to be filled again fast.
  • Yet an alternative embodiment of this algorithm provides for memorizing the state of a sweep if ended prematurely, and thus allows continuing the sweeping from that state at the next time that feeder is idle.
  • An alternative type of algorithms for determining the currently actual resonance frequency of a vibration feeder comprises gradient control and tracking the resonance frequency instead of seeking for it.
  • this gradient control algorithm comprises continuously adjusting the vibrator frequency a small amount to each side of the present vibrator frequency. By comparing the coil current for a frequency at each side of the present frequency, it is possible to determine to which side the resonance frequency is, and thereby how to adjust the vibrator frequency in order to move towards the resonance frequency. By repeating this, the resonance frequency can be found and tracked as long as the measurements to each side are performed at short intervals.
  • the measurements to each side of the present frequency should preferably be performed at a frequency close enough to not render the vibration too inefficient, but still far enough to obtain current values that are sufficiently distinct to obtain an indication of the direction of the resonance frequency.
  • the change of vibration frequency is synchronized with the filling cycles, i.e. the emptying of the storage hopper associated with the vibration feeder, so that, e.g., a first filling cycle is performed at one frequency, the second filling cycle is performed at a second frequency according to the gradient control algorithm, the third filling cycle is performed at a third frequency according to the gradient control algorithm, etc.
  • Figure 9 comprises an example of seeking the resonance frequency by a gradient control algorithm. At first the present vibrator frequency is marked by 1, and a current measurement performed.
  • the frequency is changed a little to one side, here to a lower frequency 2, and a new current measurement performed.
  • the second measurement indicates a lower current than the first measurement, so a higher frequency 3 is selected, and a third measurement performed.
  • This measurement indicates a stronger current than the first two, and so the frequency is increased again to frequency 4, and yet again to frequency 5 and 6.
  • the current measured at frequency 6 is however lower than that at frequency 5, so the vibrator frequency is changed back to frequency 5.
  • the algorithm may, e.g., just perform a measurement at each side of the determined frequency at certain, preferably small intervals, in order to glue to the resonance frequency.
  • the vibration frequency may initially be set to, e.g., 43.8Hz.
  • a result of current measurements during 3 filling cycles may indicate an average coil current of 1.85 A for the vibration frequency of 43.8Hz.
  • the next 3 filling cycles may be performed with a vibration frequency of 44.0Hz, i.e. a step of +0.2Hz, and the average measured current may be 1.95 A, indicating that 44.0Hz is closer to the resonance frequency than 43.8Hz.
  • the vibration frequency is changed by another step of +0.2Hz to 44.2Hz, and current measurements are performed during the next 3 filling cycles. This may give an average current of 1.96 A, which is slighter higher than the result for a frequency of 44.0Hz, and thus indicating 44.2Hz being slightly closer to the resonance frequency.
  • the vibration frequency may be changed by another step of +0.2Hz to 44.4Hz, whereby the average current during 3 filling cycles may be 1.93 A, i.e. less than at the frequency of 44.2Hz and indicating that the resonance frequency is closer to 44.2Hz, than 44.2Hz.
  • the optimal vibration frequency to use is thus 44.2Hz.
  • the current measurements should preferably be performed continuously, and at certain intervals should the vibration frequency be by changed by alternately -0.2Hz or +0.2Hz and current measurements be made at those frequencies.
  • the algorithm may thus, e.g., cause a sequence of 3 filling cycles at 44.2Hz, 3 filling cycles at 44.0Hz, 3 filling cycles at 44.2Hz and 3 filling cycles at 44.4Hz to be repeated until either 44.0Hz or 44.4Hz proves more efficient than 44.2Hz.
  • the determination of the resonance frequency according to the gradient control method is performed several times, and preferably a certain number of past results are memorized in order to allow establishing an average, e.g. covering the last few minutes.
  • a new value for the resonance frequency is determined by means of an averaged gradient control algorithm as described above about once or twice an hour during full operation of the multihead weigher, without influencing the vibration efficiency notably at any time, but on the contrary, in fact increasing the altogether efficacy of the multihead weigher over short time, such as a day, as well as over long time, such as a month.
  • the gradient control algorithm as described above may move the vibrator frequency farther and farther to one side, until a change is measured and the direction to the resonance frequency thus can be determined, or until a predetermined end point of the frequency range is reached, and the measurements must be performed to the other side instead.
  • the approximate amount of content, e.g. particulate products, currently processed by each feeder may be determined.
  • the coil current I co ⁇ ⁇ there exists a correlation between the coil current I co ⁇ ⁇ and the amount of products W prodUct on the feeder.
  • the more product on, e.g., the linear feeder the less is the coil current.
  • An example of this correlation is illustrated in figure 10.
  • the correlation between the coil current and the amount of particulate products in the linear feeders, at a certain frequency, can, among other things, be used to ensure a more equal dosage of partial portions which is a condition for high speed and for obtaining the desired target weight of the output packages of the multihead weigher.
  • the vibration delay 91 for vibrating products into the storage hopper and the storage hopper delay 92 for waiting for the weighing hopper to be ready cause typically at least 2 weighing cycles to pass before the regulated vibration is experience by the weighing hopper, and the feedback can trigger the appropriate regulation.
  • the traditional vibration intensity or vibration time regulation is problematic because there is a time delay of minimum 2 filling cycles before changes, if any, are discovered.
  • the coil current is used as a regulative parameter in controlling the vibration intensity and/or vibration time, thus making it possible to obtain information about the changes in the layer of particulate products on a certain feeder before the products reach the corresponding hopper. If the coil current suddenly drops, at the same working frequency, an increase in the layer of products on the feeder is indicated, and thereby, it is possible to adjust, in that case turn down, the vibration before an overloading of the hoppers takes place.
  • Figure 12 illustrates a preferred embodiment of the present invention comprising a coil current based vibrator intensity regulation. As in figure 11 , the starting point is the desired weight 93 of product that is moved into the weighing hopper for each cycle.
  • the preferred embodiment of figure 12 however further adjusts the vibrator intensity and/or vibrator time according to a representation of the current product layer on the feeder determined by means of the coil current detector and/or processing unit 74. Thereby any product layer variations may affect the vibration intensity immediately.
  • the coil current which indicates the layer of product on the feeder when the vibration frequency is fixed, as well as the weighing hopper feedback, it is, e.g., possible to adjust the changes of the layer of products before the weighing hopper becomes overloaded.
  • the present embodiment allows turning the vibration intensity completely up during start-up when, often, only a little amount of product is available, as it is possible to monitor the amount of product entering the vibrator feeder by means of the current measurement, and thus turn down the vibration intensity again, before the hoppers get overloaded.
  • the option of monitoring changes in the amount of product on each vibration feeder may further be utilized for determining when a piece has dropped from the vibration feeder into a hopper, and by using this information as input for a counter or other monitoring means, to count the number of pieces in each hopper.
  • the processing means of the multihead weigher may control the portioning on the basis of weight, number of pieces or a combination of thereof, e.g. by choosing hoppers that together contain, e.g., 10 pieces and best match a desired target weight, e.g.
  • the step of determining when a piece is added or subtracted from a vibration feeder, i.e. placed on the feeder or dropped into a hopper, may be performed by observing significant changes in the amount of product on the feeder. If the product pieces are of a significant weight, a significant change in weight indicates a change in number of pieces. If the approximate average weight of the pieces is known, e.g. as an interval, e.g. 80-120 g, it is further possible to determine the number of pieces that is added or subtracted simultaneously, and in certain embodiments and nature of product pieces and their variation even determine the number of pieces currently placed on the vibration feeder.
  • the extra information concerning the amount of particulate products on the feeders provides the possibility of performing a much more sophisticated control of the vibrators than possible by using the weight in the weighing hopper only as feedback for the vibrator control.
  • the machine will automatically increase the layer of product particles in order to have product particles in all linear feeders. Feedback may be given to an operator, or an automatic control of the in- feeder may be provided.

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Abstract

The invention relates to a method of operating a multihead weigher (10) comprising at least one vibration driven feeder (11, 12) comprising at least one vibrator (110, 120), said method comprising measuring electrical properties of said at least one vibrator, and on the basis thereof determining at least one control parameter of said multihead weigher. The invention further relates to a multihead weigher comprising at least one (10) vibration driven feeder (11, 12), each vibration driven feeder comprising at least one vibrator (110, 120), characterised in that said multihead weigher further comprises at least one measuring means (73) for determining electrical properties of said at least one vibrator (110, 120).

Description

MULTIHEAD WEIGHER AND METHOD OF OPERATING SUCH MULTIHEAD WEIGHER
Field of the invention
The present invention relates to the field of multihead weighers, in particular multihead weighers comprising vibration driven feeders.
Background of the invention
Multihead weighers, also sometimes referred to as combination weighers or associative weighers, provide an advantageous way of dividing a particulate product, typically food products such as peas, shrimps, meat balls, etc., or hardware products such as nuts, washers, rivets, etc., into portions according to predetermined weight criteria. Typically the particulate products are delivered to a central part of the multihead weigher, wherefrom it is randomly divided by a, typically vibrating, dispersion feeder and distributed to several weighing hoppers or other weighing means by means of, typically vibrating, linear feeders and possibly storage hoppers. The weighing hoppers are typically emptied into a collection chute and the particulate products delivered from there to post processing, e.g. packaging. In order to achieve product portions that meet the predetermined weight criteria, only those weighing hoppers that in combination come closest to meeting such criteria are emptied for each portion. A central control unit monitors each weighing hopper, and possibly further weighing and measuring units, in order to control for each portion which hoppers' content should contribute and thereby control the establishment of portions meeting the criteria as accurately as possible.
The cost effectiveness of a multihead weigher highly depends on the speed by which the particulate product is distributed to the weighing hoppers, i.e. the speed by which any previously emptied weighing hopper is ready to contribute to an additional product portion, and/or the amount that it is filled with in a certain amount of time. Fast filling of empty weighing hoppers gives the central control unit more weighing hoppers to choose from to combine accurately sized portions, it facilitates using fewer weighing hoppers for each combination as they get filled by a larger amount in the same time, it facilitates fewer weighing hoppers overall, i.e. fewer "heads", and it facilitates a fast system. Fast distribution of material to the weighing hoppers thus facilitates cheaper, faster and more accurate multihead weighers. The distribution speed, however, highly depends on the various feeders, especially the linear feeders, but also the dispersion feeders and any other links in the distribution chain.
A typical 14-head multihead weigher operating at medium speed, e.g. 40 - 80 drops per minute, will normally be set to use 4 weighing hoppers per drop. The 4 weighing hoppers used at the first drop will not be ready for use at the next drop, i.e. there will normally be 10 hoppers to choose from. The number of different possible combinations with 4 hoppers with a total of 10 available hoppers is 210. If the product distribution become weaker and the machine in average will have to use, e.g., 5 hoppers in stead of 4 because the hoppers are less filled, there will only be, e.g., 9 hoppers to choose from. The number of different possible combinations with 5 hoppers with a total of 9 available hoppers is only 126, which is a 40% reduction in the number possible combinations. This will at first inflict on the accuracy of the machine of reaching the target weight and thereby the customer's "giveaway". If the product distribution is weakened further the number of different possible combinations will decrease further and the multihead weigher will not be able to find a portion within the allowed limits and may no longer be able to keep the speed, as it has to wait for more hoppers to be filled.
If the accuracy of the hopper portions fluctuates reasonably, e.g. as a result of an uneven product layer, the accuracy of the multihead weigher will at first be reduced and secondly, the speed will be reduced.
The feeders in a multihead weigher are typically utilizing vibrators to move the particulate products. As mentioned above, the possible product distribution speed is important in multihead weighers, and vibration driven feeders provide the fastest product movement only at a certain vibration frequency, their resonance frequency. Hence, in order to optimize a vibration driven feeder, either the resonance frequency or the vibration frequency is changed until they substantially match. It is, however, a well-known phenomenon that the resonance frequency of a vibrator may fluctuate over time, and thereby destroy the optimization. Both short-term and long-term reasons cause this. Short-term reasons comprise, e.g., moisture, temperature and load variations, etc., and long-term reasons comprise, e.g., permanent change of material characteristics, especially spring characteristics, due to wear, moisture, friction, dirt accumulation etc.
Most prior art multihead weighers make use of simple vibrators with fixed vibration frequency. If the vibration of such vibrators becomes nonoptimal due to mismatch between the resonance and vibration frequencies, the only optimization possibility consists in changing the resonance frequency. As the resonance frequency is closely related to the mechanical characteristics of the vibrator, it is typically not possible to modify easily, and certainly not at runtime, as it typically involves changing a reference load, a centre of gravity, a spring constant, etc. Hence, optimization is realistically only possible with regards to long-term frequency changes. During daily use the impact of the short-term changes are therefore sought to be minimized by performing an optimization aimed at an average resonance frequency. Thereby the fixed frequency vibration feeders work at nonoptimal, but best possible, frequencies most of the time, if properly optimized regarding long-term changes, e.g. each 3 months.
A few prior art multihead weighers make use of more advanced vibrators with adjustable vibration frequency, e.g. by changing the frequency at which the current alternates. If the vibration of such vibrators becomes nonoptimal due to mismatch between the resonance and vibration frequencies, a regulation may be performed by changing the resonance frequency as described above by changing mechanical characteristics, or by changing the adjustable vibration frequency, or both. The optimization is thus much easier for these vibrators, as it may be done by changing electrical parameters, such as the pulsed current frequency, instead of mechanical characteristics. Finding the resonance frequency, however, still requires a technician with both experience and skills. Whereas it, thus, theoretically should be possible to optimize regarding short-term changes as well as long-term changes as electrical parameters may actually in most systems be changed at runtime, it, however, appear still not possible. This is because it is not possible to know how the resonance frequency changes during time, and therefore impossible to know which vibration frequency is optimal at a given time. Hence, adjustable frequency vibrators highly facilitates the optimization process, as the technician can quickly try different vibration frequencies until the optimal frequency is determined, and this is a great improvement in multihead weighers which often comprises 14, 20, 24 or even more heads, each with an independently changed resonance frequency. But they still do not provide a method of dealing with the short-term changes.
It is an object of the present invention to provide a method for facilitating operation of a multihead weigher with improved efficiency and precision.
It is an object of the present invention to provide a system and method for automatically or semi-automatically determining the most optimal vibration frequency of a vibration driven feeder of a multihead weigher, preferably also at runtime.
It is an object of the present invention to provide a system and method for automatically or semi-automatically adjusting the vibration frequency of a vibration driven feeder of a multihead weigher, preferably also at runtime, in order to maintain a most optimal vibration frequency.
It is an object of the present invention to provide a multihead weigher wherein the feeder control may at least partly be based on the current product load. Summary of the invention
The present invention relates to a method of operating a multihead weigher 10 comprising at least one vibration driven feeder 11, 12 comprising at least one vibrator 110, 120, said method comprising measuring electrical properties of said at least one vibrator, and on the basis thereof determining at least one control parameter of said multihead weigher.
According to the present invention, an advantageous method for operating a multihead weigher is provided, whereby efficiency and precision is improved and maintained due to adaptive, automatic or semi-automatic, control based on measurements performed within the system. The present invention may be used with any multihead weigher or similar equipment that utilizes vibration driven feeders for distributing products that have been or are to be processed, in order to make the distribution process more effective, or, alternatively, to better control the distribution process.
When said at least one control parameter comprises at least one control parameter of said at least one vibration driven feeder 11, 12, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the method controls the vibration driven feeders in order to achieve improved feeding efficiency and/or better control of the product distribution.
When said at least one control parameter comprises the vibration frequency of at least one of said vibrators 110, 120, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the method involves controlling the vibration frequency of the vibrators. As the efficiency of a vibration driven feeder is related the vibration frequency and its difference to the resonance frequency of the feeder, the efficiency may be controlled by controlling the vibration frequency.
When said determining at least one control parameter comprises determining an optimal vibration frequency for at least one of said vibrators 110, 120 on the basis of said electrical properties, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, an optimal vibration frequency is determined from the measured electrical properties.
When said determining an optimal vibration frequency for at least one of said vibrators 110, 120 comprises measuring said electrical properties of said at least one vibrator at different vibration frequencies, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the vibrators are operated at different frequencies and electrical properties measured, before an optimal frequency is determined.
When said optimal vibration frequency substantially corresponds to the resonance frequency of said vibration driven feeder 11, 12, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the optimal vibration frequency is the resonance frequency of the feeder, at which the maximum product movement may be obtained.
When said optimal vibration frequency substantially corresponds to the resonance frequency of said vibration driven feeder 11, 12 when loaded with an average amount of particulate products, an advantageous embodiment of the present invention is obtained.
As the resonance frequency of a feeder partly depends on its weight distribution, and as different amounts of products cause different weight distributions, the resonance frequency varies with varying product load. Hence, in a preferred embodiment of the present invention, the optimal vibration frequency is determined as the resonance frequency when the feeder is loaded with an average amount of products. The resonance frequency and the average amount of products may be determined at long or short intervals, e.g. once a week or several times a minute, according to the operating conditions and variation of load.
When at least one of said vibration driven feeders 11, 12 comprises a linear feeder 12, an advantageous embodiment of the present invention is obtained.
In a preferred embodiment of the present invention, the vibration driven feeders controlled by the method are linear feeders, preferably each associated with a storage hopper and a weighing hopper. The present invention may be used with great advantage for controlling linear feeders, both in order to optimize efficiency of product movement by optimizing the vibration frequency, and in order to precisely control product movement by controlling the vibration frequency, e.g., according to current product load, the current portioning requirements, etc.
When at least one of said vibration driven feeders 11, 12 comprises a dispersion feeder 11, an advantageous embodiment of the present invention is obtained.
In a preferred embodiment of the present invention, one of the vibration driven feeders is a dispersion feeder distributing the products to the linear feeders. By the method of the present invention, it is possible to optimize the efficiency of the dispersion feeder and thereby improve the distribution to the linear feeders. Embodiments with several dispersion feeders are also within the scope of the present invention, as well as any other kind of vibration driven feeders. When said at least one vibrator 110, 120 comprises a coil 40, 50, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the vibrators are operated by means of a coil, and preferably a core and an armature. Other kinds of vibrators facilitating determining a relationship between resonance frequency and electric properties are, however, also within the scope of the present invention.
When said measuring electrical properties comprises measuring the current in said coil 40, 50, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the coil current is measured as it relates to the efficiency and vibration frequency of the vibrator. It is noted that any method or means for measuring the coil current is within the scope of the present invention, and such current measuring techniques are widely known within the art, including measuring voltages over resistors coupled in serial with the coil, using current measuring transformers, using dedicated current measuring circuits or detectors, or any other suitable, current measuring means.
When said measuring at different vibration frequencies comprises sweeping through an interval of vibration frequencies, an advantageous embodiment of the present invention is obtained.
According to an embodiment of the present invention, the current is measured at several different vibration frequencies during a sweep, whereby the optimal vibration frequency may be determined. It is noted that any scheme of frequency sweeping is within the scope of the present invention, including linear sweeps at regular or irregular intervals, logarithmic sweeps, or non-linear sweeps where, e.g., the frequency alternates between high and low frequencies, etc. When said interval of vibration frequencies comprises a predetermined interval, an advantageous embodiment of the present invention is obtained.
According to an embodiment of the present invention, a predetermined interval of frequencies forms basis for the sweep and determination of the optimal frequency. This is advantageous when the optimal frequency is relatively fixed and may be relatively well estimated before the sweep, e.g. on the basis of data sheets or tests performed during development.
When said interval of vibration frequencies is determined adaptively on the basis of said measurements, an advantageous embodiment of the present invention is obtained.
In a preferred embodiment of the present invention, the sweep interval is determined adaptively on the basis of previous measurements, or on the basis of a few trial measurements performed prior to the sweep.
When said measuring at different vibration frequencies comprises measuring at different frequencies within a dynamic interval comprising an estimated optimal vibration frequency, an advantageous embodiment of the present invention is obtained.
In a preferred embodiment of the present invention an estimated optimal vibration frequency forms basis for determining which frequencies should be tried in order to determine the true optimal vibration frequency. The estimated optimal frequency may be determined from the manufacturer of the vibrator or the multihead weigher, or may, preferably, be determined on the basis of previous measurements.
When said method is repeated in order to adaptively maintain said at least one control parameter at an optimal value, an advantageous embodiment of the present invention is obtained. According to a preferred embodiment of the present invention, the control parameter, e.g. the optimal vibration frequency or the product load, is determined repeatedly in order to detect changes thereof. It can be shown that such changes comprise long- term changes over months due to, e.g., wear of springs, etc., and short-term changes over minutes or hours due to, e.g., temperature and moisture changes, load variations, etc.
When said method is repeated in order to adaptively maintain an optimal vibration frequency, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the optimal vibration frequency is tracked by determining it repeatedly.
When said method is repeated continuously, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the method is carried out continuously as it in a preferred embodiment does not reduce the efficiency of the feeders and weigher. On the contrary, by determining the optimal control parameters during production use of the weigher, realistic and optimal parameters are determined.
When said method is repeated at predetermined intervals, an advantageous embodiment of the present invention is obtained.
In an alternative embodiment, the control parameters according to the present invention are determined at intervals, e.g. once an hour, once a day, once a week, etc.
When said method is repeated at predetermined operational states of said multihead weigher, an advantageous embodiment of the present invention is obtained. In an alternative embodiment, the control parameters according to the present invention are determined at predetermined operational states, e.g. during maintenance checks, cleaning, certain states of normal use, etc.
When said method is repeated at predetermined operational states of at least one of said vibration driven feeders 11, 12, an advantageous embodiment of the present invention is obtained.
In an alternative embodiment, the control parameters according to the present invention are determined at predetermined operational states, e.g. immediately after a hopper has been emptied, etc.
When said estimated optimal vibration frequency comprises a recently determined optimal vibration frequency, an advantageous embodiment of the present invention is obtained.
In a preferred embodiment of the present invention, the estimated optimal frequency is estimated on the basis of one or more recently determined optimal frequencies.
When said estimated optimal vibration frequency comprises a predetermined frequency, an advantageous embodiment of the present invention is obtained.
In an alternative embodiment, the estimated optimal frequency is predetermined, e.g. on the basis of tests performed by the manufacturer.
When said measuring at different vibration frequencies comprises coarsely sweeping through a first interval of vibration frequencies and determining the most optimal vibration frequency of those, then finely sweeping through a second interval of vibration frequencies, whereby said second interval is narrower that said first interval, and said first interval at least partly overlaps said second interval, an advantageous embodiment of the present invention is obtained. In a preferred embodiment of the present invention, a first, coarse frequency sweep is made in order to determine an interval in which to find the optimal vibration frequency, and subsequently a second, finer frequency sweep is through the most likely interval determined by the coarse sweep. In an alternative embodiment, even further, finer and finer frequency sweeps are made within narrower and narrower intervals.
When said optimal vibration frequency is determined as the frequency where said measured current in said coil 40, 50 is largest, an advantageous embodiment of the present invention is obtained.
According to the present invention, the optimal vibration frequency, i.e. the resonance frequency, may be determined as the vibration frequency that causes the highest coil current.
When said method is initiated by a user, an advantageous embodiment of the present invention is obtained.
In an embodiment of the present invention a user, e.g. an operator, cleaning assistant or maintenance technician, may initiate the procedure of determining the optimal control parameters. In preferred embodiments the procedure is carried out automatically during normal use, and no user-interaction is normally required. It may however be advantageous to allow a user to initiate the process, e.g. for initial installation, testing, etc. In embodiments where the process is only carried out at certain intervals or operational stages, a user initiation may be the preferred embodiment.
When said at least one vibration driven feeder 11, 12 is empty while said method is carried out, an advantageous embodiment of the present invention is obtained.
Measuring the optimal control parameters while the feeders are empty may provide information about the multihead weigher, the feeders and vibrators and their state of wear, etc., and may form basis for determining an estimated resonance frequency for the feeders for use as basis for the run-time measurements. Hence, by running an empty test at certain intervals, the general maintenance state of the multihead weigher may be monitored.
When said vibration driven feeder 11, 12 is in use while said method is carried out, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the method is carried out during normal use, i.e. with load on the feeders. Thereby production need not to be halted, and optimal, realistic control parameters are determined.
When said multihead weigher comprises at least one dispersion feeder 11 and at least 10, preferably at least 14, and even more preferably at least 24 linear feeders 12, an advantageous embodiment of the present invention is obtained.
When said at least one vibrator 110, 120 is driven by low power while said measuring electrical properties of said at least one vibrator is performed, an advantageous embodiment of the present invention is obtained.
According to an embodiment of the present invention, the method may be carried out on loaded feeders at a power level too low to operate the feeder. Thereby the optimal vibration frequency may be determined without moving any products.
When said driven by low power comprises driving said at least one vibrator 110, 120 at a power low enough to substantially not move any particulate products comprised by said vibration driven feeder 11, 12, an advantageous embodiment of the present invention is obtained.
When said measuring at different vibration frequencies comprises performing measurements and changing vibration frequencies according to a gradient control algorithm, an advantageous embodiment of the present invention is obtained. According to a preferred embodiment of the present invention, the optimal control parameters are initially determined and tracked by controlling the vibration according to a gradient control algorithm that is preferably carried out continuously during normal use. Thereby the most optimal parameters, and thereby the most efficient and precise control of the weigher, are continuously determined, without reducing the efficiency of the weigher at any time.
When said gradient control algorithm comprises rules for finding and tracking said optimal vibration frequency, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the gradient control algorithm comprises rules for initially determining the optimal vibration frequency, as well as rules for tracking it during use.
When according to said gradient control algorithm the vibration frequency is alternately at predetermined intervals changed up and down relative to the last known optimal vibration frequency in order to detect changes in the optimal vibration frequency, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the gradient control algorithm causes the vibration frequency to vary between the optimal frequency, a frequency a little lower and a frequency a little higher, according to a predetermined or adaptively determined scheme. Thereby any drift of the optimal frequency may be detected immediately, the new optimal frequency be used as primary, centre frequency, and a large-scale sweep or other means for finding the optimal frequency will not be necessary until the tracking has been paused, e.g. due to maintenance, etc.
When said determining at least one control parameter comprises determining the current product load on at least one of said vibration driven feeders 11, 12 on the basis of said electrical properties, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the product load currently located on each feeder may be determined from the measurements. Thereby an advantageous tool for improving the control of multihead weighers is provided, as knowledge about the product load may be derived into information about the amount of incoming products, the distribution of products, the efficiency of the feeders, the risk of overloading the storage hoppers, etc.
When the vibration frequency of at least one of said vibration driven feeders 11, 12 is controlled on the basis of said current product load, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the product load is used as a regulation parameter for controlling the feeder efficiency by controlling the vibration frequency. Thus, detection of a large amount of products on a certain feeder may cause the efficiency of that feeder to be reduced to a less-optimal degree by operating it at a non-optimal vibration frequency, in order to reduce the risk of overloading the associated storage hopper. Similarly, detection of a small amount of products, possibly in combination with a parameter indicating the associated storage hopper and/or weighing hopper is more or less empty, may cause the feeder to be operated at increased efficiency, e.g. at the optimal vibration frequency, in order to speed up distribution of products on that feeder.
When a change in the number of product pieces on at least one of said vibration driven feeders 11, 12 is determined on the basis of said current product load, an advantageous embodiment of the present invention is obtained.
When the products processed by the multihead weigher are relatively large pieces, e.g. pieces of poultry, the product amount change detected when a piece drops from the feeder or is placed on the feeder may be so significant that it can be used for determining the number of products added to or removed from the feeder. With such products, according to a preferred embodiment, the method of the present invention may be used for determining the number of pieces delivered to the hoppers, and, in turn, for facilitating portioning the products on the basis of numbers as well as, or alternative to, weight.
When the number of product pieces on at least one of said vibration driven feeders 11, 12 is determined on the basis of said current product load, an advantageous embodiment of the present invention is obtained.
With product pieces of very little weight variation it is possible, according to an embodiment of the present invention, to determine the number of pieces currently placed on the feeder.
When determining the current product load on at least one of said vibration driven feeders 11, 12 on the basis of said electrical properties comprises measuring the current in at least one coil 40, 50 comprised by said at least one vibrator 110, 120, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the amount of products may preferably be determined from measuring the current through the coil of the vibrator, as, at a fixed vibration frequency within a period short enough to assume that the resonance frequency depends primarily on the product layer, the coil current will decrease with increasing product load.
When said vibration driven feeders 11, 12 are controlled on the basis of said current product load so as to prevent overload of storage hoppers 13 or weighing hoppers 14 of said multihead weigher, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the current amount of product on a feeder may be used for controlling its feeding speed, e.g. by operating it at a non-optimal vibration frequency, in order to prevent overload of the associated storage hopper.
When said multihead weigher establishes portions of products at least partly based on said determined change in number of product pieces or said number of product pieces on said vibration driven feeders 11, 12, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the knowledge about the number of products distributed by the feeders determined on the basis of the coil current, is used for facilitating the multihead weigher to establish portions with a predetermined number of product pieces, as well as, or alternative to, a predetermined weight.
The present invention further relates to a multihead weigher 10 comprising at least one vibration driven feeder 11, 12, each vibration driven feeder comprising at least one vibrator 110, 120, characterised in that said multihead weigher further comprises at least one measuring means 73 for determining electrical properties of said at least one vibrator 110, 120.
According to the present invention, an improved and advantageous multihead weigher is provided, which facilitates improved efficiency and precision due to means for operating the vibration driven feeders in the most optimal and/or precise mode. As both efficiency and precision of a multihead weigher depend on the feeders' ability to distribute products efficiently and precisely to the hoppers, it is important that the optimal vibration conditions for the feeders are known. The present invention provides means for determining optimal vibration conditions, as well as knowledge about the amount of product being distributed by the feeders.
When said at least one vibrator 110, 120 comprises at least one coil 40, 50, an advantageous embodiment of the present invention is obtained. According to a preferred embodiment of the present invention, the vibrators are operated by means of a coil, and preferably a core and an armature. Other kinds of vibrators facilitating determining a relationship between resonance frequency and electric properties are, however, also within the scope of the present invention.
When said measuring means 73 comprises at least one current detector 73 for measuring the current in at least one of said at least one coil 40, 50, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the coil current is measured as it relates to the efficiency and vibration frequency of the vibrator. It is noted that any kind of current detector suitable for measuring the coil current is within the scope of the present invention, and such current detectors are widely known within the art, including measuring voltages over resistors coupled in serial with the coil, using current measuring transformers, using dedicated current measuring circuits or detectors, or any other suitable, current measuring means.
When said multihead weigher comprises at least one processing unit 74 for determining an optimal vibration frequency of said at least one vibrator 110, 120 on the basis of said electrical properties, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, a processing unit is provided for processing the measurements and determining the optimal vibration frequency. Any kind of processing means are within the scope of the present invention. The multihead weigher may comprise a processing unit for each feeder or monitored feeder, or it may comprise one or more processing units capable of monitoring and controlling more than one feeder each. The processing unit may form part of, or simply be, the processing means also used by the multihead weigher for monitoring and controlling the hoppers and establishing portions of predetermined number or weight. When said multihead weigher comprises at least one processing unit 74 for determining the current product load on said at least one vibration driven feeder 11 , 12, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, a processing unit is provided for processing the measurements and determining the current product load of the feeder. Any kind of processing means are within the scope of the present invention. The multihead weigher may comprise a processing unit for each feeder or monitored feeder, or it may comprise one or more processing units capable of monitoring and controlling more than one feeder each. The processing unit may form part of, or simply be, the processing means also used by the multihead weigher for monitoring and controlling the hoppers and establishing portions of predetermined number or weight.
When at least one of said vibration driven feeders 11, 12 is a dispersion feeder 11 and at least 10, preferably at least 14, and even more preferably at least 24 of said vibration driven feeders 11, 12 are linear feeders 12, an advantageous embodiment of the present invention is obtained.
When said multihead weigher comprises control means for operating said multihead weigher according the above-described method, an advantageous embodiment of the present invention is obtained.
According to a preferred embodiment of the present invention, the multihead weigher comprises means, e.g. its major processing means, or the processing units mentioned above, for carrying out the operation method described above. The drawings
In the following, the invention will be described with reference to the drawings where
fig. 1 illustrates in general the concept of a multihead weigher, fig. 2 illustrates an embodiment of a multihead weigher, fig. 3 illustrates in more detail an embodiment of a feeding mechanism for one head of a multihead weigher, fig. 4 illustrates an embodiment of a vibration-driven linear feeder of a multihead weigher, fig. 5 illustrates an embodiment of a vibration-driven dispersion feeder of a multihead weigher, fig. 6 illustrates a vibrator controller for a vibration-driven feeder, fig. 7 illustrates the how the coil current correspond to the vibration frequency, fig. 8 illustrates an adaptive vibrator controller according to a preferred embodiment of the present invention, fig. 9 illustrates a gradient control algorithm according to a preferred embodiment of the present invention, fig. 10 illustrates an example of a correlation between the product amount and the coil current, fig. 11 illustrates a traditional vibrator intensity regulation, and fig. 12 illustrates a vibrator intensity control according to a preferred embodiment of the present invention.
Detailed description
Figure 1 illustrates the general concept of a multihead weigher 10. The particulate product to be divided into portions of desired sizes is delivered by an in-feeder 17, which may be any means of delivering the product, e.g. a conveyor band, a vibration feeder, a delivery hopper or chute, manual delivery by shovel or bag, etc. The particulate product is delivered to a, typically vibrating, dispersion feeder 11, wherefrom it is divided by random dispersion to a number of, typically vibrating, linear feeders 12, typically corresponding to the number of heads of the multihead weigher 10. At the end of each linear feeder 12, the particulate product is typically collected in a storage hopper 13, also occasionally referred to as a feeding hopper, and when the corresponding weighing hopper 14 is empty, delivered as a portion to the weighing hopper 14, which determines the weight of the portion. A central processing unit monitors the different weights determined by the different weighing hoppers 14, and when a number of weighing hoppers contain portions which in combination amount to an acceptable weight, it triggers those weighing hoppers to substantially simultaneously deliver their portions into a collection chute 15, 16, wherefrom the combined portion of acceptable weight may delivered to postprocessing means 18, e.g. packaging means, sorting means, quality control, etc., possibly directly subsequently to leaving the chute 15, 16. In an alternative embodiment, the storage hoppers 13 are omitted, and the particulate product delivered to the weighing hoppers 14 directly from the linear feeders 12. By providing the storage hoppers 13 as intermediate collectors is however facilitated that full portions may be delivered to the weighing hoppers immediately after they are emptied, except in the few cases where a particular weighing hopper is emptied at intervals too short for the corresponding storage hopper 13 to collect a significant portion of particulate product. In other alternative embodiments even further storage hoppers, or other intermediate feeders, hoppers, etc., may be provided to further increase the effect and swiftness of the multihead weigher. In alternative embodiments there may be provided more or less intermediate feeding means than there are heads, i.e. weighing hoppers 14, in the multihead weigher. In such an embodiment a linear feeder 12 may, e.g. deliver to two or more storage hoppers 13. In yet alternative embodiments, more than one level of weighing hoppers and division or combination of portions may be provided, in order to achieve increased precision or faster flow.
Figure 2 is an example of an embodiment of a multihead weigher 10. It comprises a central, vibrating dispersion feeder 11, from where a particulate product may be distributed to a number of vibrating linear feeders 12. At the end of each linear feeder is a storage hopper 13 for temporarily collecting portions of the product. Below each storage hopper 13 is a weighing hopper for determining the weight of a portion of the product. When the weighing hoppers 14 are emptied, the products are catched by a chute 15, 16 or combination of skids and a chute, and delivered to packaging means or transportation means located below the chute. As the multihead weigher 10 of figure 2 comprises 24 weighing hoppers 14, it is said to have 24 heads.
Figure 3 illustrates in more detail an example of an embodiment of a feeding mechanism for one head of a multihead weigher 10, seen from the side. It comprises a vibrating dispersion feeder 11 , which is common to all the heads of the multihead weigher. The dispersion feeder 11 is preferably conical, so particulate products placed on the feeder is transported to its outer rim by a combination of vibration and gravity, but any means for distributing the particulate product to the feeders of the different heads are within the scope of the present invention. Figure 3 further comprises a vibrating linear feeder 12 for receiving a part of the particulate product delivered to the dispersion feeder. The linear feeder 12 is typically mounted so that a small part of it extends under the dispersion feeder 11 to ensure that the entire particulate product is catched, and with its bottom surface being slightly inclined downwards away from the dispersion feeder. Thereby vibration and gravity cause the particulate product to travel away from the dispersion feeder. The linear feeder 12 is furthermore typically provided with feeder sidewalls 128 to ensure that the particulate product does not fall off the sides of the feeder. This is more clearly seen in figure 2. It is noted, that any type and design of a feeder suitable for delivering particulate products from a dispersion feeder to a storage- or weighing hopper, is within the scope of the present invention. When the particulate product leaves the far end of the linear feeder 12, it is typically collected in storage hopper 13, before being delivered to a weighing hopper 14. The hoppers are arranged with suitable means for fastening, weighing, emptying, etc., according to a typical multihead weigher embodiment, and any hopper means and hopper supporting and control means are within the scope of the present invention.
Figure 3 further comprises a dispersion feeder vibrator 110 and a linear feeder vibrator 120, which in a preferred embodiment are both mounted to a mounting plate 30 of the multihead weigher 10. Both vibrators 110, 120 comprise an electromagnet 111, 121 provided with an iron core, for establishing an alternating magnetic field when supplied with alternating, or preferably pulsed, current, and an armature 112, 122 arranged at a small distance from the electromagnet, and substantially perpendicular to the magnetic force established by the electromagnet. The armature is made of a material subject to reaction to a magnetic force, typically a ferromagnetic material. The electromagnets are preferably driven by pulse width modulated vibrator controls, or similar types of controls, in which it is possible to alter the pulsed current frequency by which the vibrators are driven. The electromagnets 111, 121 are preferably each securely fixed to a base 117, 127, mounted on the mounting plate 30. A number of flexible springs 113, 114, 115, 123, 124, are provided for keeping the armature in position. To the same flexible springs are also fixed feeder mounts 116, 126, which thus move together with the armatures. At the feeder mounts are mounted the dispersion feeder 11 and linear feeders 12, respectively, so that they also move in correspondence with the armatures.
Figure 4 illustrates in more detail an embodiment of a linear feeder vibrator 120 of a multihead weigher. It comprises a base 127 on which an electromagnet 121 is mounted. The electromagnet preferably comprises an iron core 41 and a coil 40 of insulated, electrically conducting wire. When a pulsed current is provided to the coil, a magnetic field which reciprocates as illustrated by the arrow 42 is established. To the base is also fastened the ends of one or more flexible springs 123, 124, preferably two leaf springs, preferably made of laminated epoxy and glass sheets, e.g. as obtainable from 3M under the trade name Scotchply or from Red Sheet Electric Company under the trade name Cyply, but alternatively made of any suitable material. At the other end of the springs are fastened a feeder mount 126 and an armature 122. The armature is made of a material which reacts to magnetic force, and because of the flexible mounting, it will reciprocate in accordance with the reciprocation of the magnetic field established by the electromagnet, and thereby cause the feeder mount 126 to reciprocate or vibrate substantially as indicated by the arrow 43. The reciprocation frequency is directly proportional to the frequency of the alternating or pulsed current, whereas the power of the vibrating movement is determined by several parameters, such as how well the armature engages with the magnetic field, the type, condition and mounting of the springs, the weight of the parts mounted to the springs, e.g. the linear feeder, the spring constant of the system, etc. The entire vibrating part of the system, i.e. the springs, armature, feeder mount, feeder and possible contents placed thereon, determine a resonance frequency at which the highest vibrating power is established from a certain amount of electrical power. At other frequencies the efficacy of the system is less optimal.
Figure 5 illustrates in more detail an embodiment of a dispersion feeder vibrator 110 of a multihead weigher. It comprises a base 117 on which an electromagnet 111 is mounted. The electromagnet preferably comprises an iron core 51 and a coil 50 of insulated, electrically conducting wire. When a pulsed current is provided to the coil, a magnetic field which reciprocates as illustrated by the arrow 52 is established. To the base is also fastened the ends of one or more flexible springs 113, 114, 115, preferably three leaf springs, preferably made of laminated epoxy and glass sheets, e.g. as obtainable from 3M under the trade name Scotchply or from Red Sheet Electric Company under the trade name Cyply, but alternatively made of any suitable material. At the other end of the springs are fastened a feeder mount 116 and an armature 112. The armature is made of a material which reacts to magnetic force, and because of the flexible mounting, it will reciprocate in accordance with the reciprocation of the magnetic field established by the electromagnet, and thereby cause the feeder mount 116 to reciprocate or vibrate substantially as indicated by the arrow 53, i.e. by a combination of vertical and rotational movement. The reciprocation frequency is directly proportional with the frequency of the pulsed or alternating current, whereas the power of the vibrating movement is determined by several parameters, such as how well the armature engages with the magnetic field, the type, condition and mounting of the springs, the weight of the parts mounted to the springs, e.g. the dispersion feeder, the spring constant of the system, etc. The entire vibrating part of the system, i.e. the springs, armature, feeder mount, feeder and possible contents placed thereon, determine a resonance frequency at which the highest vibrating power is established from a certain amount of electrical power. At other frequencies the efficacy of the system is less optimal.
Hence, the dispersion feeder and the linear feeders of a multihead weigher each determine an individual resonance frequency at which they work most efficiently, and as described above, the resonance frequency for a certain feeder depends on several parameters. Because those parameters change over time, the resonance frequency fluctuates. Over relatively long time, typically months, the resonance frequency changes because of permanent changes such as, e.g. spring characteristics changing due to wear, moisture, friction, dirt accumulation, etc. Over short time, e.g. hours or even minutes, the resonance frequency may change because of momentary changes such as, e.g., moisture, temperature and load variations, etc.
Figure 6 illustrates a common driving concept for vibrators. It comprises an electromagnet 111, 121 to be driven by an alternating or preferably pusled current in order to establish vibration of an armature, and a vibrator controller 60 for establishing the driving current. The controller 60 comprises a driver signal generator 61, e.g. an AC generator, a pulse width modulator, etc., and an amplifier 62, e.g. a transistor, a switch-mode amplifier, etc. The driver signal generator 61 establishes a small-signal representation of a desired driving signal, and the amplifier 62 steps up the signal to an appropriate level. The driver signal generator 61 is preferably adjustable as regards the frequency of the driver signal. Thereby the vibration frequency of the corresponding feeder may at any time be changed to the currently actual resonance frequency of that feeder, preferably independent of any of the other feeders, provided the actual resonance frequency is known or can be determined, e.g. by trial and error.
Due to the possible momentary variations of the resonance frequency of a certain feeder, typically due to temperature variations or different load during a working day, the determination of the actual resonance frequency and corresponding regulation of the vibration frequency should, however, preferably be performed as often as possible, preferably several times an hour, or even continuously.
The resonance frequency of the vibrator can be determined as the frequency at which the coil current is strongest and thus, where the system is provided with the most effect. The coil current is a manifestation of the job performed by the vibrator; the higher the amplitude on the vibrator, the higher the current through the vibrator coil. Figure 7 illustrates a typical correspondence between the driver signal frequency fdnver and the current through the coil of the electromagnet lcoι\. The resonance frequency fresonance of the system is the driver signal frequency at which the coil current is strongest. In a typical vibrator, the resonance frequency may, e.g., be 44.2Hz, and the coil current at the resonance frequency be, e.g., 1.96A. A significant change in the coil current may typically be observed within a change of IHz of vibration frequency. Thus, within in the present example, the coil current may have dropped to, e.g., 1.73 A at a vibration frequency of 43.6Hz, and 1.78A at 45.0Hz. It is noted, however, that any kinds and embodiments of vibrators with any resonance frequency and current/frequency-relationships are within the scope of the present invention.
According to a preferred embodiment of the present invention, it is thus possible to determine the resonance frequency of a vibrator by altering the driver frequency, i.e. the frequency of the pulsed current driving the electromagnet, while the current through the coil is measured simultaneously. While measuring, the amount of content, e.g. a particulate product, on the feeder should if possible, preferably be kept at an approximately constant level, e.g. empty, in order to not influence the resonance frequency. Figure 8 illustrates a preferred embodiment of the present invention. It comprises an electromagnet 111, 121 which drives a vibrating feeder. It further comprises an adaptive vibrator controller 70 for establishing the driving pulsed current for the electromagnet. The adaptive controller 70 comprises an adjustable frequency driver signal generator 71, e.g. an AC generator, a pulse width modulator, etc., and an amplifier 62, e.g. a transistor, a switch-mode amplifier, etc. The adjustable frequency driver signal generator 71 establishes a small-signal representation of a desired driving signal, and the amplifier 62 steps up the signal to an appropriate level. In an alternative embodiment the generator may establish a large-signal representation, in which case no amplification is needed. The shape of the driving signal is preferably a pulse width modulated signal (PWM signal), but may be any kind of alternating or pulsed signal such as, e.g., square, triangle, sawtooth or sine waves, pulse density modulated signals, etc., and it may be analog or digital. The type of amplifier is insignificant as long as it is suitable for amplifying the kind of driver signal established by the generator to a level suitable for driving the vibrator. The adjustable frequency driver signal generator 71 is adjustable as regards the frequency of the driver signal. Thereby the vibration frequency of the corresponding feeder may at any time be changed to the currently actual resonance frequency of that feeder. The adaptive vibration controller 70 further comprises a current detector 73 for measuring the current through the coil. A representation of the current, e.g. a voltage level or a digital value proportional to the measured current value, is received by a processing unit 74, which processes the measured current and accordingly adjusts the driver signal frequency by controlling the adjustable frequency driver signal generator 71. The processing unit 74 may be any kind of means suitable for receiving a measured value or signal and on the basis thereof establishing a control value or signal, e.g. a microprocessor, a digital signal processor, a programmable gate array, etc. In an embodiment of the present invention, the processing unit merely maps measured current values into appropriate frequency control values. In a preferred embodiment of the present invention, the processing unit comprises means for performing different control algorithms, e.g. one or more algorithms for finding the resonance frequency for initialization or at certain intervals, one or more algorithms for tracking the resonance frequency during normal use, and possibly further algorithms for test purposes, for certain uses of the multihead weigher, for considering further parameters, e.g. amount of contents, etc. In a preferred embodiment, the processing unit further comprises user input means for enabling a user to select a desired control algorithm or setting control parameters, and communication means for enabling the processing unit to receive commands, e.g. algorithm selection or parameter settings, or transmit values or commands, from/to a central processing unit of the multihead weigher, a central control computer, the other adaptive vibrator controllers, etc.
Hence, an adaptive vibrator controller 70 is provided, which comprises feedback in order to automatically optimise the driver signal frequency, preferably according to the currently actual resonance frequency of the vibrator, and thereby optimise the overall system efficacy.
By providing a multihead weigher with adaptive vibration controllers according to a preferred embodiment of the present invention, it is possible to determine the optimal driver frequency for each feeder, e.g. by sweeping through a frequency range, to continue driving the feeders at their respective resonance frequencies, e.g. by gradient control, as well as to determine the approximate amount of content, e.g. particulate products, currently processed by each feeder.
Different algorithms may be provided for determining the currently actual resonance frequency of a vibration feeder according to an embodiment of the present invention. One type of algorithm comprises sweeping through the frequency range in which the resonance frequency is expected to be, and simultaneously monitor the coil current. The sweeping may be performed by one thorough sweep, or by performing several sweeps with frequency values of increasing resolution but in frequency bands of decreasing width. The sweep method is most useful for initialization purposes, for testing once in a while, or when other, more optimal algorithms of the processing unit have lost track of the resonance frequency. Because of the relatively long time needed by sweep algorithms, and because of the very ineffective vibration produced at many of the tested frequencies, and hence for relatively long time, the use of a sweep algorithm during normal productive use of a multihead weigher is in many embodiments unacceptable. Hence the sweep algorithms may be available for manual selection by a technician, operator or cleaning crew, or for automatic performance at certain intervals, e.g. during cleaning or maintenance breaks, etc. Therefore the sweep algorithms are in most circumstances not optimal for counteracting short-term variations of the resonance frequency, whereas they are excellent for automatically dealing with long-term variations over weeks or months.
In an embodiment of the present invention, the adaptive vibrator control comprises modified sweep algorithms that are better suited for more frequent use and use during productive operation of the multihead weigher. The amplification by the amplifier 62 may, e.g., be decreased or turned off during sweep, so that the magnetic field established is not powerful enough to cause a significant vibration, but still enabling the current detector to measure a current through the coil. Thereby the product that possibly lies on the feeder is not affected by the sweep, and the sweep may be performed faster as the mechanical reciprocation is far less significant and the current measurement thus faster responding to adjustments. According to an embodiment of the present invention, a sweep algorithm is performed at the different feeders at different times during normal productive use. In a multihead weigher with, e.g., 24 weighing heads, and thus typically the same number of linear feeders, the sweeps for the resonance frequencies may be performed sequentially for the different feeders, and the productivity is thus only reduced by 1/24 during sweeping. Such sequential sweeping by different vibrator controllers may, e.g., be controlled by a central processing unit of the multihead weigher, or by the vibrator controllers communicating with each other.
A further modified sweep algorithm optimised for use during normal productive operation of the multihead weigher comprises performing sweeps for feeders whose corresponding weighing hoppers are being reset or not being relevant for combination in the current packing cycle, and therefore not requiring optimal product movement at first. An alternative embodiment of this algorithm causes a sweep to be started for each feeder for which the corresponding weighing hopper complies with the above, and just halt the sweep prematurely if it turns out that the weighing hopper is to be used and thus have to be filled again fast. Yet an alternative embodiment of this algorithm provides for memorizing the state of a sweep if ended prematurely, and thus allows continuing the sweeping from that state at the next time that feeder is idle.
An alternative type of algorithms for determining the currently actual resonance frequency of a vibration feeder according to preferred embodiments of the present invention comprises gradient control and tracking the resonance frequency instead of seeking for it. In a preferred embodiment this gradient control algorithm comprises continuously adjusting the vibrator frequency a small amount to each side of the present vibrator frequency. By comparing the coil current for a frequency at each side of the present frequency, it is possible to determine to which side the resonance frequency is, and thereby how to adjust the vibrator frequency in order to move towards the resonance frequency. By repeating this, the resonance frequency can be found and tracked as long as the measurements to each side are performed at short intervals. The measurements to each side of the present frequency should preferably be performed at a frequency close enough to not render the vibration too inefficient, but still far enough to obtain current values that are sufficiently distinct to obtain an indication of the direction of the resonance frequency.
In a preferred embodiment, the change of vibration frequency is synchronized with the filling cycles, i.e. the emptying of the storage hopper associated with the vibration feeder, so that, e.g., a first filling cycle is performed at one frequency, the second filling cycle is performed at a second frequency according to the gradient control algorithm, the third filling cycle is performed at a third frequency according to the gradient control algorithm, etc. This prevents reduction of the efficiency of the weigher due to the continuous frequency adjustment, as it actually, on the contrary, ensures optimal efficiency at all times. Figure 9 comprises an example of seeking the resonance frequency by a gradient control algorithm. At first the present vibrator frequency is marked by 1, and a current measurement performed. Then the frequency is changed a little to one side, here to a lower frequency 2, and a new current measurement performed. The second measurement indicates a lower current than the first measurement, so a higher frequency 3 is selected, and a third measurement performed. This measurement indicates a stronger current than the first two, and so the frequency is increased again to frequency 4, and yet again to frequency 5 and 6. The current measured at frequency 6 is however lower than that at frequency 5, so the vibrator frequency is changed back to frequency 5. By decreasing the size of the frequency changes it is possible to determine the resonance frequency with a higher precision. When the resonance frequency is determined with acceptable precision, the algorithm may, e.g., just perform a measurement at each side of the determined frequency at certain, preferably small intervals, in order to glue to the resonance frequency.
In an embodiment of the present invention the vibration frequency may initially be set to, e.g., 43.8Hz. A result of current measurements during 3 filling cycles may indicate an average coil current of 1.85 A for the vibration frequency of 43.8Hz. The next 3 filling cycles may be performed with a vibration frequency of 44.0Hz, i.e. a step of +0.2Hz, and the average measured current may be 1.95 A, indicating that 44.0Hz is closer to the resonance frequency than 43.8Hz. Hence, the vibration frequency is changed by another step of +0.2Hz to 44.2Hz, and current measurements are performed during the next 3 filling cycles. This may give an average current of 1.96 A, which is slighter higher than the result for a frequency of 44.0Hz, and thus indicating 44.2Hz being slightly closer to the resonance frequency. Consequently, the vibration frequency may be changed by another step of +0.2Hz to 44.4Hz, whereby the average current during 3 filling cycles may be 1.93 A, i.e. less than at the frequency of 44.2Hz and indicating that the resonance frequency is closer to 44.2Hz, than 44.2Hz. The optimal vibration frequency to use is thus 44.2Hz. In order to monitor when the optimal frequency drifts, the current measurements should preferably be performed continuously, and at certain intervals should the vibration frequency be by changed by alternately -0.2Hz or +0.2Hz and current measurements be made at those frequencies. While the optimal frequency is 44.2Hz, the algorithm may thus, e.g., cause a sequence of 3 filling cycles at 44.2Hz, 3 filling cycles at 44.0Hz, 3 filling cycles at 44.2Hz and 3 filling cycles at 44.4Hz to be repeated until either 44.0Hz or 44.4Hz proves more efficient than 44.2Hz.
It is noted, however, that any kinds and embodiments of vibrators with any resonance frequency and current/frequency-relationships, and any particular embodiment of the gradient control algorithm are within the scope of the present invention.
In a preferred embodiment of the present invention, the determination of the resonance frequency according to the gradient control method is performed several times, and preferably a certain number of past results are memorized in order to allow establishing an average, e.g. covering the last few minutes. Thereby the really fast varying parameters such as the amount of product on the feeder may be eliminated from the resonance frequency determination. In a preferred embodiment, a new value for the resonance frequency is determined by means of an averaged gradient control algorithm as described above about once or twice an hour during full operation of the multihead weigher, without influencing the vibration efficiency notably at any time, but on the contrary, in fact increasing the altogether efficacy of the multihead weigher over short time, such as a day, as well as over long time, such as a month.
If the gradient control algorithm as described above is unable to measure a difference to either side of the present frequency, it may move the vibrator frequency farther and farther to one side, until a change is measured and the direction to the resonance frequency thus can be determined, or until a predetermined end point of the frequency range is reached, and the measurements must be performed to the other side instead.
Evidently, numerous combinations and rules for managing a gradient control algorithm may be established within the scope of the present invention, e.g. rules for choosing a starting value, rules for deciding whether to increase or decrease frequency and by which amount, rules for establishing an averaged measurement, rules for determining when the resonance frequency is considered changed, etc. All such rules and combinations thereof, and variants of the gradient control algorithm, and any other algorithms for tracking the resonance frequency are within the scope of the present invention.
According to yet an alternative embodiment of the present invention, also the approximate amount of content, e.g. particulate products, currently processed by each feeder, may be determined. At a given driver signal frequency there exists a correlation between the coil current Icoιι and the amount of products WprodUct on the feeder. Hence, the more product on, e.g., the linear feeder, the less is the coil current. An example of this correlation is illustrated in figure 10.
The correlation between the coil current and the amount of particulate products in the linear feeders, at a certain frequency, can, among other things, be used to ensure a more equal dosage of partial portions which is a condition for high speed and for obtaining the desired target weight of the output packages of the multihead weigher.
Traditionally, regulation of vibrators in order to increase or decrease product movement according to the actual demands, is based on feedback from the weighing hopper, as illustrated in figure 11. The starting point is the desired weight 93 of product that is moved into the weighing hopper for each cycle. This value is compared with the actual weight 96 in the weighing hopper, and the difference 94 is used as intensity or time controlling input to the vibrator controller 61, which thereby increases or decreases the vibration intensity or vibration time 95 accordingly.
However, due to the vibrator transfer function 90, the vibration delay 91 for vibrating products into the storage hopper and the storage hopper delay 92 for waiting for the weighing hopper to be ready, cause typically at least 2 weighing cycles to pass before the regulated vibration is experience by the weighing hopper, and the feedback can trigger the appropriate regulation. Hence, the traditional vibration intensity or vibration time regulation is problematic because there is a time delay of minimum 2 filling cycles before changes, if any, are discovered. When a change in the layer of the particulate products has been acknowledged, e.g. by overloading of a weighing hopper, the problem is already a fact in that the weighing hopper is overloaded and at the same time, the storage hopper is most likely also overloaded, as the storage hopper, in consideration of speed, is filled by the vibrators before the weighing result is known. The result of a sudden rise in the layer of product particles will consequently be an overload of a storage hopper and a weighing hopper, during at least 2 cycles.
In a preferred embodiment of the present invention, the coil current is used as a regulative parameter in controlling the vibration intensity and/or vibration time, thus making it possible to obtain information about the changes in the layer of particulate products on a certain feeder before the products reach the corresponding hopper. If the coil current suddenly drops, at the same working frequency, an increase in the layer of products on the feeder is indicated, and thereby, it is possible to adjust, in that case turn down, the vibration before an overloading of the hoppers takes place. Figure 12 illustrates a preferred embodiment of the present invention comprising a coil current based vibrator intensity regulation. As in figure 11 , the starting point is the desired weight 93 of product that is moved into the weighing hopper for each cycle. This value is compared with the actual weight 96 in the weighing hopper, and the difference 94 is used as intensity or time controlling input to the vibrator controller 71, which thereby increases or decreases the vibration intensity or vibration time 95 accordingly. Contrary to the traditional regulation of figure 11, the preferred embodiment of figure 12 however further adjusts the vibrator intensity and/or vibrator time according to a representation of the current product layer on the feeder determined by means of the coil current detector and/or processing unit 74. Thereby any product layer variations may affect the vibration intensity immediately. By using the coil current, which indicates the layer of product on the feeder when the vibration frequency is fixed, as well as the weighing hopper feedback, it is, e.g., possible to adjust the changes of the layer of products before the weighing hopper becomes overloaded. Thereby, it is possible to avoid having to reject overloaded hoppers, i.e. empty the overloaded hoppers, e.g. into a rejection channel, at the expense of accuracy and speed. Furthermore, the present embodiment allows turning the vibration intensity completely up during start-up when, often, only a little amount of product is available, as it is possible to monitor the amount of product entering the vibrator feeder by means of the current measurement, and thus turn down the vibration intensity again, before the hoppers get overloaded.
In embodiments where the portioning is based primarily on number of pieces instead of weight, e.g. where the particulate products comprise relatively large particles, e.g. pieces of chicken or other poultry, etc., the option of monitoring changes in the amount of product on each vibration feeder according to the above-described embodiment, may further be utilized for determining when a piece has dropped from the vibration feeder into a hopper, and by using this information as input for a counter or other monitoring means, to count the number of pieces in each hopper. Thereby the processing means of the multihead weigher may control the portioning on the basis of weight, number of pieces or a combination of thereof, e.g. by choosing hoppers that together contain, e.g., 10 pieces and best match a desired target weight, e.g. 1 kg, to combine into a portion. The step of determining when a piece is added or subtracted from a vibration feeder, i.e. placed on the feeder or dropped into a hopper, may be performed by observing significant changes in the amount of product on the feeder. If the product pieces are of a significant weight, a significant change in weight indicates a change in number of pieces. If the approximate average weight of the pieces is known, e.g. as an interval, e.g. 80-120 g, it is further possible to determine the number of pieces that is added or subtracted simultaneously, and in certain embodiments and nature of product pieces and their variation even determine the number of pieces currently placed on the vibration feeder.
Generally, the extra information concerning the amount of particulate products on the feeders provides the possibility of performing a much more sophisticated control of the vibrators than possible by using the weight in the weighing hopper only as feedback for the vibrator control. Furthermore, by comparing the coil current measurements from all feeders in the multihead weigher locally, it is possible to analyze whether the layer of products is evenly distributed in the machine. If the layer of products is not evenly distributed, the machine will automatically increase the layer of product particles in order to have product particles in all linear feeders. Feedback may be given to an operator, or an automatic control of the in- feeder may be provided.
The reference signs used in the description and drawings comprise
10 multihead weigher
11 dispersion feeder
12 linear feeder
13 storage hopper
14 weighing hopper
15, 16 skids and/or chute
17 in-feeder
18 post-processing means
30 mounting plate
40, 50 coil
41, 51 iron core
42, 52 reciprocating magnetic force
43, 53 vibration of feeder mount
60 vibrator controller
61 driver signal generator
62 amplifier
70 adaptive vibrator controller
71 adjustable frequency driver signal generator
73 current detector
74 processing unit
90 vibrator transfer function
91 vibration delay
92 storage hopper delay
93 desired weight
94 weight difference
95 vibration intensity and/or vibration time
110, 120 feeder vibrator
111, 121 electromagnet
112, 122 armature
113, 114, 115, 123, 124 flexible springs 116, 126 feeder mount 117, 127 base
128 feeder sidewall
J driver frequency of driver signal
Jresonance resonance frequency lcoil current through the coil

Claims

Patent Claims
1. Method of operating a multihead weigher (10) comprising at least one vibration driven feeder (11, 12) comprising at least one vibrator (110, 120), said method comprising measuring electrical properties of said at least one vibrator, and on the basis thereof determining at least one control parameter of said multihead weigher.
2. Method of operating a multihead weigher (10) according to claim 1, whereby said at least one control parameter comprises at least one control parameter of said at least one vibration driven feeder (11, 12).
3. Method of operating a multihead weigher (10) according to claim 1 or 2, whereby said at least one control parameter comprises the vibration frequency of at least one of said vibrators (110, 120).
4. Method of operating a multihead weigher (10) according to any of the claims 1 to 3, whereby said determining at least one control parameter comprises determining an optimal vibration frequency for at least one of said vibrators (110, 120) on the basis of said electrical properties.
5. Method of operating a multihead weigher (10) according to claim 4, whereby said determining an optimal vibration frequency for at least one of said vibrators (110, 120) comprises measuring said electrical properties of said at least one vibrator at different vibration frequencies.
6. Method of operating a multihead weigher (10) according to claim 4 or 5, whereby said optimal vibration frequency substantially corresponds to the resonance frequency of said vibration driven feeder (11, 12).
7. Method of operating a multihead weigher (10) according to any of the claims 4 to 6, whereby said optimal vibration frequency substantially corresponds to the resonance frequency of said vibration driven feeder (11, 12) when loaded with an average amount of particulate products.
8. Method of operating a multihead weigher (10) according to any of the claims 1 to 7, whereby at least one of said vibration driven feeders (11, 12) comprises a linear feeder (12).
9. Method of operating a multihead weigher (10) according to any of the claims 1 to
8, whereby at least one of said vibration driven feeders (11, 12) comprises a dispersion feeder (11).
10. Method of operating a multihead weigher (10) according to any of the claims 1 to
9, whereby said at least one vibrator (110, 120) comprises a coil (40, 50).
11. Method of operating a multihead weigher (10) according to any of the claims 1 to
10, whereby said measuring electrical properties comprises measuring the current in said coil (40, 50).
12. Method of operating a multihead weigher (10) according to any of the claims 5 to 11, whereby said measuring at different vibration frequencies comprises sweeping through an interval of vibration frequencies.
13. Method of operating a multihead weigher (10) according to claim 12, whereby said interval of vibration frequencies comprises a predetermined interval.
14. Method of operating a multihead weigher (10) according to claim 12, whereby said interval of vibration frequencies is determined adaptively on the basis of said measurements.
15. Method of operating a multihead weigher (10) according to any of the claims 5 to 14, whereby said measuring at different vibration frequencies comprises measuring at different frequencies within a dynamic interval comprising an estimated optimal vibration frequency.
16. Method of operating a multihead weigher (10) according to any of the claims 1 to 15, whereby said method is repeated in order to adaptive Iy maintain said at least one control parameter at an optimal value.
17. Method of operating a multihead weigher (10) according to any of the claims 4 to 16, whereby said method is repeated in order to adaptively maintain an optimal vibration frequency.
18. Method of operating a multihead weigher (10) according to claim 16 or 17, whereby said method is repeated continuously.
19. Method of operating a multihead weigher (10) according to claim 16 or 17, whereby said method is repeated at predetermined intervals.
20. Method of operating a multihead weigher (10) according to claim 16 or 17, whereby said method is repeated at predetermined operational states of said multihead weigher.
21. Method of operating a multihead weigher (10) according to claim 16 or 17, whereby said method is repeated at predetermined operational states of at least one of said vibration driven feeders (11, 12).
22. Method of operating a multihead weigher (10) according to any of the claims 15 to 21, whereby said estimated optimal vibration frequency comprises a recently determined optimal vibration frequency.
23. Method of operating a multihead weigher (10) according to any of the claims 15 to 21, whereby said estimated optimal vibration frequency comprises a predetermined frequency.
24. Method of operating a multihead weigher (10) according to any of the claims 5 to 23, whereby said measuring at different vibration frequencies comprises coarsely sweeping through a first interval of vibration frequencies and determining the most optimal vibration frequency of those, then finely sweeping through a second interval of vibration frequencies, whereby said second interval is narrower that said first interval, and said first interval at least partly overlaps said second interval.
25. Method of operating a multihead weigher (10) according to any of the claims 11 to 24, whereby said optimal vibration frequency is determined as the frequency where said measured current in said coil (40, 50) is largest.
26. Method of operating a multihead weigher (10) according to any of the claims 1 to
25, whereby said method is initiated by a user.
27. Method of operating a multihead weigher (10) according to any of the claims 1 to
26, whereby said at least one vibration driven feeder (11, 12) is empty while said method is carried out.
28. Method of operating a multihead weigher (10) according to any of the claims 1 to
27, whereby said vibration driven feeder (11, 12) is in use while said method is carried out.
29. Method of operating a multihead weigher (10) according to any of the claims 1 to 28, whereby said multihead weigher comprises at least one dispersion feeder (11) and at least 10, preferably at least 14, and even more preferably at least 24 linear feeders (12).
30. Method of operating a multihead weigher (10) according to any of the claims 1 to 29, whereby said at least one vibrator (110, 120) is driven by low power while said measuring electrical properties of said at least one vibrator is performed.
31. Method of operating a multihead weigher (10) according to any of the claims 1 to 30, whereby said driven by low power comprises driving said at least one vibrator (110, 120) at a power low enough to substantially not move any particulate products comprised by said vibration driven feeder (11, 12).
32. Method of operating a multihead weigher (10) according to any of the claims 5 to
33. whereby said measuring at different vibration frequencies comprises performing measurements and changing vibration frequencies according to a gradient control algorithm.
33. Method of operating a multihead weigher (10) according to claim 32, whereby said gradient control algorithm comprises rules for finding and tracking said optimal vibration frequency.
34. Method of operating a multihead weigher (10) according to claim 32 or 33, whereby according to said gradient control algorithm the vibration frequency is alternately at predetermined intervals changed up and down relative to the last known optimal vibration frequency in order to detect changes in the optimal vibration frequency.
35. Method of operating a multihead weigher (10) according to any of the claims 1 to
34, whereby said determining at least one control parameter comprises determining the current product load on at least one of said vibration driven feeders (11, 12) on the basis of said electrical properties.
36. Method of operating a multihead weigher (10) according to claim 35, whereby the vibration frequency of at least one of said vibration driven feeders (11, 12) is controlled on the basis of said current product load.
37. Method of operating a multihead weigher (10) according to claim 35 or 36, whereby a change in the number of product pieces on at least one of said vibration driven feeders (11, 12) is determined on the basis of said current product load.
38. Method of operating a multihead weigher (10) according to any of the claims 35 to 37, whereby the number of product pieces on at least one of said vibration driven feeders (11, 12) is determined on the basis of said current product load.
39. Method of operating a multihead weigher (10) according to any of the claims 35 to 38, whereby determining the current product load on at least one of said vibration driven feeders (11, 12) on the basis of said electrical properties comprises measuring the current in at least one coil (40, 50) comprised by said at least one vibrator (110, 120).
40. Method of operating a multihead weigher (10) according to any of the claims 35 to 39, whereby said vibration driven feeders (11, 12) are controlled on the basis of said current product load so as to prevent overload of storage hoppers (13) or weighing hoppers ( 14) of said multihead weigher.
41. Method of operating a multihead weigher (10) according to any of the claims 37 to 42, whereby said multihead weigher establishes portions of products at least partly based on said determined change in number of product pieces or said number of product pieces on said vibration driven feeders (11, 12).
42. Multihead weigher (10) comprising at least one vibration driven feeder (11, 12), each vibration driven feeder comprising at least one vibrator (110, 120), characterised in that said multihead weigher further comprises at least one measuring means (73) for determining electrical properties of said at least one vibrator (110, 120).
43. Multihead weigher (10) according to claim 42, wherein said at least one vibrator (110, 120) comprises at least one coil (40, 50).
44. Multihead weigher (10) according to claim 43, wherein said measuring means (73) comprises at least one current detector (73) for measuring the current in at least one of said at least one coil (40, 50).
45. Multihead weigher (10) according to any of the claims 42 to 44, comprising at least one processing unit (74) for determining an optimal vibration frequency of said at least one vibrator (110, 120) on the basis of said electrical properties.
46. Multihead weigher (10) according to any of the claims 42 to 45, comprising at least one processing unit (74) for determining the current product load on said at least one vibration driven feeder (11, 12).
47. Multihead weigher (10) according to any of the claims 42 to 46, wherein at least one of said vibration driven feeders (11, 12) is a dispersion feeder (11) and at least 10, preferably at least 14, and even more preferably at least 24 of said vibration driven feeders (11, 12) are linear feeders (12).
48. Multihead weigher (10) according to any of the claims 42 to 47, comprising control means for operating said multihead weigher according the method of any of the claims 1 to 41.
PCT/DK2007/050074 2006-06-19 2007-06-14 Multihead weigher and method of operating such multihead weigher WO2007147419A1 (en)

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