EP2547909B1

EP2547909B1 - Dosing pump with control device of the piston stroke

Info

Publication number: EP2547909B1
Application number: EP11721379.3A
Authority: EP
Inventors: Roberto Proietti De Marchis
Original assignee: ETATRON D S SpA
Current assignee: ETATRON D S SpA
Priority date: 2010-03-17
Filing date: 2011-03-16
Publication date: 2016-01-06
Anticipated expiration: 2031-03-16
Also published as: PL2547909T3; IL239085A; EA024119B1; CN102803726A; IL221644A0; DK2547909T3; ES2567407T3; WO2011114365A8; WO2011114365A1; IL221644A; ITRM20100119A1; UA111469C2; EP2547909A1; US20120321485A1; EA201290916A1; JP5984304B2; BR112012021837A2; IL239085A0; CN102803726B; JP2013522531A

Description

The present invention basically relates to the sector of electromechanical-actuation metering pumps for metering liquids.
Said metering pumps are used for metering additives of various sorts, by means of a regulation of a volumetric type of the fluid pumped at each cycle. They are devices that are widely used in numerous both industrial and domestic applications, such as for example:

treatment of drinking water;
public and private waterpools;
car-wash facilities;
laundries;
galvanic equipment;
chemical industry;
cooling towers;
fertirrigation;
agro-alimentary industry;
etc.

Entering into the detail of operation of a generic metering pump, it should be noted that metering occurs by means of a pulsating movement of a mechanical member present in the so-called pump body, referred to as "membrane", which, pushed by a piston, injects the liquid to be metered into the purposely provided delivery passage of the pump body. The liquid to be metered is then introduced into the system to be treated, overcoming the counterpressure present in the piping in which it is injected.
The key element that must provide the force necessary in order for the liquid to be metered to be effectively mixed with the liquid to be treated is hence the piston, which constitutes the component designed to move the membrane that sucks in the liquid in the pump body from the intake pipe and injects it into the delivery pipe in a cyclic way.
Said piston is actuated by an electromagnet, which hence constitutes a fundamental part of the pump. The electromagnet is made up of a fixed part, housed in which is the armature, and a mobile part - referred to as "plate" - fixed with respect to the piston.
It should be noted that the plate constitutes, in effect, the closing element for the magnetic flux of the electromagnet, said flux having as effect that of recalling said plate to the remaining fixed part of the electromagnet, thus producing a displacement.
In other words, the electromagnet hence enables conversion of electrical energy into mechanical energy to obtain work and move the liquid.
The electrical and mechanical characteristics of the metering pump hence depend upon how said electromagnet is designed, driven, and controlled. The electronic card present in the pump will do nothing but supply the electromagnet and manage the electrical energy supplied thereto in the best way.
It is thus clear that the better said control is performed the higher the efficiency of the metering pump. This is a very important factor when, once the metering to be performed on the system to be treated has been established, it is necessary to perform said metering in a continuous way and with the minimum consumption of electrical energy.
Certain applications in effect require the metering pump to deliver small amounts of liquid at each injection, hence requiring a high number of injections so as to be able to meter the volume required for carrying out the treatment. Said target can be achieved by regulating the piston stroke causing that given the same injections, the volume of liquid can be properly metered via regulation of the aforesaid stroke. The present invention thus falls within the field of closed-loop regulation systems, where the input variable, which in the case of the present invention is the current supplied to the electromagnet, is regulated by the system through a feedback based upon control of appropriate physical quantities that can be brought down to the variable to be controlled: the injection volume.
The progress in time of the metering pump has followed the evolution of the electromagnet, as well as the study of the various driving and control systems. The target of said studies has been to prevent the following drawbacks:

waste of energy due to the fact that in the past applied to the electromagnet was a voltage pulse of a fixed duration, whatever the force necessary for overcoming the counterpressure of the system. In effect, the waste of energy is the greater the lower the pressure of the system present in which is the liquid to be treated and consequently the lower and the thrust necessary for overcoming said counterpressure;
excessive heating of the apparatus and in particular of the electromagnet due to the fact that in the past the energy to be supplied was not controlled on the basis of the force to be overcome according to what is described in the previous point, with consequent drop in performance due to an increase in the resistance of the armature over the medium-to-long term;
reduced service life caused by medium-to-high operating temperatures both of the electronic components and of the electromagnet;
the need to have mechanical means for calibration of the piston stroke to obtain the desired pump capacity, which results in the need to introduce shim washers inside the electromagnet;
the need to have mechanical systems for regulation of the stroke that limit only mechanically the piston stroke and hence reduce the injection without proportionally reducing the electrical energy supplied to the equipment: this fact entails a constancy in the levels of energy consumption even when the delivery of the product is minimal, given the same injections. Equipment of this sort is not characterized by high levels of efficiency.

Known from US 2009 0206184 is a system for injection into the combustion chamber regarding control of a fuel injector by using a sensor that detects the displacement of a mobile piston that slides in an accessory channel and, which is distinct from the needle valve. The purpose of the system is to monitor and process the signal of displacement of the accessory piston in order to control operation of the injector in the case of any malfunctioning (blocking in ON or OFF position), in the cases of deterioration of the geometries of the nozzles, of the injector, or of the chamber, in the cases where it is necessary to modify the shape of the injection pulse. In US 2009 0206184 , the displacement of the piston is subordinate to the pressure difference existing between the two ends of the piston itself: the top one is subject to the supply pressure generated upstream of the injection system; the bottom one is subjected by the pressure in the "pre-injection" chamber, the volume of which depends upon the geometries involved. The aforesaid pressure difference obviously depends upon the supply pressure, the supply frequency, and the state of the injector. Three states of the injector may in fact be distinguished, namely, closed ("OFF"), intermediate ("blocking"), and open ("ON"), which determine, respectively, the following situations: OFF: the nozzles are closed by the needle, the pre-injection chamber is in communication with the supply, and the ends of the slidable piston are subject to one and the same pressure; blocking: the nozzles are still closed, and the supply ducts are geometrically closed, separating the pre-injection chamber from the supply; ON: the path to the nozzles is free, the pre-injection chamber does not communicate with the supply, and the mobile piston is displaced downwards following upon the negative pressure that is generated upon opening of the port of the nozzles.
The present invention presents a series of substantial differences as compared to what is described in the injection system of US 2009 0206184 where, irrespective of the type of injector or of control exerted, the supply pump is completely separate from the injector. In fact, in the present invention the pumping action and the injection action are carried out by means of one and the same device comprising a controlled-energization magnet, a piston, and a diaphragm. The latter presses the liquid into a duct, opening/closing of which is ensured by specific valves that operate exclusively for the fluid-dynamic effect.
In addition, the document No. US 2009 0206184 describes a control device obtained by means of an auxiliary piston, which displaces as the conditions of injection vary. Its displacement is detected by a specific sensor. The displacement datum is processed both to modify the conditions of supply upstream and to restore volumes and pressure in the pre-injection chamber.
According to the present invention, instead, by measuring the variation of impedance that is generated in the electrical circuit itself of the magnet as the position of the pumping piston varies, it is possible to determine the position of the piston itself and, from this, to control the energy involved in order to guarantee the pre-set flowrate of fluid. In other words, it is possible to carry out control of the displacement of the device by means of a measurement of its electrical characteristics, without the need to resort to - indirect measurements of other mobile elements.
Finally, in the document No. US 2009 0206184 the position of the auxiliary piston, in a certain configuration, can be corrected by means of an additional actuator. The displacement datum detected by the sensor is in this case processed also for restoring the volume and pressure in the pre-injection chamber. In the present invention, instead, by processing the measurement of the impedance of the electrical circuit of the magnet, it is possible to vary the position of the pumping piston and hence guarantee the pre-set flowrate of fluid. It is thus possible to carry out control of the displacement of the device by acting directly on the pumping piston without resorting to additional actuators in the pre-injection chamber.
In conclusion, in an attempt to find an analogy between the two systems, we can state that: whereas the injection system of US 2009 0206184 envisages as distinct elements the pumping device, the injectors, the auxiliary piston, the movement sensor, and the actuator on the auxiliary piston and performs the control on the basis of the signal detected with the device that is additional (sensor + actuator) with respect to the pumping element, the innovative system described in the present patent application manages just the pumping device by means of measurement and electrical control of the magnet; namely, the pumping assembly is simultaneously sensor and actuator, without any additional elements.
Also known from the document No. WO 2007/007365 is a metering pump, which, in relation to the variation of inductance, generates a corresponding variation on the curve of the current and identifies, through the analysis carried out by the electronic circuit, the point of contact between the plate and the core of the electromagnet. In this way, the device identifies the end-of-travel of the piston and can thus interrupt activation of the electromagnet, preventing a useless waste of energy and hence of heat that jeopardizes the performance of the product.
Also the present invention envisages a measurement of the current, which it uses for calculating the impedance, but, unlike what is indicated in WO 2007/007365 , is not limited to intercepting the point of arrival of the plate and hence the end-of-travel, but sets in relation the value of the inductance with the position assumed by the piston during its stroke and decides, on the basis of the settings entered by the user, in which position to block the piston stroke. Through the present invention it is possible to control with centesimal precision the piston stroke and hence the capacity of the pump for each single injection of additive.
The document No. DE 20 2005 013089U regards an electronic-metering pump that carries out a control on the piston stroke in order to determine the necessary amount of energy thereof to cause the device to perform work properly. From an analysis of said document, it follows that it regulates the piston stroke through a mechanical system and, with the aid of an optical sensor, detects the stroke set by the user, and sends the datum to an electronic circuit that activates the electromagnet with a control signal, having an energy content commensurate with the work that is to be performed by the pump.
Unlike the document No. DE 20 2005 013089U , the present invention is not limited to controlling the current on the electromagnet in order to prevent useless waste of energy, leaving it to the mechanical system to control the piston stroke, but activates the solenoid with a precise amount of current, controls advance thereof through the calculation of the impedance, and decides blocking thereof in a well-defined position that can be set through calibration of the electronic circuit. In other words, in the present invention no mechanical control of the pump capacity is present, but everything is entrusted to an electronic system that activates the electromagnet, controls end-of-travel of the piston, and hence determines the pump capacity with centesimal precision.
The document No. WO 03/023226 describes an electromagnetic metering pump for infusion of medical substances in the human body, wherein the device is supplied by a battery and is not connected to the electrical mains supply: for this reason, the invention described in said document regards a system that guarantees a correct metering with the least use of electrical energy, to the advantage of lengthening the service life of the battery.
From an analysis of WO 03/023226 it may be noted that at the instant in which the pump is supplied, through actuation of a switch and of a control signal, a capacitor that functions as accumulator charges in a given period of time and remains in said state until a control circuit activates, via the control signal 34 and a switch, a solenoid, which determines actuation of the injection delivered by the pump. The principle might seem trivial, but in actual fact controlling in a "smart" way charging and discharging of the capacitor and drawing off, at the moment of actuation, the minimum energy indispensable for proper operation of the pump, a considerable energy saving is achieved.
In conclusion, the system described in WO 03/023226 controls the voltage and the evolution of the current in order to determine the right amount of energy to be applied to the solenoid.
Unlike WO 03/023226 , the present invention is based on the control of the stroke in order to modify instantaneously the injection volume of the pump. In the case in point, the invention is not limited to determining whether the end-of-travel has been reached, but a control is made in real time on what happens in the magnet, the impedance is calculated, and the exact point at which the piston stroke is to cease is decided. According to a peculiar characteristic of the present invention, the solenoid performs not only the function of actuator but also that of sensor of the system.
The main purpose of the present invention is to overcome all the drawbacks listed above by providing a new, latest-generation, metering pump with low energy consumption and high performance.
The above has been obtained, according to the invention, by providing an innovative regulation of the stroke (dynamic variation of the injection volume) based upon control of the value of the impedance of the electromagnet, which is advantageously a quantity with low sensitivity to external factors such as, for example: operating temperature, mechanical wear, and supply voltage.
The metering pump of the above sort is designed to replace the existing apparatuses by improving the current performance and reducing the levels of energy consumption to a minimum, at the same time guaranteeing an accurate metering based upon control of the piston stroke without the use of mechanical devices that are subject to wear, but through the measurement of physical quantities depending exclusively upon the geometry of the electromagnet.
A better understanding of the invention will be obtained from the ensuing detailed description and with reference to the attached figures, which illustrate, purely by way of non-limiting example, a preferred embodiment of the invention.

Figure 1A is an axial section of a metering pump forming the subject of the present invention with the electromagnet mounted thereon, and with an enlarged detail that shows the air gap.
Figure 1B illustrates an enlarged detail of Figure 1A showing the electromagnet.
Figures 2, 3, 4 and 5 show, respectively, as many positions during displacement of the plate towards the core at the instant when a potential difference is applied across the coil; said displacement can be defined as "piston stroke". As the piston stroke varies, the value of inductance of the electromagnet changes.

The metering pump according to the invention is basically constituted by three fundamental elements: the electronic card, the electromagnet, and the pump body.
The core of the system is the electromagnet, which, appropriately governed by the electronic card and recalled by a spring, actuates a piston in a pulsed mode in a range preferably of between 0 and 360 pulses per minute.
The inventive idea underlying the present invention consists in actuating the magnet gradually with small voltage increments, whilst simultaneously said electronic system, measures the current that traverses the solenoid in order to obtain the corresponding value of impedance thereof, which is directly associated to the position of the piston along the maximum stroke envisaged. Through a potentiometric or digital regulation, which can be set directly by the operator via the interface of the electronic card, it is possible to select with centesimal precision, the exact point of arrest of the piston, i.e., its maximum stroke. Consequently, the pump that is described can vary with extreme precision the amount of additive injected for each single actuation of the electromagnet, without the aid of particular and costly mechanical regulations or of further sensors and electronic feedback.
Operation of the metering pump that is described envisages that the mobile part of the electromagnet 1, referred to as "plate" 2, will be displaced until it closes the magnetic flux present in the core of the electromagnet, thus causing displacement of the piston 3 fixed with respect to the plate 2. Said displacement, as has been said, is defined as "stroke".
From the electrical standpoint, said electromagnet is nothing but an inductor constituted by a winding housed on a ferromagnetic material, of a defined geometry, in which an air gap is present, which tends to close with displacement of the plate 2.
Said air gap is the distance (L1, L2, L3, L4) between the fixed part and the mobile part of the electromagnet, which coincides with the "stroke", whilst the mobile part (plate fixed with respect to the piston) displaces, causing variation of the value of inductance on account of a variation of the mechanical characteristics of the inductor itself and in particular of the variation of the physical parameter that in literature is defined as "magnetic reluctance".
In the light of what has been described above, it is possible to determine with certainty a relation between the displacement of the plate 2, and hence of the piston 3, and the variation of the inductance as a function of said displacement. The reason for this is that the quantities involved can be brought down to those typical of inductors, i.e., to the number of turns of the winding, to the cross section of the iron core, to the length of the air gap, etc., and hence can be calculated mathematically.
The present invention is consequently based upon the possibility of relating a measurement of inductance of the electromagnet 1 - and hence of a physical parameter depending only and exclusively upon geometries of production and constructional parameters that are not subject to drift of any type - with a displacement of the piston 3.
It is known that the impedance of an electromagnet is characterized by a resistive factor typical of the copper that constitutes the winding, by an inductive factor that results from the number of turns, and by the geometry of the iron itself.
Operation of the apparatus will be characterized by the measurement of the impedance of the electromagnet Z = V/I at constant voltage (V = constant) and by the measurement of the instantaneous current (I) every millisecond for a given maximum time, for example of 100 ms, equal to the duration of a typical pulse that is supplied to the electromagnet, constructed to obtain for example a travel of one millimetre: a hundred values of instantaneous impedance corresponding to the displacement are thus obtained, consequently a providing centesimal measurement thereof.
For what has been said, the formula that expresses the evolution of the current in the impedance referred to in the previous point, with which it is possible to represent the equivalent electrical circuit of the electromagnet, is: $i (t) = (V / R) * [1 - e^{- (R / L) t}]$

where:

R: is the pure resistance of the armature;
L: is the inductance of the electromagnet (which varies in time with the displacement of the plate);
V: is the constant voltage applied;
e: is Napier's constant equal to 2.7182.

The resistance R remains practically constant but for minor variations as a function of the temperature which can in any case be corrected. Should it be desired to regulate the stroke with extreme precision, it would be sufficient to insert a temperature sensor in order to correct the value of resistance R of the electromagnet 1 and measure the impedance thereof more accurately.
It may thus be appreciated that the variation of the inductance L modifies the evolution of the current and is mathematically linked to the constructional geometries of the electromagnet 1 so that, by performing a simple sampling of the current for each electromagnet at constant voltage and measuring the impedance thereof just once and upon first turning-on, it is possible to characterize it, and the sampled values will repeat in all the operating conditions of the electromagnet, indicating, for the reasons mentioned above, the displacement of the piston 3 and of the plate 2 fixed with respect thereto.
According to the invention, the measurement of the impedance is not affected by the pressure of the system to which the apparatus is connected nor by the variations of said pressure in so far as the force applied to the electromagnet 1 does not depend upon the system counterpressure but rather upon the stroke that the piston that determines the amount of liquid injected has to perform.
By measuring instantaneously the impedance it will also be possible to block the plate at a certain point of its stroke or else maintain long pulses of displacement so as to facilitate the outflow of the liquid to be pumped increasing the hydraulic efficiency, especially in the case where viscous liquids are treated.
Each electromagnet will hence be characterized by a table of its own in which the value of impedance will be correlated to the stroke and managed by the microcontroller.
According to a peculiar characteristic of the invention, the electromagnet 1 is connected to the electronic microcontroller card, which is designed to detect, in real time, the position of the piston 3 as a function of the variations of the values of the impedance of the electromagnet 1 and hence to regulate, according to the requirements, the stroke of said piston 3 and consequently the amount of liquid that is injected instantaneously by the pump. The greater the stroke that the piston 3 performs, the higher the capacity of the metering pump.
During the stroke of the piston 3, fixed with respect to the plate 2, the impedance of the electromagnet 1 varies, since corresponding to a variation of the stroke is a variation of the reluctance of the magnetic circuit of the electromagnet and hence of the inductance thereof. From this it may be inferred that, according to the invention, by controlling the impedance the stroke is controlled, as has been said above.
In the example of embodiment that is described purely by way of example, upon initial start-up of the apparatus during the testing phase, in the factory, the electronic circuit supplies a first pulse, which causes the piston 3 to perform its entire stroke, and through measurements of current and voltage in the electromagnet 1, performed at intervals of one millisecond, stores in its own internal memory the values of the corresponding inductance, together with the actual value of stroke of the piston.
Given that the inductance is a quantity that varies only as some quantities of a physical and mechanical nature that can determine minor variations of the stroke vary, it is evident that the pump could require further calibrations to be carried out during the life cycle of the pump itself.
The microcontroller will then receive as external input the desired position of the piston stroke, associated to a precise capacity of the metering pump, and will govern the power circuit connected to the electromagnet in such a way as to apply a pulse train able to generate the intermittent electromagnetic field that attracts and releases the plate 2, which, being fixed with respect to the piston 3, which is in turn fixed to the diaphragm 4, displaces the latter inside the pump body 5, and consequently the liquid agent is pumped in the aqueous solution.
Each time during displacement of the piston and hence of the plate, the impedance of the electromagnet will vary point-by-point and hence via its detection the stroke performed is determined; this will provide the microcontroller of the device with the possibility of identifying the point of arrest of the piston itself.
Once the pump has been re-set, in order for the pump capacity to be such as to satisfy the required need, the electronic circuit will manage the current as a function of said request within the admissible levels of ambient temperature envisaged for proper operation of metering pumps.
In this way, advantageously avoided is the consumption of more energy than what is necessary for the work required of the metering pump, and the levels of performance remain unaltered for long time with beneficial effects on the metering and on the service life of the equipment.
The metering pump equipped with said control may likewise be connected via serial port to a remote computer for enabling intervention on operation of the pump itself.
A first advantage in the use of the device described so far is represented by the possibility of regulating the piston stroke 3 as a function of the work that the metering pump must effectively perform, and hence of the amount of liquid that it must inject. This entails not only a saving in terms of energy, which is limited to the levels effectively necessary for said work, but also a saving in economic terms for the user, due to the fact that a precise metering that is constant over time prevents a useless waste of additive.
A second advantage consists in the possibility of knowing, with a precision in the order of hundredths of millimetre, the position of the piston 3 during its stroke via the electronic control card.
A third advantage is represented by the fact that it is possible to configure the piston stroke 3 during programming of the microcontroller. This consequently entails a further dual advantage: elimination of mechanical calibration means necessary for proper definition of the piston stroke and economic saving due to said elimination. With the same configuration of the product there is the possibility of producing metering pumps with different technical characteristics.
Yet a further advantage of the invention is that the metering pump comprising the electromagnet so far described works at temperatures lower than those at which a traditional metering pump works, thus preventing an excessive heating, due to the fact that the limitation of the piston stroke does not occur with mechanical means without decreasing the energy yielded to the electromagnet, but only limiting said energy.
A further advantage of the metering pump results from the possibility, proper to the new system, of implementing two functions defined as follows:

1. "UNDERLOAD": detection of the absence of liquid or additive inside the pump body during normal operation of the device;
2. "OVERLOAD": detection of a possible obstruction or over-pressure of the delivery line of the pump during normal operation of the device.

Said situations (UNDERLOAD and OVERLOAD) can be detected thanks to the possibility, on the part of the electronic device, to evaluate both the position of the piston and its rate of displacement.
In the case of UNDERLOAD, the absence of liquid or additive inside the pump body causes a considerable increase of the rate of displacement of the piston, and the microcontroller, through control - in real time - of the variation of the inductance, detects said situation and, on the basis of the settings entered in programming step, signals the state of alarm immediately or following upon a given number of pulses. In the case of UNDERLOAD, the system can envisage a procedure of re-priming that will start supply of a given number of injections, for a definite and programmed period of time.
The OVERLOAD function enables the microcontroller of the electronic device to detect lack of metering irrespective of the presence of appropriate external devices of a known type (flow sensor): an obstruction of the delivery line of the pump, whether partial or total, slows down the rate of displacement of the piston, said slowing-down being a function of the degree of the obstruction; the electronic circuit of the pump acquires said data and, once a given period of time has elapsed, sets itself in a condition of alarm, indicating the state of OVERLOAD.
It should be noted that described in the Italian patent No. 1343207 , filed in the name of the present applicant and entitled "POMPA DOSATRICE DOTATA DI MEZZI DI AUTOREGOLAZIONE DELLA POTENZA ASSORBITA" ("METERING PUMP EQUIPPED WITH MEANS FOR SELF-REGULATION OF THE ABSORBED POWER") is a metering pump that controls, through a switch, the piston stroke and takes away supply to the coil in the proximity of the end-of-travel, thus eliminating a useless waste of energy.
In addition, described in the patent application No. RM2009A000537 , presented in Italy by the present applicant and entitled "DISPOSITIVO DI CONTROLLO DELLA CORSA DEL PISTONE DI UNA POMPA DOSATRICE" ("DEVICE FOR CONTROLLING THE STROKE OF THE PISTON OF A METERING PUMP") is a metering pump provided with a capacitive sensor connected to the electromagnet, which does not simply control end-of-travel of the piston, but also verifies instantaneously, on the basis of the distances between the plates of the capacitor, the position of the piston and, through the control performed by an electronic card, defines and establishes the exact amount of additive to be metered.
Unlike the technical solutions just referred to, the present invention bases control of the position of the piston 3 on the measurement of the inductance of the electromagnet 1; in this way, the end-of-travel switch and the capacitive sensor are eliminated.
In other words, the electromagnet 1, in addition to performing the function of actuator of the pump, also functions as inductive sensor through which the position of the piston 3 and hence the capacity of the pump is detected and managed.
Finally, it is known that other producers of electronic-metering pumps have sought to achieve the same target through control of the current that traverses the electromagnet, but the results obtained have not witnessed proper and adequate operation of the pump.
The present invention has been described and illustrated in a preferred embodiment, but it is evident that the person skilled in the branch may make modifications and/or variations thereto without thereby departing from the sphere of protection of the present industrial patent right. For example, it is possible to envisage for the electromagnet appropriately sized windings, and cores having geometries and solutions that minimize the dispersions of the magnetic field with beneficial effects on the efficiency and on the reduction of heat losses.
List of the references used in the figures:

1. ELECTROMAGNET
2. PLATE
3. PISTON
4. DIAPHRAGM
5. PUMP BODY

Claims

A system comprising a metering pump and a control device for controlling the stroke of a piston (3) of the metering pump comprising an electromagnet (1) with an armature and being characterized in that a pumping part comprising the piston (3) is managed by means of measurement and control of the electromagnet (1) in such a way that the pumping part is simultaneously sensor and actuator, for this purpose said electromagnet (1) being equipped with a fixed plate and a mobile plate (2) fixed with respect to said piston (3) in such a way that to each displacement of the piston there corresponds a different distance between said plates and hence a different value of the impedance of the electromagnet itself; said electromagnet being connected to a microcontroller electronic card of said metering pump, which is designed to detect, in real time, the position of the piston (3), by controlling the variations of the values of impedance (Z) of the electromagnet caused by the variations of inductance, which depends only upon the geometrical characteristics of the electromagnet (1); wherein Z = V/I, at constant voltage (V = constant), and wherein the formula that expresses the evolution of the current in the impedance, with which it is possible to represent the equivalent electrical circuit of the electromagnet (1), is: $i (t) = (V / R) * [1 - e - (R / L) t]$

where:
R is the pure resistance of the armature;

L is the inductance of the electromagnet (1), which varies in time with the displacement of the plate (2) and therefore of the piston (3) fixed to said plate;

V is the constant voltage applied;

e is Napier's constant equal to 2,7182.
The system according to the preceding claim, characterized in that, in order to program the microcontroller in such a way that associated to each variation of inductance of the electromagnet is a piston stroke and consequently a different capacity of the metering pump, it is envisaged to apply across the electromagnet (1) a pulse train, which generates an intermittent magnetic field that attracts and releases a plate (2), which, since it is fixed with respect to the piston (3), which is in turn fixed to a diaphragm (4) of the metering pump, displaces the latter inside the pump body (5), and consequently a liquid agent is pumped into the system on which the pump is installed; thus obtaining that to each value of inductance detected by the microcontroller there corresponds a precise piston stroke, the value of which is stored in the memory of the microcontroller itself.
The system according to Claim 1, characterized in that in order to keep the capacity of the metering pump constant, the electronic card sends a pulse for measuring the inductance upon first turning-on of the pump itself in order to correlate the piston stroke to the inductance itself of the electromagnet; said first pulse being designed to cause the piston (3) to perform an entire stroke, in such a way that the microcontroller can measure the current in the electromagnet (1) at intervals of one millisecond, storing in its own internal memory said values together with the piston stroke, simultaneously obtaining the impedance of the electromagnet.
The system according to Claim 1, characterized in that the microcontroller is designed to regulate the piston stroke (3) by modulating the impulsive current at a fixed frequency on the electromagnet on the basis of the values detected of the inductance as compared with those contained in the internal memory of the microcontroller that relate them to the piston stroke.
The system according to Claim 1, characterized in that said microcontroller is designed to regulate the piston stroke (3) by modulating the current that is required for the electromagnet (1) in order to displace the piston (3).
The system according to Claim 1, characterized in that said microcontroller is designed to verify and evaluate the rate of displacement of the piston (3) in order to detect the state of UNDERLOAD, i.e., detect the absence of additive inside the pump body during normal operation of the device.
The system according to Claim 1, characterized in that said electronic device is designed to verify and evaluate the rate of displacement of the piston (3) in order to detect the state of OVERLOAD, i.e., detect a possible obstruction or over-pressure of the delivery line of the pump during normal operation of the device.
The system according to one of preceding claims, characterized in that it is equipped with a serial port for connection to a computer or other similar electronic device for remote acquisition of the information gathered by the microcontroller and/or intervention on operation of the pump itself.
The system according to Claim 1, characterized in that, albeit envisaging a measurement of the current that uses for the calculation the impedance, it is not limited to intercepting the point of arrival of the plate (2) and hence the end-of-travel, but said microcontroller electronic card is designed to set in relation the value of the inductance with the position assumed by the piston (3) during its stroke and to determine, on the basis of the settings made by the user, in which position to block the piston stroke; thus obtaining the possibility of controlling with centesimal precision the piston stroke (3) and hence the capacity of the pump for each single injection of additive.
The system according to Claim 1, characterized in that said microcontroller electronic card controls the current on the electromagnet (1) in order to prevent any useless waste of energy and activates the solenoid thereof with a precise amount of current, controlling the advance through the calculation of the impedance and determining blocking thereof in a precisely defined position that can be set through calibration of the electronic circuit, without any mechanical system for controlling the pump capacity.
The system according to Claim 1, characterized in that in order to control the piston stroke (3) so as to modify instantaneously the injection volume of the metering pump, said microcontroller electronic card determines whether the pre-set end-of-travel has been reached and controls, in real time, what occurs in the electromagnet (1) calculating the impedance and determining the exact point at which the piston stroke (3) must cease; thus obtaining that the solenoid of the electromagnet (1), in addition to performing the function of actuator, functions also as sensor.
The system according to Claim 8, characterized in that the metering pump is basically constituted by three fundamental elements: an electronic card, an electromagnet (1), and a pump body with piston (3), wherein said electromagnet (1), appropriately actuated by the electronic card and recalled by a spring, actuates the piston (3) in a pulsed mode in a range preferably comprised between 0 and 360 pulses per minute.
The system according to the preceding claim, characterized in that said electronic card actuates the electromagnet (1) gradually with small increments of voltage whilst it simultaneously measures the current that traverses the solenoid of said electromagnet in order to obtain therefrom the corresponding value of impedance, which is directly associated to the position of the piston (3) along the maximum stroke envisaged; through a potentiometric or digital regulation, that can be set directly by the operator via the interface of the electronic card, it being possible to select with centesimal precision the exact point of arrest of the piston, i.e., its maximum stroke, it thus being possible to vary with extreme precision the amount of additive injected for each single actuation of the electromagnet, without the aid of particular and costly mechanical regulations or of further sensors and electronic feedback.