WO2016051285A1

WO2016051285A1 - Magnetostrictive transducer

Info

Publication number: WO2016051285A1
Application number: PCT/IB2015/050338
Authority: WO
Inventors: Marco Carrara; Giovanni CAPRONI; Giovanni ZILIANI
Original assignee: Sensor Systems S.R.L.
Priority date: 2014-09-29
Filing date: 2015-01-16
Publication date: 2016-04-07

Abstract

The present invention relates to a magnetostrictive transducer, comprising: a magnetostrictive waveguide forming a measurement probe; excitation means coupled to said - waveguide for generating an excitation pulse forming a magnetic field which surrounds said waveguide; - magnetic positioning means coupled to said waveguide so as to perform a displacement in relation to said waveguide to generate an acoustic pulse in the waveguide at the point in which said magnetic positioning means are located as a reaction to a magnetic field surrounding the waveguide; - signal conversion means coupled to the waveguide and suitable to produce an output signal in response to the interaction of said conversion means with said acoustic pulse. the transducer being characterised in that said magnetostrictive waveguide comprises a magnetostrictive wire made of at least 99% pure nickel.

Description

DESCRIPTION

"MAGNETOSTRICTIVE TRANSDUCER"

[0001] This invention relates to a magnetostrictive transducer, in particular for position measurements.

[0002] As is known, this type of transducer is essentially an electro-magneto-mechanical device that, in its operation, involves some important energy exchange effects between different states of matter.

[0003] In the linear version which is the object of the present invention, the transducer consists essentially of a wire made of material with ferromagnetic properties. In this type of material, there is a series of magnetic domains, each consisting of many randomly oriented atoms. The presence of an external magnetic field acts on these domains, forcing them to orient themselves in the direction of its lines of force. This interaction between the external magnetic field and the magnetic domains of the material is due to the magnetostrictive effect. This effect causes a dimensional change of the material in a positive or negative sense, along the direction of the applied magnetic field. In some materials, such as iron and cobalt, the magnetostrictive effect causes them to lengthen while, in other materials, such as nickel, there is a dimensional contraction.

[0004] By sending a strong current pulse, with a duration of several microseconds, through the wire, a circular magnetic field is immediately obtained throughout its entire length. The transducer includes a cursor, integrally fixed to the object whose position one wants to measure, that slides along the wire. The transducer cursor interacts with this magnetic field by means of a magnet with which, the cursor is provided. Following the magnetostriction, a torquing pulse is generated in the wire that is propagated at high speed as an ultrasonic wave towards its two ends. This effect, related to the magnetic anisotropy present in the wire, is called the Wiedemann effect. While the ultrasonic pulse directed towards the outside is suitably damped and absorbed, the one in the opposite direction is the echo signal used for the determination of the position of the cursor. The speed with which the ultrasonic wave propagates along the wire is about 3000 m/s, corresponding to about 3μ≤ per mm. . The position of the cursor is determined by measuring, with suitable resolution, the time between the sending of the current pulse and the return echo. The detection of the ultrasonic wave is possible as a. result of the Villari effect, which is the inverse manifestation of the magnetostrictive effect. In this case, it is the mechanical stress action exerted on the wire by the ultrasonic pulse that causes the variation of the magnetic properties of the wire. At one end of the magnetostrictive wire, one places a signal-detection sysjtem, known as pick-up, which comprises a detection coil arranged coaxially to the wire itself. The coil collects this variation of the magnetic properties of the wire, generating at its ends, due to Faraday's law, an electrical echo pulse.

[0005] In general, the operation and technical characteristics of a magnetostrictive transducer are related to the following factors:

[0006] - The material of the magnetostrictive wire and its microscopic structure;

[0007] - The signal-detection pick-up;

[0008] - The system for fixing the magnetostrictive wire and damping the ultrasonic reflections;

[0009] - The electronic signal-conditioning circuit.

[0010] With regard to the magnetostrictive wire, the material commonly used is ALLOY 902, known under the trade name of Nispan C. It is essentially a quaternary alloy of Ni, Cr, Ti and Fe with a specific weight of about 8kg/dm³. Its main characteristic is related to the stability of the conduction velocity of the ultrasonic echo in the temperature range between -40 °C and 100 °C, where it maintains the error within a range of 5 ppm/°C. However, the processing of the material, consisting in the methods of extrusion, hot or cold, and the heat treatment of the wire, are fundamental elements for allowing the transducer to maintain good performance stability over time. Today, the wire is supplied to transducer manufacturers already treated and ready to be put into production. The most used wire diameters are between 0.4 and 0.7 mm.

[001 1] The proper heat treatment gives the Nispan C wire the characteristics of a spring with good hardness and stiffness. While, on the one hand, these features allow obtaining low attenuation of the echo signal, even over long return paths, they also involve the disadvantage of greater difficulty in attenuating the bounce signals generated by the change of acoustic impedance in the terminal fixing zones fixing and, therefore, the inability to . compress the length of these terminal zones below certain values. Another disadvantage of this type of wire is the inability to make welds with the usual tin alloys without the use of special liquid adjuvants.

[0012] As regards the signal pick-up, the production method used today that allows extracting the greatest signal amplitude, requires the use of a thin blade welded on the magnetostrictive wire in an orthogonal position by means of a capacitive discharge. The thin blade constitutes the central core of a winding of copper wire on the outside of which is placed a small field-generator magnet. Since the ultrasonic return echo is torsional, the mechanical pulse that this echo impresses on the thin blade by its passage, determines a significant variation in the flux generated by the magnet and consequently an important electrical signal. Depending on the various constructive solutions, the amplitude of the echo signal obtained is between 200 and 400mV.

[0013] An important drawback of this solution is generated by the difficulty of handling the wire in the various stages of assembly due to the projection of the sensitive blade. In addition, the magnetic screen of the pick-up, which is essential for eliminating errors generated by the presence of any external magnetic fields, is difficult to create and bulky.

[0014] A variant embodiment of the pick-up system, which is simpler but less sensitive, involves the use of an electric detection winding placed coaxially on the magnetostrictive wire. The signal obtained from this winding is related to the variation of the magnetic properties generated by the passage, at high-speed, of the torsional echo pulse in the magnetostrictive wire that forms the core of the electric detection winding (Villari effect) . Even using a winding with a high number of turns and a strong excitation current, the signal extracted from such a pick-up with a core wire made of Nispan C does not exceed the peak value of 30mV.

[0015] Despite the lower sensitivity, the advantages of simplicity of construction and reduction of the bulk of the shield make this detection one of the most used.

[0016] However, when increased precision is required, with the use of Nispan C wire, the only possible choice is a pick-up with high output signal. In fact, since the remaining electrical is normally compressed within a basic band of amplitude, the greater signal/noise ratio that a high detection signal carries with it, consequently also determines an increase in the precision of the system.

[0017] As mentioned above, an important characteristic of the transducer is linked to the mode of fixing the magnetostrictive wire at both its ends. In fact, the ultrasonic energy inside it has a high intensity and, in order ■ to -adopt the highest interrogation frequency compatible with the echo times, it is necessary to rapidly reduce the value. Various solutions to this problem have been proposed but all can be traced to the use of attenuating materials of different densities that seek to adapt, with greater or lesser success, the acoustic impedance of the termination with that of the sensitive wire. [0018] In any case, the need to give the wire a fixed anchorage, which is essential in a position measurement, requires the availability of final over-travel space that, depending on the constructive method, is in the order of 30-60mm.

[0019] As regards the electronic signal conditioning part, the only existing problems are related to the containment of the bulk of its housing, especially in output solutions that require redundant circuits with SIL2 or SIL3 performances applied to transducers inserted in hydraulic cylinders and with the need for reduced rear bulk.

[0020] The object of the present invention is to propose a magnetostrictive transducer capable of increasing the advantages of the current technical solutions described above while, at the same time, reducing the disadvantages .

[0021] This object is achieved with a magnetostrictive transducer according to claim 1. The dependent claims describe preferred embodiments of the invention.

[0022] In accordance with claim 1, there is proposed a magnetostrictive transducer comprising a magnetostrictive waveguide forming a measurement probe. Excitation means are coupled to said waveguide for generating an excitation pulse forming a magnetic field which surrounds said waveguide. Magnetic positioning means are coupled to said waveguide so as to perform a displacement in relation to said waveguide to generate an acoustic pulse in the waveguide at the point in which said magnetic positioning means are located as a reaction to a magnetic field surrounding the waveguide. Signal conversion means are coupled to the waveguide and are suitable to produce an output signal in response to the interaction of said conversion means with said acoustic pulse.

[0023] According to one aspect of the invention, the magnetostrictive waveguide comprises a magnetostrictive wire made of at least 99% pure nickel.

[0024] More preferably, such magnetostrictive wire is obtained by a mechanical plastic deformation treatment by stretching of a starting wire in at least 99% pure nickel.

[0025] In one embodiment, said magnetic positioning means comprise a magnetic cursor sliding along the waveguide.

[0026] In one embodiment, the transducer comprises timer means operatively connected to excitation means to measure the echo time elapsing between the generation of said excitation pulse and the reception of said acoustic pulse, so as to determine the position of said magnetic positioning means along the waveguide.

[0027] In an embodiment, the transducer comprises magnetic reference means coupled to the waveguide in a fixed position and suitable for generating a reference acoustic pulse in the waveguide as a reaction to the magnetic field that surrounds the waveguide. These timer means are suitable to measure the reference time elapsing between the , generation of said excitation pulse and the reception of said acoustic reference pulse, the position of said magnetic positioning means along the waveguide being derivable from the ratio of the difference between the reference time and the echo time and the reference time.

[0028] In an embodiment, the' signal conversion means comprise an electric winding placed coaxially to the waveguide and suitable to generate an electrical pulse in response to an acoustic pulse passing through said coil.

[0029] In an embodiment, the conversion means are positioned at one end of the waveguide and the magnetic reference means are positioned at the opposite end of the waveguide .

[0030] This invention is the result of a search for a material that possessed a greater magnetostrictive coefficient or, in any case, such that, by using a coaxial pick-up, as will be described below, one could extract an output signal significantly higher than Nispan C.

[0031] In the light of numerous tests, pure nickel (at least 99%, preferably > 99.6%) proved to be the most suitable material for the application in a magnetostrictive position transducer.

[0032] The best results in terms of output signal amplitude have been obtained by previously subjecting the nickel wire to a mechanical treatment of plastic deformation by stretching .

[0033] The stretching action is slow and progressive, and ends at the critical point in which, through a load cell, one detects a significant subsidence of mechanical tension immediately followed by a rapid increase due to stiffening of the structure. The value of the mechanical tension that corresponds to this action is not fixed being primarily related to the diameter of the wire, the type of processing, hot or cold, it was subjected to during its production, and the total length in stretching. The characteristics acquired by pure nickel wire subjected to such treatment, show a remarkable improvement of its performance in terms of magnetostrictive effect and excellent conduction of the ultrasonic pulse.

[0034] Without being linked to any scientific theory, it is believed that the plastic deformation of the pure nickel wire resulting from its severe mechanical stretching, has the effect of changing the internal crystallographies structure of the material in an irreversible manner. The critical point, reached in the stretching, corresponds to a re-arrangement of the magnetic domains that lead the material to a new equilibrium of its internal energy states by determining, in this way, a significant increase of the parameters of magnetic coercivity and magnetic anisotropy. The coercivity is responsible for an increase in the magnetic hysteresis cycle, while the anisotropy is related to the difference shown by the material between its preferred direction of magnetization and that of greater resistance. The combination of these two parameters is directly responsible for the increase of sensitivity of the magnetostrictive effect in the face of the action of the external magnetic field and the increase of the variation of magnetic permeability (Villari effect) following the passage of the ultrasonic wave that generates the electrical echo signal.

[0035] Comparative testing carried out with exactly the same current excitation parameters, the same coaxial pick-up system and equal magnetic cursor, on the same length of pure nickel and Nispan C wire, showed for the first a peak value 200mV, corresponding to 10 times the 20mV obtained from the second.

[0036] The characteristics and advantages of the transducer according to the invention will, in any case, be evident from the following description of its preferred embodiments, provided by way of non-limiting example, with reference to the attached figures, wherein:

[0037] - Figure 1 is a perspective view that schematically illustrates the main elements of the transducer according to the invention;

[0038] - Figure 2 is a block diagram of the transducer;

[0039] - Figure 3 is an axial section of the transducer in a practical embodiment; and

[0040] - Figure 4 is the graph of. several electrical signals of the transducer.

[0041] In these drawings, reference numeral 1 denotes a magnetostrictive transducer according to the invention as a whole.

[0042] The transducer 1 comprises a nickel wire 10 of high purity (at least 99%, preferably ≥ 99.6%) forming a measurement probe. The nickel wire 10 forms, with an electric return wire 12, an electrical circuit 13 of the transducer operatively connected to excitation means suitable to generate an electrical excitation pulse 16 to be sent to the electrical circuit 13. The excitation pulse 16 forms a magnetic field that surrounds the nickel wire 10.

[0043] In an embodiment, the electrical excitation pulse 16 is generated by a microprocessor 18 and is amplified by an amplifier circuit 20.

[0044] Coaxially to the nickel wire, a magnetic cursor ring 22 is slidably positioned integral with_. the object whose position is to be measured. The cursor ring 22 is suitable to generate an acoustic pulse, i.e., a torsional ultrasonic pulse, in the nickel wire 10 at the point where said cursor ring 22 is located, in response to the magnetic field surrounding the nickel wire 10. In the continuation of the description, the acoustic pulse that returns towards the end of the nickel wire from which the excitation pulse 16 departed, is also called an echo pulse .

[0045] This acoustic pulse noise is detected by a pick-up circuit 24 coupled to the nickel wire 10 and suitable to produce an output signal in response to interaction with the acoustic echo pulse.

[0046] In the continuation of the description, the term " (portion of) proximal end" refers to the (portion of the) end of the transducer 1 from which the excitation pulse departs and towards which the ultrasonic echo pulse returns.

[0047] In a preferred embodiment, the pick-up circuit 24 comprises a detection coil 26 wound coaxially around a portion of the proximal end of the nickel wire 10.

[0048] Therefore, this end portion of the nickel wire 10 constitutes the core of the coil 26. As explained in the introduction to this description, the signal obtained from this winding is related to the variation of the magnetic properties generated by the passage, at high- speed, of the torsional echo pulse in the portion of the wire itself that constitutes the core of the detection coil (Villari effect). In the continuation of the description, the signal detected by the detection coil generated by the ultrasonic echo will be defined echo signal.

[0049] In accordance with a preferred embodiment, at the termination of the nickel wire opposite to the end portion around which is wound the detection coil 26 is fixed a magnetic reference means 30 used to obtain a temperature compensation, as will be described below.

[0050] The detection coil 26 is operatively connected to an amplifier circuit 28 for the amplification of the echo signal. This amplifier circuit 28 is operatively connected to a comparator circuit 32, in its turn operatively connected to a logic circuit 34 controlled by the microprocessor 18.

[0051 ] Inside the microprocessor, there is a high-frequency timer for the high-resolution measurement of the echo times correlated to the positions of the magnetic cursor 22 and the magnetic reference means 30, as will be described below.

[0052] An analogue output block 36 is managed by the microprocessor 18 to provide an output voltage in the range 0-lOV or a DC current in the range 0-20/4-20mA as a function of the absolute position of the magnetic cursor 22.

[0053] A digital output block 38 is always managed by the microprocessor 18 and allows it to communicate with external intelligent systems.

[0054] In a preferred embodiment illustrated in Figure 3, at least the signal conditioning circuits are integrated in an electronic board 40 placed around, or otherwise in contact with, the pick-up circuit 24. This latter comprises a container 42, for example substantially cylindrical with axis coincident with the axis of the nickel wire, in which the detection coil 26 is housed. This container 42 is made of a material with high magnetic permeability and is shielded from any external magnetic fields by means of a screen 44.

[0055] The electronic board 40 and the pick-up circuit 24 are placed in a housing 46, for example provided with a threaded connector 46', coaxial with the end portion of the nickel wire 10, for the connection of the transducer to an apparatus inside of which the component whose position is to be measured moves. [0056] The electronic board 40 is also connected to the control circuitry, for example the logic circuit 34, and to the microprocessor 18, by means of an electrical connection cable 48, for example provided with an 12 connector.

[0057] In an embodiment, the nickel wire 10 and the return wire 12 are inserted in a flexible sheath 50, for example made of silicone-coated fibreglass, which supports and centres the nickel wire 10.

[0058] The flexible sheath 50 is inserted into a rigid support and insulation tube 52, for example made of plastic. The magnetic reference means 30 is also housed in the distal end of this rigid tube 52.

[0059] In an embodiment, the rigid tube 52 is in its turn inserted into a container tube 54, for example made of AISI 316 stainless steel so as to be suitable to withstand high pressures (> 350 bar) .

[0060] In a preferred embodiment, the proximal end of the container tube 54 is inserted into the threaded connector 46' of the housing 46, to which it is fixed for example by welding.

[0061] The distal end of the container tube 54 is closed by a plug 56, for example made of AISI 316 steel. This plug 56 is welded to the container tube 54 with which it supports the high pressure. [0062] The measurement cycle begins with a START pulse 16, generated by the microprocessor 18, having, for example, a duration of a few microseconds. The power of the START pulse is amplified by the power amplifier circuit 20 and it is sent to the electric circuit 13 formed by the nickel wire 10 and return wire 12. The high current sent instantly creates, along the nickel wire 10, a circular magnetic field that interacts with the magnetic fields of the magnetic cursor ring 22 and the magnetic reference means 30.

[0063] Due to the magnetostrictive effect, at the point of separation of the lines of magnetic flux of the cursor ring and reference means, two torsional ultrasonic pulses 60, 70 are generated in the nickel wire that are propagated at high speed towards the ends of the wire itself. While the pulses that move towards the outside are appropriately absorbed and damped, the opposite ones are collected by the detection coil 26 and respectively constitute the position signal of the cursor position and the reference. The propagation speed of the echo pulse along the nickel wire is about 3000 m/s, corresponding to about 1 mm every 333 ns . By accumulating the number of periods of the system clock, the position can normally be obtained with a resolution of approximately 45μπι which, for normal applications, for example for the measurement of the position of a piston inside a cylinder, is amply sufficient. Applications that require higher resolutions can be obtained by increasing the clock frequency or through the use of specific integrated circuits.

[0064] The echo signals 60, 70 derived from the coil of the pick-up have a peak amplitude of about 200mV. They are amplified by the amplifier circuit 28 and sent to the comparator circuit 32. Each time a pre-set value is exceeded, the comparator circuit 32 determines the switching of the latches contained in the logic circuit 34 and the sending of start and stop counting signals 62, 72 to the microprocessor 18.

[0065] As mentioned above, the compensation system adopted to eliminate the thermal drift of the transducer provides for the positioning of magnetic reference means 30 in the terminal part of the nickel wire at a known and fixed distance ^' from the detection coil. The echo signal 70 generated by this reference magnet is used as a measurement reference according to the following operating method. The time between the excitation pulse 16 (Start) and the stop signal 72 given by the magnetic reference means 30 is indicative- of the total counting period.

[0066] The counting pulse 80, which represents the totalized count between the first stop signal 62 generated by the echo signal 60 generated by the movable cursor 22, and the stop signal 72 generated by the echo signal 70 generated by the reference magnet, compared to the total period to which the known distance corresponds, is the value of the position measurement made independent of the temperature.

[0067] In other words, the position of the cursor 22 is derived from the ratio between the value of the counter pulse 80 accumulated in the timer and the total period existing between the start pulse 16 and the echo of the reference magnet 30. This ratio is maintained independently of the temperature, since it only a function of the cursor position relative to the reference .

[0068] The advantages associated with the use of the pure nickel wire treated by us can be summarized as follows:

[0069] - ease of provisioning;

[0070] - lower purchase cost compared to Nispan C;

[0071] - absence of thermal cycles;

[0072] - welding the nickel wire with the usual tin alloys;

[0073] - the ultrasonic energy in the wire is much smaller than that retained by Nispan C. This avoids the need to introduce damping systems that, by their nature, are not very reproducible for standardized production. This allows reducing initial area of over-travel to only the dimensions of the pick-up circuit and the wire can be rigidly welded to it without any damping. In the final zone, the adaptation of the acoustic impedance with the wire may, for the same preceding reason, be implemented with an over-travel of less than 30mm. The reduction of dead over-travel areas is much appreciated in measurement applications of the stroke inside hydraulic cylinders;

[0074] - high detection signal. The increase of the signal leads to an increase of the repeatability of the system and a lower itter of the measurement; .

[0075] - the nickel wire has a very low retention of the magnetic field and, consequently, less hysteresis of the measurement as a function of the direction of movement of the cursor .

[0076] To the embodiments of the transducer according to the invention, a skilled person, to satisfy contingent requirements, may make modifications, adaptations and replacements of members with others functionally equivalent, without departing from the scope of the following claims. Each of the characteristics described as belonging to a possible embodiment can be achieved independently from the other embodiments described.

Claims

1. Magnetostrictive transducer, comprising:

- a magnetostrictive waveguide (10) forming a measurement probe ;

- excitation means (18, 20) coupled to said waveguide for generating an excitation pulse forming a magnetic field which surrounds said waveguide;

magnetic positioning means (22) coupled to said waveguide so as to perform a displacement in relation to said waveguide to generate an acoustic pulse in the waveguide at the point in which said magnetic positioning means are located as a reaction to a magnetic field surrounding the waveguide;

signal conversion means (24, 26) coupled to the waveguide and suitable to produce an output signal in response to the interaction of said conversion means with said acoustic pulse;

the transducer being characterised in that said magnetostrictive waveguide comprises a magnetostrictive wire (10) made of at least 99% pure nickel.

2. Transducer according to claim 1, wherein said magnetostrictive wire (10) is obtained by a mechanical plastic deformation treatment by stretching of a starting wire in at least 99% pure nickel.

3. Transducer according to any of the previous claims, wherein said magnetic positioning means comprise a magnetic cursor (22) sliding along the waveguide (10) .

4. Transducer according to any of the previous claims, comprising timer means (18) operatively connected to said excitation means to measure the echo time elapsing between the generation of said excitation pulse and the reception of said, acoustic pulse, so as to determine the position of said magnetic positioning means along the waveguide .

5. Transducer according to claim 4, comprising magnetic reference means' (30) coupled to the waveguide in a fixed position and suitable to generate a reference acoustic pulse in the waveguide as a reaction to the magnetic field surrounding the waveguide, said timer means being suitable to measure the reference time elapsing between the generation of said excitation pulse and the reception of said acoustic reference pulse, the position of said magnetic positioning means along the waveguide being derivable from the ratio of the difference between the reference time and the echo time and the reference time.

6. Transducer according to any of the previous claims, wherein said signal conversion means comprise a detection coil (26) placed coaxially to the waveguide and suitable to generate an electrical pulse in response to an acoustic pulse passing through said coil.

7. Transducer according to claim 5, wherein said conversion means (24) are positioned at one end of the waveguide and in which said magnetic reference means (30) are positioned at the opposite end of the waveguide.