WO1993004349A1

WO1993004349A1 - Magnetostrictive pressure sensor

Info

Publication number: WO1993004349A1
Application number: PCT/GB1992/001551
Authority: WO
Inventors: Michael David Crossfield
Original assignee: Scientific Generics Limited
Priority date: 1991-08-23
Filing date: 1992-08-21
Publication date: 1993-03-04
Also published as: AU2466292A

Abstract

A piece of magnetostrictive material (5) is arranged as a diaphragm, or is mounted on a separate diaphragm (3), so as to be stressed by a pressure which is to be sensed. The stress will alter the magnetic properties of the piece of magnetostrictive material (5), reducing its permeability and increasing its coercivity. These changes may be detected by applying an alternating magnetic field to the piece of magnetostrictive material (5), and determining the phase and/or amplitude of harmonics in the induced field emitted by the piece of magnetostrictive material (5). In this way, no contact is required between the sensor and the detection circuitry, and therefore they can move relative to each other. In this way, the sensor can be mounted on a vehicle wheel for sensing vehicle tyre pressure, while the detection circuitry is mounted on the vehicle body. In one embodiment the sensor is mounted so as to seal a hole in a vehicle wheel (35), with the piece of magnetostrictive material (5) outside the pressurised volume but subject to the tyre pressure through the hole in the wheel. In this way the sensor can be mounted on the wheel (35) rather than on the tyre (39), without the metal of the wheel (35) interferring with the transmission of magnetic fields between the piece of magnetostrictive material (5) and coils (13, 15, 27) of the detection circuitry. In another embodiment, particularly suited to situations where the sensor does not move relative to the detection circuitry, a coil (45) is wound around the piece of magnetostrictive material (5), so that changes in the permeability of the piece of magnetostrictive material (5) in response to pressure alter the inductance of the coil (45).

Description

MAGNETOSTRICTIVE PRESSURE SENSOR

The present invention relates to the sensing of pressure. It has particular application to the sensing of fluid pressures, and may be used, for example, in sensing the air pressure in a vehicle tyre, although it may be used for other fluids, both liquid and gas.

Various types of pressure sensing arrangement are known. In many of them a closed envelope containing a fluid at a standard pressure or a vacuum is deformed to an extent dependent on the pressure of an external fluid. This deformation controls the position of a mechanically coupled pointer. In other sensors, deformation or some other change in the property of a material in response to pressure is converted an electrical signal for signal processing.

In some circumstances it is not practical to provide electrical wire connections between a pressure sensor and a signal processor, for example where the wires would have to pass through the wall of a sealed enclosure or where the sensor moves relative to the signal processor. An example of such a situation is the sensing of pressure inside the vehicle tyre while the wheel is rotating. It is possible to provide a commutator arrangement to the pass signals between the rotating wheel and the car body, but such arrangements tend to be expensive and complex and the transmission of signals through the moving contact may not always be reliable. Additionally, signal wires will have to pass through the tyre or the wheel in some way which does not weaken them or reduce air-tightness.

Accordingly, systems have been proposed in which the pressure information is passed from a sensor to a signal processor without mechanical connection, for example via a radio link. Such systems can be bulky and costly, owing to a need to provide the system with a data transmitter and receiver. Additionally, where a source of power is needed at a pressure sensor, this will typically have to be in the form of a battery, which will have a finite life, so that either the battery must be replaced from time to time or, if this is not possible, the pressure sensor itself will have a finite period of operation. A vehicle tyre pressure sensor is disclosed in JP-A-2-197405 (Yamaha Corp). This pressure sensor has a tubular body which passes through a hole in the vehicle wheel, and the tube is sealed by a diaphragm, so that one side of the diaphragm is exposed to atmospheric pressure from outside the wheel and the other side of the diaphragm is exposed to the inflation pressure of the tyre. A distortion sensor detects distortion of the diaphragm. The distortion sensor appears to be an electric strain gauge.

A different type of pressure sensor arrangement is disclosed in GB-A-1530952 (Exxon Nuclear Company Incorporated). This is concerned with monitoring pressure inside a nuclear fuel rod, and it^" proposes a pressure sensor comprising a cylinder of magnetostrictive material which is exposed to the pressure inside the fuel rod and is distorted thereby. An interrogation coil is arranged to be placed around the cylinder of magnetostrictive material at a precise repeatable location as close as possible to midway along the cylinder, so that the inductance of the coil is affected by the permeability of the magnetostrictive cylinder. This permeability is in turn affected by the stress applied to it by the pressure inside the fuel rod.

The detection circuit GB-A-1530952 comprises the detection coil and an equivalent coil placed around a comparison sensor which is not exposed to the pressure in the fuel rod, and both of these coils are driven at a common frequency from constant current sources. The output voltages, which will be impedance dependent, are provided to a differential amplifier, and the output of the differential amplifier is summed with a signal of equal amplitude but opposite phase to produce a null combined signal. The null combined signal is provided to a further amplifier. Small changes in the voltage generated from the sensor coil will cause the combined signal input to the further amplifier to cease to be null, and these changes will be amplified by the further amplifier. These amplified signals are multiplied by a further reference signal having a phase selected so that the product signal contains only the voltage generated by the reactive component of the impedance of the sensor coil and does not contain any component derived from the resistive component. Consequently, the output signal provides a measure of changes in the reactance of the sensor coil, and hence changes in the permeability of the magnetostrictive cylinder.

A system which detects the effect of the permeability of a piece of magnetostrictive material on the inductance of a coil requires a very close coupling between the coil and the piece of magnetostrictive material, and this appears to be the reason why the coil in GB-A-1530952 is placed around the magnetostrictive cylinder as close as possible to midway along it. In this way, the piece of magnetostrictive material is enabled to act as the inductor core.

Various types of sensors using amorphous metal alloys, including high magnetostriction alloys, are reviewed in "Review on Recent Advances in the Field of Amorphous-Metal Sensors and Transducers", K. Mohri, IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984. Figure 1 of this document lists a pressure sensor as an application of an alloy having high magnetostriction, under the heading "Ultrasonics". A torque sensor, a force sensor and a knock sensor are listed under the heading "Stress Magnetic". In the body of the document, a pressure sensor is proposed in which ultrasonics are generated through two coils and propagated to a third in which the two waves cancel out when an external pressure is absent. The presence of the pressure unbalances the arrangement leading to an output at the third coil. The document also refers to an oil pressure sensor using an amorphous circular diaphragm fixed on a ferrite core with an 0-ring.

Embodiments of the present invention can be provided which do not require any physical connection between a pressure sensor and a signal processor for processing a signal representative of sensed pressure or pressure change, although the present invention is not limited to such embodiments . Such an embodiment can be provided which requires no power source at the pressure sensor, and does not require complicated circuitry at the sensor.

In one aspect the present invention provides a pressure sensor comprising a piece of material which is magnetostrictive, in the sense that at least one of its magnetic permeability and its magnetic coercivity varies with applied stress or deformation, provided in an arrangement to stress or deform the piece of material in response to applied pressure.

In another aspect the present invention provides a pressure sensing system comprising a pressure sensor as described above and means to input a magnetic signal, preferably an alternating magnetic signal, to the piece of magnetostrictive material, and to detect a property of an output signal which is dependent on the permeability or the coercivity of the piece of magnetostrictive material.

In a further aspect, the present invention provides a method of sensing pressure in which a piece of magnetostrictive material is arranged to be stressed or deformed by the pressure to be sensed, a magnetic field is applied to the piece of magnetostrictive material, and an output signal is detected having a property dependent on the permeability or the coercivity of the piece of magnetostrictive material.

The magnetostrictive material in the present invention is preferably 10 micrometres thick or less, for example of the order of 1 micrometre thick, and may be provided as a thin film or foil. There are many arrangements by which the piece of magnetostrictive material may be arranged to be subject to stress or deformation in response to applied pressure. For example, in the case of a sensor for detecting the pressure inside a vehicle tyre, the piece of magnetostictive material may be applied to the tyre, preferably at its side wall, and the slight deformation of the tyre in response to the internal pressure will stress the magnetostrictive material. Accordingly, another aspect of the present invention provides a vehicle tyre bearing a piece of magnetostrictive material.

In another arrangement, a device is provided in which a piece of magnetostrictive material forms or is provided on a flexible diaphragm one side of which is exposed to the fluid whose pressure is to be sensed and the other side of which is exposed to a reference pressure. For example, the diaphragm may seal a cavity, and if the sensor is placed so that the side of the diaphragm which is facing away from the cavity is exposed to the fluid whose pressure is to be sensed, the sealed cavity can contain a vacuum or a quantity of fluid to provide the reference pressure. Alternatively, the inside of the cavity can be connected to the fluid whose pressure is to be sensed and the ambient fluid (eg. air) can provide the reference pressure.

In another arrangement, a pressure sensor may define two volumes, separated by a diaphragm comprising or bearing a piece of magnetostrictive material. One of the cavities is sealed to provide a reference while the other contains the fluid whose pressure is to be sensed.

In one embodiment of the means for detecting the pressure sensed by the pressure sensor, an alternating magnetic field is applied to the magnetostrictive material so as to reverse its saturated magnetic state repeatedly, and the changes in the magnetic flux density in the magnetostrictive material are detected, for example by detecting an electrical current induced by the changes in flux density. The phase difference between the applied magnetic field and the induced flux density will vary depending on the coercivity of the piece of magnetostrictive material, and the shape of the waveform of the induced flux density will depend on the permeability of the piece of magnetostrictive material. The shape of the waveform can be detected by monitoring the amplitudes of harmonics of the basic frequency.

In an embodiment of the present invention, a coil is provided around the piece of magnetostrictive material and the variation of the inductance of the coil is used as a measure of the sensed fluid pressure. Where the coil will not be moved relative to the signal processor, the coil can be connected directly to it, providing a pressure sensor of simple and compact construction. Where relative movement is required between the coil and the signal processor, the coil wound around the piece of magnetostrictive material can be provided as part of a resonant circuit, to be excited by a varying frequency magnetic field. The induced current in the resonant circuit can be detected inductively, and will show a peak when the applied magnetic field is at the resonant frequency. The resonant frequency will vary with the inductance of the coil, which in turn is affected by the magnetic properties of the magnetostrictive material.

Embodiments of the present invention, given by way of non limiting example, will now be described with reference to the accompanying drawings, in which: Figure 1(a) shows the B-H hysteresis curve for a magnetostrictive material, Figure 1(b) shows an applied alternating magnetic field, Figure 1(c) shows the magnetic flux induced by that field in a piece of magnetostrictive material, and Figure 1(d) shows the e.m.f. induced in a nearby conductor by the changing flux density of Figure 1(c);

Figure 2 is a view of a pressure sensor embodying the present invention;

Figure 3 is a section of the sensor of Figure 2;

Figure 4 is a section of a modification of the sensor of Figures 2 and 3;

Figure 5 is a top view of another sensor embodying the present invention;

Figure 6 is a circuit diagram for a first detector circuit for use in the present invention;

Figure 7 shows schematically an arrangement of coils for use with the circuit of Figure 6; Figure 8 shows a circuit diagram for a second detector circuit for use with the present invention;

Figure 9 shows schematically a coil arrangement for use with the circuit of Figure 8;

Figure 10 shows a circuit modification for the circuits of Figures 6 and 8;

Figure 11 shows a configuration of a sensor embodying the present invention with a detection circuit arranged to sense a vehicle tyre pressure;

Figure 12 shows a further sensor embodying the present invention;

Figure 13 shows another sensor embodying the present invention;

Figure 14 shows yet another sensor embodying the present invention;

Figure 15 is a view corresponding to Figure 11 of a second configuration for sensing a vehicle tyre pressure, using the embodiments of Figures 13 and 14;

Figure 16 is a top view of another embodiment of the present invention;

Figure 17 is a section through the embodiment of Figure 16;

Figure 18 is a Fourier analysis of the signal detected in a test of a piece of magnetostrictive material at (a) no load and (b) 2.5 kg load; and

Figure 19 is a view similar to Figure 18 showing the Fourier analysis on an enlarged scale around the 12th harmonic at (a) no load, (b) 1 kg load, (c) 2.5 kg load and (d) 3.5 kg load.

The principle of operation of an embodiment of the present invention will first be described with reference to Figure 1. Figure 1(a) shows the hysteresis curve for a typical magnetostrictive material. In the figure, H represents applied magnetic field strength, and B represents the flux density in the magnetostrictive material. The values of H for which B is zero are known as the coercivity, He. As can be seen from the figure, as H increases in either direction, the value of B reaches a limit, which is the saturated flux density for the material. The slope of the graph in the region where B changes from one saturated value to the other is the permeability of the material.

In practice, the permeability which is actual observed for a piece of material, known as its "external" permeability, will depend on the shape of the piece of material. A sphere of any ferromagnetic material has a relative permeability (compared with the permeability of a vacuum) of close to 3. An infinitely long thin rod has the maximum permeability, known as the "intrinsic" permeability, and the value of this varies from material to material. The intrinsic relative permeability of a magnetostrictive material is typically very high and may for example be greater than 100,000. The coercivity is typically very low. It may for example be 3A/m.

In Figure 1(a) the continuous line shows schematically the property of a typical magnetostrictive material which is not under any stress. The width of the hysteresis loop has been exaggerated for clarity. If the material is stressed, its properties change so that the coercivity He is increased, and the permeability is decreased, as shown in broken lines on Figure 1(a). By using a very thin piece of magnetostrictive material, a permeability close to the intrinsic permeability can be obtained, and the effect of stressing such a piece of material on its magnetic properties can be very marked.

Figure 1(b) shows a sine wave alternating magnetic field, being applied to the magnetostrictive material whose properties are depicted in Figure 1(a). Figure 1(c) depicts schematically the flux density in the piece of magnetostrictive material while the alternating magnetic field is applied. As in Figure 1(a), the solid line shows the case for an unstressed piece of magnetostrictive material and the broken line shows the case for a stressed piece of magnetostrictive material.

Considering first the material in its unstressed condition, the flux density is saturated for most of the input magnetic field values, and it changes very abruptly from a saturated magnetic flux density in one direction to a saturated magnetic flux density in the reverse direction as the applied field strength passes through a small range of values around the coercivity. Then the flux density remains at this saturated level until it reverses again. Accordingly, the plot of the flux density is very close to a square wave. Owing to the hysteresis of the material, the phase of the flux density square wave lags slightly behind the phase of the applied magnetic field, as the flux density does not pass through zero until the applied field reaches the value of the coercivity.

When the piece of magnetostrictive material is stressed, its permeability decreases and its coercivity increases . The decreasing permeability means that the induced flux density will change more gradually over a longer period of time, and therefore its graph is less like a square wave. Additionally, the increase in coercivity means that the induced flux density passes through zero at a greater time lag after the instant when the applied magnetic field passes through zero, that is to say the phase difference between the induced flux density and the applied magnetic field increases. Both of these effects can be seen from the broken line in Figure 1(c).

Figure 1(d) shows schematically the e.m.f. induced in a nearby conductor by the changing flux density of Figure 1(c). It is essentially an alternating spike waveform, having a sharpness dependent on the degree of approximation of the flux density waveform to a square wave, and a phase determined by the phase of the flux density waveform. Accordingly, if the alternating magnetic field of Figure 1(b) is input, a signal as shown in Figure 1(d) can be detected in a nearby conductor. By comparing the phase of the input signal of Figure 1(b) with the phase of the detected signal of Figure 1(d), the coercivity of the piece of magnetostrictive material can be detected. By detecting the amplitude of harmonics in the detected signal of Figure 1(d) which will change with the shape of the waveform, the permeability of the piece of magnetostrictive material can be detected. Both the permeability and the coercivity will provide measures of the degree of stress applied to a piece of magnetostrictive material. Accordingly, if a pressure sensor is arranged so that the pressure to be sensed stresses a piece of magnetostrictive material, these effects may be used to detect the pressure.

Figure 2 shows a pressure sensor embodying the present invention, and Figure 3 shows the pressure sensor of Figure 2 in section. The sensor of Figures 2 and 3 comprises a rigid box 1, which is sealed by a diaphragm 3, which is in turn covered with a thin film, a few micrometres thick, of a magnetostrictive material 5. Suitable materials include the "2605" range of alloys sold under the trade mark "METGLAS" by Allied Signal Corporation, Metglas Products, 6 Eastmans Road, Parsippany, New Jersey 07054, United States of America. Magnetostrictive alloys are also sold by Vacuumschmelze GmbH, Werk Hanau, Gruner Weg 37, 6450 Hanau, Germany. Magnetostrictive materials tend to be extremely strong and stiff, and even a very thin layer of magnetostrictive material 5 can contribute significantly to the stiffness of the diaphragm 3.

The volume inside the box 1 is sealed, and therefore contains a fixed quantity of fluid. As pressure outside the box 1 varies, the diaphragm 3 and layer of magnetostrictive material 5 will be deformed by the pressure difference between the inside of the box 1 and the outside of the box 1, and this deformation of the magnetostrictive material 5 can be detected as discussed with reference to Figure 1. The box 1 can be made of any convenient material, such as non-magnetic stainless steel or synthetic resin.

If the magnetostrictive material 5 is obtained as a sheet or a strip of a suitable thickness, it can be bonded to the diaphragm 3 using glue. Alternatively, a piece of the magnetostrictive material can be used as a sputter target in a sputter deposition process to deposit a film of it directly onto the diaphragm 3. An advantage of sputter depositing is that the film of magnetostrictive material 5 deposited on the diaphragm 3 can be very thin, for example about 1 micrometre thick. By contrast, METGLAS 2605, referred to above, is typically supplied as a ribbon 12 micrometres thick.

Figure 4 shows an alternative to the structure of Figures 2 and 3, in which the layer of magnetostrictive material 5 may be used to close the box 1 without any separate diaphragm 3. This avoids the need to provide a bond between the diaphragm 3 and the layer of magnetostrictive material 5 which can resist possible delamination owing to the bending of the laminated structure of the diaphragm 3 and the magnetostrictive layer 5. However, the arrangement of Figures 2 and 3 can be advantageous as it allows the whole surface of the magnetostrictive layer to be used for bonding. The arrangement of Figure 4 requires a suitably strong and permanent bond to be provided between the layer of magnetostrictive material 5 and the end surface of the side wall of the box 1.

A very thin film of magnetostrictive material 5, e.g. about 1 micrometre thick, suffers greater deformation for a given pressure difference and also its permeability will tend to be close to its intrinsic permeability, and this increased sensitivity means that a material having a less marked magnetostrictive property can be used. This may be advantageous, as the materials such as METGLAS 2605 which demonstrate very high magnetostriction are typically alloys containing high percentages of iron and liable to corrosion. If the alloy composition is changed to reduce the proportion of iron and increase the proportion of cobalt, the corrosion resistance tends to increase but the degree of magnetostriction tends to decrease. Accordingly, if the sensor is designed so that a less magnetostrictive material is usable, an alloy having greater corrosion resistance than the high iron alloy may be used.

The box 1 is shown as cylindrical in Figure 2, but this shape is not essential.

Figure 5 shows a top view of another sensor embodying the present invention. This sensor is generally similar to the sensor of Figure 2 , except that the layer of magnetostrictive material 5 does not cover the whole of the diaphragm 3, but is provided as an elongate strip extending generally diametrically across the diaphragm 3. This embodiment is convenient to manufacture when the magnetostrictive material 5 is provided as a narrow ribbon, as a length of the ribbon may be cut to form the strip to be fastened to the diaphragm 3. This shape is also advantageous if the magnetostrictive material 5 is supplied with a thickness of the order of 10 micrometres or above. It has been found in practice that for a diaphragm more than 2 or 3 cm across the shape of the layer of magnetostrictive material 5 does not greatly affect its permeability if the layer is no more than about 1 micrometre thick, but the effect of shape becomes increasingly significant as the thickness increases above about 1 micrometre. If the layer is 10 micrometres or more thick a long thin strip has a significantly higher permeability than a disc. Accordingly, where the layer of magnetostrictive material 5 is of the order of 10 micrometres thick, or thicker, the strip shape of Figure 5 is preferable to the disc shape of Figure 2.

Typically, a strip 15 micrometres thick, 0.5 millimetres wide, and 50 mm long will be suitable.

In practice, a strip 12 micrometres thick, 10 millimetres wide and 25 millimetres long has been used satisfactorily, although this shape does not give the best possible performance as its external permeability is not as close to the intrinsic permeability of the material as could be obtained with a narrower, longer or thinner strip.

The strip should be thin; preferably its thickness is no more than one tenth of its width and preferably its thickness is no more than one thousandth of its length. Its cross-section should be small in relation to its length; preferably the square of its length is at least 1,000 times its cross-sectional area, preferably at least 5,000, more preferably at least 10,000 and most preferably at least 100,000.

Figure 6 shows the circuit for a detector for use with any of the sensors of Figures 1 to 5. In the circuit an oscillator 7 generates a sine wave signal at a chosen high frequency. An analog frequency divider 9 divides the output of the oscillator 7 by a predetermined integer, corresponding to the number of the harmonic which will be used to detect changes in the properties of the layer of magnetostrictive material 5 owing to changes in the pressure applied to it. An amplifier 11 amplifies the output of the analog frequency divider 9, and supplies the amplified signal to an interrogator coil 13. The signal applied by the amplifier 11 to the interrogator coil 13 will be a sine wave. As is indicated schematically in Figure 6, the interrogator coil 13 is positioned opposed to the sensor, and it emits an alternating magnetic field as shown in Figure 1(b). A detector coil 15 is also positioned opposing the sensor, and as explained with reference to Figure 1, the waveform of Figure 1(d) is induced in the detector coil 15. The output of the detector coil 15 is supplied to a bandpass filter 17, which filters the signal to pass only a predetermined harmonic of the frequency of the signal supplied to the interrogator coil 13. The properties of the analog frequency divider 9 and the bandpass filter 17 are chosen so that the selected harmonic has the frequency of the output from the oscillator 7. The output from the bandpass filter 17 is amplified in an amplifier 19 and is supplied to a phase detector 21, which also receives the output from the oscillator 7.

As is well known in the phase detector art, the phase detector 21 compares the two signals input to it, and outputs two signals, known as I and Q. The I signal has an amplitude corresponding to the amplitude of the component of the signal received from amplifier 19 which is in phase with the reference signal supplied from oscillator 7. The Q signal has an amplitude corresponding to the amplitude of the quadrature component of the signal input from amplifier 19. The vector sum of the amplitudes of the I and Q signals provides a measure of the amplitude of the detected harmonic component, and the ratio between the amplitudes of the I and Q signals provides a measure of the tangent ("tan") of the phase angle between the detected harmonic and the output of the oscillator 7.

The I and Q signals are provided to a processor 23, which uses the properties of the I and Q signals to determine changes in either or both of the permeability and the coercivity of the piece of magnetostrictive material 5 in the sensor, using the effect of these on the amplitude and phase of the selected harmonic as explained with reference to Figure 1. From this, the processor 23 can monitor the pressure applied to the piece of magnetostrictive material 5, and it provides an output pressure signal to an output device 25. Depending on the desired use of the pressure sensor, the output device may be a display, a printer, or further circuitry which responds to the pressure signal in some predetermined manner. Where the sensor is used as a low tyre pressure warning device in a vehicle, the processor 23 may compare the detected pressure with a preset threshold level, and the pressure output signal may indicate whether the detected pressure is above or below the preset reference. In this case, the output device 25 may include a lamp or other alarm to notify the driver of the vehicle when the pressure signal indicates that the sensor pressure is below the preset threshold.

The physical arrangement of the coils 13,15 is shown schematically in Figure 7. The interrogator coil 13 is provided in a simple loop shape, whereas the detector coil 15 is provided in a figure-of-eight shape. This arrangement provides very good rejection by the detector coil 15 of signals induced in it directly by the interrogator coil 13. The coils could be connected the other way around so that the simple loop is the detector coil 15 and the figure-of-eight is the interrogator coil 13. In order to ensure that there is coupling between the piece of magnetostrictive material 5 and the figure-of-eight coil, the sensor must be positioned to be opposed to one of the lobes of the figure-of-eight, as is shown schematically in Figure 7. The relative arrangement of the coils 13, 15 and the piece of magnetostrictive material 5 should be such that the piece of magnetostrictive material 5 is magnetised along its easy axis.

Figure 8 shows an alternative to the circuit of Figure 6. In Figure 8, both the amplifier 11 providing the interrogation signal and the bandpass filter 17 receiving the detected signal are connected to a common coil 27, which replaces both the interrogator coil 13 and the detector coil 15. The common coil 27 can be provided as a simple loop, with the sensor physically arranged to be opposed to it, as shown schematically in Figure 9. Accordingly, the size of the coil system in the circuit of Figure 8 can be reduced as compared with the circuit of Figure 6, and the circuit is simplified in that only one coil is required. However, it will be noted that the output of the amplifier 11 is directly coupled to the input of the bandpass filter 17, with the result that the fundamental frequency used to interrogate the sensor will be input to the bandpass filter 17 with a very high amplitude. Accordingly, the bandpass filter 17 should have a very high rejection ratio for the fundamental frequency compared with the frequency of the selected harmonic. The remainder of the circuit of Figure 8 is the same as the circuit of Figure 6, and will not be described further.

The circuits of Figures 6 and 8 are suitable for detecting the pressure applied to the sensor from the phase of the selected harmonic of the detected signal. However, use of the amplitude of the detected harmonic may be less reliable in these circuits as the amplitude can be altered by factors other than changes in the permeability of the piece of magnetostrictive material 5, such as variations in the degree of magnetic coupling between the piece of magnetostrictive material 5 and the coils and variations over time in the gain of the amplifiers 11,19, and the bandpass filter 17. In order to compensate for such variations, and allow the circuit to detect changes in pressure from changes in the permeability of the piece of magnetostrictive material 5 even when other factors are varying the amplitude of the detected signal, the circuits of Figures 6 and 8 are preferably modified as shown in Figure 10.

In Figure 10 a supplementary bandpass filter 17', supplementary amplifier 19' and a supplementary phase detector 21' are provided in parallel with the main bandpass filter 17, main amplifier 19 and main phase detector 21. The detected signal from the detector coil 15 or the common coil 27 is supplied to both the main bandpass filter 17 and the supplementary bandpass filter 17', and the I and Q signals from both the main phase detector 21 and the supplementary phase detector 21' are supplied to the processor 23. A supplementary analog frequency divider 9' is connected to the output of the oscillator 7, and the main phase detector 21 receives the output from the oscillator 7 as a comparison phase signal, whereas the supplementary phase detector 21' receives the output from the supplementary analog frequency divider 9' as the phase comparison signal. The main analog frequency divider 9 may receive as its input either the output from the oscillator 7 or the output from the supplementary analog frequency divider 9' .

In the circuit of Figure 10 the main bandpass filter 17 is designed to pass the frequency of the output of the oscillator 7, whereas the supplementary bandpass filter 17' is designed to pass the frequency of the output from the supplementary analog frequency divider 9'. In this way, the detection circuitry detects the phase and amplitude of two different harmonics in the detected signal. The outputs from the main phase detector 21 relate to a relatively high order harmonic and the outputs from the supplementary phase detector 21' relate to a relatively low order harmonic. The change in the waveforms of Figures 1(c) and (d) as the permeability increases have a greater effect on the amplitude of the higher order harmonics than on the amplitude of the lower order harmonics. Accordingly, the relative amplitudes of different harmonics also changes. In the modification of Figure 10, the processor 23 is enabled to compare the amplitudes of different order harmonics, in place of detecting the absolute amplitude of one harmonic. The result of this comparison is less susceptible to factors other than the permeability of the piece of magnetostrictive material 5.

If it was desired to provide a detection circuit which responded only to variations in the amplitude of the detected signal and not to variations in its phase, the phase detector 21 is not necessary. However, it is convenient to provide it as its output will enable both amplitude and phase information to be used. If only phase information is to be used, it is in principle possible to provide a circuit which detects the phase of the entire induced signal of Figure 1(d), or which uses the fundamental frequency. However, the arrangement of Figures 6 and 8 using a bandpass filter 17 and detecting a harmonic of the induced signal, reduces the effects of coupling between any parts of the interrogation signal from the frequency divider 9 and the amplifier 11 into the detection part of the circuit, which could alter the detected phase, and additionally the phase angle of a harmonic frequency changes by a greater amount than the phase angle of the fundamental frequency for a given time delay, so that the phase angle of a harmonic frequency provides a more sensitive measure of changes in the coercivity of the piece of magnetostrictive material 5.

Typically, harmonics of orders 5 to 20 will be used in the illustrated detection circuits, although harmonics of orders from 2 to 100 will be usable in many circumstances.

The frequency of the interrogating signal output by amplifier 11 to the interrogator coil 13 or the common coil 27 can be selected from a wide range. The coercivity of a magnetic material is generally frequency related, and increases with frequency. The coercivity at low frequencies provides a more sensitive response to stress applied to the piece of magnetostrictive material 5. However, in order to provide sufficient detected signal for the coercivity or permeability of the piece of magnetostrictive material to be detected, several cycles of the input interrogation signal are required. Therefore, if the input interrogation signal has a low frequency, for example 1 Hz, a relatively long time is required to obtain a pressure reading. This will not be acceptable in some applications. Additionally, it can be difficult to provide effective filtering of the signals at the these frequencies.

Where the coils of the detection circuitry do not move relative to the sensor, interrogation frequencies in the range 20Hz to 3000Hz, preferably 200 Hz to 300 Hz, will typically be satisfactory. However, where the sensor is moving relative to the coils, for example if the sensor is mounted on or in a wheel of a vehicle to rotate with it while the coil is mounted on an adjacent non-rotating part of the vehicle, the sensor will only be adjacent the coil to be cJosely coupled thereto intermittently, and for relatively short periods. Accordingly, the input interrogation frequency must be sufficiently high to provide several cycles during each period in which the sensor is closely coupled to the coils. Typically for a vehicle tyre pressure sensor, an interrogation frequency of several kHz, for example 3 to 10 kHz, will be suitable.

Figure 11 shows a pressure sensor embodying the present invention in a configuration for measuring the air pressure inside a vehicle tyre. A hub assembly 29, stationary with respect to the vehicle body, carries a rotating wheel hub 31. A brake disc 33 and a wheel 35 are both mounted on the hub 31. A brake calliper 37 is mounted for action against the brake disc 33. A pneumatic tyre 39 is fitted to the wheel 35.

A pressure sensor is fastened securely to the sidewall of the tyre 39, so as to be inside the pressurised volume enclosed by the tyre. The sensor may be any of those illustrated in Figures 2 to 5, and is mounted with the box 1 secured to the tyre and the layer of magnetostrictive material 5 exposed to the pressurised air within the tyre. A coil unit, comprising the interrogator and detector coils 13,15 of a circuit as shown in Figure 6 or the common coil 27 of a circuit as shown in Figure 8 is mounted on the stationary hub assembly 29 at a position to be opposite the pressure sensor once per revolution of the wheel 35 and tyre 39. The remainder of the circuit may be mounted with the coil unit, or the signals to and from the coil unit may be conveyed to the remainder of the circuit mounted elsewhere.

As the wheel 35 rotates, the sensor will pass the coil unit once a revolution, and at this time the pressure within the tyre, as detected by stress in the layer of magnetostrictive material 5, can be detected in the manner discussed above. If it is desired, a plurality of sensors may be provided spaced around the sidewall of the tyre 39, so that the pressure within the tyre can be detected a corresponding plurality of times per revolution of the wheel 35.

In an alternative to the arrangement of Figure 11, the sensor can be provided merely by a strip or piece of magnetostrictive material 5, as shown in Figure 12, bonded directly to the sidewall of the tyre 39. In this arrangement, the tyre 39 itself acts as the diaphragm, and the air outside the tyre provides the pressure reference. As the air pressure inside the tyre varies, the tyre sidewall will change shape slightly, stressing the piece of magnetostrictive material 5.

The sidewall of the tyre 39 is also deformed by pressure from the road surface as it passes through the bottom position in its rotation. Accordingly, it is most convenient to mount the coil unit away from the bottom of the wheel, so that the tyre sidewall does not experience this additional deformation as it passes the coil unit.

It may be preferable in some circumstances to fix the sensor to the wheel 35 itself, rather than to the tyre

39. This avoids any difficulties there may be in securing the box 1 of the sensor to the flexible sidewall of the tyre 39, and also avoids the consequence that the sensor would be lost whenever the tyre 39 is changed, and that a special tyre equipped with the sensor would have to be fitted if the pressure sensing arrangement was to continue to operate after a tyre change. However, it may not be possible to detect the state of the piece of magnetostrictive material 5 if there is metal between the sensor and the coils 13,15 or 27 of the detection circuit, because such metal will tend to interfere with the magnetic fields applied to the piece of magnetostrictive material 5 and emitted by it. In modern vehicle tyres, the reinforcing plies in the sidewall are often now synthetic fibres, rather than metal cords, and so this problem can be avoided in the arrangement of Figure 11. However, where the wheel 37 is metal, steps should be taken to avoid interference by the metal wheel with the magnetic fields in arrangements in which the sensor is mounted on the wheel. For this reason, further embodiments of the sensor as illustrated in Figures 13 and 14, may be preferable when the sensor is to be mounted on the wheel 35.

In Figures 13 and 14, the layer of magnetostrictive material 5 is shown but the diaphragm 3 is not shown, for clarity. However, it should be understood that in these sensors there may be a separate diaphragm 3 bearing a disc of magnetostrictive material 5 as shown in Figures 2 and 3, a disc of magnetostrictive material 5 acting as a diaphragm as shown in Figure 4, or a diaphragm 3 bearing a strip of magnetostrictive material 5 as shown in Figure 5, in the place where the magnetostrictive material 5 is shown in Figures 13 and 14.

The arrangement of Figure 13 is similar to the arrangements of Figures 2 to 5, except that the box 1 has a threaded boss 41, and a bore 43 passes through the threaded boss 41 to connect the volume inside the box 1 with the environment to which the end face of the threaded boss 41 is exposed. In use, a wheel to which the sensor of Figure 13 is to be fitted is pierced by a threaded hole, and the sensor of Figure 13 is attached to the outside of the wheel by screwing the threaded boss 41 into the hole. In this way, the sensor seals the hole in the wheel so that the air pressurising the tyre does not escape, and the volume inside the box 1 communicates with the pressurised volume enclosed by the tyre through the bore 43, although the sensor itself is mounted outside the wheel. Therefore the volume inside the box 1 is pressurised to the tyre pressure, and the piece of magnetostrictive material 5 will be stressed by the difference in pressure between the tyre pressure inside the box 1 and the atmospheric pressure outside it. Since the piece of magnetostrictive material 5 is outside the wheel, a coil unit can be mounted to be opposed to the piece of magnetostrictive material 5 for sensing the pressure without the metal of the wheel passing between them.

Figure 14 shows a modification of the sensor of Figure 13, in which the box 1 extends beyond the diaphragm formed by or bearing the piece of magnetostrictive material 5, so that the box 1 contains a first volume which is in communication with the environment through the bore 43, and a second volume which is sealed and is not in communication with the environment or the bore 43. The second volume provides a standard reference volume the pressure of which is not affected by variations in atmospheric pressure, while the first volume is pressurised to the tyre pressure in the manner described with reference to Figure 13.

An example of a configuration for sensing the tyre pressure of a vehicle wheel using a sensor according to Figure 13 or Figure 14 is shown in Figure 15. Most parts of Figure 15 are the same as in Figure 11, and the description of these will not be repeated. In Figure 15 a threaded hole has been formed in the main circumferential surface of the wheel 35 which supports the tyre 39, and a sensor as shown in Figure 13 or Figure 14 has been attached to the radially inner side of the wheel by screwing its threaded boss 41 into the hole. Thus, as can be seen in Figure 15, the box 1 of the sensor (and therefore the piece of magnetostrictive material 5) is outside the pressurised volume contained by the tyre 39. However, the interior of the box 1 is in communication with the pressurised volume through the bore 43. Accordingly, the piece of magnetostrictive material 5 is stressed by the inflation pressure of the tyre 39. The coil unit comprising the interrogator coil 13 and detector coil 15 or the common coil 27, is mounted on a piece of the stationary hub assembly 29 which opposes the circumferential surface of the wheel 35. As can be seen from Figure 15, the material of the wheel does not come between the sensor and the coil unit, so that even if the wheel is of metal this does not interfere with the transmission of magnetic fields between the coil unit and the piece of magnetostrictive material 5. The configuration of Figure 15 is also advantageous in that the sensor is mounted away from the region where a person works when replacing the tyre 39, so that the sensor is less likely to be damaged during the removal and replacement of the tyre 39. Additionally, if it is ever necessary to replace the sensor, this can be done without removing the tyre 39 since it is accessible from outside the pressurised volume.

In cases where the wheel 35 is made of a material which does not unduly obstruct the passage of magnetic fields, the sensor can be screwed into the hole in the wheel from the other side, so that the box .1 is inside the pressurised volume, provided that the sensor of Figure 13 is used. In this case, the interior of the box would be at atmospheric pressure by the action of the bore 43, while the environment outside the box 1 would be at the tyre pressure. The sensor of Figure 14 cannot be used in this way as there would then be no way of applying the tyre pressure to the piece of magnetostrictive material 5.

Figure 16 shows the top view of a further embodiment of the present invention, and Figure 17 shows a section through the embodiment of Figure 16. In this embodiment, a strip of magnetostrictive material 5 mounted on the diaphragm 3 is used as a core for a coil 45. Stress on the piece of magnetostrictive material 5 will vary the inductance of the coil 45. It should be noted that the inductance of the coil 45 is changed by the changes in the magnetic properties of the piece of magnetostrictive material 5, rather than by relative movement between the piece of magnetostrictive material 5 and the coil 45.

The inductance of the coil 45 can be sensed in a variety of ways. If it is desired to avoid a permanent connection between the detection circuitry and the sensor, as in the configurations of Figures 11 and 15, the coil may be connected across a capacitor to create a tuned circuit having a resonant frequency dependent on the capacitance of the capacitor and the inductance of the coil 45. A variable frequency alternating magnetic field can be applied to the inductor 45, to induce oscillations in the tuned circuit. Alternatively the tuned circuit can be connected to another coil or circuit for coupling a received alternating magnetic field to the tuned circuit. The resonant frequency of the tuned circuit can be determined by varying the frequency of the applied magnetic field over a range covering expected resonant frequencies of the tuned circuit. The oscillations in the tuned circuit will pass through a maximum when the applied field is at the resonant frequency, and the tuned circuit will absorb maximum energy from the interrogator coil at this frequency. By monitoring the frequency at which maximum energy absorption occurs, the resonant frequency can be detected, and from this the pressure applied to the diaphragm 3 can be deduced.

Alternatively, in applications where a permanent connection to the sensor is acceptable, for example when monitoring the fluid pressure in a supply line, the inductance of the coil 45 can be monitored by any of a variety of methods which will be familiar to those skilled in the art.

The sizes and spacings of various components will depend on the intended application of the sensor. As an example, in laboratory tests a 2 cm diameter circular sensor made from a 1 micrometre thick layer of a cobalt-based alloy was subjected to an alternating magnetic field from an interrogator coil 13 of 20 amp-turn and 15 cm diameter, 10 cm from the sensor. An 8 cm diameter detector coil 15 successfully detected the field emitted by the magnetostrictive material in response to the applied field.

In another test, a sample of plastic film 15 mm thick and 60 mm x 15 mm in area supported a cobalt-nickel-iron amorphous alloy layer 1 micrometre thick. This was stretched by applying loads of up to 3.5 kg. An interrogation alternating magnetic field was applied, with a frequency of 5kHz, via an interrogator coil 13, and the signal induced in a detection coil 15 was subjected to Fourier analysis and displayed, with particular attention being given to the 12th harmonic. The coils were configured as shown in Figure 7. The results are shown in Figures 18 and 19. In these figures, the amplitude is shown in a log scale, so that a constant distance change in the amplitude direction of the graph represents a constant factor change in the amplitude of the signal.

Figure 18(a) shows the Fourier analysis of the detected signal at zero load. Figure 18(b) shows the Fourier analysis of the detected signal at 2.5 kg load. It can be seen from these figures both that the amplitude of harmonics decreases with increasing harmonic number, and that the effect of load on the amplitude is greater for the higher order harmonics.

Figure 19 shows a part of the Fourier analysis of the detected signal, around the 12th harmonic, with an expanded horizontal (frequency) axis. Figure 19(a) shows the signal at no load. Figure 19(b) shows the signal at 1 kg load. Figure 19(c) shows the signal at 2.5 kg load. Figure 19(d) shows the signal at 3.5 kg load. It can be seen by comparing Figure 19(a) with Figure 19(d) that the amplitude of the 12th harmonic decreased by over 20dB, i.e. it was reduced by a factor or more than 10, at a load of 3.5 kg compared with the zero load condition. Each vertical axis division in Figures 18 and 19 represents a change of lOdB.

The load applied to a piece of magnetostrictive material in a pressure sensor will depend on the area over which the sensor pressure is applied. By a suitable choice of materials it should be possible to create a sensor having a diaphragm of the order of 1 cm diameter in which the amplitude of the 12th harmonic in the detected signal would change by at least 10% for a change in applied pressure of 1 lb per square inch (approx. 7 kPa). Thus, it can be seen that a usable vehicle tyre pressure sensor can be made at a convenient size.

Various embodiments of the present invention have been described for the purpose of illustration. They should not be taken to be limiting, and various modifications and alternatives will be apparent to those skilled in the art.

Claims

1. A pressure sensing system comprising: a pressure sensor comprising a piece of magnetostrictive material arranged to be stressed by the pressure to be sensed, thereby affecting its magnetic properties; and a signal extraction means for applying an alternating magnetic field to saturate the piece of magnetostrictive material, detecting the alternating magnetic flux induced thereby in the piece of magnetostrictive material, and providing an output based on at least one parameter of said magnetic flux which varies with variation in a magnetic property of the piece of magnetostrictive material.

2. A system according to claim 1 in which the said at least one parameter comprises at least one of: (a) the phase of the waveform of the alternating magnetic flux or a harmonic thereof; (b) the amplitude of a harmonic in the waveform of the alternating magnetic flux; and (c) the difference between the phase or the amplitude of a plurality of harmonics in the waveform of the alternating magnetic flux.

3. A system according to claim 1 in which the said magnetic property comprises permeability or coercivity.

4. A system according to any one of claims 1 to 3 in which the signal extraction means comprises coil means for applying the said alternating magnetic field and for generating an e.m.f. induced by the said alternating magnetic flux, signal generating means for supplying an alternating electric signal to the coil means for generating the said alternating magnetic field, and detection means for receiving the said e.m.f. and generating the said output therefrom.

5. A system according to claim 4 in which the detection means comprises phase detector means.

6. A system according to claim 4 or claim 5 in which the detection means comprises bandpass filter means.

7. A system according to any one of claims 4 to 6 in which the coil means comprises a first coil coupled to the signal generating means, for applying the said alternating magnetic field, and a second coil coupled to the detection means for generating the said e.m.f.

8. A system according to any one of claims 4 to 6 in which the coil means comprises a coil coupled both to the signal generating means and to the detection means.

9. A pressure sensor in which a piece of magnetostrictive material is stressed by the pressure to be sensed, thereby affecting its magnetic properties, the square of the length of the piece of magnetostrictive material being at least 1000 times its cross-sectional area.

10. A sensor according to claim 9 in which the piece of magnetostrictive material is an elongate strip.

11. A sensor according to claim 9 in which the width of the piece of magnetostrictive material is the same order of magnitude as its length.

12. A sensor according to claim 11 in which the piece of magnetostrictive material is a disc.

13. A sensor according to any one of claims 9 to 12 in which a diaphragm comprising or coupled to the piece of magnetostrictive material is arranged to be subject to the pressure to be sensed on one side thereof and a reference pressure on the other side thereof.

14. A sensor according to any one of claims 9 to 13 in which the diaphragm closes a sealed chamber which provides the reference pressure.

15. A sensor according to any one of claims 9 to 13 in which the diaphragm bounds an unsealed chamber which is in communication with the exterior of the sensor, whereby the sensor may seal an aperture in a member so that the interior of the chamber is in communication by virtue of the aperture with fluid on one side of the member.

16. A sensor according to claim 15 in which the side of the diaphragm away from the chamber is in communication with the exterior of the sensor whereby it may be in communication with fluid on the other side of the said member.

17. A sensor according to claim 15 in which the side of the diaphragm away from the chamber bounds a further, sealed, chamber which provides a reference pressure.

18. A sensor according to any one of claims 15 to 17 having attachment means for attaching it to a member to seal an aperture therein, and the unsealed chamber is in communication with the exterior of the sensor through the attachment means.

19. A sensor according to any one of claims 13 to 18 in which the piece of magnetostrictive material is mounted on the diaphragm.

20. A vehicle wheel for fitting to a pneumatic tyre to define a volume, the wheel having a hole through a part thereof which defines the said volume and a pressure sensor mounted to seal the hole, the pressure sensor comprising a piece of magnetostrictive material arranged to be stressed by the pressure in the volume, thereby to affect its magnetic properties.

21. A vehicle wheel according to claim 20 in which the sensor comprises a diaphragm which is subject to the pressure in the volume and which comprises or is coupled to the piece of magnetostrictive material.

22. A vehicle wheel according to claim 21 in which the sensor comprises a chamber bounded by the diaphragm, the diaphragm being on one side of the said part of the wheel and the interior of the chamber being in communication with the other side of the said part through the hole.

23. A vehicle wheel according to claim 22 in which the diaphragm is outside the said volume and the interior of the chamber communicates with the said volume through the hole.

24. A vehicle tyre having a pressure sensor mounted on its inner surface, in which pressure sensor a piece of magnetostrictive material is stressed by the pressure in the tyre, thereby affecting its magnetic properties.

25. A vehicle tyre having a piece of magnetostrictive material fixed to it to be stressed by distortion of the tyre wall due to pressure inside the tyre.

26. A vehicle equipped with a pressure sensing system according to any one of claims 1 to 8.

27. A method of detecting the degree of stress in a piece of magnetostrictive material in a pressure sensor, which piece of magnetostrictive material is stressed by the pressure to be sensed, the method comprising: applying an alternating magnetic field to the piece of magnetostrictive material with a peak magnetic field strength sufficient to saturate magnetically the piece of magnetostrictive material, detecting the alternating magnetic flux induced in the piece of magnetostrictive material by the alternating magnetic field, and deriving an output from at least one parameter of said magnetic flux which varies with variation in the degree of stress of the piece of magnetostrictive material.

28. A method according to claim 27 in which the said at least one parameter comprises at least one of: (a) the phase of the waveform of the alternating magnetic flux or a harmonic thereof; and (b) the amplitude of a harmonic in the waveform of the alternating magnetic flux.

29. A method according to claim 27 in which the said at least one parameter comprises a parameter of a harmonic in the waveform of the alternating magnetic flux.

30. A method according to any one of claims 27 to 29 in which the step of detecting the alternating magnetic flux comprises generating an e.m.f. from the alternating magnetic flux, and the step of deriving an output comprises analysing at least one parameter of the e.m.f.

31. An apparatus for interrogating a pressure sensor comprising a piece of magnetostrictive material arranged to be stressed by the pressure to be sensed, thereby affecting its magnetic properties, the apparatus comprising: interrogation means for generating an alternating magnetic field to be applied to the piece of magnetostrictive material of the pressure sensor and detecting the alternating magnetic flux induced thereby in the piece of magnetostrictive material to generate an output signal in accordance therewith, and signal analysis means for analysing the signal output by the interrogation means to detect the phase and/or the amplitude, in the waveform of the alternating magnetic flux, of a harmonic of the frequency of the alternating magnetic field or to detect the difference in the said phase and/or amplitude between a plurality of said harmonics, thereby to provide an indication of the degree to which the piece of magnetostrictive material is stressed.

32. Apparatus according to claim 31 in which the signal analysis means comprises filter means to filter the frequency of alternating magnetic field from the output signal of the interrogation means.

33. Apparatus according to claim 31 or claim 32 in which the signal analysis means comprises phase detector means for receiving the said harmonic or one of the said harmonics and a signal from the interrogation means having a set phase relationship with the alternating magnetic field.

34. Apparatus according to any one of claims 31 to 33 in which the interrogation means comprises signal generating means outputting a first signal at the frequency of the or one of the said harmonics and a second signal which is used in generating the alternating magnetic field, the signal generating means comprising oscillator means and both the first and the second signals being derived from the output of the oscillator means.

35. Apparatus according to claim 34 in which at least the second signal is derived from the output of the oscillator means via frequency divider means.

36. Apparatus according to any one of claims 31 to 35 in which the interrogation means comprises a coil for receiving an alternating electric signal and generating the alternating magnetic field therefrom and for receiving the alternating magnetic flux and generating an e.m.f. therefrom.

37. Apparatus according to any one of claims 31 to 35 in which the interrogation means comprises a first coil for receiving an alternating electric signal and generating the alternating magnetic field therefrom and a second coil for receiving the alternating magnetic flux and generating an e.m.f. therefrom.

38. Apparatus according to claim 37 in which the first and second coils are mounted in close proximity to each other and are wound so as to minimise coupling from the first coil to the second coil.