WO2007116218A1

WO2007116218A1 - Measuring physical quantities

Info

Publication number: WO2007116218A1
Application number: PCT/GB2007/001325
Authority: WO
Inventors: Serghei Sandacci
Original assignee: Sensor Technology Limited
Priority date: 2006-04-11
Filing date: 2007-04-10
Publication date: 2007-10-18
Also published as: GB0607300D0

Abstract

A method and apparatus for measuring physical quantities using amorphous ferro-magnetic sensor elements 1, 1'. Reliance is placed on the change in switching field which causes such materials to exhibit the Barkhausen effect seen as the material is subjected to stress. In a preferred form of sensor two sensor elements 1, 1' of the same material are used, which have been pre-stressed to different degrees.

Description

MEASURING PHYSICAL QUANTITIES

This invention relates to measuring physical quantities. In particular it relates to the measurement of strain be this linear strain or rotational strain/torque and the measurement other quantities which may be determined as the result of monitoring the stress or strain on a sensing element.

There is a general desire to be able to produce effective measurement apparatus, for example, strain or torque measuring devices which are cost-effective and if possible, contacts-less.

It is an aim of the present invention to provide apparatus and methods for allowing such measurements.

In the present invention use is made of sensing elements comprising amorphous ferro-magnetic material. Such amorphous ferro-magnetic material may, for example, be in the form of microwires, ribbons or thin films. Such structures can be created so as to exhibit bi-stable magnetic switching behaviour. Moreover, it has been realised by the applicants that the sensitivity of such switching behaviour to tensile or compressive stress can allow use of such materials in methods and apparatus for measuring physical quantities.

In the present specification mention is made of the Barkhausen effect. This effect is a well known effect in the field of magnetic materials and is described, for example, in W. Brown "Micromagnetics ", New York: Wiley, (1963). A brief explanation of the Barkhausen effect is also given below making reference to Figure 1.

Ferro-magnetic materials have small magnetic areas with random magnetic orientation which are called magnetic domains and which are separated by high- energy walls. In general, the movements of the domain walls in ferro-magnetic materials are very complex and difficult to analyse due to the stochastic nature of process. However, in some cases, for example for ferro-magnetic materials with large positive magnetostriction, the magnetisation process can be characterised by a magnetic Barkhausen discontinuity (MBD), which occurs when the material is subjected to an alternating magnetic field. A Barkhausen discontinuity takes place when an applied magnetic field exceeds a predetermined threshold value called the switching field (Hsw)- When this occurs, fast moving domain walls will generate a short burst of magnetic flux, which can be detected by, for example a pick-up coil or magnetic field sensor, located in a close proximity to a specimen. In amorphous ferro-magnetic materials where the microcrystalline structure is very weak or does not exist at all, the switching field can be influenced by magnetoelastic energy. This is one of the main components determining the energy equilibrium of a specimen of amorphous ferro-magnetic material. In turn, magnetoelastic energy is a function of applied and intrinsic stress. Figures 1 shows a specimen of amorphous ferromagnetic material 1 in an applied magnetic field H and in a saturation state, i.e. fully magnetised in the direction of the applied field H. The specimen has a domain structure comprising a main domain Dl in the middle and closure domains D2 at each end. The main domain Dl is aligned with the applied field H and the closure domains D2 are aligned against the field H. An energy equilibrium exists. The applicants believe that as stress is applied to such a specimen, the closure domains D2 move and change size/shape as illustrated by the dotted lines in Figure 1. The energy equilibrium needs to be maintained. If the direction of the applied field H is reversed, then at some level of applied field H, the main domain Dl and closure domains D2 will switch in magnetic orientation giving rise to the Barkhausen effect mentioned above. The field strength at which this occurs is of course the switching field Hsw- It is believed that as the closure domains alter in size and shape this causes the value of the required switching field Hsw to change.

In general the switching field of an amorphous specimen can be expressed as follows:

3 y

Hsw^ ^{~ λ}s (^σa + ^σr ) ^cos α (Eq- 1)

where a is the angle between the magnetisation direction (direction of applied

field H) and the axial direction of the sample (i.e. a = 0 in Figure 1), λ_s is a

saturation magnetostriction constant, σ_a is the applied stress and σ_r is the residual internal stress. The residual internal stress σ_r is effectively a constant for

any single specimen, it is the inherent stress in the material specimen itself resulting from the manufacturing processes/treatment of the specimen.

According to one aspect of the present invention there is provided apparatus for measuring a physical quantity comprising: a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; a magnetic field source for applying a varying magnetic field to a region occupied by the sensor; a detector for detecting bursts of magnetic flux which are generated by a magnetic

Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and determining means for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.

The determining means may comprise an analyser. The determining means or analyser may be configurable to determine and output values in a selectable way. The determining means or analyser may be a dedicated component arranged to perform one function - for example to determine if the physical quantity passes a threshold value. The determining means or analyser may be arranged to output an indicator having a value that is indicative of the value of the physical quantity. The value may, for example, be output as a signal to another component or output via a display. In other situations the determining means or analyser may be arranged to output an indicator which although dependent on the measured value of the physical quantity, is not directly representative of the measured value of the physical quantity. Such an indicator might, for example, be an alarm signal, which might be generated as a signal to a user (for example a light or an audible signal) or sent to another component. The apparatus may, for example, be arranged to generate such an indicator if the physical quantity changes in value or passes a threshold value.

The apparatus may be arranged for monitoring times at which bursts of magnetic flux are detected. The apparatus may be arranged for monitoring magnitudes of applied magnetic field at which bursts of magnetic flux are detected.

Preferably the apparatus is arranged so that the varying magnetic field is an oscillating magnetic field arranged to trigger the Barfchausen effect in the sensor element both as the field is increasing one direction and as the field is increasing in the opposite direction. Preferably the apparatus is arranged so that the

Barkhausen effect is triggered twice in the sensor element in each cycle of the varying magnetic field. The varying magnetic field may be sinusoidal in time, but other waveforms, such as square waves, sawtooths, triangular waves and so on are possible. The detector may be arranged to detect the bursts of flux generated by the Barkhausen effect both as the magnetic field increases in one direction and as the magnetic field increases in the opposite direction. The determining means/analyser may be arranged for determining a value for the physical quantity in dependence on the bursts of flux detected both as the magnetic field increases in one direction and as the magnetic field increases in the opposite direction.

The determining means/analyser may be arranged for determining the magnitude of applied magnetic field at which bursts of magnetic flux are detected. The determining means/analyser may be arranged for determining an average of the magnitude of applied magnetic field at which a burst of magnetic flux is detected as the applied magnetic field increases in one direction and the magnitude of applied magnetic field at which a burst of magnetic flux is detected as the applied magnetic field increases in an opposite direction.

The sensor may comprise a second amorphous ferro-magnetic sensor element. The second sensor element may be a reference sensor element. The second sensor element may be arranged so as to avoid it experiencing changes in stress in response to changes in the physical quantity. The second sensor element may be arranged to be subjected to changes in stress in response to changes in the physical quantity in a different way than the first sensor element is arranged to be subjected to changes in stress in response to changes in the physical quantity. The or each amorphous ferro-magnetic sensor element may comprise a portion of amorphous ferro-magnetic microwire.

The or each amorphous ferro-magnetic sensor element may be pre-stressed.

The apparatus may be arranged so that during operation, the sensor element remains under tension even when a compressive stress is applied to the sensor during measurement.

The second amorphous ferro-magnetic sensor element may be pre-stressed by a different amount from the first amorphous ferro-magnetic sensor element.

The first and second amorphous ferro-magnetic sensor elements may be of the same material.

The first and second amorphous ferro-magnetic sensor elements may be taken from the same batch of microwire.

The detector maybe arranged to detect the bursts of flux generated by a Barkhausen effect in the magnetisation process of the second sensor element under action of the varying magnetic field.

The determining means/analyser may be arranged for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux from both the first and second sensor elements. The determining means/analyser may be arranged for determining a value for the physical quantity in dependence on a differencing operation performed in respect of the detected bursts of magnetic flux from both the first and second sensor elements.

The determining means/analyser may be arranged for determining a value for the physical quantity in dependence on a difference between the magnitude of applied magnetic field at which the detected burst of magnetic flux from the first sensor element is detected and the magnitude of applied magnetic field at which the detected burst of magnetic flux from the second sensor element is detected.

The determining means/analyser may be arranged for determining a value for the physical quantity in dependence on a difference between the time at which the detected burst of magnetic flux from the first sensor element is detected and the time at which the detected burst of magnetic flux from the second sensor element is detected.

According to another aspect of the present invention there is provided a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity.

The sensor may comprise a second amorphous ferro-magnetic sensor element. The or each amorphous ferro-magnetic sensor element may comprise a portion of ferro-magnetic amorphous microwire. The first amorphous ferro-magnetic sensor element may be pre-stressed. The second amorphous ferro-magnetic sensor element may be pre-stressed and optionally by a different amount than that which the first is pre-stressed. Preferably the second sensor element is one of: arranged so as to avoid it experiencing changes in stress in response to changes in the physical quantity; and arranged to be subjected to changes in stress in response to changes in the physical quantity in a different way than the first sensor element is arranged to be subjected to changes in stress in response to changes in the physical quantity.

According to a further aspect of the present invention there is provided a method of measuring a physical quantity comprising the steps of: providing a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; applying a varying magnetic field to a region occupied by the sensor; detecting bursts of magnetic flux which are generated by a magnetic Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.

The method may comprise the further step of outputting an indicator having a value that is indicative of the value of the physical quantity. The method may comprise the further step of outputting an indicator, which although dependent on the measured value of the physical quantity, is not directly representative of the measured value of the physical quantity.

The amorphous ferro-magnetic sensor element may be pre-stressed and the method may comprise the step of subjecting the sensor to compressive stress to measure a quantity whilst the sensor element remains in tension due to the degree ofpre-stressing.

According to another aspect of the present invention there is provided apparatus for measuring a physical quantity comprising: a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; a magnetic field source for applying a varying magnetic field to a region occupied by the sensor; a detector for detecting bursts of magnetic flux which are generated by a magnetic

Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and an analyser for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.

For the sake of brevity not all of the optional features described above are set out after each aspect of the invention, however it is to be understood that each of the optional features defined above in relation to one aspect of the invention are equally applicable as optional features for the other aspects of the invention where

context allows.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 schematically shows an amorphous ferro-magnetic specimen in an applied magnetic field;

Figure 2 schematically shows an apparatus for measuring a physical quantity;

Figure 3 is a plot showing the magnetic field applied to a specimen in the apparatus of Figure 2 and an output representing magnetic flux, as detected by the apparatus shown in Figure 2;

Figure 4 is a plot showing the magnetic field applied to a specimen in the apparatus shown in Figure 2 and an output representing magnetic flux detected by the apparatus of Figure 2 under different prevailing conditions;

Figures 5a to 5d schematically show different forms of sensor, which may be used in an apparatus of the type shown in Figure 2;

Figure 6 schematically shows apparatus that may be used in pre-stressing microwire for use in a sensor of an apparatus of the type shown in Figure 2; and Figures 7, 8 and 9 show various plots indicating the properties of example portions of micro wire to illustrate the effects of pre-stressing microwires for use in apparatus of the type shown in Figure 2.

Figure 2 schematically shows apparatus for measuring a physical quantity, for example, strain, rotary strain or torque, pressure or similar quantities. The apparatus comprises a sensor S comprising a specimen of amorphous ferromagnetic material 1 as a sensing element. The specimen 1, which, acts as, a sensing element is of the same type as the specimen 1 shown in Figure 1. In the present embodiment the sensing element 1 comprises a portion of ferro-magnetic amorphous microwire. This microwire may, for example, be manufactured in the form of wire in a glass coating by what is known as the Taylor-Ulitovskiy method see for example G. Taylor, "Process and apparatus for making filaments ", US Patent 1,793,529 (1931) and A. Ulitovsky, I. Maianski, A. Avramenco, "Method of continuous casting of glass-coated microwires ", Patent No. SU 128427 (1960). As one example the wire may have a composition Of(FeCgCOo₁I)_7SSi₁₅Bi_O. Such wire is commercially available. It will be appreciated that in other examples different amorphous ferro-magnetic materials may be used and they may be in the form of ribbons or thin films as mentioned above.

The sensor element 1 of this type is chosen so as to exhibit bi-stable magnetic switching behaviour in applied magnetic field and so that this bi-stable magnetic switching behaviour is sensitive to applied stress. In the present embodiment the sensing element 1 is a microwire with a diameter of approximately 20μm and a length of approximately 20mm. It will be appreciated that suitable sensing elements may be chosen as a matter of design choice depending on the application in which the sensor is to be used, the sensitivity which is required and so on. The use of microwire is preferred because of the magnetic properties which can be obtained in such wires. It has been found that the use of such wires can produce a well defined and "large" Barkhausen response. It is thought that with such a wire the domain structure can more closely approach the ideal shown in Figure 1 because with a wire (which is a round object in cross section), there will typically be no edge domains as there would be in a ribbon or thin film. It is believed that this can help in producing a well defined and "large" Barkhausen response.

The apparatus comprises means for applying a magnetic field 3 to the region in which the sensor S is located. The means for applying magnetic field 3 comprises an AC source 31 and a pair of driving coils 32. The driving coils 32 are disposed either side of the location of the sensor S and serve to provide a substantially spatially constant magnetic field in the region of the sensor S. In the present embodiment the driving coils 32 are in a so-called Helmholtz configuration, but other arrangements of driving coils 32 may be used and other arrangements for generating magnetic fields may also be used.

The apparatus comprises a detector 4 for detecting signals generated by the sensor S and an analyser unit 5 for analysing the signals detected by the detector 4. In the present embodiment, the detector 4 comprises a pick-up or receiver coil 41 which is connected to an amplifier 42, the output of which in turn is connected to a band pass (or notch) filter 43. The output of the band pass filter 43 is connected to the analyser unit 5 which has another input connected to a sync output of the AC source 31 used to generate the applied magnetic field.

The sensor S comprises an elastic substrate 11 to which the sensing element 1 is mounted. The elastic substrate 11 may be of silicon and similar to such substrates used in conventional thin film strain gauges. The sensing element 1 is mounted to the elastic substrate 11 in such a way that if the elastic substrate 11 is subjected to compressive or tensile strain (or more particularly if the surface of that substrate 11 to which the sensing element 1 is mounted is subjected to compressive or tensile strain), the sensing element 1 itself is also subjected to compressive or tensile strain or to put this another way it is subjected to compressive or tensile stress. Arranging for the sensing element 1 to experience stress as the elastic substrate 11 is stressed is achieved by bonding each end of the sensing element 1, i.e. the ends of the microwire, to the elastic substrate 11.

(Sensing elements can also be directly attached via suitable adhesive or a plastic insulating membrane that can be adhesively attached to objects to be sensed. The sensing element can be also coated with protective layer of elastic seal against environmental conditions i.e. corrosion).

In operation, an alternating magnetic field H is applied to the sensor S by the drive coils 32 under the control of the AC source 31. Provided that the magnitude of the alternating magnetic field is correctly chosen this causes the sensing element 1 to omit bursts of magnetic flux due to the Barkhausen effect described above when the applied magnetic field H reaches the switching field Hsw- The applied magnetic field H and the bursts of magnetic flux generated by the sensor S are picked up by the receiving coil 41, amplified by the amplifier 42, and fed into the band pass filter 43. The frequency selection of the band pass filter is chosen so that signals due to the applied magnetic field are filtered out and the signal passed on to the analyser unit 5 is basically representative of the bursts of flux caused by the Barkhausen effect alone.

Figure 3 shows a plot of the applied magnetic field H and the filtered detected magnetic flux B_F as output by the band pass filter 43 to the analyser unit 5. This plot shows a single cycle of the applied magnetic field H and two flux bursts BH₁ and BH₂ which have been generated by the Barkhausen effect having occurred in the sensing element 1. The first burst occurred as the switching field Hsw was reached for the sensing element with the applied magnetic field H having one sense and the second burst when the switching field Hsw was reached in the opposite sense.

The analyser unit comprises a Digital Signal Processor or FPGA (Field Programmable Gate Array) and is arranged to digitise and analyse the sync signal fed in from the AC source 31 and the output of the band pass filter 43 to determine the value of the applied field H at which the Barkhausen effect occurred. That is to say, to determine the value of Hsw- It will be appreciated that as Hsw is proportional to the applied stress as illustrated by Equation 1, it is possible to calibrate the analyser unit 5 to give an indication of the stress which sensor S was subjected to at the point when measurement was taken. Of course continuous and real time measurements may be taken each time the Barkhausen effect occurs. Furthermore, the apparatus in principle may be arranged and calibrated to be used in the measurement of strain, torque, pressure or any other physical quantity, which will cause the sensor to experience stress.

At present it is envisaged that the frequency of the applied magnetic field H will be in the range of 0.1 Hz to 1OkHz - this frequency range being selected to take into account the dynamic properties of the ferro-magnetic sensing element 1. It will be appreciated that the possible sampling rate for measuring the stress to which the sensor S is being subjected is governed by the frequency of the applied field.

Theoretically, the magnitude of the switching field Hsw with the magnetic field in one direction will be identical to that with that of the magnetic field H in the opposite direction. In practice, however, there are likely to be offsets which mean that the magnitudes of the switching field Hsw determined during one cycle of the alternating magnetic field H will differ from one another. Averaging the determined values for the switching field Hsw for two adjacent, and therefore opposite measurements, can remove the effect of many types of offset. Thus the analyser unit 5 in the present embodiment is arranged to carry out such averaging as part of its calculation process. The Earth's magnetic field is one possible cause of an offset. It will be appreciated that in some cases it may not be necessary to actually determine the value of the switching field Hsw provided that the required information concerning the desired physical property can be determined. That is to say, there may be a direct calibration between, for example, the time during the cycle of applied magnetic field that the flux burst is seen and the value of the quantity, which is to be measured. Thus the analyser unit 5 may be arranged to monitor the timing of flux bursts caused by the Barkhausen effect and use these timings to indicate the value of the quantity, which is to be measured.

Once a physical quantity has been determined by the analyser unit 5 this may be displayed to the user via a display or output as a signal to be fed to another component for further use. In some embodiments the final calculation of a value for a physical quantity may be conducted in a device which is separate from the analyser unit.

It will be noted that the present system is a contact-less one in that there is no physical contact between the sensor S and the interrogation and analysis apparatus 3, 4, 5. Furthermore, the sensor S requires no self-contained power source, it merely being a component that reacts to the applied magnetic field H. This means that the sensors S themselves may be produced very cheaply and used in a wide variety of circumstances where a need to provide a power source and/or direct physical contact with the sensor would cause a difficulty or make operation impossible. The operation and sensitivity of the sensor S and apparatus as a whole can be improved by a variety of techniques, some of which are explained in more detail below.

The sensor S, in particular the elastic substrate 11, will typically be mounted to an object, the strain or stress of which is to be monitored. It will be appreciated that in such a case, the elastic substrate 11 is mounted to the object so that as the object experiences stress this is correspondingly passed on to the substrate 11 and hence sensing element 1. Thus the sensor S may, for example, be mounted to a shaft, bolt, or other component, the stress or strain of which is to be measured, or to the surface of a tyre wall where the pressure in the tyre is to be measured (the pressure in the tyre of course, being related to the stress/strain in the wall of the tyre).

Figure 5a shows an alternative form of sensor S, which is mounted on an object O, the tensile or compressive strain of which is to be monitored. The sensor S comprises an elastic substrate 11, on which is mounted a first sensor element 1 which is the same as the sensor element 1 described above in relation to Figure 2 and has both of its ends mounted to the elastic substrate 11. The sensor S also comprises a second sensor element 1 ' which again is a piece amorphous ferromagnetic microwire, but which in this case has only one of its ends mounted to the elastic substrate 11. In this situation the second sensing element 1 ' acts as a reference element. Because only one of its ends is fixed to the elastic substrate 11, if the object O and hence elastic substrate 11 is subjected to stress (and hence strain) although the first sensing element 1 will also experience this stress, the second sensing element 1 ' will not.

Where such a sensing element S is used in an apparatus of the type shown in Figure 2, whilst the switching field Hsw required to change the magnetisation of the first sensing element 1 will change with the stress applied to the object O, the switching field Hsw of the second sensing element 1 ' will be unaffected by stress applied to the object O.

Figure 4 shows a plot against time of applied magnetic field H and filtered detected magnetic flux B_F as seen at the output of the band pass filter 43 in the apparatus of Figure 2 where the sensor S has the form shown in Figure 5a. Here there are a total of 8 flux bursts, which are illustrated as having been produced by the sensing elements 1, 1 '. It will be noted that four of flux bursts BH₁, BHR₁, BH₂ and BH_R2 are shown in solid lines whereas four of the flux bursts BH_1C, BH₁T, BH₂C and BH₂χ are shown in dotted lines. This is because not all of these flux bursts would occur at one point in time, but would rather occur in accordance to the prevailing conditions. BH₁ and BH₂ are flux bursts generated by the first sensing element 1 when there is no applied compressive or tensile stress on the object O. Flux bursts BH_R1 and BHR₂ are produced by the second sensing element 1 ' whether or not there is tensile or compressive stress exerted on the object O. On the other hand BHic and BH₂C are flux bursts, which would be generated when there is a compressive stress exerted on the object O and flux bursts BHIT and BH_2T would be generated if there were tensile stress exerted on the object O.

With this type of sensor used in the apparatus of Figure 2, the analysing unit 5 is arranged to use the flux bursts BH_R1 and BH_R2 from the second sensing element 1 ' as reference points. That is to say the time of, or value of the H-field at which the flux bursts from the first sensor element 1 occur may be measured relative to the time of, or value of the H-field at which the flux bursts from the second sensor element 1 ' occur. Thus during the positive going part of the driving magnetic field H the difference between the time of, or H field value at the burst BH, BH_1C or BHI_T and the burst BHR₁ may be used and during the negative going part of the field H the difference between the burst BH₂, BH_2C or BH₂T and the burst BHR₂ may be used. Such a differencing in technique allows constant and time varying offsets due to, for example, the Earth's magnetic field or temperature to be taken into account, thus providing greater accuracy and/or sensitivity.

Again, of course, appropriate calibration can be used to allow the analyser unit 5 to use these measured values and differences to provide an indication of the value of the physical quantity which is to be measured.

In this case although the sensor S as shown in Figure 5a is set up for measuring tensile or compressive strain, the value generated by the analyser unit 5 may be a strain measurement, a stress measurement, a pressure measurement or a measurement of any other physical quantity which is proportional to tensile or compressive strain.

Figures 5b to 5d show alternative forms of sensor S which may be helpful in measuring particular quantities in particular circumstances.

Figure 5b shows a sensor S which comprises first and second sensing elements 1, 1 ' each mounted to respective elastic substrates 11, where the elastic substrates 11 are mounted on opposing sides of a member M. This sensor is useful where it is desired to measure the bending force on a member M in a direction transverse to the surfaces on which the sensing elements 1, 1 ' are mounted.

Figure 5c shows a sensor S which is arranged for measuring rotational strain or torque on a body B. Here two sensing elements 1, 1' are mounted to an elastic substrate 11 at right angles to one another and each at 45° to the axis of rotation of the body B. Each end of each of the sensing elements 1, 1 ' is fixed to the elastic substrate 11 such that as the body B twists one of the sensing elements will be compressed and the other tensioned.

Figure 5d shows an alternative sensor 5 for use in measuring the torque or rotational strain exerted on a body B. Here again there are two sensor elements 1, 1'. Each is mounted to a respective piece of elastic substrate 11, and here the sensor elements 1,1 ' are mounted parallel with one another and on opposing faces of the body B. Both ends of each of the sensing elements 1, 1' are mounted to the elastic substrates 11.

Each of the elements 1,1 ' is arranged at 45° to the axis of rotation of the body. Again here as torque is applied to the body B one of the elements 1, 1' will experience compression and one tension. However because each element 1,1 ' can have the same orientation relative to the applied field and any ambient offset fields, it is an easier matter to subtract the effects of offsets by using differencing techniques than is the case for the sensor shown in Figure 5c where the sensing elements are perpendicular to one another.

Each of the sensors shown in Figures 5b to 5d may be used in apparatus of the type shown in Figure 2, the analyser unit 5 being suitably configured and calibrated to allow measurement of the desired quantity.

It should be noted that there are other forms of detector 4 which may be used for detecting magnetic flux generated by the sensor S. In particular two detecting coils may be used; one for detecting the magnetic field generated by the driving coils 32 and the other for measuring the magnetic field at the sensors which will be a superposition of that generated by the sensor S and that generated by the driving coils 32. Where such a two coil technique is used the signal picked up by the coil used just to measure the applied magnetic field is subtracted from that used to detect the combined field in the region of the sensor. However, in the present embodiment the use of two receiver coils is avoided by use of the band pass filter 43 which works well to the filter out the driving signals as the frequency of the signals generated by the sensor are spaced well away from the frequency of the applied magnetic field. In further alternatives other types of magnetic field sensors (besides coils) may be used for example magneto-resistive or magneto-impedance magnetic field sensors.

As will be noticed in the plot shown in Figure 4, there is separation between the ("resting") flux bursts BHi and BH₂ which are generated by the first sensing element 1 and the flux bursts BH_R1 and BHR₂ which are generated by the second sensing element 1 '. hi the sensors used in generating the plot shown in Figure 4 this separation was achieved by using two different types of micro wire for the different sensing elements 1, 1'.

This separation in response of the sensing elements 1, 1 ' is useful to ease determination of from which sensing element a bursts of flux is being generated.

However, the use of two different types of micro wire (or indeed two different types of any other class of "sensing material") in the sensor S to achieve this separation has disadvantages, hi particular, the characteristics of the microwires will be different. For example, they are likely to have different temperature coefficients, different stress sensitivity and linearity and also different sensitivity to any external magnetic field. This makes compensation for these different effects difficult and reduces the accuracy of the results which might be achieved and/or makes it more difficult and expensive to produce a device giving the desired sensitivity and accuracy.

A further issue with the sensing elements 1, 1' of the type described above is that when the sensor S is put under a compressive stress, the sensing elements 1, 1 ' are also put under compression. It has been found that the behaviour of the sensing elements 1, 1 ' under compression is different from that when under tension. That is to say, the response to stress and the linearity of that response is different. Generally speaking when the sample is under compression the response is less linear.

The Applicants have developed alternative systems which aim to address these issues.

Simply stated the idea is to pre-stress the microwire used for the sensing elements 1, 1'. Figure 6 shows schematically an apparatus which may be used for pre- stressing (or equivalently pre-straining) microwire to be used in sensing elements

1, 1'.

A length of microwire 1 is attached to a frame F having a declining surface, the microwire 1 being attached at the higher end of the declining surface and running along that surface. The remote end of the microwire 1 is loaded with a load L of a known weight. This puts the microwire 1 under tension and strains the microwire. Two pieces of elastic substrate 11a and 1 Ib are then attached to the wire 1 whilst it is in the stressed condition. It will be seen that this gives rise to a sensor S of the general type described above. The wire may be adhesively attached to the elastic substrates 11a, 1 Ib in the stressed condition, ie under tension. Other fixing methods might be used. One particular possibility is the encapsulation of microwires into a substrate as the substrate is in the process of solidification from a liquid state. Example materials for use as substrates include polyamide and other polymers having suitable elastic properties. It will be noted that the elastic substrate has to hold the microwire under the pre-stressed tension and also be able to move in such a way as to vary the stress on the microwire 1 as the sensor S is subjected to different stresses during its use in measuring a physical quantity.

By pre-stressing the wire in this way, a much more linear response may be achieve when measuring physical quantities. This is because the apparatus may be arranged so that even when the sensor S is being subjected to a compressive force due to the prevailing conditions which it is to measure, the portion of microwire 1 which is being used to measure that quantity is still under tension compared with its rest state.

Figure 7 shows a plot of switching field against applied torque for a portion of wire which has been pre-stressed l_p and a portion of wire which has not been pre- stressed I_n. This illustrates the difference in response which can thereby be obtained. The table below shows an example of the type of strain and resulting stress which might be applied to a microwire 1 in this pre-straining technique using an apparatus of the type shown in Figure 6. In this instance, the microwire used had a diameter of 8 μm.

In the table, it will be noted that the figure given for applied weight is a weight in grams of the load L applied using the apparatus of the type shown in Figure 6. The figures given for strain are those which were measured as each weight was applied to the sample wire. The value for stress is approximate and calculated. It is noted that for microwires such as this, the Youngs modulus can, in most cases, be found only by experiment. The remaining column in the table shows how the switching field (ie the value of applied magnetic field at which a burst of flux is generated by the Barkhausen effect) varies with the pre-stressing to which the microwire is subjected.

From this it can be seen that as a different degree of pre-stress/strain is applied to the microwire, its response to the magnetic field, ie its value of switching field, varies.

This leads to another possibility in producing sensors which has been used by the Applicant. That is to say rather than using two sensing elements 1, 1' which are made of different pieces of microwire, the same type of microwire may be used and indeed different portions of the same length of microwire may be used for two sensing elements 1, 1' with one of these being pre-stressed to a greater degree than another. In this way it is possible to obtain the separation in response as desired and mentioned above in relation to Figure 4, without using different materials.

Thus, for example, a sensor of the type shown in Figure 5d may be used where the first sensing element 1 has been pre-stressed by applying a weight of 5 grams as the load L on apparatus of the type shown in Figure 6 and the second sensor element 1 ' has been obtained by applying a weight of 15 grams as the load. In such a case the two wires 1, 1 'will have different "rest" switching fields so that their respective bursts of magnetic flux will be separated in the same kind of way as shown in Figure 4. In this type of arrangement with two pieces of the same wire being pre-stressed by different degrees and used in the sensor, various advantages can be achieved: a) the temperature co-efficient is the same for both sensor elements 1, 1' because they have the same composition and react to temperature in a similar way; b) the sensitivity for both sensor elements is to all intents and purposes, identical for the same reason; c) it is easier to match the pair of wires 1, 1 ' for use in the differential measurement system by pre-stressing them to different degrees than it is to look for a match between two different wires having different compositions with different geometrical and physical parameters. To put this another way, by taking the same piece of wire and pre-stressing it to a different degree then it is in the hands of the designer of the sensor S to achieve the characteristics which they require. Where alternative methods are used which rely on finding different types of wire to achieve the desired characteristics, eg separation between their switching fields, much more reliance is placed on the availability and properties of existing wires.

Figure 8 is a plot showing the dependence of switching field on the angular position of the body B (or shaft) of a set-up as shown in Figure 5d where, as described above, the first of the sensing elements 1 is obtained by applying a load of 5 grams and the second 1 ' is obtained by applying a load of 15 grams. As can be seen, the dependence on angular position of the shaft is opposite for these two sensors 1, 1 '. This is because they are on opposing sides of the body B, but otherwise they are closely matched to one another, ie symmetrical, such that when the difference between the two responses is taken, the differential response is almost a completely straight line, as also indicated in the plot.

Figure 9 is a plot of switching field against applied torque showing the response of the first sensor element 1 when in a positive magnetising field and also the response of the first sensing element 1 when in a negative magnetising field. That is to say the plot shows the switching field of the sensing element 1 when in the two different halves of the cycle of the applied magnetic field, as is similarly shown in Figure 4 for the sensor of Figure 5a. The plot in Figure 9 also shows the differential response of the first sensing element 1 where it can be seen that a high degree of linearity is obtained by taking the difference between the responses in the two halves of the cycle.

Further as mentioned above, the reaction of the second sensing element 1 ' will be very similar to that of the first sensing element 1 (although shifted in its rest response due to the different degree of pre-stressing (pre-straining)) so that when differential measurements are taken between the response of the first sensing element 1 and second sensing element 1 ' (or indeed additive readings are taken) most potential sources inaccuracy and differences in response will be cancelled or can be ignored. This can dramatically simplify the process of obtaining accurate measurements using the apparatus.

Claims

CLAIMS:

1. Measurement apparatus for measuring a physical quantity comprising: a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; a magnetic field source for applying a varying magnetic field to a region occupied by the sensor; a detector for detecting bursts of magnetic flux which are generated by a magnetic Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and determining means for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.

2. Measurement apparatus according to claim 1 in which the amorphous ferro-magnetic sensor element comprises a portion of ferro-magnetic amorphous microwire.

3. Measurement apparatus according to claim 1 or claim 2 in which the amorphous ferro-magentic sensor element is pre-stressed.

4. Measurement apparatus according to claim 3 in which the apparatus is arranged so that during operation, the sensor element remains under tension even when a compressive stress is applied to the sensor during measurement.

5. Measurement apparatus according to any preceding claim in which the sensor comprises a second amorphous ferro-magnetic sensor element.

6. Measurement apparatus according to claim 5 when dependent on claim 3 or claim 4 in which the second amorphous ferro-magnetic sensor element is pre- stressed.

7. Measurement apparatus according to claim 6 in which the second amorphous ferro-magnetic sensor element is pre-stressed by a different amount from the first amorphous ferro-magnetic sensor element.

8. Measurement apparatus according to anyone of claims 5 to 7 in which the first and second amorphous ferro-magnetic sensor elements are of the same material.

9. Measurement apparatus according to claim 8 when dependent on claim 2 in which the first and second amorphous ferro-magnetic sensor elements are taken from the same batch of micro wire.

10. Measurement apparatus according to any one of claims 5 to 9 in which the second sensor element is arranged: a) so as to avoid it experiencing changes in stress in response to changes in the physical quantity; or b) so as to be subjected to changes in stress in response to changes in the physical quantity in a different way than the first sensor element is arranged to be subjected to changes in stress in response to changes in the physical quantity.

11. Measurement apparatus according to anyone of claims 5 to 10 in which the detector is arranged to detect bursts of flux generated by a Barkhausen effect in the magnetisation process of the second sensor element under action of the varying magnetic field.

12. Measurement apparatus according to any one of claims 5 to 11 in which the determining means is arranged for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux from both the first and second sensor elements.

13. Measurement apparatus according to claim 12 in which the determining means is arranged for determining a value for the physical quantity in dependence on a differencing operation performed in respect of the detected bursts of magnetic flux from both the first and second sensor elements.

14. Measurement apparatus according to claim 12 or claim 13 in which the determining means is arranged for determining a value for the physical quantity in dependence on a difference between the magnitude of applied magnetic field at which the detected burst of magnetic flux from the first sensor element is detected and the magnitude of applied magnetic field at which the detected burst of magnetic flux from the second sensor element is detected.

15. Measurement apparatus according to any one of claims 12 to 14 in which the determining means is arranged for determining a value for the physical quantity in dependence on a difference between the time at which the detected burst of magnetic flux from the first sensor element is detected and the time at which the detected burst of magnetic flux from the second sensor element is detected.

16. Measurement apparatus according to any preceding claim arranged so that the varying magnetic field is an oscillating magnetic field arranged to trigger the Barkhausen effect in the sensor element both as the field is increasing in one direction and as the field is increasing in the opposite direction.

17. Measurement apparatus according to claim 16 in which the apparatus is arranged so that the Barkhausen effect is triggered twice in the sensor element in each cycle of the varying magnetic field.

18. Measurement apparatus according to claim 16 or claim 17 in which the detector is arranged to detect the bursts of flux generated by the Barkhausen effect both as the magnetic field increases in one direction and as the magnetic field increases in the opposite direction.

19. Measurement apparatus according to any one of claims 16 to 18 in which the determining means is arranged for determining a value for the physical quantity in dependence on the bursts of flux detected both as the magnetic field increases in one direction and as the magnetic field increases in the opposite direction.

20. Measurement apparatus according to any preceding claim which is arranged for monitoring times at which bursts of magnetic flux are detected.

21. Measurement apparatus according to any preceding claim which is arranged for monitoring magnitudes of applied magnetic field at which bursts of magnetic flux are detected.

22. Measurement apparatus according to any preceding claim in which the determining means is arranged for determining the magnitude of applied magnetic field at which bursts of magnetic flux are detected.

23. Measurement apparatus according to claim 22 when dependent on claim 16 in which the determining means is arranged for determining an average of the magnitude of applied magnetic field at which a burst of magnetic flux is detected as the applied magnetic field increases in one direction and the magnitude of applied magnetic field at which a burst of magnetic flux is detected as the applied magnetic field increases in an opposite direction.

24. Measurement apparatus according to any preceding claim in which the determining means is arranged to output an indicator having a value that is indicative of the value of the physical quantity.

25. Measurement apparatus according to any preceding claim in which the determining means is arranged to output an indicator which although dependent on the measured value of the physical quantity, is not directly representative of the measured value of the physical quantity.

26. A sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in a physical quantity.

27. A sensor according to claim 26 in which the amorphous ferro-magnetic sensor element comprises a portion of ferro-magnetic amorphous microwire.

28. A sensor according to claim 26 or claim 27 comprising a second amorphous ferro-magnetic sensor element.

29. A sensor according to claim 28 in which the first amorphous ferromagnetic sensor element is pre-stressed, and the second amorphous ferromagnetic sensor element is pre-stressed by a different amount.

30. A sensor according to claim 28 or claim 29 in which the second sensor element is one of: arranged so as to avoid it experiencing changes in stress in response to changes in the physical quantity; and arranged to be subjected to changes in stress in response to changes in the physical quantity in a different way than the first sensor element is arranged to be subjected to changes in stress in response to changes in the physical quantity.

31. A method of measuring a physical quantity comprising the steps of: providing a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; applying a varying magnetic field to a region occupied by the sensor; detecting bursts of magnetic flux which are generated by a magnetic Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.

32. A method according to claim 31 comprising the further step of outputting an indicator having a value that is indicative of the value of the physical quantity.

33. A method according to claim 31 or 32 comprising the further step of outputting an indicator, which although dependent on the measured value of the physical quantity, is not directly representative of the measured value of the physical quantity.

34. A method according to any one of claims 31 to 33 in which the amorphous ferro-magnetic sensor element is pre-stressed and the method comprises the step of subjecting the sensor to compressive stress to measure a quantity whilst the sensor element remains in tension due to the degree of pre-stressing.

35. Measurement apparatus for measuring a physical quantity comprising: a sensor comprising an amorphous ferro-magnetic sensor element arranged to be subjected to changes in stress in response to changes in the physical quantity; a magnetic field source for applying a varying magnetic field to a region occupied by the sensor; a detector for detecting bursts of magnetic flux which are generated by a magnetic Barkhausen effect in the magnetisation process of the sensor element under action of the varying magnetic field; and an analyser for determining a value for the physical quantity in dependence on the detected bursts of magnetic flux.