WO1994019665A1

WO1994019665A1 - Magnetostrictive position detector

Info

Publication number: WO1994019665A1
Application number: PCT/GB1994/000316
Authority: WO
Inventors: Andrew Nicholas Dames
Original assignee: Scientific Generics Limited
Priority date: 1993-02-16
Filing date: 1994-02-16
Publication date: 1994-09-01
Also published as: GB9303047D0; AU6009094A

Abstract

To determine the relative position of two members, for example the position of a dial (104) on an axle (118), a magnetic bias element (122) and two magnetostrictive resonators (124, 126) are provided. The bias element (122) rotates with the dial (104) while the resonators (124, 126) remain stationary. Accordingly, the magnetic bias applied to the resonators (124, 126) varies as the dial (104) rotates. To read the position of the dial (104), an alternating magnetic field of appropriate frequency is applied to the resonators (124, 126) and the resulting resonance is detected. Reading is carried out at least twice, with a different value of an additional magnetic bias being applied to the resonators (124, 126) during each reading. By subtracting the signals obtained from a given resonator during the first and second readings, a difference signal is obtained. The difference signal from each resonator (124, 126) is then used to determine the angular position of the dial (104).

Description

Magnetostrictive Position Detector The invention relates to the field of indicating a variable such as the position of a movable member. For example, the invention may be applied to the determination of the position of the dials in the meter such as a water, gas or electricity meter or to linear or to rotary encoders in general.

Systems are known which use magnetostrictive resonators and bias elements for applying a magnetic bias field thereto for indicating the relative positions of two members. In such systems , an interrogating alternating magnetic field is applied when it is desired to read the positions of the members, and the positions are determined on the basis of the detected signal which has been modified by the magnetostrictive resonators in such a way that depends upon the relative positions of the bias elements and magnetostrictive resonators themselves. In order to be of use in practice, these systems must overcome the problem that the amount of the interrogating alternating magnetic field which is coupled into the magnetostrictive resonator is proportional to the magnetic bias applied by the bias element, and that the amount of resonance coupled out of the magnetostrictive resonator is also proportional to the magnetic bias, thereby rendering the amplitude of the signal detected in the reading device proportional to the square of the applied magnetic bias. This has a particular disadvantage in rotary systems since the relationship between position and detected signal is many-to-one, and accordingly it is not possible to determine a unique position from a given detected signal value.

WO92/12401, assigned to the present applicant and incorporated herein by cross-reference, discloses two techniques for overcoming this problem. In the first technique, a magnetostrictive resonator is caused .to resonate with a unique frequency signature for each relative position of the members. In the second technique, a plurality of magnetostrictive resonators are used and different combinations of resonators are caused to resonate depending upon the relative positions of the two members.

These prior art devices and techniques suffer from the disadvantage that a plurality of magnetostrictive resonators and/or bias elements are required, making construction complex and expensive.

It is an object of the present invention to overcome or alleviate one or more of these problems.

In one aspect, the invention comprises a method of indicating the value of a variable, in which a magnetostrictive resonator is magnetically biased with a bias dependent upon the variable, and an additional magnetic bias is applied.

In embodiments, the additional magnetic bias may be applied to resolve ambiguities.

In a second aspect, the invention comprises a method of indicating the value of a variable, in which a magnetostrictive resonator is magnetically biased with a bias dependent upon the variable, and first and second alternating magnetic fields are applied.

In a third aspect, the invention comprises use of such a method for reading a meter.

In a further aspect, the invention comprises a novel device for use in such a method, comprising magnetostrictive resonator means and means for applying a magnetic bias in dependence upon the variable.

In yet a further aspect, the invention comprises a device for use in such a method comprising means for applying an additional magnetic bias, and in another aspect comprises a device for use in such a method comprising means for applying first and second alternating magnetic fields. The invention also comprises processing means for use in such a method comprising means for processing a signal modified by a magnetostrictive resonator in dependence upon an applied magnetic bias which is dependent upon the variable and an applied additional bias to derive the value of the variable.

Embodiments of the invention have the advantage that a small number of magnetic resonators and a small number of magnetic bias elements are required. In some embodiments, two resonators and a single bias element may be employed. In other cases, a single bias element may be used.

A further advantage of some embodiments is that it is possible to obtain a signal which is proportional to the magnetic bias applied in dependence upon the variable.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which:

Figs. 1, 2, 3a and 3b are diagrams illustrating the electromagnetic/magnetostrictive principles utilised in the preferred embodiments of the invention;

Fig. 4 diagrammatically illustrates a remotely readable meter, such as a water meter, comprising six dials, according to a presently preferred embodiment of the invention;

Fig. 5 is a view in the direction of arrow A in Fig. 4 showing one dial of the meter;

Figs. 6a-6i illustrate the detected signal from one magnetostrictive resonator for different angular positions of the dial shown in Fig. 5;

Fig. 7 illustrates the detected signals from one magnetostrictive resonator when an additional magnetic bias is applied during reading;

Fig. 8A and Fig. 8B illustrate the processed difference signals derived from the first and second magnetostrictive resonators respectively;

Fig. 9 illustrates a technique for applying an alternating additional bias;

Fig. 10 is a diagram illustrating an arrangement for remotely reading the meter of Figs. 4 and 5;

Fig. 11 is a diagrammatic perspective view, partly cut away, showing a further embodiment of the invention which is applicable to linear rather than rotary encoders;

Fig. 12 shows an example of patterns of magnetisation which may be provided on the linearly movable member shown in Fig. 11; and

Figs. 13a and 13b show a modified arrangement of an embodiment of the invention.

Electromagnetic/Magnetostrictive Principles Fig. 1 shows a magnetostrictive element 12 in the form of a strip of magnetostrictive material disposed adjacent a rectangular plate 14 of hard material magnetised to act as a magnetic biasing element for the magnetostrictive element 12. The magnetic bias element 14 has south and north poles 24, 26 respectively at each end, and produces a magnetic field to which the magnetostrictive element 12 is subjected as indicated by the arrows.

As is known, with the bias element 14 magnetised in the same way as a simple bar magnet, that is with a north pole at one end and a south pole at the other end as illustrated in Fig. 1, the magnetostrictive element 12 is stressed by the resulting magnetic field in such a way that if the magnetostrictive element 12 were subjected to an external interrogating alternating magnetic field at a frequency equal to the natural (fundamental) frequency of the magnetostrictive element 12, that element would mechanically vibrate and produce a detectable, regenerated, modified alternating magnetic field having the same frequency. This effect can be understood from consideration of Fig. 2, which comprises two curves illustrating the way in which the sensitivity of the magnetostrictive element 12 to the applied interrogating magnetic field varies as a function of the strength of the biasing field produced in the element 12 by the magnetisation of the bias element 14. In Fig. 2, curve A is a plot of the strain produced in a magnetostrictive element against field H applied to the element. Thus, at the origin, where the field H is zero, there is no strain. If the field H is increased to a value S or -S, the magnetostrictive element becomes saturated and further increase in the field (in either direction) does not produce any further strain. Curve B shows that the sensitivity of the device to the applied field increases linearly with increasing strength of the bias field H and thus the strength of the magnetism applied to the magnetostrictive element 12 is chosen to provide a bias field towards the upper end of the sensitivity curve. For example, the field represented by the arrows in Fig. 1 might have a value HI or -HI as shown in Fig. 2. As described above, when the magnetic bias field applied to the magnetostrictive element 12 is as shown in Fig. 1, the element 12 will resonate at its fundamental, or natural, frequency in response to an applied interrogating field at that frequency. Fig. 3a illustrates a situation in which the magnetic pattern in bias element 14 comprises north pole regions 26 near to the centre of the strip and south pole regions 24 towards to the ends of the strip. Arrows 28 and 30 in Fig. 3A indicate the magnetic lines of force arising in the magnetostrictive element 12 from this pattern of magnetic bias. As shown by the direction of these arrows, the resulting field with which the magnetostrictive element 12 is biased is directed to the right in the right-hand portion of the strip 12 and to the left in the left-hand portion. Accordingly, when an interrogating alternating magnetic field of twice the natural frequency of the magnetostrictive strip 12 is applied, the two halves of the strip 12 will resonate in phase opposition to each other at a frequency equal to twice the natural frequency. Such resonance, which is a mechanical vibration, will produce a detectable regenerated magnetic field twice the natural frequency of the magnetostrictive strip 12. In such a case, if an interrogating alternating magnetic field equal to the natural frequency of the magnetostrictive strip 12 were applied, the regenerated magnetic field at that natural frequency would have an amplitude approximately equal to zero.

Fig. 3b shows a situation similar to that illustrated in Fig. 3a. In Fig. 3b, the magnetostrictive element 12 is again biased in opposite directions in each half thereof, but in this case, as shown by the direction of arrows 32 and 34, the element is biased to the right in the left- hand portion of the strip 12 and to the left in the right-hand portion. Again, with this pattern of applied magnetic bias, when an interrogating magnetic field twice the natural frequency of the magnetostrictive strip 12 is applied, the two halves thereof resonate in phase opposition to each other at a frequency equal to twice the natural frequency. Accordingly, a detectable regenerated magnetic field at twice the natural frequency of the strip 12 is produced. As in the case illustrated in Fig. 3a, when an alternating magnetic field equal to the natural frequency of the magnetostrictive strip 12 is applied, the regenerated magnetic field produced at the natural frequency has an amplitude approximately equal to zero.

Embodiments of the Invention

A presently preferred embodiment of the invention in a meter, for the reading of the positions of the dials thereof, will now be described with reference to Figs. 4 to 10. Referring to Fig. 4 and Fig. 5, there is shown a remotely readable meter 100 in which the invention is embodied. This meter comprises a conventional sensing mechanism 102 (not shown in detail), for example for sensing water flow, and six dials 104, 106, 108, 110, 112, and 114 driven by the sensing mechanism 102 through a conventional linkage which is not shown, but which is diagrammatically represented by broken lines 116.

The dials 104 to 114 are mounted for rotation on a common axle 118, and each of the dials is marked with the digits 0 to 9 around its periphery. The dials are interconnected and driven by drive means (not shown) so as to form a six decade counter, the six dials representing respectively the digits of a six-digit number in a conventional manner. An apertured window plate 120 is provided, through which the value representing the current rotary position of each dial can be visually inspected. The meter 100 is mounted in a conventional housing (not shown).

As can be seen with respect to dial 104, the centre of each dial is substantially hollow, and for each dial a wheel hub 122 which rotates in conjunction with the associated dial is provided on the common axle 118. In this embodiment, each wheel hub is made of ferrite-loaded plastic, which is magnetised as will be described below. Adjacent the periphery of each wheel hub are fixed two magnetostrictive resonators 124, 126, which remain stationary as the associated wheel hub and dial rotate. The magnetic field of the wheel hub 122 has the effect of magnetically biasing the magnetostrictive resonators 124, 126. In a presently preferred embodiment, each magnetostrictive resonator comprises a ferrite rod approximately 8mm to 12mm long and 1mm in diameter. Preferably, the ferrite used has a high magnetostriction. It has been found that magnetostrictive resonators of this type are advantageous in systems in which the magnetostrictive resonators are immersed in fluid, for example water meters, since their low surface area to cross-section ratio minimises fluid damping.

Each ferrite-loaded plastic wheel hub 122 is magnetised after assembly of the appropriate dial 104 to 114, the pattern of magnetisation being such as to produce a dipole along a defined diameter, that is, a north pole and a south pole at opposite ends of a diameter. In order to reduce coupling effects in the system, the wheel hub 122 is constructed such that the minimum magnetic material is placed as close to each magnetostrictive resonator 124, 126 as possible. Accordingly, the wheel hub 122 is magnetised about its periphery only. The wheel hub 122 is magnetised so as to provide a suitable field strength at the magnetostrictive resonators, for example 2500 amp/m.

In order to reduce the effect of external magnetic fields and to provide maximum magnetic coupling between the magnetised wheel hub 122 and each of the magnetostrictive resonators 124, 126, each resonator 124, 126 has a curved configuration which corresponds to the curvature of the wheel hub 122. Further, the gap between the periphery of the wheel hub 122 and the resonators 124, 126 is made small, for example 0.5mm or less. In a preferred embodiment, each magnetostrictive resonator 124, 126 is mounted using a pin (not shown) through its centre. It has been found in practice that such a mounting technique does not have an adverse effect on the mechanical vibration of the magnetostrictive resonators during operation.

A reading coil 128, for example comprising 10 to 20 turns of 0.1mm diameter wire, is provided beneath the window plate 120. The magnetostrictive resonators 124, 126 are located so as to be symmetric with respect to coil 128, thereby ensuring that the coil gives equal coupling to both resonators, and are arranged so that their centres are sited on orthogonal radii of the wheel hub 122.

The principle of operation of the above-described meter system will now be explained with reference to Figs. 6 to 9 .

In operation, each of the dials 104 to 114 rotates as the variable to be indicated, such as the amount of water used, changes. Accordingly, the angular orientation of the wheel hub 122 with respect to the magnetostrictive resonators 124, 126, and hence the biasing magnetic field experienced by the resonators 124, 126, changes.

As in the systems described in O92/12401, when it is desired to remotely read the meter, an interrogating alternating magnetic field is applied via the coil 128. As will be clear, there are 12 magnetostrictive resonators within the meter system 100. In order to be able to discriminate the resonant signal from each resonator, each resonator is constructed to have a slightly different length from the others, thereby providing a unique fundamental frequency of vibration. For example, the resonators are provided with different lengths in the range 8.6mm to 12mm such that each resonator resonates at a different fundamental frequency in the range 290 kHz to 208 kHz. The lengths of the magnetostrictive are so arranged that the frequency spacing between adjacent resonant frequencies is approximately 3%.

In order to read the value on each dial, it is necessary to apply the interrogating magnetic field at the natural frequency of the first magnetostrictive resonator for that dial and at the natural frequency of the second magnetostrictive resonator, and to detect the regenerated signal from each resonator. Accordingly, to read all six dials, it is necessary to apply the interrogating alternating magnetic field at 12 different frequencies. It is possible to apply the interrogating alternating magnetic field at the required frequencies in a number of different ways. For example, during interrogation, each frequency of the interrogating field may be transmitted in turn with a listening interval between. Alternatively, the frequency of the interrogating field could be swept through all relevant frequencies and there would thereafter be a listening interval for listening for regenerating signals. In this case, a sweep must, on the one hand, be slow enough to ensure that there is sufficient time for a response to be generated at each frequency and, on the other hand, the sweep must be completed in a time sufficient that all elements will still be resonating during the listening period. As a further alternative, an interrogating field having components at all the required frequencies could be transmitted simultaneously in a single burst, followed by a listening interval.

Since the principle of operation is the same with respect to each dial, the operation of a single dial, namely dial 104, will be described.

Fig. 6a shows the amplitude of the detected signal from the magnetostrictive resonator 126 as the angular position of the dial 104 varies from 0° to 360°. The signal shown in Fig. 6a is the detected signal regenerated by the magnetostrictive resonator 126 at its fundamental frequency, and can be understood by considering the magnetic field patterns present in the magnetostrictive resonator 126 for different angular positions of the wheel hub 122 as shown in Figs. 6b to 6i.

When the wheel hub is at 0° or 360° as shown in Fig. 6b, the biasing magnetic field pattern experienced by the resonator 126 is the same as that shown in, and described with reference to. Fig. 3a. That is, the two halves along the length of the magnetostrictive resonator 126 are biased in oppositely opposing directions. Accordingly, as described previously, when interrogated with a signal of frequency equal to the fundamental frequency of the magnetostrictive resonator 126, the regenerated signal will have an amplitude approximately equal to zero.

The magnetic bias field applied to the magnetostrictive element 126 when the hub 122 is rotated by 45° is shown in Fig. 6c. In this case, the magnetostrictive resonator 126 experiences a resultant bias along its length in the direction indicated by the arrow 140. The amplitude of this bias is less than the maximum that the magnetostrictive resonator 126 will experience (Fig. 6d) and accordingly the amplitude of the regenerated signal shown in Fig. 6a is less than the maximum value; it is in fact approximately one-half of the maximum value since, as described earlier, the amplitude of the regenerated signal is proportional to the square of the applied bias, in this case resulting in the amplitude of the detected regenerated signal having a (sine)² format.

Fig. 6d illustrates the case in which the magnetostrictive resonator 126 experiences the maximum applied magnetic bias from the wheel hub 122. Accordingly, the amplitude of the regenerated signal shown in Fig. 6a has a maximum at this angle (90°).

In Fig. 6e, the dial has rotated by 135°, and the magnetic bias experienced by the magnetostrictive resonator 126 has the same direction and amplitude as in the case where the dial had rotated by 45° (Fig. 6c). Accordingly, the amplitude of the regenerated signal is the same at angles of 45° and 135°. In Fig. 6f (180° rotation) , the magnetic bias experienced by the magnetostrictive element 126 is the same as that shown in, and described with reference to. Fig. 3b. Accordingly, the amplitude of the detected regenerated signal at the fundamental frequency of the element 126 is approximately zero for this angle.

Figs. 6g, 6h and 6i illustrate the magnetic bias experienced by the magnetostrictive resonator 126 for angles of rotation of 225°, 270° and 315° respectively. For the case of 225° (Fig. 6g) the amplitude of the applied magnetic bias is the same as that for 45° (Fig. 6c) but the direction is opposite. However, because the amplitude of the detected signal is proportional to the square of the applied magnetic bias, the value of the detected signal at 225° is the same as that at 45°. Similarly, the amplitude of the detected signal at 270° is the same as that at 90°, and the amplitude of the detected signal at 315° is the same as that at 135°.

Thus, the magnetic bias experienced by the magnetostrictive resonators 124, 126 has a periodic format, which, in this embodiment, is substantially a sine wave. Accordingly, the detected regenerated signal is an analogue signal and has a (sine)² format, as shown in Fig. 6a. It will be clear from a consideration of Fig. 6a, that for a given detected signal amplitude, it is not possible to determine a unique angle of rotation unless rotation angles are restricted to a 90° segment, for example 0° to 90°. For example, the signal has the same amplitude at angles of 45°, 135°, 225° and 315°, and therefore it is not possible to distinguish between these angles.

The way in which this problem is overcome in the presently preferred embodiment will now be described.

When it is desired to read a dial, reading is carried out at least twice, and for each reading an additional magnetic bias is applied to the magnetostrictive resonators 124, 126. The additional magnetic bias has a different value and/or direction for each reading. For example, for the first reading, an additional magnetic bias of suitable strength, for example 100 to 1000 A/m is applied via the coil 128 in the direction shown by arrow 142 in Fig. 5, while for the second reading an additional magnetic bias of equal amplitude is applied via coil 128 in the opposite direction, that is in the direction indicated by arrow 144 in Fig. 5.

The effect of the additional biases will now be described. As described previously, the detected regenerated signal amplitude is proportional to the square of the applied magnetic bias. Accordingly, if the additional bias is represented by +delta and -delta, then the amplitude of the detected regenerated signal is proportional to (bias + delta)² for the first reading and (bias - delta)² for the second reading. Subtracting these signals gives a resultant signal which is proportional to (4 x bias x delta) .

Referring again to Figs. 6b to 6i, it will be seen that when an additional bias is applied in the direction shown by arrow 142 in Fig. 5, the resultant magnetic bias experienced by the magnetostrictive resonator 126 increases for angles between 0° and 180° and decreases for angles between 180° and 360°. Accordingly, the amplitude of the detected regenerated signal is that shown by the solid line 150 in Fig. 7.

Similarly, when the additional bias is applied in the direction indicated by the arrow 144 in Fig. 5, the resultant magnetic bias experienced by the magnetostrictive resonator 126 decreases for angles between 0° and 180°, but increases for angles between 180° and 360°. Accordingly, the amplitude of the detected regenerated signal in this case is shown by the dotted line 152 in Fig . 7 .

Fig. 8A shows the resultant difference signal for magnetostrictive resonator 126 obtained when signal 152 shown in Fig. 7 is subtracted from signal 150, that is when the detected regenerated signal obtained using the second value of the applied addition bias is subtracted from the detected regenerated signal obtained using the first value of the applied additional bias. As can be seen, the resultant difference signal shown in Fig. 8A is bipolar, having the form of a sine curve.

Since the signal shown in Fig. 8A still does not provide a one-to-one relationship between amplitude and angle (for example, the amplitude is the same as both 0° and at 180°), it is necessary to carry out further processing to determine the exact angular position of the dial. (It will be apparent to the skilled reader that further processing is not required if angles of rotation between only 90° and 270°, ie. a 180° range, are of interest since the relationship between amplitude and angle in the sine curve for these angles is a one-to-one relationship. The skilled reader will also appreciate that the same effect, ie. a one-to-one relationship over a 180° range, can be achieved using one value of the applied additional bias (rather than two as in the case above) and two magnetostrictive resonators. )

Suitable further processing is carried out using the signal from the second magnetostrictive resonator 124. Signals are obtained from resonator 124 in the same way as those obtained and processed from resonator 126, described above. However, since resonator 124 is displaced by an angle of rotation of 90° with respect to resonator 126, the resultant difference signal from resonator 124, obtained by subtracting the two detected regenerated signals obtained with different additional magnetic biases, is shifted by 90° with respect to the resultant difference signal obtained from magnetostrictive resonator 126. Thus, while the resultant signal from resonator 126 is, in this embodiment, a sine wave as shown in Fig. 8A, the resultant signal from magnetostrictive resonator 124 is a cosine signal, as shown in Fig. 8B.

Accordingly, using the information in the resultant difference signal from resonator 126 and in the resultant difference signal from resonator 124, it is possible to determine the angular position of the dial uniquely. For example, while the amplitude of the resultant signal from magnetostrictive resonator 126 is zero at 0° and 180°, the amplitude of the resultant signal from magnetostrictive resonator 124 is 1 at 0° and -1 at 180°, and accordingly it can be determined whether the dial is at the 0° position or the 180° position.

In a presently preferred embodiment, the additional bias is applied by applying a low frequency varying current to the coil 128. The frequency of the applied current, and hence of the applied additional bias, is low compared to, or of the same order as, the inverse of the decay time of the resonance of the magnetostrictive resonators 124, 126. The application of the additional bias plus the interrogating alternating magnetic field and the subsequent detection of the resulting regenerated signal are then carried out in a "pulse-echo" format, as will now be described with reference to Fig. 9.

Curve 160 in Fig. 9 represents one cycle of the alternating current applied to the coil 128. During the time interval Txl and Tx2, an alternating magnetic field of frequency equal to that of the fundamental frequencies of the magnetostrictive resonators 124, 126 is applied via the coil 128. During time periods Rxl and Rx2, the regenerated signal from each magnetostrictive resonator 124, 126 is detected. Thus, during time period Txl, an additional bias having a given magnitude and direction is applied to resonators 124, 126, while in time period Tx2, an additional bias of equal magnitude but opposite direction is applied to the resonators 124, 126. When an additional magnetic bias and an alternating magnetic field are applied at the same time, the resultant signal is an asymmetric alternating magnetic field.

Fig. 10 shows a self-contained, hand-held reading unit 164 for remotely reading the meter system 100 in the manner described above. The reading unit 164 comprises a high frequency drive system 166, an additional magnetic source 168, a decoder 170 and a data store 172, all under operational control of a control unit 174. The reading unit 164 further comprises a power supply 176 and a coil 178 mounted behind a magnetically transparent window 180. The portable reading device 164 may be carried by the person whose job it is to read the meter 100.

Reading can be achieved by placing the reading device 164 close to the meter 100. However, in the arrangement shown in Fig. 10, a connecting coil 182 is arranged to providing coupling between the coil 128 of the meter system 100 (not shown in Fig. 10) and the coil 178 of the portable reading device 164. The provision of connecting coil 182 makes it possible, for example, for the meter 100 to be located in a building, for example a house, or underground, and for the reading to be made remotely.

The control unit 174 controls the HF drive unit 166, additional bias source 168 and decoder 170 so as to interrogate the meter 100 and process the detected regenerated signals in the manner described above. The value indicated by the dials 104 to 114 of the meter 100 may then be stored in the data store 172.

The identity of the meter 100 may also be stored in the data store 172. For example, an,identity tag (not shown) as described in WO92/12401 and WO92/12402, which is assigned to the present applicant and incorporated herein by cross-reference, may be mounted on the meter 100 enabling the identity of the meter to be automatically determined and recorded by the portable reading device 164. The identity tag may, for example, comprise a hollow tray made of magnetically transparent material containing a magnetostrictive element similar to the element 12 of Fig. 1 and a hard magnetic element similar to the element 14 of Fig. 1. The magnetostrictive element is free to vibrate within the tray and the bias element is encoded in a manner described in WO92/12401 and WO92/12402 with a magnetic pattern which biases the magnetostrictive element to resonate at a combination of its harmonic frequencies or its fundamental and harmonic frequencies chosen to indicate the identity of the particular meter.

A second embodiment of the invention will now be described with reference to Figs. 11 and 12, which illustrate a linear encoder.

The linear encoder according to this embodiment of the invention comprises a linearly movable elongate strip 200 whose position is to be monitored or detected, a stationary transducer unit 202 positioned adjacent the strip 200 for monitoring or detecting the position thereof, and a control and display unit 204 connected to the transducer unit 202 for processing signals therefrom and producing a display of the position of the strip 200.

The transducer unit 202 comprises a housing 206 which contains four magnetostrictive resonator units 208, 210, 212 and 214 and two reading coils 216 and 218. Each magnetostrictive resonator unit 210 to 214 comprises a casing which contains a rectangular magnetostrictive strip element, similar to element 12 of Fig. 1, and a hard magnetic element, similar to the element 14 of Fig. 1, for applying a magnetic bias to the magnetostrictive element. The magnetostrictive element is free to mechanically vibrate within the casing. The casing is wholly made of a magnetically transparent material, such as a synthetic plastics material.

Known magnetostrictive materials may be used for the magnetostrictive elements. Examples are amorphous, spin- melt ribbon such as that sold under the trade mark "METGLAS 2826MB" or grain-oriented silicon transformer steel. The material chosen preferably has a high magnetic permeability with a high magnetostrictive coupling. The hard magnetic bias elements may be made of any variety of hard magnetic materials. Examples include magnetic stainless steel, nickel, ferrite or mild steel. Alternatively, the hard magnetic material may comprise a non-magnetic substance with a magnetic coating thereon, such as slurry-formed ferrite as used in magnetic tapes and magnetic disks

The reading coils 216, 218 are connected to a central distribution terminal 220 which receives signals from, and transmits signals to, the control and display unit 204 via a cable 222.

The linearly movable strip 200 has two tracks 224, 226 thereon. Track 224 is a course position indicating track, and recorded thereon is a course magnetic bias pattern as will be described below with reference to Fig. 12. Track 226 is a fine position indicating track, and recorded thereon is a fine magnetic bias pattern, as will be described below with reference to Fig. 12.

Magnetostrictive resonator units 212 and 214 are positioned transversely above course position indicating track 224, and magnetostrictive resonator units 208 and 210 are positioned transversely above fine position indicating track 226.

The principle of operation of this linear encoder embodiment is the same as the first rotary-meter embodiment described above. Thus, as shown in Fig. 12, recorded on track 224 there is a magnetic bias pattern having, in this embodiment, a substantially sinusoidal waveform. The amplitude and direction of the recorded magnetic bias at different positions along the track 224 is indicated by the direction and length of the arrows shown on the track. The magnetic bias is arranged so as to bias the magnetostrictive elements in resonator units 212 and 214 along their length. Similarly, on track 226, there is also recorded a magnetic bias pattern having a sinusoidal waveform. The spatial frequency (wavelength) of the magnetic bias pattern in track 226 is greater than that of the spatial frequency of the waveform on track 224. For example, in this embodiment, there are four cycles of the magnetic bias waveform on track 226 for every one cycle of the magnetic bias waveform on track 224.

Magnetostrictive resonator units 212 and 214 have a spatial separation in a direction along track 224 corresponding to one-quarter of the wavelength of the magnetic bias pattern recorded on track 224. Thus, for example, the spacing of the resonator units 212 and 214 corresponds to the spacing between points 230 and 232 on the magnetic bias pattern shown in Fig. 12. In this way, the signals produced by the magnetostrictive elements in resonator units 212 and 214 are 90° out of phase. This is the same situation as with magnetostrictive resonators 124 and 126 in the first embodiment.

Similarly, magnetostrictive resonator units 208 and 210 have a spatial separation along track 226 corresponding to one-quarter of the wavelength of the magnetic bias pattern recorded on track 226. Thus, for example, the separation corresponds to the distance between point 234 and 236 on track 226 shown in Fig. 12.

Reading coil 216 is used to apply additional magnetic biases together with interrogating alternating magnetic fields at the natural frequencies of the magnetostrictive elements in units 212 and 214, and to detect the fields regenerated by these magnetostrictive elements. Reading coil 218 performs the same functions with respect to magnetostrictive resonator units 208 and 210. The frequency of the applied interrogating alternating magnetic field depends upon the parameters of the magnetostrictive elements in the units 208 to 214. For example, for magnetostrictive elements made from METGLAS 2826MB having lengths in the range 4mm to 9mm, suitable frequencies would lie in the range 550 kHz to 240 kHz. A suitable value for the applied additional magnetic bias in this case would be, for example, 100 to 1000 A/m which is applied in either direction substantially along the length of the magnetostrictive elements via coils 216 and 218.

Accordingly, as will be clear to the skilled reader, the system may be operated in the same way as the first embodiment to produce four resulting difference signals, that is: a sine difference signal and a cosine difference signal from the magnetostrictive elements in units 212 and 214; and a sine difference signal and a cosine difference signal from the magnetostrictive elements in units 208 and 210. The difference signals from the magnetostrictive elements in units 212 and 214 are processed to derive an unambiguous but low accuracy value for the position of the movable member 200, and the difference signals from the magnetostrictive elements in units 208 and 210 are processed to derive an ambiguous but relatively high accuracy value for the position of the movable member 200. These signals may then be combined to produce an unambiguous and relatively high accuracy indication of the position of movable member 200. A number of modifications which may be applicable to either or both of the embodiments described above will now be described.

In the first embodiment described, the magnetostrictive resonators 124, 126 remain stationary while the magnetic bias means, that is the wheel hub 122, rotates. It is also possible to provide a system in which the magnetostrictive resonators rotate while the magnetic bias means remain stationary. Such a system would a e the advantage of reducing unwanted torques and simplifying the provision of the magnetic bias means; one bias means, common to all of the dials 104 to 114, could be provided.

In another modification, the wheel hub 122 may be omitted and the inside surface of the dial 104 magnetised instead. In this case, the resonators 124, 126 would be provided as close to the inner surface of the dial as possible.

The magnetostrictive resonators 124, 126 provided in the first embodiment comprise cylindrical ferrite rods having a curved configuration corresponding to the curvature of the wheel hub 122. As an alternative, it is possible to provide magnetostrictive resonators having a straight configuration rather than a curved one. The disadvantage with a straight configuration is that the distance between the magnetostrictive resonator and the periphery of the wheel hub 122 is larger than with a curved configuration. As described earlier, an increase in the gap between the wheel hub 122 and the magnetostrictive resonator may increase the susceptibility of the system to external magnetic fields and reduce the magnetic coupling between the wheel hub 122 and each of the magnetostrictive resonators 124, 126. It is possible to reduce the gap when straight magnetostrictive resonators are used by providing a pole cap at each end of each resonator. However, it has been found that such a configuration is still inferior to the curved magnetostrictive resonators described with respect to the first embodiment.

Instead of mounting the magnetostrictive resonators 124, 126 via a pin through their centre as described with respect to the first embodiment, it is possible to provide them in a mounted magnetically transparent housing, in which they are free to vibrate.

In the first and second embodiments, magnetostrictive resonators having different lengths, and hence different fundamental frequencies of vibration, are provided so that the signal from each resonator can be distinguished from the others. As described in WO92/12401, it is possible to achieve different fundamental frequencies of vibration by providing elements having the same length but with different mass added to each element at its ends, for example by lateral projections. The addition of mass to a magnetostrictive element in this way has the effect of reducing its fundamental frequency of vibration.

In the first and second embodiments, it is necessary for each magnetostrictive resonator to have a different fundamental frequency of vibration since a single detection coil (128 in the first embodiment and 216, 218 in the second embodiment) is provided for a plurality of resonators. However, it is possible to provide a separate coil for each magnetostrictive resonator, in which case it is then not necessary for the magnetostrictive resonators to vibrate at different frequencies since the signal from each resonator can be discriminated from the others on the basis of the coil in which the signal is detected.

In the embodiments described, an additional magnetic bias of equal amplitude but opposite direction is applied during the first and second reading steps. However, it is only necessary for the additional magnetic bias to be different during each reading step. For example, the applied additional bias could be +delta in the first step and +2 delta in the second step, or it could be +delta in the first step and -2 delta in the second step. As an alternative, no additional bias could be applied during one of the reading steps. That is, a reading could be taken without the application of any additional magnetic bias during the first step, and a reading with an applied additional magnetic bias could be taken during second step. As the skilled reader will appreciate, the subsequent processing of the detected regenerated signals would need to be modified slightly to produce the resulting different signal.

As a further modification, the additional magnetic bias need not be applied as an AC signal as described previously, but could be applied as a DC signal.

In the first embodiment described, magnetically induced torques which cause inaccuracies in the detected positions of the dials may be formed between (i) the rotating wheel hub 122 and the magnetostrictive resonators 124, 126, (ii) the rotating wheel hubs of adjacent dials, for example dial 104 and dial 106, (iii) an external magnet and the wheel hubs 122 in the meter 100, and (iv) the coil 128 and the wheel hubs 122. Possible ways of reducing these torques include providing smaller magnetostrictive resonators 124, 126 and reducing the field strength of the magnetic field of the wheel hub 122. In addition, it is possible to provide magnetic screens between the magnetostrictive resonators 124, 126 and around the whole meter 100. It is also possible to gear together dials which represent the lowest and least important digits in the six-digit number indicated by the meter 100, for example, dials 112 and 114. Thus, the drive force needed is geared down and torques are reduced.

While the magnetostrictive resonators 124, 126 are provided inside the dial 104 in the first embodiment, it is possible to provide them on the outside of the dial although this has the disadvantage that magnetic torques, and hence inaccuracies, are increased.

In the linear encoder embodiment described above, the tracks 224 and 226 may be provided as an integral part of the linearly movable member 200 or as separate components to be attached to the member 200.

It would also be possible to omit the track 226 and magnetostrictive resonator units 208 and 210 where only a course indication of absolute position is needed. Alternatively, the track 224 and resonator units 212 and 214 could be omitted and reliance placed upon the high resolution track 226. In that case, where an absolute indication position of the member 200 is needed, means could be provided to keep a count of incremental movements of the member 200 from a start position.

In the embodiments so far described, the signal from a second resonator is used to resolve ambiguities since the relationship between amplitude and angle in the difference signal obtained from the first resonator (the sine curve shown in Fig. 8A) is not one-to-one. As an alternative, it is possible to use a single magnetostrictive resonator and to derive values of the difference signal separated by a time period in which the relative position of the resonator and magnetic bias means has changed. For example, in the first embodiment, if a first value of the difference signal is obtained as zero, it is not possible to determine whether the angle of rotation is 0° or 180° as described previously. However, by waiting for a time period sufficient for the angle of rotation to change by a small amount and then taking further readings to derive a second value of the difference signal it is possible to distinguish between these positions; consideration of Fig. 8A will show that if the second value of the difference signal is larger than the first value, the original position must have been 0°, while if it is lower than the first value, it must have been 180°. Two further arrangements in which a single magnetostrictive resonator is used will now be described. In these arrangements, quadrature signals are produced using the single resonator rather than using two resonators to produce such signals as in the first and second embodiments described.

Fig. 13, which corresponds to Fig. 5 and in which components identical to those in the first embodiment have the same numbers, shows the first modified arrangement. In this arrangement, magnetic bias means 250 rotates about axle 118 as dial 104 rotates, while magnetostrictive resonator 126 remains stationary.

The magnetic bias means 250 comprises two orthogonal magnetic dipoles. These dipoles are provided, for example, by electromagnets 256 and 258 arranged at 90° to each other. The electromagnets 256, 258 may be activated and deactivated, by control means (not shown), independently of each other.

The principle of operation of the system is similar to that of the first embodiment. In operation, when it is desired to read the position of dial 104, one of the electromagnets, for example 256, is energised thereby creating a magnetic dipole. The other electromagnet remains unenergised so that there is no magnetic field to interfere with the field of the first dipole. An alternating magnetic field at a frequency corresponding to the fundamental frequency of the magnetostrictive resonator 126 is then applied via the coil 128. The regenerated modified field is then detected. This procedure is then repeated with an additional magnetic bias applied in the direction of arrow 260. As will be clear to the skilled reader, on the basis of the detected signals, it will not be possible to distinguish angular positions separated by 180°, for example the positions shown in Figs. 13A and 13B. Accordingly, in order to be able to determine the unique angular position of the dial 104, after the readings above have been made, the electromagnet 256 is deenergised and the electromagnet 258 is energised instead, thereby forming a magnetic dipole at 90° to the dipole previously formed by electromagnet 256. Reading steps are then performed in the manner described above, that is, with no additional magnetic bias applied and with an additional magnetic bias applied in the direction of arrow 260.

When the position cf the dial 104 is as shown in Fig. 13A, the magnetic bias applied to the magnetostrictive resonator 126 by the electromagnet 258 is the same as that shown in Fig. 6d. Accordingly, when the additional magnetic bias is applied via coil 128 in the direction shown by arrow 260, the resultant magnetic bias experienced by the resonator 126 increases, so that the detected regenerated signal also increases in amplitude. When the position of dial 104 is as shown in Fig. 13B, the magnetic bias applied to the magnetostrictive element 126 by the electromagnet 258 is the same as that shown in Fig. 6h. Accordingly, the resultant magnetic bias experienced by the resonator 126 when the additional magnetic bias is applied in direction 260 decreases, and hence the amplitude of the detected regenerated signal when the additional bias is applied also decreases. A unique angular position of the dial 104 can therefore be determined using this method.

Thus, in this system, relative position is determined using a single resonator and producing quadrature signals modified by an additional magnetic bias applied in one direction. Clearly the values of the applied additional bias do not need to be the same. Also, the system can be arranged so that the bias means 250 remains stationary while the resonator 126 moves relative to it.

A second modified system using a single magnetostrictive resonator and in which quadrature signals are produced will now be described. This modified systems has the same arrangement as that shown in Figs. 4 and 5 for the first embodiment, but in this case the magnetostrictive resonator 124 is not provided. As described previously, when the magnetostrictive resonator 126 is bir.sod as shown in Fig. 6b or Fig. 6f, its resonance at its fundamental frequency is a minimum, effectively zero, but it does resonates at a frequency equal to twice its natural frequency, that is at its second harmonic. When biased as shown in Figs. 6b and 6f, the amplitude of the detected regenerated field is a maximum when the magnetostrictive resonator 126 is interrogated with an alternating magnetic field at a frequency equal to twice the fundamental frequency, but is a minimum when the resonator is interrogated with an alternating magnetic field at its fundamental frequency.

Similarly, when the magnetostrictive resonator 126 is biased as shown in Figs. 6d and 6h, the amplitude of the detected regenerated signal is a maximum when an alternating magnetic field at a frequency equal to the fundamental frequency of the resonator is applied, and the amplitude of the detected regenerated signal is a minimum when an alternating magnetic field at twice the fundamental frequency of the resonator is applied.

Accordingly, the detected regenerated signal obtained by applying an alternating magnetic field at the fundamental frequency is in quadrature to the detected regenerated signal obtained by applying an alternating magnetic field at twice the natural frequency. Since the coil 128 is asymmetric with respect to the resonator 126, as shown in Fig. 5, additional magnetic biases applies via the coil 128 will contain components which modify the resonance at the fundamental and second harmonic frequencies. Accordingly, as the skilled reader will appreciate, additional magnetic biases applied in the directions 142 and 144 in ways described previously with respect to the first embodiment will modify the quadrature signals in such a way that the angular position of the dial 104 can be determined uniquely.

In the second modified system described above, the position of the dial is thus determined using a single magnetostrictive resonator and detecting the regenerated signal produced at at least two frequencies.

It would be possible to construct a device in accordance with the invention for indicating the value of a variable without any moving parts. Electrical means could be provided for producing different magnetic patterns by energising a set of electromagnetic coils as a function of the value of the variable to be indicated. Magnetostrictive elements biased by the magnetic pattern could be arranged to resonate at different amplitudes according to the magnetic biasing pattern produced.

Claims

CLAIMS;

1. A method of indicating the value of a variable comprising: moving magnetostrictive resonator means and magnetic bias means relative to each other in such a way that the magnetic bias applied to said resonator means by said bias means varies as a function of said variable; applying an additional magnetic bias to said resonator means; applying an alternating magnetic field of a predetermined frequency to said resonator means for causing said resonator means to resonate at said predetermined frequency in dependence upon the resultant magnetic bias experienced by said resonator means of said bias and said additional bias; detecting the resonance of said resonator means; and deriving, from the detected resonance signal,- said value.

2. A method according to claim 1, wherein the magnetic bias applied to said resonator means by said bias means has a periodic format.

3. A method according to claim 2, wherein said periodic format is substantially a sine wave.

4. A method according to any preceding claim, wherein said magnetostrictive resonator means comprises first and second magnetostrictive resonators, and said value is derived from the resonance signal detected from said first resonator and the resonance signal detected from said second resonator.

5. A method according to claim 4, wherein said first and second resonators have different fundamental frequencies of vibration, and the step of applying an alternating magnetic field comprises the steps of applying an alternating magnetic field at the fundamental frequency of the first resonator and an.alternating magnetic field at the fundamental frequency of the second resonator.

6. A method according to claim 4 or 5 when dependent upon claim 2 or 3, wherein said first and second magnetostrictive resonators are arranged so that the magnetic bias applied to the first resonator is substantially 90° out of phase with the magnetic bias applied to said second resonator.

7. A method according to any preceding claim, wherein the step of applying an additional magnetic bias comprises the steps of applying an additional magnetic bias having first and second values.

8. A method according to claim 7, wherein one of said first and second values is zero.

9. A method according to claim 7, wherein said first and second values have substantially the same magnitude but are applied in substantially opposite directions.

10. A method according to any preceding claim, wherein said additional magnetic bias has the form of an alternating signal.

11. A method according to any preceding claim, wherein the steps cf applying an alternating magnetic field and detecting the resonance of said resonator means are performed as a pulse-echo technique.

12. A method according to any preceding claim, wherein the step of deriving, from the detected resonance signal, said value includes the step of deriving a signal proportional to the magnetic bias applied to said resonator means by said magnetic bias means.

13. A method according to claim 12 when dependent upon claim 7, wherein the step of deriving a signal proportional to the magnetic bias comprises the step of subtracting the resonance signal detected from said resonator means when said additional magnetic bias of the second value is applied from the resonance signal detected from said resonator means when said additional magnetic bias of the first value is applied.

14. A method according to any preceding claim, wherein said resonator means and magnetic bias means are moved relative to each other in a rotary manner.

15. A method according to any of claims 1 to 13, wherein said resonator means and magnetic bias means are moved relative to each other in a linear manner.

16. A method according to any preceding claim, wherein the steps of applying an additional magnetic bias, applying an alternating magnetic field, and detecting the resonance of said resonator means are performed remote from said resonator means via at least one coil coupling signals to and from said resonator means.

17. A device for use in a method of indicating the value of a variable according to any preceding claim, comprising magnetostrictive resonator means and magnetic bias means for biasing said resonator means with a magnetic bias which has a substantially sine wave format and which varies as a function of said variable.

18. A device for use in a method of indicating the value of a variable according to any preceding claim, comprising magnetostrictive resonator means and magnetic bias means arranged to be movable relative to each other in such a way that the magnetic bias applied to said resonator means by said bias means varies as a function of said variable and causes said resonator means to resonate at a predetermined frequency with an amplitude which varies in an analogue manner as said resonator means and said magnetic bias means move relative to each other in response to an alternating magnetic field at said predetermined frequency.

19. A device according to claim 17 or claim 18, wherein said resonator means and said magnetic bias means are arranged to be movable relative to each other in a rotary manner.

20. A device according to claim 19, wherein said magnetic bias means comprises a magnetic dipole.

21. A device according to claim 19 or 20, wherein said magnetic bias means comprises a substantially cylindrical member.

22. A device according to claim 21, wherein said cylindrical member is magnetised around its outer periphery only.

23. A device according to claim 21, wherein said cylindrical member is magnetised around its inner periphery only.

24. A device according to any of claims 17 to 23, wherein said magnetostrictive resonator means comprises first and second magnetostrictive resonators.

25. A device according to any of claims 19 to 24, wherein said magnetostrictive resonator means comprises a ferrite rod.

26. A device according to any of claims 19 to 25, wherein said magnetostrictive resonator means has a curved configuration.

27. A device according to claim 26 when dependent upon claim 22 or claim 23, wherein the curvature of said magnetostrictive resonator means corresponds to the curvature of said cylindrical member.

28. A device according to claim 27, wherein said magnetostrictive resonator means and said magnetic bias means are arranged such that the gap between the magnetised periphery of said cylindrical member and a surface of said magnetostrictive resonator means is less than 0.5mm.

29. A device according to any of claims 19 to 28, comprising a meter.

30. A device according to claim 29, wherein said meter is a water meter.

31. A device according to claim 30, wherein said variable is the accumulated total water flow.

32. A device according to any of claims 29 to 31, comprising a plurality of dials, wherein each dial has associated therewith its own magnetostrictive resonator means and its own magnetic bias means.

33. A device according to claim 32, wherein each dial is substantially hollow and the associated magnetostrictive resonator means are provided within a given dial.

34. A device according to any of claims 29 to 33, wherein said meter also provides a visual indication of the value of said variable.

35. A device according to any of claims 29 to 34, further comprising a magnetostrictive meter-identity-element.

36. A device according to any of claims 17 to 35, further comprising at least one coil for coupling signals to and from said magnetostrictive resonator means.

37. A device according to claim 36, when dependent upon claim 32, wherein a single coil, common to a plurality of resonator means, is provided.

38. A device for use in a method of indicating the value of a variable according to any of claims 1 to 16 comprising: means for applying an additional magnetic bias to a device comprising magnetostrictive resonator means and magnetic bias means for biasing said resonator means with a bias dependent upon said variable; means for applying an alternating magnetic field of a predetermined frequency to said resonator means for causing said resonator means to resonate at said predetermined frequency in dependence upon the resultant magnetic bias experienced by said resonator means of said bias and said additional bias thereby modifying said alternating magnetic field; means for detecting said modified field; and processing means for deriving, from the detected modified field, said value.

39. A device according to claim 38, wherein said processing means comprises means for subtracting the resonance signal detected from said resonator means when an additional magnetic bias of a first value is applied from the resonance signal detected from said resonator means when an additional magnetic bias of a second value is applied.

40. A device according to claim 38 or claim 39, further comprising means for storing said value of the variable.

41. A device according to any of claims 38 to 40, further comprising means for reading a magnetostrictive identity element.

42. A method of indicating the value of a variable comprising the steps: magnetically biasing magnetostrictive resonator means with a bias dependent upon said variable; applying an additional magnetic bias to said resonator means; applying an alternating magnetic field of a predetermined frequency to said resonator means for causing said resonator means to resonate at said predetermined frequency in dependence upon the resultant magnetic bias experienced by said resonator means of said bias and said additional bias; detecting said field after modification by said resonator means; and deriving, from the detected modified field, said value.

43. A method according to claim 42, wherein said magnetostrictive resonator means comprises a single magnetostrictive resonator.

44. A method according to claim 43, wherein the step of applying an alternating magnetic field comprises the steps of applying an alternating magnetic field at first and second predetermined frequencies.

45. A method according to claim 43, wherein the step of magnetically biasing said resonator means with a bias dependent upon said variable comprises applying a first bias field and a second bias field orthogonal to said first field.

46. A method according to claim 45, wherein said first and second bias fields are applied at different times.

47. Use of a method according to any of claims 1 to 16 and 42 to 46 for reading a meter.

48. A device for use in a method of indicating the value of a variable according to any of claims 1 to 16 and 42 to 47, comprising means for processing data representing a signal modified by a magnetostrictive resonator in dependence upon the resultant of a magnetic bias applied to said resonator in dependence upon said variable and an additional magnetic bias applied to said resonator, to derive said value.

49. A method of indicating the value of a variable comprising: varying, as a function of said variable, the relationship between magnetostrictive resonator means and a magnetic field so as to vary the effect of said field on said resonator means, said variation of said effect being ambiguously related to said variable; applying a first interrogating alternating magnetic field to said resonator means to produce in said resonator means a first response dependent upon said effect; applying a second interrogating alternating magnetic field to said resonator means which differs from said first interrogating field, so as to produce in said resonator means a second response dependent upon said effect, said second response differing from said first response and said first and second responses together providing an unambiguous indication of the value of said variable; and detecting said first and second responses.

50. Apparatus for indicating the value of a variable comprising: magnetostrictive resonator means; magnetic field producing means arranged for producing a magnetic field which affects said magnetostrictive resonator means; means for varying the effect of said field on said resonator means, the variation of said effect being ambiguously related to said variable; means for applying a first interrogating alternating magnetic field to said resonator means to produce in said resonator means a first response dependent upon said effect; means for applying a second interrogating alternating magnetic field to said resonator means which differs from said first interrogating field, so as to produce in said resonator means a second response dependent upon said effect, said second response differing from said first response and said first and second responses together providing an unambiguous indication of the value of said variable; and means for detecting said first and second responses.