GB2175096A - Electromagnetic transducer assemblies and means for determining relative speed and/or configuration using such assemblies - Google Patents

Abstract

A first component is caused to produce a varying magnetic field. At least one second component detects this field and from the nature of the variation, information about the relative position, configuration or rotation of the components is derivable. The first component may have a rotary shaft 10 carrying a magnetisable bar 14 fixed at an angle; and an exciting coil 16 for inducing magnetism in the bar. If the coil carries an a.c. current and the shaft rotates, the bar produces a gyrating field. This induces in a detector coil 22 a varying EMF whose characteristics depend on the rotation rate and on the relative disposition of the coil. With a plurality of detector coils, detailed information about relative positions and orientations is derivable from the EMFs. Either the detector coil or coils, or the arrangement for producing the gyrating field may be mounted on an earthworking mole. <IMAGE>

Description

SPECIFICATION Electromagnetic transducer assemblies for determining relative speed and/or configuration using such assemblies The present invention relates to electromagnetic transducer assemblies and means for determining relative speed and/or configuration using such assemblies. It may be applied to detecting rotation and/or determining the position and/or orientation of a body. In another aspect it relates to remote control apparatus, and assemblies with remotely controllable mobile elements, e.g. soil piercing tools or 'moles'.

Broadly, in a first aspect the invention involves causing rotation of a transducer to produce a synchronously varying magnetic field; and detecting the field at one or more angular locations. Preferably the field has an axis which generally defines an acute angle with the rotation axis, so that the field "gyrates" about the axis.

Preferably the field varies with time, e.g. being induced by an alternating current, and the varying field is inductively picked up by a detector. Generally the degree of inductive coupling between detector and field is a function of the angular position of the transducer relative to the detector.

In one form, a rotatable body is rotationally fast with a transducer comprising an elongate ferromagnetic element which extends at an angle to the rotation axis; and an exciting coil may be arranged with its axis having at least a component coaxial with the rotation axis such that it can induce the ferromagnetic element to be magnetised longitudinally. The field then set up by the magnetised element has its axis in alignment with the element, but at an angle to the axis of rotation. This field therefore gyrates about the rotation axis in synchronism with the rotation. The gyratory field can be detected. One form of detector or receiver comprises a detector coil, which may have a ferromagnetic core.Such a detector coil is preferably arranged with respect to a said exciting coil so that there is substantially no direct inductive coupling between them (in the absence of the ferromagnetic element).

Preferably the detector is arranged so that the degree of coupling to the field of the transducer varies between substantially zero and a maximum during rotation. Preferably a said exciting coil is supplied with alternating current so that the induced fieid of a said ferromagnetic element varies, and the resultant varying flux is detected.

In one preferred arrangement, the rotation of a shaft is detected by means of a ferromagnetic element fast with it, an exciting coil magnetically coupled to the element, and a detector coil. Preferably the exciting coil is coaxial with the shaft. Preferably it is stationary. (Alternatively the exciting coil may rotate with the shaft, e.g. being wound on the ferromagnetic element. It may then be energised via slip rings.) Preferably the axis of the detector coil is in a direction parallel to a tangent to the exciting coil and substantially in a radial plane of the exciting coil. One alternative is for the detector to be symmetrical about the rotation axis, with its own axis parallel with the plane of the exciting coil. Preferably the axes of the ferromagnetic element, the shaft and the exciting coil approximately intersect at a point.Preferably the shaft is non-magnetic and non-conducting, at least in the vicinity of the exciting coil and the ferromagnetic element.

In such a preferred arrangement, when the exciting coil is supplied with alternating current and the shaft rotates, the fluctuating induced field of the ferromagnetic element produces a varying magnetic flux within the detector coil whose mean amplitude during a single rotation of the shaft, goes through a first maximum, a minimum (substantially zero), a second maximum (of different phase relationship to the excitation from the first), and a second minimum. Thus the detector coil can provide an output signal related to the rotation of the shaft. A plurality of detector coils distributed angularly about the shaft can give a multiphase output, from which information about the angular position of the shaft can be derived. Indeed, a plurality of detectors can provide data about the position and orientation of the shaft.In another type of arrangement, a body is provided with a transducer assembly which includes a rotary shaft. Use of detectors as indicated above makes it possible to determine the location and orientation of the body.

Some embodiments of the invention will now be described in greater detail with reference to the accompanying drawings in which: Fig. 1 is a side view of apparatus embodying the invention; Fig. 2 is a front elevation thereof; Figs. 3a to d are views similar to Fig. 1 for explaining the operation; Fig. 4 is a graph of the detector output; Figs. 5 and 6 are views similar to Fig. 1 but showing alternative embodiments; Fig. 7a is a view like Fig. 2 but showing a toroidal detector; Fig. 7b is a side view of the Fig. 7a apparatus with the detector sectioned; Figs. 8a and b are views like Figs. 7a and b but showing a further embodiment; Fig. 9 is a graph for explaining the operation of the Fig. 8 embodiment; Fig. 10 is a vertical section showing a mole assembly embodying apparatus according to the invention;; Fig. 11 is a perspective view of a second mole assembly embodying apparatus according to the invention; Figs. 1 2a and 1 2b are views in elevation and axial section of a toroidal transducer or aerial; Fig. 13 is an axial section of a toroidal transducer according to Fig. 12 further provided with a longitudinal coil; Fig. 14 is a view similar to Fig. 13 but showing a modified embodiment; Fig. 15 is an axial section through a practical embodiment of a transmitter assembly for a mole; and Fig. 16 is a detail of the Fig. 15 embodiment in a different orientation.

As shown in Figs. 1-3, a shaft 10 whose rotation is to be investigated is, at least in the illustrated portion, non-magnetic and an electrical insulator. A bore 12 passes through the axis of rotation at an acute angle. A ferromagnetic element 14 (suitably of ferrite) is secured symmetrically in the bore, so as to project by equal amounts on either side, and thus not to affect the balance of the shaft 10. A stationary exciting coil 16 is coaxial with the shaft 10, and is mounted about it so that its central radial plane contains the inter-section of the axes of the shaft 10 and the element 14. The central opening 18 of the coil 16 is large enough to permit the free rotation of the shaft 10 and element 14. Conductors 20 lead to an AC source. A detector 22 comprises an elongate coil 24 wound about a ferrite core in the form of a cylindrical rod 26.Its axis is parallel to a tangent to the exciting coil 16 or shaft 10, and is in the central plane of the exciting coil 16.

In operation, an AC current (suitably with a frequency in the range 1kHz to 150kHz) is supplied to the exciting coil 16. During one half-cycie, the current induces the ferrite element 14 to become a bar magnet, with north and south poles at opposite ends. Of course, in the next half-cycle, the poles are reversed.

Thus the element 14 becomes in effect a continually reversing bar magnet. The magnetic field set up by element 14 has its axis in alignment with it, but at an angle to the rotation axis of shaft 10, so that the field gyrates about shaft 10 at its speed of rotation. This field is coupled to the detector 22, the degree of coupling varying with the angular position.

(From another point of view, the induced bar magnet can be resolved into axial and radial components (relative to the shaft 10). The radial component provides a fluctuating magnetic field, which rotates with the shaft 10, and which is coupled to the detector 22, the degree of coupling varying with the angular position.) Referring to Fig. 3, the coupling is at a maximum when the element 14 is in a plane parallel to the detector 22 (Figs. 3a and 3c), and substantially zero in positions at 90" thereto (Figs. 3b and 3d). In fact the signals induced in the detector coil 24 are of opposite phase (in relation to the exciting signal in the exciting coil 16) in the positions of Figs. 3a and 3c. The exciting signal 29 and the output signal from the coil 24 are shown in Fig. 4.

The full line 30 and the broken line 32 show the signal modulation envelope for one revolution. This envelope portion has two bulges B,B' and a node N. In the first bulge B the signal current 31 in the detector is in phase with the exciting signal 29, whereas in the second bulge B' it is in antiphase. This phase relationship can be detected and used to produce an output signal bearing phase information, e.g. as shown by line 30. This signal is quasi-sinusoidal, the actual shape depending on the angle of the element 14 to the shaft 10; and the modulation frequency being simply related to the rate of rotation of the shaft 10. It will be readily apparent that the output from a single detector 22 can thus provide an input for a remote tachometer 39 which produces a display indicative of rotational speed.

If further detectors 40a,b (Fig. 2) are positioned similarly to the detector 22 but at 1200 intervals, the modulation of the signals induced in the three detectors will constitute a three-phase pattern, from which the angular position of the shaft can be accurately computed over a full 360". A pair of detectors with 90" separation will similarly give a twophase result, and of course other multiphase systems can be produced analogously.

It is preferred for the, or each, detector coil 24 to be tangential to the shaft 10 and in the plane of the exciting coil 16, as shown in Figs. 1 to 3, particularly since this simplifies the output signal by avoiding direct coupling to the exciting coil 16. However, other positions may be used. For example, Fig. 5 shows a detector 40c whose centre is still in the plane of the exciting coil 16, but whose axis is parallel to that of the shaft 10. This will give an output which varies in magnitude but not in phase, the maximum corresponding to the null position of Fig. 3b. Fig. 6 shows a detector 40d whose centre is on the axis of the shaft 10, and whose axis is normal to it.

This will have an output in which both phase and magnitude vary.

As already explained, a single detector can be used to determine the rotational speed of the shaft 10, and a plurality of detectors can be used to provide data about its angular position. Additionally or alternatively, a plurality of detectors can be used to calculate the orientation of the shaft relative to the detectors, if this varies. Thus a body may be provided with a transducer assembly comprising a rotatable shaft 10, a ferromagnetic element 14 and an exciting coil 16 generally as described above. The gyrating field produced thereby can be detected at predetermined location(s) by one or more remote detectors. The gyratory motion of the field means that the depth of modulation and the phase properties of a detected signal are indicative of the orientation of the detector relative to the transducer assembly.If that assembly includes a fixed de tector (e.g. arranged as in Fig. 6, and suitably mounted on a housing of the shaft 10), then comparison of the phasing of the output of that detector with that of a remote detector allows their relative orientations to be determined easily. Angular differences about the rotation axis (of the shaft) correspond to phase shifts.

The use of an alternating current of high frequency permits a large rate of variation of flux to be produced in a detector, so that detection is possible from standstill to high speeds, and over substantial distances. Furthermore, the skewness of the induced magnet produces the gyrating field whose properties make further types of determination possible.

Figs. 7 and 8 show apparatus similar to that of the earlier figures, but with a different form of detector. In place of the rod-shaped detector elements 26,40 etc. use is made of a detector 41 of toroidal form. As shown in Fig.

7, a toroidal ferromagnetic core 43 is occupied completely by a continuous evenly spaced winding 44. The arrowed chain-dotted lines L indicate how the flux induced in rod 14 divides itself into two parallel paths through the toroidal core, to induce EMFs which will produce a maximum voltage at diametral tappings x,x' in alignment with rod 14. Rotation of shaft 10 will reduce the voltage at these taps to null after 90 rotation; further rotation will increase the voltage, but with the opposite phase sense with reference to the frequency of a.c. applied to exciting coil 16. The amplitude of the voltage will be found to follow a sinusoidal relationship with rotation angle (more accurately than with previous embodiments).It will also be found that three tappings 120O spaced at x,y and z will produce three-phase (delta) voltages between them.

If four taps are made at 90O spacings as shown in Fig. 8a in winding 45 at x,y,x' and y', the signal voltages measured at diametral taps x,x' and y,y' will follow sinusoidal curves displaced by 90" from each other. While the measured signals comprise the a.c. carrier frequency in coil 16 modulated by the rod rotation in the manner shown in Fig. 4, it is more convenient to show them after demodulation by conventional electronic techniques (effected by signal processing means 48), to produce d.c. levels varying in magnitude with the amplitude of the modulation, and varying in polarity with the relative phase of the modulated signal to the supply frequency. This result is shown in Fig. 9 for the construction of Fig. 8.

It will be apparent that the two curves correspond to the sine and cosine of rotation angle. Conventional electronic techniques can be applied to these (by the processing means 48) to derive analog or digital data corresponding to the absolute angular position of the shaft.

The device will therefore behave in the same way as a conventional brushless resolver, but with much simpler construction. The conventional resolver has a wound two-phase stator and single-phase wound rotor, and also incorporates a rotary transformer to provide the rotor excitation, both necessitating precision construction and winding. The subject device is not only simpler and cheaper to make, but lends itself much better to miniaturisation, having only a single stator assembly of toroidally-wound core and internal exciting coil, and a non-wound rotor of ferrite rod or equivalent laminated steel construction. It will also be apparent that the Fig. 7 construction could substitute for a three-phase brushless synchro.

Apparatus for electromagnetic surveying, location and tracking of movable objects out of sight will now be described with reference to Figs. 10 and 11.

Reference has already been made to the possibility of using the device to determine the relative orientations of the rotating shaft and a distant detector. A specific example of this is in electromagnetic surveying of relative positions and orientations which cannot be determined by optical or similar means, e.g.

when the object of the survey is hidden from view underground. If a transmitting element comprising the rotating transducer and exciting coil of Fig. 1 is mounted at a known point and orientation, a receiving element comprising typically a wound ferrite rod attached to the object which is to be located or tracked will have a signal induced in it at the frequency applied to the exciting coil (reference frequency), but which is modulated at the rate of rotation. The depth of modulation relative to the reference frequency signal and amplitude will vary with the relative orientations of transmitter rotation axis and receiver rod axis in a manner which is capable of mathematical analysis and computation.A second transmitter located at another known position will similarly enable the relative orientation of the receiver to it to be computed, so that the two readings enable the actual position of the receiver to be deduced by triangulation. If required to be computed simultaneously, different reference frequencies can be used for each transmitter, with appropriate filtering of the receiver signals. More than two transmitters can of course be used to increase the confidence level in the computed result. The application is exemplified in Fig. 10 in relation to the simple monitoring of the straight-line motion of a soil piercing tool, and in Fig. 11 to guidance of a steerable version of the same.

In Fig. 10, a piercing tool ("mole") 50 may be designed to follow a straight line path as indicated by the arrow, driven by fluid power via hose 55. A typical use would be to enable a pipe or cable to be inserted through the ground below a road, without disturbance of road surface or traffic flow. However, variations in soil consistency or the encountering of unexpected obstacles may cause the mole to veer from the straight path, with undesirable consequences if undetected.

If the mole has a receiving element 51 fixed to it, and the transmitter assembly 52 mounted in the launch-pit with its shaft in accurate alignment with the required hole to be drilled, the signal detected by the receiving element and fed back by cable 60 alongside the pneumatic or hydraulic power feed to the mole should comprise the reference frequency only, declining in amplitude with distance increase, but with zero modulation as long as the mole and receiver axis remain coincident.

If the mole deviates from this straight line path by an angle of divergence d, the signal from receiving element 51 will be modulated at the rotation frequency. The depth of modulation will increase with d. The phase of this modulation will vary with the angle p, which is the deviation direction it is taking about the required hole axis. If the transmitter is provided with a detector reference coil 53 of known orientation about the rotation axis, comparison of the phase of signal 51 with the phase of signal 53 (by signal processing means 62) will enable the angle p to be determined. This system is therefore capable of drawing attention to a degree of divergence beyond a prescribed limit, and also of indicating the direction of the divergence.

Referring now to Fig. 11, this mole is designed and constructed to incorporate a facility for remote power steering. It has a pair of rod-shaped receiver elements 51,54. These will generally be elongate coils on ferromagnetic cores. For ease of computation they are preferably mutually perpendicular, with one of them (51) accurately aligned with the mole body. There may be a plurality of mutually spaced transmitters. The illustrated example uses three (T1, T2 and T3). Each of them is positioned in a precisely known position, inclination and orientation by normal surveying techniques. The longitudinal receiver element 51 receives from each transmitter a respective signal which bears modulation related to relative attitude and orientation of a respective transmitter.Analysis of the two signals (by computer 70) will then enable vectors V1, V2 and V3 representative of the position relative to respective transmitters to be computed.

Thus the true location of the mole can be computed and compared with the required trajectory, and a correction signal given as necessary to the remote steering system 64.

(The transmitters can be energised successively, or, if they employ different frequencies and/or rotation rates, simultaneously.) However, for a steering system to function correctly, it is important to know of any variation of the roll axis of the mole. If it is provided with fins 64 to steer left or right, for example, rolling of the mole through 90 would cause these to steer up or down instead.

To provide this information, a second receiving element 54 is required, with its axis normal to the axis of the longitudinal element 51. If the axis of 54 is horizontal in the starting attitude of the mole, for example, any modulated signal received by it will have a similar phase reference to that received by another receiver element 66 fixed horizontally at a chosen position. If the mole rolls in the course of its traverse, the phase difference between signals from 54 and 66 will be related to the roll angle, and the steering system adjusted to correct this by appropriate means.

The description and illustrations of the location and tracking system examples have been based for convenience on the concept of fixed transmitters and moving receivers. It will be obvious that their relationships can be interchanged, i.e. the transmitter attached to the movable object, and receiver elements installed in precisely known locations and attitudes. In this case a single transmitter frequency may suffice for simultaneous computation of position vectors from several receiver stations.

Moles and such like objects may be remotely powered, e.g. by compressed air, so any electromagnetic element attached has to be of a design configuration which allows for the power line, such as a fluid power coupling hose, to pass through. There may in any case be cables carrying power and/or data. The result is that only an annular volume is generally available for the transmitting or receiving equipment.

When it is desired to provide separate information about position and roll, two separate receiving or transmitting channels are necessary. These can be provided by two wound aerials fixed at right-angles to each other as in Fig. 11, but this arrangement has two limitations: (a) It does not readily provide for passage of a fluid power line etc. through it; (b) The ferromagnetic rods, being in close proximity to each other, have a certain amount of interaction, tending to give local distortion of the field which is being sensed or created by the other.

Figs. 12-14 show aerial or transducer assemblies which can overcome both these limitations. They employ annular configurations of the active material. Substantially the whole of the common ferromagnetic core material within each sensing winding can be used, thus avoiding or ameliorating such local interaction and field distortion.

Starting with the roll attitude sensing or transmitting requirement, Fig. 12 shows an aerial 78 which, like all radio aerials, may be used to transmit or receive signals. It comprises a toroid of ferromagnetic material 80, typically of spirally-wound steel strip of the appropriate thickness and magnetic properties for the application, on which is a toroidal winding 82 terminating at a and b. This in turn comprises two nominally equal sections in series, covering 1800 each, (one on either side of the diameter AB in Fig. 12a) and arranged so that their EMFs are additive when subjected to an external alternating magnetic field in the vertical direction indicated by the broken lines 84 in Fig. 12a.It will be clear that the resulting flux in the core will divide through each half of the toroidal core; a similar effect will result if this element is used as a transmitter. Energising terminals a,b with a d.c. or a.c. voltage will develop magnetic poles at points A and B, in a manner similar to the historical Gramme-ring armature, producing a field in space of orientation A-B. If the energisation uses alternating current, the field too will alternate. This can induce EMF signals in receiving aerials of appropriate orientation (not shown). Thus the aerial 78 serves as a transmitter. In this mode, points C and D will be magnetically neutral. It will be similarly apparent that the aerial 78 can be used as a receiver. Indeed, Figs. 7 and 8 show such an aerial 43,44 serving that function. An external field of horizontal orientation C-D will result in a null output at terminals a and b.The signal output will therefore vary approximately sinusoidally with the relative orientation of the element to the external alternating field, with maximum for the A-B direction and minimum for C-D. The phase of the output will also change relative to the field source as rotation of the element and field relative to each other causes the output to pass through the null condition. The signal amplitude and phase can therefore be processed by suitably designed electronic systems to give information about the relative angular orientations of receiver element and external field source, generally as already described with reference to Figs. 7 to 9. Similarly, a transmitting element of this configuration will cause the orientation of its external field to vary with roll angle, which can then be sensed and interpreted by suitable remote receiver systems.

For axial position sensing or transmission, the embodiment of Fig. 13 shows the same toroidal element 78 further provided with a simple longitudinal coil 86 wound on a suitable former 88 surrounding the toroidal winding. It will be obvious that in the receiver application, an external a.c. field in the horizontal or axial direction as indicated by the broken lines 90 will be concentrated in the ferromagnetic core along the toroid, and will induce an EMF in the axial winding 86, but not in the toroidal winding 82. The induced EMF will be at a maximum when field direction and coil axis coincide. As the external field direction moves away from this axis, the EMF will reduce to a null when they are at right-angles to each other. Further relative rotation will increase the EMF, but with opposite phase sense relative to the a.c. field source.

In the transmitting application, energising the longitudinal coil 86 at terminals c and d will produce a field originating and oriented with the coil axis, the signal from which can be processed by suitable remote receiver systems to recover positional information about the relative attitudes and separation of transmitter and receiver.

While the principle has been illustrated using a complete toroidal core, the same advantages may be gained beneficially by separating the toroidal core and windings into two halves, as shown in Fig. 14. The quasi-toroidal core is in two halves 80a,80b with respective winding portions 82a,82b. By exposing the core ends at positions A and B, the development or sensing of external fields in the A-B direction is enhanced. In addition, the most effective portion of the toroidal winding is the central part, so that it may be found more efficient in material usage and performance to provide short central windings 82a and 82b only, again connected in series. The removal of ferromagnetic material at the exposed points A and B will have little diminution effect on the coupling to longitudinal winding 86, but the overall cross-section of the structure will be more oval than circular.

A simpler and cheaper way to produce a modified toroidal core with an enhanced ability to behave as a 2-pole structure in the A-B axis is to take a simple tubular core (like the core 80 in Figs. 12 and 13) and to drill two diametrically opposite series of diametrically extending holes 90,92 (Fig. 12b) through it, along its length (at A and B in Fig. 12). (Of course there would not then be windings 82 in the regions of the holes 90,92.) It will be clear from the foregoing that the entire ferromagnetic content of the toroidal core is coupled to both sets of windings, thus minimising interaction problems associated with separate wound aerial rods of ferrite or other suitable material. It will also be apparent that the structure provides a clear cylindrical space within for passage of piping or other cables.

Figs. 15 and 16 show a presently preferred embodiment of a gyratory field transmitter 100 suitable for use in monitoring a mole, e.g.

generally as shown in Fig. 10 or Fig. 11. The transmitter comprises a transducer assembly 102, a nonmetallic shaft 104, and means 106 for rotating the shaft 104.

The transducer assembly 102 is fundamentally the same as those described with reference to Figs. 7 to 9. However it has been found preferable for the ferromagnetic element 108 to be a laminated steel core, since this can transmit higher power. For ease of construction the element 108 is formed as a pair of parallel portions each formed of a stack of rectangular laminations. To mount these, the shaft 104 has a pair of angled slots 110 (see Fig. 16) in a central portion 112. The two portions of the element 108 are clamped together about the shaft 104 by means of clamping studs 114 that pass through apertures in the end regions of the element 108 and are engaged by nuts.

The exciting coil 116, wound on a spool 118, is shaped so as to embrace the element 108 closely. It is therefore necessary in assembling the device to mount the element 108 after the coil 116. The two portions can easily be slid into their respective slots 110 and then mutually connected by studs 114. In order to balance the element 108 about the rotor 104, its position is finely adjustable by variable links extending from the studs 114 to the shaft 104. Each link comprises a nonmetallic tie bar 120 which extends between the portions of the element 108, and an elongate nonmagnetic stud 122 which extends from the tie bar 120 through the shaft 104 to an adjustable extent. Each stud 122 carries a nonmetallic balance weight 124 to balance the forces produced by the element 108 when the shaft 104 rotates.

The rotating means 106 comprise a motor 126 coupled to the shaft 104, which is jour nalled in bearings 128 mounted in nonmagnetic end plates 130, between which are mounted tubular housing portions 132,134 which engage and locate the coil spool 118.

Receivers 136 for providing a reference signal are formed as longitudinal coils wound on ferrite rods, and mounted in radial cavities in one end plate 130.

The electronic systems required to capture the data from the gyrating fields of the transmitting elements from the receiver signals have not been described, as many different techniques and circuits can be devised by those skilled in the art, and are as used in guidance systems involving radio transmissions between the guided object and the receiving or controlling stations.

Of course, the apparatus and methods (and parts thereof) described above can be applied in different situations and combinations and for different purposes from those specifically exemplified.

Claims (16)

1. A transducer for producing a varying magnetic field, comprising: a rotatable shaft having an axis of rotation; an elongate ferromagnetic element mounted to said shaft so as to extend at an acute angle to its axis of rotation; and an exciting coil disposed in relation to the elongate ferromagnetic element so that passage of a current through the coil induces the elongate ferromagnetic element to be longitudinally magnetised with a direction of magnetisation which depends on the direction of the current in the coil; whereby a gyrating mag netic field is producible by rotating the shaft while passing an alternating current through the coil.

2. A transducer according to claim 1, wherein the exciting coil is substantially coax ial with the axis of rotation of the rotatable shaft.

3. A transducer/receiver assembly comprising a transducer according to any preceding claim; and at least one receiver which is capable of producing an electrical signal when subjected to a varying magnetic field.

4. A transducer/receiver assembly according to claim 3, wherein at least one said receiver comprises a detector coil and is mounted in relation to the exciting coil so that there is substantially no direct inductive coupling between them.

5. A transducer/receiver assembly according to claim 3, which comprises a plurality of said receivers mounted at respective different angular positions relative to the axis of rotation of the shaft.

6. A transducer/receiver assembly according to claim 3, wherein at least one said receiver comprises a toroidal coil preferably mounted substantially coaxially with the axis of rotation of the shaft.

7. A transducer/receiver assembly according to any of claims 3 to 6, further including signal processing means coupled to the or at least one said receiver to receive a said electrical signal therefrom, and adapted to derive, from said signal, data relating to the rotation of the shaft; and to display said data.

8. A transducer/receiver assembly according to any of claims 3 to 7, wherein said receiver comprises a toroidal coil which has a plurality of taps at different angular locations of the coil whereby a multiphase electrical signal is derivable.

9. Apparatus for monitoring the location and orientation of a body, comprising a transducer/receiver assembly according to any of claims 3 to 8, said assembly comprising a transmitter assembly and a receiver assembly which are relatively displaceable; and wherein one of said transmitter assembly and receiver assembly is adapted to be mounted to said body.

10. Apparatus according to claim 9, wherein the transmitter assembly includes a reference receiver which is fixed relative to the transducer and which is capable of producing an electrical signal when subjected to a varying magnetic field due to the transducer; the apparatus further including data processing means coupled to the reference receiver and to at least one receiver of the receiver assembly to receive respective signals from them; said data processing means being adapted to compare said signals.

11. Apparatus according to claim 9 or 10 wherein the receiver assembly is adapted to be mounted to said body; and wherein the transmitter assembly comprises at least two said transducers at spaced locations.

12. Apparatus according to any of claims 9 to 11 wherein the receiver assembly comprises two receivers, each said receiver having a magnetic axis, the axes of said two receivers being in substantially perpendicular planes.

13. Apparatus according to any of claims 9 to 12 wherein the receiver assembly comprises a toroidally or quasitoroidally wound coil.

14. Apparatus according to claim 15 wherein the receiver assembly comprises said toroidally or quasi-toroidally wound coil and a longitudinally wound coil, said coils being coaxial.

15. Apparatus for monitoring the location and orientation of a body, comprising a transmitter assembly and a receiver assembly which are relatively displaceable; wherein one of said transmitter assembly and receiver assembly is adapted to be mounted to said body; and wherein the transmitter assembly comprises at least one coil assembly which comprises a toroidally or quasi-toroidally wound coil and a longitudinally wound coil, said coils being coaxial.

16. A transducer for producing a varying magnetic field, a transducer/receiver assembly, or apparatus for monitoring the location and orientation of a body, substantially as any herein described with reference to and as illustrated in the accompanying drawings.