WO2005119179A1

WO2005119179A1 - Magnetic displacement detector

Info

Publication number: WO2005119179A1
Application number: PCT/GB2005/002115
Authority: WO
Inventors: Michael John Farrow
Original assignee: Howard, Mark, Anthony
Priority date: 2004-05-29
Filing date: 2005-05-27
Publication date: 2005-12-15
Also published as: GB0412122D0

Abstract

A device for measuring the position of a first body relative to a second body comprising: a first body which further comprises an irregular magnetic pattern.; a second body which further comprises two or more sensors which detect the magnetic pattern on the first body at two or more points; whereby a mathematical comparison of the outputs from the detectors will be uniquely indicative of the position of the first body relative to the second body.

Description

MAGNETIC DISPLACEMENT DETECTOR

Field of the Invention

This invention relates to a magnetic displacement detector, operable to measure the displacement of two relatively moveable bodies which has particular, but not exclusive, relevance to measuring the position of pistons in cylinders.

Review of the Art known to the Applicant

Various forms of encoder have been used to measure the position of two relatively moveable bodies. A common form, especially in hydraulic cylinders and hydraulic control systems, is the magnetostrictive encoder. Such encoders are widely available from companies such as Balluff GmbH of Gelnhausen-Roth, Germany. Such encoders typically comprise a magnet embedded in the hydraulic cylinder's piston and a strip of magnetostrictive material extending along the length of the cylinder. The magnetostrictive strip is contained within a second cylinder. As the piston and magnet assembly moves along the length of the magnetostrictive strip it causes a change in the pulse:echo timing of a signal transmitted along the magnetostrictive strip. These changes can be measured electronically to indicate the piston's position along the cylinder. Such encoders typically require the magnetostrictive strip to be held and protected within a carefully welded steel structure - so as to withstand the very high pressure and harsh environment inside hydraulic cylinders. Furthermore, the piston aid piston rod must be bored out, sometimes over lengths of several metres, in order to accommodate the encoder's own cylinder. The encoder's electronics also requires sophisticated temperature compensation hardware and software. Consequently, the application of such magnetostrictive encoders is only practical where high costs can be justified.

Patent US 5,359,288 discloses a magnetic field position detector that uses a Hall effect sensor and a magnetically patterned telescopic part. The planar geometry of the telescopic part, featuring a top and bottom side, is not suitable for most cylmdrical geometries because many pistons rotate as they moves along the cylinder. Any relative rotation of the detector's two main parts would introduce gross inaccuracies in to the position measurement described in US 5,359,288.

Patent US 4,836,578 discloses a digital position coding technique for automotive dampers whereby several magnetic targets are used to generate a Gray code from a relatively small number of detectors. This technique is suitable for relatively coarse position measurement but is unsuitable for high resolution, analogue position detection.

Patent US 4,471,304 discloses a magnetic position encoder that uses a regular magnetic pattern designed for a cylindrical geometry. The main invention described in this patent can only measure position incrementally. Absolute position measurement is not possible without a calibration step at each power up - thus making the invention unsuitable for many applications such as automobile suspension position measurement. Absolute position measurement is enabled by the addition of a second magnetic pattern parallel to the first, but this addition also requires the magnetic patterns and piston rod to be constrained so as not to rotate relative to the magnetic sensors. Such mechanical constraints to motion make this invention unsuitable for many industrial applications. Furthermore, geometries for rotary, curvi-linear etc. geomteries are not taught.

Patent EPO 0 235 750 describes a magnetic encoder similar to US 4,471,304 in which a periodic or regular magnetic pattern on a first body moves relative to a second body. The second body has pairs of magnetic sensors arranged in such a way to indicate the position or speed of the first body relative to the second. The device is only suitable for incremental rather than true position measurement.

Other devices are known which use magnetic fields as a means of determining the position of a first body relative to a second body, see EPOO 227 333; EPOO 048 983; DE10007011; US6,246,233; DE19991008361; GB2,157,831; GB2,056,692; US5,455,509; US5,608,317; US5,399,967; US4,898,027; US5,363,034; US5,242,280 and US4.484.391.

US patent 5,455,509 discloses means for mounting magnetic sensors to a cylinder.

Devices are also known which utilise non-magnetic means of determi ng the position of a first body relative to a second body: GB 1,290,093 discloses a mechanical means;

US4,756,229 utilises ion implantation; US5,404,516 uses sinusoidal signals; US5,943,639 utilises a correction circuit through which a pair of sine wave signals are detected;

US5, 706,219 interpolates a cyclic analogue signal; US5,663,643 uses three phase signals;

US6,664,535 and US6,781,694 disclose optical displacement sensing devices;

US6,772,087 describes a position measuring device which utilises two sets of rotary encoders which generate two phase signals; US 6,496,266 discloses a system which utilises bi-phase sine wave signals; US6,157,188 discloses a system with two members moveable relative to each other and which includes a code track transducer and at least one fine wavelength transducer.

Patent US4,586,576 discloses magnetic means for detennining the deformation of members as well as strain, internal stress or external forces applied to the member.

The present invention encompasses the concept of a low cost and robust device to detect the relative positions of objects such as pistons within cylinders with high levels of accuracy.

In a preferred embodiment, the device comprises a first body with a generally irregular magnetic pattern and a second body comprising magnetic sensors spaced along the measurement axis and able to detect the local magnetic field produced by the first body such that the comparison of the sensor readings is indicative of the true position of the first body relative to the second.

Summary of the Invention

In its broadest aspect, the invention provides a device for measuring the position of a first body relative to a second body comprising: a first body which further comprises an irregular magnetic pattern; a second body which further comprises two or more sensors which detect the magnetic pattern on the first body at two or more points; whereby a mathematical comparison of the outputs from the detectors will be uniquely indicative of the position of the first body relative to the second body.

Preferably a series of magnetic poles spaced along the first body form the irregular pattern.

Preferably irregular variations in magnetic field strength provide the magnetic pattern without the need for multiple changes h the polarity of the magnetic pattern.

Preferably the magnetic pattern is comprised of two regularly spaced magnetic patterns which together form an irregular pattern.

Preferably the magnetic pattern is selected from the group comprising a helix, a sphere and a 2-dimensional plane.

Preferably one or more annular magnetic flux concentrating arrangements are provided to reduce the variability in the signal strength to the sensor. Such variations in signal strength can arise due to variations in the position of the magnetic pattern relative to the sensors in axes other than the axis of measurement.

Preferably displacement of the first body relative to the second body is a rotary displacement. Preferably a magnetic pattern with a fine pitch is over written with a second magnetic patter of longer pitch, a first pair of sensors detect changes in the first magnetic pattern to give a fine pitch measurement and a third sensor detects the longer pitch magnetic pattern and the readings from the sensors are combined to give a fine resolution measurement of the relative displacement of the first body relative to the second body.

Preferably the third sensor is a Hall effect sensors employing a plate large enough to null out detection of the fine pitch magnetic pattern.

Preferably the two or more sensors are replaced by an alternating current inductive sensor comprising respective transmit and receive windings which co-operate such that the varying magnetic pattern beneath the electrical intermediate device causes a varying degree of inductive coupling between the transmit and receive windings which is indicative of the displacement of the first body relative to the second.

Preferably the sensor further comprises an amorphous metal component and measurement of the metal components resonant frequency is indicative of the position of the two bodies.

Brief Description of the Drawings

In the accompanying drawings;

Figure 1 shows a sectional view of a hydraulic actuator with a cylindrical form of a known form of encoder.

Figure 2 shows a sectional view of a hydraulic actuator with a cylindrical form of a known fonn of encoder with a regular or periodic magnetic pattern.

Figure 3 shows a sectional view of a hydraulic actuator incorporating a device embodying the present invention. Figure 4 shows a schematic of the magnetic pattern's field strength at the sensor and sensor outputs of the device of figure 3.

Figure 5 shows a sectional view of a yoke surrounding the rod of a hydraulic actuator for use with a cylindrical form of the encoder of figure 3.

Figure 6 shows a sectional view of a hydraulic actuator with a cylindrical form of the figure 3 encoder with an irregular magnetic pattern comprising multiple regular patterns

Figure 7 shows a schematic of the sensors and microprocessor electronic circuit.

Figure 8 shows a schematic of the rotary form of the encoder.

Figure 9 shows a plan and side elevation of an alternative form of the encoder employing an AC rather than DC type of magnetic sensor.

Figure 10 shows a sectional view of a system used to encode an irregular magnetic pattern on to a rod.

The Embodiments of Figures 1 and 2 (Prior Art)

Figure 1 shows a sectional view of a hydraulic actuator with a cylindrical form of a known type of magnetic position encoder. The piston and piston rod assembly constitute a first body which forms the moving part or target. The target is made from a hard magnetic material, able to retain a magnetic pattern over a long period of time. The target carries a simple magnetic pattern [1] and is arranged to move along the axis of the stator or cylinder [2] under the influence of fluid pressure on either side of the piston. The magnetic field pattern changes from a north to a south pole along the measurement axis. Attached to the cylinder [2] are two magnetic field sensors [3]. The sensors [3] are sufficiently close to the magnetic field that they use the highest possible magnetic flux density without sensing any anomalies from surface defects. The magnetic pattern is arranged so that any mathematical comparison of the outputs from the two sensors is uniquely indicative of the position of the piston relative to the cylinder. It should be noted that it is often advantageous for pistons and piston rods to rotate inside the cylinder, especially in fluid actuation systems such as hydraulic cylinders so as to avoid mechanical over constraint. Such combined rotary and linear motion prevents the use of many of the more traditional displacement measurement systems. However, this arrangement is generally unaffected by rotation since the poles of the piston and piston rod assembly are generally uniform around the circumference of the assembly at any given point along the measurement axis. Nevertheless, the measurement resolution is relatively low and generally unsuitable for any application requiring significant physical displacement.

In order to improve the measurement resolution of this known form of magnetic position encoder, multiple poles may be used within the magnetic pattern. Figure 2 shows a sectional view of a hydraulic actuator with a cylindrical form of a known encoder similar to that described in patent US 4,471,304. The magnetic field pattern regularly alternates between north and south poles along the measurement axis. The distance between the poles is a uniform distance of X. Attached to the cylinder [2] are two magnetic field sensors [3]. The sensors [3] are spaced apart along the measurement axis by a distance of <X, preferably X/2. The magnetic pattern is arranged so that any mathematical comparison of the outputs from the two sensors is indicative of the incremental, rather than absolute, displacement of the piston relative to the cylinder.

Description of the Preferred Embodiment

The present invention permits such encoders (also known as detectors) to measure a unique or absolute position, rather than incremental displacement. In this invention the distance between some of the poles varies along the measurement axis to form an irregular pattern. Figure 3 shows a sectional view of a hydraulic actuator with a cylindrical form of the encoder. The magnetic pattern [1] consists of alternating north and south poles of minimum pitch distance X. The sensors are displaced along the measurement axis by a distance of less than X, preferably X/2. The magnetic pattern [1] is arranged so that any mathematical comparison of the outputs from the two sensors [3] is uniquely indicative of the true position of the piston relative to the cylinder. Figure 4 shows a graph of an example magnetic field strength (H) pattern measured along the measurement axis of a sensor arranged in a similar fashion to that shown in Figure 3. h this embodiment there is a minimum distance X between north and south poles and the sensors are displaced by X along the measurement axis. The graph is supplemented with two further graphs of the readings [Si & S ] from each of the two magnetic fields sensors [3]. The sensors output the signals shown as the magnetic field pattem passes each sensor's own measurement point. It should be noted that this specific magnetic field pattern is for illustration purposes only. As will be appreciated by those skilled in the art, a variety of patterns may be used to achieve an irregular magnetic pattern. For example, the strength of magnetic field encoding may vary in an irregular manner without the polarity of the field changing. In the particular example shown in Figure 4, three sets of north-south-north poles are included in the pattern. It should be noted that the term 'set' specifically refers here to the individual magnetic pattern from the centre line of one north pole through a south pole to the centre line of a second north pole along the measurement axis. The pole distance in the first set is 4X, 6X in the second and 2X in the third. As can be seen from the graphs, any mathematical comparison of the two sensor outputs would be unique at any point along the measurement axis. As such, the comparison of the magnetic field sensor readings relative to an electronically stored look up table of sensor readings relative to axial displacement is indicative of the absolute position of the encoder's magnetic field pattem relative to the sensors. More usefully, the comparison is indicative of the position of one body to which the pattern is attached relative to a second body to which the magnetic field sensors are attached.

It will be appreciated by those skilled in the art that magnetic fields are subject to some variability. Such variability derives from temperature effects, batch variations from one batch of magnetised targets to another and, in particular, from displacements of the target relative to the stator in axes other than the measurement axis, h the example of the hydraulic actuator this non-measurement axis displacement could be due to wobble of the piston rod within the cylinder and guides. In an extreme, variation of the signal strength could lead to erroneous measurement value being returned by an encoder arrangement such as that described in Figure 3. Such variations due to non-measurement axis displacements can be minimised by the use of a yoke or flux concentrating arrangement. Figure 5 shows a yoke arrangement in which a magnetically permeable yolk [10] surrounds a cylindrical target [1] such as those shown in Figure 3. Without the yoke, any normal displacement of the target [1] away from the sensor [3] would tend to reduce the signal strength and hence introduce variability. Such effects will be minimised due to the flux concentrating and averaging effects of the yoke. Preferably, the yoke is made from a magnetically soft material such as soft iron so that no magnetic effects are retained by the yoke, which would otherwise cause erroneous measurement.

Figure 7 shows an electrical schematic of the sensing system. Each of the sensors is provided with a power supply. Hall effect sensors are well suited to the invention and typically such sensors require a 5N DC supply with about 10 milliamps current. It should be noted that two sensors may be embodied within a single element by the provision of two or more Hall plates on an integrated circuit. The output from each sensor is fed to a microprocessor holding a look up table, or mathematical equivalent, held in software or memory. The microprocessor compares the two readings with its memory and outputs an electrical signal as an analogue of the position. Such electrical output can be in variety of forms such as a 0 to 5N DC analogue signal, a pulse width modulated signal, RS232 etc.

Preferably the microprocessor, sensor and ancillary electronics are located on a common printed circuit board. FR4 grade circuit board is well suited to the harsh environments, such as hydraulic actuators or suspension dampers, where such displacement encoders will find application.

Figure 8 shows a plan view of a rotary form of the encoder. Three sets of north-south- north poles are irregularly patterned around the periphery of the moving part of the encoder [1]. One set of poles has an angle of 60 degrees between the poles, the second set has an angle of 90 degrees between the poles and the third has an angle of 30 degrees between the poles. Two sensors [3a & 3b] are arranged at 30 degrees above the target and attached to the stationary part of the encoder [2]. At any angle of the pattern's rotation, the comparison of the readings from the two sensors [3 a and 3b] is unique and hence indicative of the angle of rotation.

This rotary form of the device could find application, as a means of determining the displacement of a piston along the axis of a stator or cylinder. As the piston moves along the cylinder, the piston may be made to rotate in response to its linear displacement. Thus, the degree of rotation of the piston as measured by a rotary form of the encoder system may be used to determine the displacement of the piston along the axis of the cylinder.

Modifications and Further Embodiments

Figure 6 shows an alternative arrangement of a cylindrical form of the encoder. This alternative arrangement permits fine resolution position measurement over extended lengths. The first fine pitch magnetic pattern [1] is over written with a second magnetic pattem [la] with a longer period of »X. The first pair of sensors [3] provides a fine pitch measurement. A third sensor [9] is located within a yoke [10] of approximate length X. The third sensor provides a coarse position measurement based on the longer pitch magnetic pattern. The yoke has the effect of nulling out the finer pitch magnetic pattern. The third sensor [9] that senses the coarse pitch magnetic pattern provides a low resolution but unique or absolute indication of displacement. In other words the sensor sensing the coarse magnetic pattern provides an indication of which of the fine pitches is being measured by the sensors [3] sensing the fine pitch magnetic pattern. The two sets of readings can be electronically combined so as to produce a fine resolution measurement of displacement over an extended length.

Those skilled in the art will appreciate that many forms of irregular magnetic patterns may be used. For example, a first periodic pattern of period 1/3 of full scale length may be combined or overwritten with a pattern of period 1/4 of full scale to produce an irregular pattem over the full scale. Such multiple patterns can be used to provide an unambiguous indication of position which only becomes ambiguous once the lowest common multiple of the pattern pitches is surpassed. For example, a first pattern of 6 units length when combined with a pattern of 7 units length provides unambiguous indication over a length of 42 units and ambiguous indication at distances >42 units.

An alternative to the use of a yoke arrangement is to employ differing sized Hall plates within the Hall effect sensors. Sensors with large Hall plates can be used for coarse position measurement and vice versa. Figure 9 shows a plan and side elevation of an alternative form of encoder arrangement whereby an AC rather than DC type of sensor is used, hi some instances, it may not be practical for space or other reasons to use a Hall effect or magnetoresistive sensor, hi this alternative embodiment an inductive technique utilises an inductive sensor [4] comprising a transmit winding [4b] and two receive windings [4a & 4c] spaced along the measurement axis. The windings are advantageously embodied as tracks on a printed circuit board. The windings co-operate with an electrical intermediate device [5] made from a planar piece of metal such as nickel or amorphous spin melt ribbon. The planar metal extends to beyond the periphery of the windings by around 10% of the length of the windings. Varying the magnetic pattern beneath the electrical intermediate device causes a varying degree of inductive coupling between the transmit and receive windings. As with the previously described sensors, the degree of inductive coupling and hence sensor output at any relative position of the two bodies will be unique and hence indicative of the absolute displacement.

A further modification of an AC encoder, utilising an amorphous metal, is to use the measurement of the metal component's resonant magnetostrictive frequency as indicative of the position of the two bodies. Such resonant frequencies will be determined by the position of the north and south poles relative to the metal component.

An irregular magnetic pattern may be produced by a variety of means but preferably by the use of an electromagnet. An example for magnetising a pattern on to a rod [1] is shown in section in Figure 10. Two series of coils [6 & 7] contiguously extend over the length of the pattern. The coils are preferably made from single strand, PNC insulated copper wire. The first series of coils [6] is wound in an anti-cloclcwise direction relative to the rod [1], the second clockwise [7]. An end stop or datum position [8] is shown which ensures the target object is accurately and consistently positioned relative to the coils [6 and 7]. A DC current is passed through each of the coils. The number of windings, the current and its duration need to be optimised for the host material, component size and type of sensors. The best method to determine the optimal parameter values is experimentation. For a 0.25 inch (say 6mm) diameter steel rod, 6 inches (150mm) long co-operating two Hall effect sensors 1 inch (25mm) apart, workable values were found to be 20 windings per inch; 12 windings deep in the radial direction; 20 SWG (standard wire gauge) copper wire; PNC wire insulation; inner winding diameter of 0.26 inches (6.5mm) carrying a current of 10 Amps for 10 seconds. Those skilled in the art will appreciate that the coil arrangement in Figure 10 will preferably be placed inside a larger cylindrical winding or degauss coil to remove any undesirable or disturbing extraneous fields. This degauss coil is not shown for reasons of clarity. Preferably, the degauss coil should be energised for a period of a 1-2 seconds and then allowed to decay over a period of about 10 seconds.

Using a similar kind of electromagnetic arrangement, a coarse magnetic pattern can be written over the finer pattern with a similar but coarser arrangement of windings intertwined with the first windings.

A current carrying printed circuit board or twisted pair of cables with alternating wound loops so as to produce north and south polarities may be used as a means to produce a magnetic pattern rather than a magnetised rod, disc or plate. This is generally not preferred since electrical connections are then required to both bodies and it is generally preferable to have a single electrical connection to just the stationary body.

hi order to increase the encoder's resolution over extended distances, multiple irregular patterns may be used so as to form a Gray scale or vernier effect. Such patterns may, for example, be offset from each other across or along the measurement axis. A vernier effect may be achieved by the use of a repeating pattern of pitch length n and a second repeating pattern of pitch length (n+1). A Gray scale may be achieved by the use of a coarse and fine pitch pattern.

In order to minimise electronics costs, several magnetic sensors and a microprocessor can be produced on a single integrated circuit. This is readily accomplished using modem silicon techniques where multiple Hall plates may be embodied on a single piece of silicon.

The magnetic pattern need not be considered as a series of rings along a cylindrical target or as a set of discrete poles around a circumference of a rotary target. The poles may advantageously be arranged along a helix, a sphere, 2-dimensional plane or other such arrangement.

The encoder has a wide variety of applications including but not limited to hydraulic cylinders, pneumatic cylinders, hydraulic valves, pneumatic valves, shock absorbers, dashpots, fluid dampers, friction dampers, spring dampers, fluid flow regulators, dial indicators, fluid flow encoders and position encoders for automobiles, aeroplanes, trains, machine tools, domestic appliances, industrial equipment, fitness equipment, defence equipment, control panels, environmentally sealed equipment, joysticks and weighing systems. The encoder can be used as an inexpensive replacement for inductive position encoders such as linearly variable differential transformers. There is a wide variety of applications within an automobile. For example, the position encoder can be used hi steering wheels, throttle control, pedal position measurement, fuel level measurement, ride height systems, suspension control, electronic braking systems, door and window controls, occupancy and seat instrumentation systems. The application of the invention to automotive shock absorbers is particularly important given the increasing prevalence of ride height measurement in automotive control systems.

In Summary

The present invention discloses a system to measure the displacement of two relatively moveable bodies along an axis comprising: - an irregular magnetic pattern attached to the first body with a multiplicity of north and south poles - at least two magnetic field sensors attached to the second body which are displaced from each other along the said axis and which sense the magnetic field of the said irregular magnetic pattern such that the outputs of the said magnetic field sensors are uniquely indicative of the position of the first body relative to the second.

A displacement encoder/magnetic displacement detector, in which the magnetic field sensors are Hall effect sensors. A displacement encoder in which the magnetic field sensors are magnetoresistive sensors.

A displacement encoder in which the first body is a cylinder and the second body is a piston rod.

A displacement encoder in which the magnetic pattern comprises a magnetic pattern of alternating north and south poles in which the pitch distance between poles of the same polarity varies along the measurement axis.

A displacement encoder in which the sensed variation of field strength due to relative displacement of the two bodies in axes other than the measurement axis is minimised by a magnetically permeable yolk substantially surrounding at least one of the magnetic field sensors.

A displacement encoder in which multiple magnetic sensors are produced in a single integrated circuit.

A displacement encoder in which the magnetic pattern comprises a first magnetic pattem of fine pitch alternating north and south poles and a second magnetic pattem of a coarse pitch wherein the coarse pitch is at least twice the fine pitch.

A displacement encoder in which at least one of the magnetic sensors senses the field from the magnetic pattern generated between the plates of a magnetically permeable yolk which extends along the measurement axis by a distance substantially equivalent to the distance between north and south poles of the fine pitch magnetic pattern such that the sensor substantially senses only the magnetic field caused by the coarse pitch magnetic pattern.

A displacement encoder operable to measure the relative displacement of one body constrained to move substantial^ in one direction relative to a second body comprising - a magnetic pattem which is attached to the first body which comprises at least 3 magnetic poles - at least one transmit winding energised by an alternating electrical signal attached to the second body - at least one receive winding attached to the second body - an electrical intermediate device made from a magnetically permeable material and positioned between the windings and the magnetic pattern wherein the degree of inductive coupling between the transmit and receive windings is indicative of the displacement of the first body relative to the second.

A displacement encoder operable to measure the relative displacement of one body constrained to move substantially in one direction relative to a second body comprising - a magnetic pattern which is attached to the first body which comprises at least 3 magnetic poles - an element of magnetostrictive material adjacent to the magnetic pattern - wherein the resonant frequency of the magnetostrictive element is indicative of the displacement of the first body relative to the second.

Claims

1. A device for measuring the position of a first body relative to a second body comprising: a first body which further comprises an irregular magnetic pattern; a second body which further comprises two or more sensors which detect the magnetic pattern on the first body at two or more points; whereby a mathematical comparison of the outputs from the detectors will be uniquely indicative of the position of the first body relative to the second body.

2. A device as claimed in claim 1 wherein a series of magnetic poles spaced along the first body fonn the irregular pattern.

3. A device as claimed in claim 1 wherein irregular variations in magnetic field strength provide the magnetic pattern without the need for multiple changes in the polarity of tire magnetic pattern.

4. A device as claimed in any one of the preceding claims wherein the magnetic pattern is comprised of two regularly spaced magnetic patterns which together form an irregular pattern.

5. A device as claimed in any one of the preceding claims wherein the magnetic pattern is selected from the group comprising a helix, a sphere and a 2-dimensional plane.

6. A device as claimed in any one of the preceding claims wherein one or more annular magnetic flux concentrating arrangements are provided to reduce variability in the signal strength to the sensor.

7. A device as claimed in any previous claim wherein the displacement of the first body relative to the second body is a rotary displacement.

8. A device as claimed in any preceding claim wherein a first magnetic pattem with a fine pitch is over written with a second magnetic pattern of longer pitch, a first pair of sensors detect changes in the first magnetic pattern to give a fine pitch measurement and a third sensor detects the longer pitch magnetic pattern and the readings from the sensors are combined to give a fine resolution measurement of the relative displacement of the first body relative to the second body.

9. A device as claimed in claim 9 wherein the tliird sensor is a Hall effect sensor employing a plate large enough to null out detection of the fine pitch magnetic pattern.

10. A device as claimed in any preceding claim wherein the two or more sensors are replaced by an alternating current inductive sensor comprising respective transmit and receive windings which co-operate such that the varying magnetic pattern beneath the electrical intermediate device causes a varying degree of inductive coupling between the transmit and receive windings which is indicative of the displacement of the first body relative to the second.

11. A device as claimed in claimed in claim 10 wherein the sensor further comprises an amorphous metal component and measurement of the metal components resonant frequency is indicative of the position of the two bodies.