GB2367622A

GB2367622A - Position sensor utilising the changing radiating characteristic of an antenna

Info

Publication number: GB2367622A
Application number: GB0024417A
Authority: GB
Inventors: Graham Michael Freakes
Original assignee: Transense Technologies PLC
Current assignee: Transense Technologies PLC
Priority date: 2000-10-05
Filing date: 2000-10-05
Publication date: 2002-04-10
Also published as: WO2002029443A8; GB0024417D0; AU2001292105A1; WO2002029443A1

Abstract

A position sensor for detecting the position of a conductive element (6,7,8) provided on a device whose position is to be sensed, comprises a loop antenna (3) and a radio frequency generator for generating a radio frequency signal and applying the radio frequency signal to the loop antenna. Means are provided for detecting a change in the radiating characteristic of the loop antenna which is induced by the presence of a conductive element (6) adjacent the loop antenna (3). A second loop antenna (4) may be provided offset from the first loop antenna (3) by half the diameter thereof to increase the sensitivity of the system. If the system is used as a rotary movement detector a third loop antenna (5) is preferably provided which cooperates with a single long conductive element (7) to provide an absolute position signal. Alternatively, an array of loop antennas may cooperate with a rotary encoder to provide a series of unique indications of the angular position of the rotary encoder.

Description

POSITION SENSOR This invention relates to a position sensor, and more particularly to a position sensor utilising a radio frequency device.

Position sensors, that is to say devices for sensing the position of one component relative to another, are extensively used in a wide range of industrial and commercial applications. The present invention is concerned particularly with rotary position sensors, that is to say sensors capable of measuring rotational movement of one component relative to another. The invention is not, however, limited to rotational sensors and the techniques of the invention may in certain circumstances be applied to linear sensors.

A typical rotational sensor comprises a disc secured to a rotatable component, typically a shaft, and a pickup head mounted adjacent the disc for detecting a characteristic of the disc which changes as the disc rotates. For example, the pickup may consist of a light source and a light detector positioned on opposite sides of the disc so that as the disc rotates alternate transparent and non-transparent portions of the disc pass between the light source and the light detector. In a simple device the nontransparent areas may consist of radial lines. Such a sensor will be capable of detecting and measuring rotation of the shaft but will provide no indication of the absolute position of the shaft. If an absolute position detector is required typically the disc will have a complex pattern of non-transparent areas which provides a unique combination of non-transparent areas for each of a specified number of radial segments of the disc. By positioning a multiplicity of light sources and detectors along a radial line such a disc may provide an absolute indication of the shaft position, as well as a measure of rotation.

Whilst known rotational position sensors operate well for certain applications, the extent to which the existing technology can be extended to other areas, for example use on motor vehicles, is limited. The limitations arise from a number of factors. If, for example, the position sensor makes use of one or more light sources and one or more light detectors the output is potentially degraded or corrupted by the presence of dirt. If a capacitive detector is used the output may be degraded or corrupted by the presence of water and, in any event, the detectors required are large and accordingly not appropriate to many automotive applications.

We have now developed a new position sensor which makes use of an RF device to detect the presence or absence of an adjacent conductive area. A device in accordance with the present invention is robust, small, and can be manufactured at a reasonable cost. It is accordingly highly suitable for mass applications within the automotive industry.

In accordance with one aspect of the present invention there is provided a position sensor including a detector for detecting the position of conductive elements provided on a device whose position is to be sensed, the detector comprising: a loop antenna; a radio frequency generator for generating a radio frequency signal and applying the radio frequency signal to the loop antenna; and means for detecting a change in the radiating characteristic of the loop antenna which is induced by the presence of a conductive element adjacent the loop antenna.

In a particularly preferred embodiment of the invention the loop antenna consists of a single turn having a diameter of approximately 1 mm. Such a loop antenna can be driven to radiate radio frequency energy at a frequency of approximately 1 GHz. The device will have an inherent resolution of approximately lmm and accordingly if conductive elements of approximately Imm wide having a spacing of 1 mm are provided the device will be able to detect the presence of absence of a conductive element adjacent to it.

The change in the radiating characteristic of the antenna induced by the presence of a conductive element may be determined in a number of ways. For example, the change in power radiated by the aerial may be detected as a change of voltage applied to the antenna or a change in the resonant frequency of the antenna and its associated RF drive circuit.

In a particularly preferred embodiment of the invention the antenna may be formed as a track provided on a printed circuit board. With such an arrangement, the antenna may be provided on one side of the printed circuit board and the required driver and detector circuits provided may be mounted on the opposite sides of the board. The board may be located adjacent an encoder secured to a shaft, the required conductive elements being provided on the disc. The conductive elements may also be in the form of tracks laid down on a printed circuit board or a flexible plastics substrate.

It will be noted that provided relative movement between the components being sensed is small and slow the detector and the encoder may be in physical contact with each other. If desired, a low-friction coating can be positioned on one or both of the detector and the encoder and/or a low friction washer of suitable plastics material may be positioned between the detector and the encoder.

A particularly simple position sensor capable of detecting rotational movement between a shaft and a shaft support with a resolution of 1 degree of rotation may consist of a ring of 180 conductive radially extending bars each lmm wide and each spaced apart by lmm. The total circumferential extent of such an array is 360mm. If the array is arranged on a circle the circle will have a radius of approximately 57mm.

The resolution of the preferred sensor may be enhanced by positioning two detectors circumferentially offset from each other by half of the diameter of the antennae. Such an arrangement effectively doubles the resolution of the device enabling a resolution of 0.5 degrees for an array of 57mm radius or enabling a device with a resolution of I degree to be obtained using an array having a radius of approximately 28mm.

The present invention can be utilised to provide an absolute indication of a shaft position (after a maximum of one revolution) by providing a further detector and a further conductive element which passes the further detector once each revolution. Accordingly, at start up the device will initially provide no absolute indication of the shaft position. After a maximum of one revolution, however, the further conductive element will have passed the further detector to provide a datum value for future outputs of the first and/or second detectors, thereby providing an absolute measure of shaft position.

If an absolute measurement of shaft position is required at all times the position sensor of the present invention is provided with a radially extending array of detectors and the conductive elements are arranged in an array which has a unique combination of conductive elements for each radial position around the encoder. The detectors, by detecting the presence or absence of conductive elements, will provide a unique indication of shaft position. By use of suitable decoding circuitry the combination of detector outputs may be converted to an absolute indication of shaft position.

Whilst the present invention is primarily intended to provide a robust and lowcost rotary sensor having a long life expectancy, it will be appreciated that the teaching of the present invention may be applied to the detection of linear movement.

For example, a linear array of antennae may be provided on a detector for cooperating with one or more conductive elements on a linearly moveable element to provide an indication of the position of the linearly moveable element.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein: Figure 1 shows schematically a front view of a detector using a preferred embodiment of the present invention; Figure 2 corresponds to Figure I but shows the conductive elements of an associated encoder overlying the detector; and Figure 3 shows a linear array of detectors suitable for use in an absolute rotational position encoder.

Referring firstly to Figure I the detector I comprises a base 2 of any suitable material, for example a glass fibre reinforced board. Laid down on the board are circular loops 3,4, 5 of copper. The loops form single turn loop antennae. The four ends of the loops are connected via conductive tracks passing through the board 2 to appropriate circuitry on the reverse face of the board. The circuitry on the reverse face of the board includes, for each loop 3,4, 5, an RF generator for generating an RF signal and applying it to the loops 3,4, 5. In a typical embodiment of the invention the diameter d of the loops is I mm and the RF generators operate at a frequency of I GHz.

The detector 1 is used in association with an encoder wheel which is secured to a shaft the position of which is to be detected. Figure 2 shows the detector of Figure 1 having superimposed thereon the pattern of the conductive elements of a portion of an encoder wheel. The encoder wheel may typically comprise a base of suitable nonconducting material upon which are laid down bars or tracks 6,7, 8 of a suitable conductive material, for example copper. The bars 6,7, 8 each have a width w measured in the circumferential direction substantially equal to the diameter d of the loops 3,4, 5. The respective bars 6,7, 8 are each spaced apart substantially by the same distance w. It will be appreciated that the bars 6,7, 8 are part of a large array of bars extending substantially around the entire periphery of the shaft. The radial direction of the shaft is indicated by the arrow R.

It will be noted that the bar 7 is longer than the bars 6,8 and indeed is longer than any other bar of the complete array. As a result, the loop 5 will only be covered by a bar once per revolution and may accordingly be used to establish a datum position at start up after, at most, one revolution of the shaft.

It will be noted that in the position of the components illustrated in Figure 2 the loop 3 is covered by the bar 6 and accordingly the loop 3 will provide a detection of the position of the bar 6. After a further rotation of the encoder equal to one half of the width of the bar 5 the bar 5 will completely cover the loop 4 and that bar will provide an indication of the presence of the bar 6. Accordingly, the resolution of the sensor is doubled by the provision of the second loop 4 offset circumferentially by half the diameter of the loop from the first loop 3.

In use, all three loop antennae 3,4, 5 are permanently loaded with an RF signal of substantially the resonant frequency of the loop in free air. When one of the loops is covered by one of the bars (as for example loop 3 is covered by the bar 6 of Figure 2) the radiating characteristics of the loop antenna will be changed. This change can be detected by any suitable means, for example by detecting the change in loading of the loop antenna.

The sensor described above provides, from start up, a measure of rotation of the shaft to which the encoder is connected, and after not more than one revolution, provides an absolute position indication for the shaft.

If absolute position is required from start up the detector device 9 shown in Figure 3, in association with an 8 bit encoder is preferred. The detector 9 comprises a base board 10 of suitable material and an array of 8 loop antennae 11-18. The loop antennae 11-18 are identical to each other and each comprise a single turn loop antenna laid down on the surface of the board 10 and connected to appropriate drive circuitry by means of conducting paths through the thickness of the board. The detector is arranged with the centres of the loops 11-18 lying on the radial line 19.

The detector 9 is associated with an encoder having thereon conductive tracks defining 256 unique radial combinations of track. The tracks are designed to overlie the respective antennae 11-18 so that, for each of 256 radial positions of the encoder the detector is able to detect the unique pattern of tracks. The track pattern may be encoded by any suitable means, for example simple binary encoding or Gray coding.

It will be noted that in the case of both embodiments of the invention the detector is required only to be positioned on one side of the encoder. Further, the exact spacing between the encoder and the detector is not critical. If the sensor is to be used in slow speed application, for example to detect the position of the steering column of a motor vehicle, the detector and the encoder may be in rubbing engagement. To avoid degradation of the tracks on the encoder and the loops on the detector the encoder and/or detector may be furnished with a surface coating. Additionally or alternatively a washer of non-conductive material may be positioned between the encoder and the detector.

Claims

CLAIMS: 1. A position sensor including a detector for detecting the position of a conductive element provided on a device whose position is to be sensed, the detector comprising: a loop antenna ; a radio frequency generator for generating a radio frequency signal and applying the radio frequency signal to the loop antenna ; and means for detecting a change in the radiating characteristic of the loop antenna which is induced by the presence of a conductive element adjacent the loop antenna.
2. A position sensor according to claim 1 wherein the loop antenna has a single turn.
3. A position sensor according to claim 1 or claim 2 wherein the loop antenna has a diameter of approximately 1 mm.
4. A position sensor according to any preceding claim wherein the loop antenna is a track on a printed circuit board.
5. A position sensor according to claim 4 wherein the RF generator and the means for detecting a change in the radiating characteristic in the loop antenna are mounted on the printed circuit board.
6. A rotary position sensor comprising a position sensor according to any preceding claim and an encoder adapted to be secured to a rotatable member, the encoder comprising a plurality of conductive elements, the arrangement being such that the means for detecting a change in the radiating characteristic of the loop antenna provides a signal indicative that one of the conductive elements on the encoder is adjacent the loop antenna.
7. A rotary position sensor according to claim 6 wherein the encoder is a circular or part circular disc and the conductive elements comprise radially extending strips of conductive material.
8. A rotary position sensor according to claim 7 wherein each strip of conductive material has a width, measured in the circumferential direction, which is substantially equal to the space between each adjacent pair of conductive strips measured in the circumferential direction.
9. A rotary position sensor according to any of claims 6 to 8 wherein the width of each conductive strip is substantially equal to the diameter of the loop antenna.
10. A rotary position sensor according to any of claims 6 to 9 wherein a second loop antenna is provided which is offset in the circumferential direction of the rotatable member relative to the first loop antenna by distance substantially equal to half of the diameter of the first loop antenna.
11. A rotary position sensor according to claim 10 wherein the first loop antenna and the second loop antenna have substantially equal diameters.
12. A rotary position sensor according to any of claims 6 to 11 wherein an additional conductive element is provided at one zone only of a circumferential band defined on the encoder and an additional loop antenna is provided for detecting when the additional conductive element is adjacent the additional loop antenna to provide an absolute indication of the position of the shaft.
13. A rotary position sensor according to any of claims 6 to 11 wherein the encoder has a multiplicity of circumferential zones each having conductive elements positioned at one or more discreet locations within the zone whereby each of the plurality of radial zones of the encoder has a distinctive combination of conductive elements; and wherein a multiplicity of loop antennae are provided for detecting the position of conductive elements in each of the radial zones whereby the output of the means for detecting a change in the radiating characteristic of the loop antennae provides an absolute indication of the rotary position of the rotary member.
14. A rotary position sensor according to any of claims 6 to 13 wherein the encoder and the detector are in rubbing contact with each other or are both in contact with a washer positioned therebetween.