WO1993008567A1

WO1993008567A1 - Sensor device

Info

Publication number: WO1993008567A1
Application number: PCT/GB1992/001899
Authority: WO
Inventors: Edward Watkins Williams; Mufti Mohmed Ashraf; Alfred John Alexander
Original assignee: Penny & Giles Blackwood Limited
Priority date: 1991-10-16
Filing date: 1992-10-16
Publication date: 1993-04-29
Also published as: GB9121904D0

Abstract

A position sensor device for sensing the relative positions of first and second members (100 and 102) which are relatively movable linearly or angularly, has a magnetic or magneto-optic layer (106) fixed relative to one of the members (100) and a reader (116) fixed relative to the other member (102). The layer (106) has magnetic information recorded thereon to serve as a scale which is read by the reader (116) to give an identication of relative position. In an alternative, identification information is recorded on a magneto-optic layer in magnetic bar code form to be read optically or magnetically by a suitable reader.

Description

SENSOR DEVICE

This invention relates to a sensor device and is more particularly concerned with a sensor device for sensing the relative positions of parts which are relatively angularly or linearly movable. The invention also relates to a bar code system for recording identification information.

Magneto-optic films have been previously proposed for storage of computer data thereon in digital form, see, for example, "An introduction to sputtering of magnetic and magneto-optic thin films for data recording", Williams, E.W. in Journal of Magnetism and Magnetic Materials, .9_5_ (1991) 356-364. Magneto-optic film is a medium whereon data can recorded by localised heating of the layer by a laser in the presence of a magnetic field to cause magnetic alignment in the heated region, followed by allowing the region to cool to set such magnetic alignment, thereby to produce regions where direction of the magnetic field is reversed with respect to regions which have not been subjected to localised heating. Reading recorded data from the film can be effected using a low power laser light source fitted with a polarising filter, and a light sensor disposed so as to receive the polarised light which has been transmitted through the layer, and possibly reflected back after having passed through the layer. Magneto-optic rotation of the plane of polarisation of the light takes place upon passage through the layer and differences in the plane of polarisation are detected by the light sensor to produce a digital output signal corresponding to the digital data recorded on the layer. As an alternative, it has been previously proposed to read data recorded on a magneto-optic layer directly magnetically using a magneto-resistive reading head in a similar way to that in which data on a conventional magnetic tape is read.

We have now found that sensor devices incorporating magneto-optic layers can be used as position indicators if the magneto-optic layer has data recorded thereon longitudinally of the intended direction of relative movement between the layer and an optical sensor arranged, in use, to receive reflected or transmitted light incident upon the layer from a suitable light source, so that individual data bits making up the data can be read and displayed in any suitable form to indicate the relative positions of the layer and the sensor. As an alternative, the magnetic positional data on the layer can be read magnetically. As a further alternative, where requirements for measuring position to a very high accuracy are not so important, it is possible to use a magnetic layer upon which positional data is recorded magnetically, and to read such data magnetically using, for example, a magneto-resistive head or a hall effect device.

According to one aspect of the present invention, there is provided a position sensor device comprising first and second relatively movable members, a layer having magnetic information recorded thereon corresponding to a positional scale, said layer being fixed relative to one of said first and second members, and a reader fixed relative to the other of said first and second members, the magnetic information on the layer enabling the reader to identify the relative positions of the first and second members.

The device can be used to measure the relative positions of first and second members which are relatively linearly movable, in which case the magnetic information on the layer corresponds to a linear scale. Alternatively, the device can be used to measure the relative positions of first and second members which are relatively angularly movable, in which case the magnetic information is arranged on the layer to simulate an appropriately curved scale. The layer containing the magnetic information thereon may be provided on a rigid or a flexible substrate. For example, for linear position sensing, the layer may be provided on a flexible or rigid tape, whilst for angular position sensing, the layer may be provided on a flexible or rigid disk.

Most preferably, the layer is a magneto-optic layer.

It is preferred for the reader to comprise an optical transducer arrangement disposed relative to the magneto- optic layer so as to read magnetic information on the layer optically by sensing an optical characteristic (eg polarisation) affected by the magnetic state of the point on the layer being read. For example, a laser and polariser (plane or angular) may be used to illuminate the layer, and the reader used to measure the rotation of the "e" vector of the collected light. The "e" vector is rotated by different amounts depending upon the direction and magnitude of the magnetisation of the magneto-optic layer at the point being scanned.

Alternatively, the reader may comprise a magnetic transducer arrangement positioned relative to the magneto-optic layer so as to read information thereon magnetically. For example, a magneto-resistive transducer may be used in which the electrical resistance of one or more read heads is affected by the adjacent magnetic field in the layer. Such arrangements can be sensitive to magnetic domains as small as 1 to 3μ. Alternatively, the reader may include one or more Hall effect devices which produce a voltage proportional to the magnetic field surrounding them. Currently available versions of this type of reader are applicable to situations where the magnetic domain size or separation is greater than about 30μ.

The first aspect of the present invention also resides in the use of a layer having magnetic information recorded thereon to indicate the relative positions of mutually movable first and second members.

In one convenient embodiment, the information recorded on the layer is in digital word form, preferably like a bar code, with each of the "bars" or data words being defined by a plurality of digitally coded bit regions which together make up a bar or data word uniquely identifying position and which extend in a line transversely with respect to the direction of relative movement between the first and the second members. Whilst the "bars" or data words may be equi-spaced to give a uniform scale, it is within the scope of the invention for the bars or data words to be unequally spaced apart, eg with the smallest separation being about lμ, for use in applications where this is acceptable.

The digital information in the bars or data words may be read optically using a reader comprising a single light source arranged to cause polarised light to illuminate all of the data bits in each bar or data word simultaneously, and a plurality of light sensors corresponding in number to the number of data bits in each bar or data word, each light sensor receiving light which has passed through a respective one of such data bits.

In another embodiment, the coding pattern on the layer may indicate incremental change in position. In this case, only one bit of data is needed at each longitudinal position on the layer. The present invention, therefore, contemplates the use of any coding technique, for example, single track with equal mark-space ratio for incremental applications, or multitrack with binary or Gray coding for absolute position sensing applications.

With regard to the magneto-optic layer itself, in one embodiment, the magneto-optic layer typically comprises a glass substrate upon which are provided a reflective aluminium layer, a dielectric oxide or nitride layer, a magneto-optic layer and a second dielectric layer. The dielectric layers are provided to prevent corrosion of the magneto-optic layer and act as anti-reflective coatings. Typically, the magneto-optic layer is a ternary rare earth transition metal alloy, eg. terbium- iron-cobalt. The magneto-optic layer may be provided by a sputtering operation or by chemical vapour deposition, for example metallo-organic chemical vapour deposition (MOCVD) .

In another embodiment, the magneto-optic layer is a magneto-optic multi-layer with, for example, 10 to 20 layers situated between 2 dielectric layers with an aluminium reflective layer deposited as the final layer, a transparent substrate (eg a glass substrate) being used so that the sensor can be used in either the transmissive (without reflective layer) or the reflective mode. Domain sizes in the range of 1 to 10 μ can be recorded with very high precision.

Broadly speaking, the magneto-optic layer may be one such as to rotate or otherwise modify the state of polarisation of the incident light. An example of this is an iron-terbium-cobalt layer (Fe 66wt%, Te 26wt%, Co 8wt%). Such layer may be sandwiched between a reflective layer of aluminium, gold, silver, copper or any other material acting as a reflector at the operative wavelength, and a suitably transparent protective layer which may be formed of any protective material such as an oxide, nitride or sulphide which protects the magneto- optic layer from environmental corrosion. This protective layer, or an additional layer, can optionally provide anti-reflection properties if its thickness can be made one quarter of the wavelength of the incident light.

Typically, the magneto-optic layers can be produced as described by Williams, E.W., supra. Briefly, the layers can be deposited by any thin film depositing technique, eg by any of the followingt- Chemical vapour deposition.

Radio frequency or direct current sputtering. Evaporation from elemental or compound source. Molecular beam epitaxy.

The position sensor according to the present invention has use in robotics, manipulators for dangerous processes or for medical applications, and in joysticks for simulators or other controllers.

The present invention according to a second aspect thereof relates to a bar code reading system for coding parts and products in stores, shops and the like wherein the magneto-optic layer has a bar code or series of data words recorded thereon which can be read by a compact disk-type optical reader suitably modified to be sensitive to light polarisation shift. In addition, a write facility may also be provided to write a new code or to put a new date, the name of the store or shop and to change the price of the goods, whenever this occurs.

Thus, in accordance with said second aspect, there is provided the use of a magneto-optic layer in a bar code system for presenting identification information to be read magnetically or optically, wherein the identification information to be read is recorded on the magneto-optic layer as a series of bars or data words which are spaced apart in the intended direction of relative movement between the layer and an optically or magnetically sensitive reader, each of said bars or data words extending transversely relative to said intended direction of relative movement.

In accordance with the present invention in either of its first and second aspects, the magneto-optic layer is preferably defined by one or more metallo-organic vapour deposited layers.

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

Fig. 1 is a schematic illustration of the use of the present invention in connection with a linear position sensor device,

Fig. 2 is a schematic illustration of the use of the present invention in connection with a rotary position sensor device,

Fig. 3 is a side view of a linear position sensor device used to indicate the position of a tool table slidably mounted on a fixed bed,

Fig. 4 is an end view of the device of Fig. 3,

Fig. 5 is a schematic view of a read head forming part of a linear position sensor device,

Fig. 6 is a schematic view of a write head for writing to a magneto-optic film used in a linear position sensor device,

Fig. 7 is a schematic plan view illustrating a coded magneto-optic layer forming part of an angular position sensor device,

Fig. 8 is a schematic plan view illustrating the shapes of the areas of the layer of Fig. 7 to be read at any one time,

Fig. 9 is an axial section through an angular position sensor device in accordance with the present invention in the form of a rotary shaft encoder,

Fig. 10 is a section on the line A-A of Fig. 9,

In Fig. 1, a linear position sensor device with magneto-resistive (M-R) head reader is shown. The laser heats up the magneto-optic layer in the presence of an applied magnetic field. With the field vertically upwards a zero is recorded but when the field is vertically downwards a one is recorded. The field can be provided by a small electromagnet or a permanent magnet. The resistance of the M-R reader changes when it passes over a charge in magnetic field from a "1" to a "0" digital bit. Hence the number of ones or zeros gives a measure of the distances moved by the MR head. The "1" and "0" can be written by the laser through the substrate. Hence the scale or the number of ones or zeros can be altered by changing the voltage pulse that is applied to the laser. A code or bar code can also be added by writing on the appropriate bit pattern with a laser. The bar code can be of the European standard equivalent to number the product for quality analysis.

The resolution between bits is 1 micron so a position sensor with an accuracy of 1 micron +.0.2 micron is possible.

In Fig. 2 a rotary sensor is illustrated. Each segment is coded by writing a bit pattern with the laser in the high power mode and applying a small magnetic field. For example, segment 1 could have the code 101010101 whereas segment 2 could be 1100110011. So segment 1 has a one bit spacing and segment 2 has a two bit spacing. Segment 3 would have a three bit spacing and segment 4 would have a four bit spacing. Hence each 90 degree segment is defined uniquely. Bits are then recorded on the outer tracks, one track for each scale; hence up to 100 scales can be defined and read with the laser in low power mode or with an M-R head.

Referring now to Fig. 3 and 4 of the drawings, the linear position sensor device is provided for sensing the position of a tool table 100 linearly slidably mounted on a fixed bed 102. An elongated housing 104 is fixedly mounted on one side of the table 100 so as to extend longitudinally of the direction of sliding movement of the latter. The housing 104 contains a scale 106 which is mounted on the underside of a scale carrier 108 so as to face a slot 110 in the lower surface of the housing 104. The scale is constituted by an elongate magneto- optic film having a plurality of coded tracks along its length formed by reversing the direction of magnetisation at predetermined locations in order to present an arrangement which, when the tracks are read laterally of the scale, gives a unique code by means of which the position along the scale is defined. Thus, in effect, the tracks define position-identifying strips or bars extending laterally of the scale in a similar manner to a conventional ruler with identifying lines scribed thereon to indicate the scale visually.

A cranked bracket 112 is fixed to the bed 102 and extends into the housing through the slot 110. Lip seals 114 are provided to prevent unwanted ingress of dirt into the housing 100. A read head 116 is supported on a mount 118 which is biassed upwardly by springs 120 so as to urge guide wheels 122 on the mount 118 into engagement with the sides and underside of the scale carrier 108. In so doing the read head 116 is accurately positioned relative to and adjacent the scale 106. In use the read head 116 reads the position of the table 100 relative to the bed 102 by reading the tracks on the scale 106 over a region thereof which is disposed opposite the read head 116 and indicating this position visually, eg on a display (not shown) positioned conveniently close to the tool operator.

A read head suitable for use as the read head 116 of Figs. 3 and 4 is illustrated in Fig.5. Referring now to Fig. 5, the read head comprises a laser light source 200 from which light through a convex lens 202 to be focused in field stop 204 from where it is passed via convex lens 206 through a polariser 208 which transmits light polarised at an angle of 45° with respect to the p and s polarisation directions. In other words, the p and s components of polarisation of the transmitted light are equal. The polarised light then passes into a 50% reflecting mirror cube 210 before being focused to the desired extent by a further convex lens 212 onto the region of scale 106 which lies opposite the read head and which contains a uniquely coded strip or bar to be read. The light passes through the magneto-reflective layer of the scale and is reflected back through such layer by means of a reflective layer disposed behind the magneto- reflective layer. During passage through the magneto- optic layer, the direction of magnetisation at that portion of each track traversed by the light affects the angle of polarisation of such light. If the direction of magnetisation is in one direction, the p component of polarisation is enhanced slightly at the expense of the s component, and vice versa for the opposite direction of magnetisation. The reflected light then passes via the lens 212 and into the cube 210 to be internally reflected through lens 214 into polarising, 50% reflecting cube 216 which splits the light into its p and s components and permits one of these components to pass to a first photodetec or array 218 whilst the other component is reflected to a second photodetector array 220. The angle of the polariser 208 is such that the angle of polarisation of the light reaching the polarising cube 216 is nominally at 45° to the polarisation axis of the polarising cube 216. Thus, if no rotation had taken place at the magneto-optic layer 106, equal amounts of light would be passed through the two directions of the polarising cube 216, and hence the two photodetectors would give equal signals. If the direction of polarisation of the light reflected from the magneto- optic layer 106 had been rotated by a small amount (0.5 to 1°) due to the direction of magnetisation of the layer, the intensity of light passing to one of the two photodetectors, say 218, would be increased slightly and decreased on the other photodetector 220, and vice versa for the opposite direction of magnetisation. Each photodetector in each array 218 and 220 produces an electrical output signal whose magnitude is proportional to the intensity of the light incident thereon. These electrical signals are passed to a comparator and display unit 222 where the magnitude of the electrical output signal from each photodetector in array 218 is compared with that of the corresponding photodetector in the other array 220 which has received light from the same portion of the same track to enable the direction of magnetisation and therefore the bit value at that portion to be assessed. This differential measurement arrangement mitigates the inevitable background noise and photodetector output drift effects. Thus, all the bits in the data word to be read can be read at precisely the same time whilst the read head and the layer are relatively stationary.

Referring now to Fig. 6, there is show a write head for writing to a magneto-optic layer 106. Laser 300, which is set at a rather higher power output than laser 200 used in the read head of Fig. 5, passes a thin beam of light through a light modulator 302, a beam expander 304 (constituted by convex lens 306, pin hole 308 and convex lens 310) to a focusing lens system 312. The focused beam from system 312 is thereby directed onto one portion of one of the tracks on the magneto-optic layer to heat that portion locally. At the same time, a magnetic field is applied perpendicularly of the layer by means of electromagnet coils 316 to enable the direction of magnetisation at that portion of the layer 106 to be set. The direction of magnetisation can be altered by reversing the direction of current flow through the coils 316. The layer 106 is moved past the focused laser beam so that the track currently being written can be coded. The remaining tracks are coded in a similar way so that the coding of the tracks enables unique identification of position to be recorded for subsequent reading by a read head such as that illustrated in Fig. 5.

The principle of operation is to use the focused laser beam to heat a small area of the film to a temperature above or sufficiently near its Curie temperature that the coercive force required to change or reverse the state of magnetisation of the area of the layer is below the value of the applied magnetic field. The applied magnetic field must be low enough to prevent the state of magnetisation of the unheated area surrounding the heated area from being changed.

The function of the beam expander 304 with the pin hole 308 is to produce a highly collimated beam. This in conjunction with the focusing lens system 312 must focus the beam to the desired spot size at the laser wavelength.

In one method of use, the modulator 302 may be used to switch a continuous beam laser on and off to produce the desired bit pattern on the magneto-optic layer 106 as it is moved steadily over the whole area to be written. In this case, the magnetic field can be constant and the coils 316 could be replaced by a suitable permanent magnet. Alternatively, if a suitable laser is used (eg a semiconductor laser), the laser may be switched on and off directly.

In a second method of use, the modulator 302 may be omitted completely and the magnetic field pulsed on and off (or positive or negative) by appropriate control of the coils 316 to produce the desired bit pattern as the magneto-optic layer 106 is moved over the whole area to be written.

In a third method of use, the magneto-optic layer 106 is first bulk magnetised to a defined unidirectional magnetised state over the whole area to be written. This can be effected using a very large field on a permanent magnet, and for this the laser and optics systems are unnecessary. The magneto-optic layer 106 is then mounted in the write head system described above and the laser beam pattern generated as described previously. In this method, the external magnetic field may be zero or absent and the result of heating any area of the layer above or near to its Curie temperature is to reverse its state of magnetism, this arising due to the demagnetising effect . of the unheated areas around the heated area still retaining their original magnetisation.

A typical coding arrangement is schematically represented in Fig. 7 which is for an angular position sensor device. Here, eight coded tracks A to H are provided. These tracks are annular in shape and are radially spaced apart. They are coded in a so-called gray code pattern wherein the coding representing one angular location differs from the next angular location by only one bit. The dark and light areas of the tracks A to H indicate oppositely magnetised regions. In outer track A, the dark and light areas subtend the smallest angle.

Fig. 8 shows the area corresponding to that region of each track A to H which is read at any one time. Such area corresponds to the full radial depth of the track and half the angle subtended by each of the dark and light areas of track A.

Referring now to Figs. 9 and 10, the angular encoder illustrated therein comprises a housing 400 having bearings 402 in which a shaft 402 is mounted for rotation relative to the housing 400. A backing plate 406 is secured by grub screws 408 and mounting bracket 410 to the shaft 404 within the housing 400. The backing plate 406 carries an annular scale 412 including a magneto- optic film which is coded in a similar manner to that described in Fig. 7 above. The housing 400 also fixedly mounts a read head 414 which may be similar to that described hereinabove in relation to Fig. 5. The read head can therefore read the precise angular position of the shaft 404 relative to the housing 400 at any instant.

In the arrangements as described hereinabove, a magneto- optically coded scale or pattern is readable by a reader without the need for there to be relative motion between the scale and the reader during a reading operation in a system where there is an externally accessible shaft or other member, the angular or linear position of which can affect a precisely known position of the scale relative to the reader. The reading of all the data bits in a data word at precisely the same time by the use of one or more read heads enables absolute information (eg absolute position) to be read. This is in contrast to current methods of reading and writing data on computer disks where:- 1) Such reading methods make use of steady state or slowly varying speeds of rotation of the disc, thereby generating a high frequency alternating signal in the read head. The subsequent signal processing can therefore easily separate the wanted high frequency signal from the steady state or slowly varying error signals due to temperature drift in the head or processing electronics or due to extreme fields. Current systems cannot indicate the magnetic state of the point or points under the read head if the disk is stationary relative to the read head.

2) In general, the angular position of the data on the disk is not especially significant. Whilst it is conventional to divide any track on the disk into physical or pseudo-physical sectors, the position of these sectors and the data contained in them cannot be used to indicate or measure and external shaft angle or position.

3) The read heads cannot read more than one bit of information at a time. Multiple read heads are used but they are not normally arranged synchronously to read separate bits of a single data word.

The methods and equipment described above and/or as illustrated in the accompanying drawings may be used, mutatis mutandis, to encode and read a bar code system for presenting identification information to be read magnetically or optically, wherein the identification information to be read is recorded on the magneto-optic layer as a series of bars or data words which are spaced apart in the intended direction of relative movement between the layer and an optically or magnetically sensitive reader, and wherein each of said bars or data words extends transversely relative to said intended direction of relative movement.

Claims

1. A position sensor device comprising first and second relatively movable members, a layer having magnetic information recorded thereon corresponding to a positional scale, said layer being fixed relative to one of said first and second members, and a reader fixed relative to the other of said first and second members, the magnetic information on the layer enabling the reader to identify the relative positions of the first and second members.

2. A device as claimed in claim 1, wherein the first and second members are relatively linearly movable, and the magnetic information on the layer corresponds to a linear scale.

3. A device as claimed in claim 1, wherein the first and second members are relatively angularly movable, and the magnetic information is arranged on the layer to simulate an appropriately curved scale.

4. A device as claimed in any preceding claim, wherein the magnetically recorded scale is provided on a magneto-optic layer.

5. A device as claimed in claim 4, wherein the reader comprises a magnetic transducer arrangement positioned relative to the magneto-optic layer so as to read information thereon magnetically.

6. A device as claimed in claim 4, wherein the reader comprises an optical transducer arrangement disposed relative to the magneto-optic layer so as to read information magnetically recorded on the layer optically by sensing an optical characteristic affected by the magnetic state of the information being read.

7. A device as claimed in claim 6, wherein the optical characteristic is change in polarisation of light transmitted through the magneto-optic layer.

8. The use of a layer having information magnetically recorded thereon to indicate the relative positions of mutually movable first and second members.

9. The use of a magneto-optic layer in a bar code system for presenting identification information to be read magnetically or optically, wherein the identification information to be read is recorded on the magneto-optic layer as a series of bars which are spaced apart in the intended direction of relative movement between the layer and an optically or magnetically sensitive reader, each of said bars extending transversely relative to said intended direction of relative movement.