GB2271031A - Displacement encoder - Google Patents
Displacement encoder Download PDFInfo
- Publication number
- GB2271031A GB2271031A GB9219938A GB9219938A GB2271031A GB 2271031 A GB2271031 A GB 2271031A GB 9219938 A GB9219938 A GB 9219938A GB 9219938 A GB9219938 A GB 9219938A GB 2271031 A GB2271031 A GB 2271031A
- Authority
- GB
- United Kingdom
- Prior art keywords
- track
- optical apparatus
- light
- encoded
- sensor head
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 238000006073 displacement reaction Methods 0.000 title claims description 6
- 239000011324 bead Substances 0.000 claims abstract description 15
- 239000011248 coating agent Substances 0.000 claims abstract description 6
- 238000000576 coating method Methods 0.000 claims abstract description 6
- 230000005540 biological transmission Effects 0.000 claims abstract description 5
- 230000003287 optical effect Effects 0.000 claims description 33
- 239000000835 fiber Substances 0.000 claims description 9
- 239000000463 material Substances 0.000 claims description 6
- 239000011521 glass Substances 0.000 claims description 3
- 238000010586 diagram Methods 0.000 description 2
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000007373 indentation Methods 0.000 description 1
- 238000003754 machining Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 239000013307 optical fiber Substances 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/12—Analogue/digital converters
- H03M1/22—Analogue/digital converters pattern-reading type
- H03M1/24—Analogue/digital converters pattern-reading type using relatively movable reader and disc or strip
- H03M1/245—Constructional details of parts relevant to the encoding mechanism, e.g. pattern carriers, pattern sensors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/347—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
- G01D5/34707—Scales; Discs, e.g. fixation, fabrication, compensation
- G01D5/34715—Scale reading or illumination devices
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/12—Analogue/digital converters
- H03M1/22—Analogue/digital converters pattern-reading type
- H03M1/24—Analogue/digital converters pattern-reading type using relatively movable reader and disc or strip
- H03M1/28—Analogue/digital converters pattern-reading type using relatively movable reader and disc or strip with non-weighted coding
- H03M1/282—Analogue/digital converters pattern-reading type using relatively movable reader and disc or strip with non-weighted coding of the pattern-shifting type, e.g. pseudo-random chain code
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Theoretical Computer Science (AREA)
- Optical Transform (AREA)
Abstract
A sensor head (2) is operable to direct light onto a selected portion (3) of an optically detectable coded track (4). A retroreflector (11) returns the encoded light so as to be received by the sensor head (2), the retroreflector (11) being operable to reflect obliquely incident light through 180 DEG . The track (4) may be coded such that light is encoded by transmission through the track and the retroreflector (11) may be a sheet of reflective transparent beads having a reflective coating to provide internal reflection. The apparatus allows alignment tolerances to be relaxed and reduces light losses. <IMAGE>
Description
"OPTICAL APPARATUS"
This invention relates to optical apparatus comprising an optically detectable coded track and a sensor head enabling code from a selected portion of the track to be read, and in particular but not exclusively to optical apparatus for use in a displacement transducer in which the sensor head is used to read a code indicating the relative position of the head and the track.
It is known to provide a sensor head which directs light on to a selected portion of a coded track and for a reflector to be provided such that encoded light is reflected back into the sensor head to facilitate decoding. Typically the encoded light is transmitted from the sensor head to a remote decoding station for analysis.
Such reflector means may be provided by forming the track with a planar reflective surface carrying indicia which are non-reflective so that the spacial distribution of reflective and non-reflective portions embodies the code to be read. A disadvantage of such apparatus is that imperfect angular alignment of the track and sensor head due to vibration for example will reduce the amount of reflected light collected.
Typically it is necessary to use an objective lens in proximity with the track in order to collect reflected light if significant light losses are to be avoided.
The presence of such lenses may be unacceptable in certain hostile environments.
It is also known to provide optical apparatus having light dispersing means whereby different portions of the track are illuminated by light of different wavelength to facilitate wavelength division multiplexing of the encoded signal. It would be desirable in such apparatus to dispense with lenses and utilise the dispersing means to disperse and direct light on to the track. In the absence of a light collecting lens however such an arrangement results typically in efficient collection only of light incident normally on the track and suffers substantial losses of light which is obliquely incident on the track.
According to the present invention there is provided optical apparatus comprising an optically detectable coded track, a sensor head operable to direct light on to a selected portion of the track to produce encoded light which is encoded with a signal representative of code embodied in the selected portion, and reflector means operable to reflect the encoded light so as to be received by the sensor head, wherein the reflector means comprises retroreflective means operable to reflect obliquely incident light through 1800 such that encoded light is returned to the sensor head in a direction opposite to the direction at which it is incident upon the scale.
Preferably the light is encoded by transmission through the track and the retroreflective means comprises an array of retroreflective elements spaced from the track such that the track lies intermediate the array and the sensor head.
An advantage of such an arrangement is that light from the sensor head can be focused on to the track and light transmitted through the track will be incident upon the retroreflective means in an unfocused state. The size and position of the individual retroreflective elements need then not be specified to any specific tolerance and in particular the size of the elements need not be directly related to the spatial density of coding on the track.
Conveniently the array of retroreflective elements may comprise a portion of retroreflective sheet material having transparent reflective beads distributed over a surface of the material.
Conveniently each of the beads may comprise a glass bead having a reflective coating applied to part of the bead's surface to provide retroreflection by internal reflection.
The track may be binary coded by means of a sequence of respectively opaque and transparent areas of the track. Conveniently the transparent areas of the track comprise holes.
Alternatively the track may be encoded by means of transparent areas of distinguishable transparency.
The optical apparatus may alternatively comprise retroreflecting means located on the track and comprising an array of retroreflective elements arranged such that the track is encoded in a manner determined by the spatial distribution of the elements.
Each element may comprise a groove formed in a surface of the track and defining mutually orthogonal reflective surfaces.
Alternatively the elements may comprise recesses formed in a surface of the track, each recess defining three mutually orthogonal reflective surfaces.
In each case such elements may be formed by indenting the track surface and applying a reflective coating to the reflective surfaces.
Alternatively the retroreflective elements may comprise strips of a retroreflective sheet material comprising an array of transparent beads arranged to provide retroreflection by internal reflection.
The sensor head may comprise light dispersing means whereby different portions of the track are illuminated by light of different wavelength to facilitate wavelength division multiplexing of the encoded signal.
The light dispersing means may comprise a diffraction grating. Conveniently the grating is part spherical or cylindrical to provide focusing of the light on to the surface of the track. The need for an objective lens is thereby obviated.
The track may be encoded with a pseudo random binary sequence code. Such codes are described by way of example in GB-A-2126444.
The optical apparatus may constitute a displacement transducer in which the track is movable relative to the sensor head and the selected portion of the track identifies the position of the sensor head relative to the track.
Typically the sensor head will comprise a fibre optic link transmitting encoded light to a remote decoding station.
Particular embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings of which:
Figure 1 is a schematic diagram of an optical apparatus in which light is encoded by transmission through a track;
Figure 2 is a schematic diagram of an alternative apparatus in which the track is scanned by a reciprocating mirror;
Figure 3 is a sectional view of a track having retroreflecting grooves;
Figure 4 is a perspective view of a retroreflective recess formed in a track surface; and
Figure 5 is a schematic view of a further alternative apparatus having a curved diffraction grating.
In Figure 1 an optical apparatus 1 comprises a sensor head 2 which is arranged to read code from a selected portion 3 of a coded track 4.
The track 4 is encoded with a PRBS (psuedo random binary sequence) code in which each binary 1 is represented by a respective opaque portion of the track and each binary 0 is represented by a respective hole 5.
The sensor head 2 receives light from a remotely located source (not shown) via a fibre optic link 6, the light being formed into a collimated beam 7 by a lens 8, and the beam being incident on an inclined linear diffraction grating 9.
The diffraction grating 9 deflects the beam 7 through an objective lens 10 which focuses the light on to the track 4.
The selected portion 3 of the track 4 is thereby illuminated along its length by light of progressively varying wavelength due to the dispersing effect of the grating 9.
A sheet 11 of retroreflective material is located so as to extend parallel to the track 4 and spaced therefrom such that the track lies intermediate the sheet and the objective lens 10.
Light from the beam 7 which falls on to the selected portion 3 so as to be coincident with one of the holes 5 is able to pass through the track and is then incident upon the retroreflective sheet 11.
The retroreflective sheet 11 comprises a closely packed array of glass beads each being bonded to a backing sheet and having a metallised surface portion which faces the backing sheet. Internal reflection in the bead results in light being retroreflective so as to be returned after reflection through 1800 along a line parallel to but laterally spaced from the line of incidence into the bead.
The maximum lateral displacement is approximately equal to the diameter of the beads so that beads of smaller diameter than the holes should preferably be used.
Light incident upon the retroreflective sheet therefore is reflected back through the holes 5 to be collected by the objective lens 10. Such light is encoded with spectral information representative of the code present within the selected portion 3 since only those wavelengths will be present in the reflected light which correspond to positions in the selected portion 3 at which binary 0 is represented by a hole.
The encoded light collected through the objective lens 10 is returned through the sensor head into the fibre optic link 6 and is transmitted to a remote decoding station (not shown) at which the code present in the selected portion 3 can be read. The length of the selected portion 3 is selected such that there are sufficient bits of binary information to complete a word which uniquely identifies the location of the selected portion along the track 4.
Such an arrangement may be used as a linear displacement transducer in which the track 4 and the sensor head 2 are relatively movable and an output signal at the decoding station is indicative of the relative position.
The use of the retroreflective surface thereby enhances the efficiency of the sensor head and allows its operation with light of a lower intensity, thereby improving the available length of optical fibre which can be used in the fibre optic link and contributing to an overall improvement in noise level in the system.
An alternative optical apparatus 20 is shown in
Figure 2 and will now be described using corresponding reference numerals to those of Figure 1 where appropriate for corresponding elements.
The alternative optical apparatus 20 has a sensor head 2 which directs light on to a selected portion 3 of a track 4 which is binary coded by means of holes 5.
A retroreflective sheet 11 extends parallel to and spaced from the track 4 such that the track 4 is intermediate the sensor head and the retroreflective sheet.
The sensor head 2 receives light from a light source 21 via a fibre optic link 6 from which emerges a divergent light beam 22 which is directed on to a concave mirror 23. The mirror 23 focuses the light on to the surface of the track 4 so as to illuminate a localised spot 24 which is small in size compared with the extent of a selected portion 3 of the track 4 which is to be read. The mirror 23 is reciprocatingly scanned such that the spot 24 rapidly moves backwards and forwards along the selected portion 3 to produce a serially encoded light signal constituted by light which passes through the hole 5 and is reflected from the retroreflective sheet 11.
Light reflected from the retroreflective sheet 11 returns through the hole 5 to be collected by the mirror 23 and refocused into the fibre optic link 6.
Collected light is then transmitted to a detector 25 which is coupled to the link by means of a coupling device 26.
In Figure 3 an alternative arrangement is shown in which a track 30 formed from a metal plate 31 is encoded with a PRBS code by means of grooves 32 which constitute individually retroreflective elements.
The grooves extend parallel to one another and are spaced at intervals defining the coding of the track 30 such that binary 1 is represented by the presence of a groove and binary 0 is represented by the absence of a groove.
Such a track 30 may be utilised in optical apparatus 40 of the type illustrated in Figure 5 in which a sensor head 41 comprises a plano-concave diffraction grating 42.
The grating 42 is arranged obliquely to a divergent light beam 43 emergent from a fibre optic link 44 so as to laterally deflect light on to the track 30. The track 30 is arranged such that incident light from the grating 42 is focused at points along the length of a selected portion 45 of the track. Light which is retroreflected from the grooves 32 formed in track 30 is encoded with code present within the selected portion 45 and is returned to the sensor head 41 where it is collected by the grating 42 and refocused into the fibre optic link 44 for transmission to a remote decoding station (not shown).
The optical apparatus 40 of Figure 5 may alternatively include a track 30 having reflective elements in the form of recesses 50 as illustrated in
Figure 4. Each recess 50 comprises an indentation of circular profile formed in a surface of the track and having an apex 51 formed by the convergence of first, second and third mutually orthogonal plane surfaces 52, 53 and 54 respectively.
A linear array of such recesses 50 extends along the length of the track 30. Coding of the track is effected by the presence of a recess 50 representing binary 0 and the absence of a recess representing binary 1.
In an alternative arrangement the group of adjacent recesses 50 may together constitute a retroreflective element indicating binary 0.
The retroreflective elements described above with reference to Figures 3 and 4 may be formed by indenting or machining a surface of the track and subsequent application of a highly reflective surface coating to the mutually orthogonal surfaces.
Claims (19)
1. Optical apparatus comprising an optically detectable coded track, a sensor head operable to direct light on to a selected portion of the track to produce encoded light which is encoded with a signal representative of code embodied in the selected portion, and reflector means operable to reflect the encoded light so as to be received by the sensor head, wherein the reflector means comprises retroreflective means operable to reflect obliquely incident light through 1800 such that encoded light is returned to the sensor head in a direction opposite to the direction at which it is incident upon the scale.
2. Optical apparatus as claimed in claim 1 wherein the light is encoded by transmission through the track and wherein the retroreflective means comprises an array of retroreflective elements spaced from the track such that the track lies intermediate the array and the sensor head.
3. Optical apparatus as claimed in claim 2 wherein the array of retroreflective elements comprises a portion of retroreflective sheet material having transparent reflective beads distributed over a surface thereof.
4. Optical apparatus as claimed in claim 3 wherein the beads comprise glass beads each being provided with a reflective coating to provide retroreflection by internal reflection.
5. Optical apparatus as claimed in any preceding claim wherein the track is binary coded by means of a sequence of respectively opaque and transparent areas of the track.
6. Optical apparatus as claimed in claim 5 wherein the transparent areas of the track comprise holes.
7. Optical apparatus as claimed in any of claims 1 to 4 wherein the track is encoded by means of transparent areas of distinguishable transparency.
8. Optical apparatus as claimed in claim 1 wherein the retroreflecting means is located on the track and comprises an array of retroreflective elements arranged such that the track is encoded in a manner determined by-the spatial distribution of the elements.
9. Optical apparatus as claimed in claim 8 wherein each element comprises a groove formed in a surface of the track and defining mutually orthogonal reflective surfaces.
10. Optical apparatus as claimed in claim 8 wherein the elements comprise recesses formed in a surface of the track, each recess defining three mutually orthogonal reflective surfaces.
11. Optical apparatus as claimed in any of claims 9 and 10 wherein the elements are formed by indenting the track surface and applying a reflective coating to the reflective surfaces.
12. Optical apparatus as claimed in claim 8 wherein the elements comprise strips of a retroreflective sheet material comprising an array of transparent beads arranged to provide retroreflection by internal reflection.
13. Optical apparatus as claimed in any preceding claim wherein the sensor head comprises light dispersing means whereby different portions of the track are illuminated by light of different wavelength to facilitate wavelength division multiplexing of the encoded signal.
14. Optical apparatus as claimed in claim 13 wherein the light dispersing means comprises a diffraction grating.
15. Optical apparatus as claimed in claim 14 wherein the grating is part spherical or cylindrical to provide focusing of the light on to the surface of the track.
16. Optical apparatus as claimed in any preceding claim wherein the track is encoded with a pseudo random binary sequence code.
17. Optical apparatus as claimed in any preceding claim wherein the track is movable relative to the sensor head such that the optical apparatus constitutes a displacement transducer.
18. Optical apparatus as claimed in any preceding claim wherein the sensor head comprises a fibre optic link for transmitting encoded light to a remote decoding station.
19. Optical apparatus substantially as hereinbefore described with reference to and as shown in any of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9219938A GB2271031B (en) | 1992-09-21 | 1992-09-21 | Optical apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9219938A GB2271031B (en) | 1992-09-21 | 1992-09-21 | Optical apparatus |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9219938D0 GB9219938D0 (en) | 1992-11-04 |
| GB2271031A true GB2271031A (en) | 1994-03-30 |
| GB2271031B GB2271031B (en) | 1995-07-26 |
Family
ID=10722247
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9219938A Expired - Fee Related GB2271031B (en) | 1992-09-21 | 1992-09-21 | Optical apparatus |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2271031B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2004063671A2 (en) * | 2003-01-16 | 2004-07-29 | Yaskawa Eshed Technologies Ltd | Optical encoder with cylindrical light guide |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1585561A (en) * | 1977-09-21 | 1981-03-04 | Marconi Co Ltd | Optical position sensing arrangements |
-
1992
- 1992-09-21 GB GB9219938A patent/GB2271031B/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1585561A (en) * | 1977-09-21 | 1981-03-04 | Marconi Co Ltd | Optical position sensing arrangements |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2004063671A2 (en) * | 2003-01-16 | 2004-07-29 | Yaskawa Eshed Technologies Ltd | Optical encoder with cylindrical light guide |
| WO2004063671A3 (en) * | 2003-01-16 | 2005-03-31 | Yaskawa Eshed Technologies Ltd | Optical encoder with cylindrical light guide |
| US7348543B2 (en) | 2003-01-16 | 2008-03-25 | Yaskawa Eshed Technologies Ltd. | Optical encoder with hollow light guide for indicating the angular position of a rotary shaft |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2271031B (en) | 1995-07-26 |
| GB9219938D0 (en) | 1992-11-04 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20030921 |