GB2065297A - Apparatus for detecting movement - Google Patents
Apparatus for detecting movement Download PDFInfo
- Publication number
- GB2065297A GB2065297A GB7941714A GB7941714A GB2065297A GB 2065297 A GB2065297 A GB 2065297A GB 7941714 A GB7941714 A GB 7941714A GB 7941714 A GB7941714 A GB 7941714A GB 2065297 A GB2065297 A GB 2065297A
- Authority
- GB
- United Kingdom
- Prior art keywords
- grating
- optical system
- path
- movement
- light source
- 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.)
- Withdrawn
Links
- 230000003287 optical effect Effects 0.000 claims abstract description 45
- 238000006073 displacement reaction Methods 0.000 claims description 10
- 238000012544 monitoring process Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 230000000737 periodic effect Effects 0.000 description 2
- 238000003491 array Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000001186 cumulative effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000005670 electromagnetic radiation Effects 0.000 description 1
- 238000005286 illumination Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000000873 masking effect Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 230000002093 peripheral effect Effects 0.000 description 1
Classifications
-
- 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
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optical Transform (AREA)
Abstract
Apparatus for detecting movement of a member includes light source 8 and a grating or carrier 4 coupled for movement relative to the light source in accordance with movement of the member. The grating includes portion 6 illuminated by the light source. An optical system including mirror 9 projects an image of the illuminated portion onto further portion 7 of the grating or onto a further grating, so that the image and the further portion or grating appear to photocell 13 (arranged to view the image through the further portion or grating) to move in contrary directions. The optical system is constructed to compensate, where the grating forms part of a disc (4) mounted on rotary shaft 3 for eccentric mounting of the disc on the shaft and possibly also for axial movement. In an alternative embodiment (Fig. 4) the light source and photocell are mounted on the same side of disc 4. <IMAGE>
Description
SPECIFICATION
Apparatus for detecting movement
This invention relates to apparatus for detecting, and for use in monitoring movement by employing an optical technique which involves cyclically modulating the intensity of an electromagnetic beam using a grating element arranged to move in accord with the movement to be monitored, and detecting the modulated beam.
The invention is particularly, but not exclusively concerned with the monitoring of rotary movement, the grating element in such cases deriving rotary movement from, for example a rotating shaft, the apparatus providing an output from which the instantaneous angular position and speed of the shaft may be determined.
For example, in optical scanning equipment in which a reading or a writing beam is made impinge upon a surface to be scanned in a raster fashion, a rotating reflector is commonly employed to produce the necessary angular movement of the beam. In order to ensure synchronism of the modulation of the writing beam, or of the processing of information derived using the reading beam the electronic systems employed to control such modulation or processing require the input of data concerning the instantaneous position of the reading or writing beam. This data is most conveniently derived from the rotary movement of a shaft upon which the reflector is mounted.
It is known to use a grating element providing a linear array of alternate optical slits and masks, for example in the form of spaced opaque lines inscribed upon an otherwise transparent or translucent base, and to view the moving grating through a slit fixed relative to an illumination source positioned to project a light beam through the grating, onto the slit. The intensity of the light transmitted by the slit and detectable by a photocell is accordingly modulated cyclically at a frequency dependent upon the speed of the grating relative to the slit and the spacing of the inscribed lines on the grating. In practice, a reticule having a number of opaque lines and transparent spaces is used in place of the slit to facilitate detection of the transmitted light.
However, in order to avoid problems derived from optical parallax between the reticule and the grating when these elements are of high resolution (i.e. a large number of lines per unit length in the line/space arrays), by the detector it is necessary to place the reticule very close to the grating. This requirement imposes very close manufacturing tolerances concerning for example surface irregularities on the grating and reticule and relative mounting of these elements.
Furthermore, where the grating is circular and comprises a circular array of radial lines inscribed peripherally of a base disc to be mounted on a rotary shaft, the requirements of concentricity of the grating element and the shaft are particularly strict if inaccuracies are to be avoided.
According to the present invention, therefore, there is provided apparatus for detecting
movement of a member, comprising: a light source; a grating element coupled for movement relative to the light source in accordance with movement of the member and including an optical grating, the light source being arranged to illuminate a portion of the grating; and a photoelectric cell fixed relative to the light source and arranged to view an image of the illuminated portion, formed by an optical system, through a further grating or portion of the said grating, the optical system being so arranged that the image and the further grating or portion appear to the photocell to move in contrary directions.
Where the apparatus is to detect rotary movement of the member about a rotational axis, the grating element preferably comprises a disc coupled, nominally coaxially with said rotational axis, for rotation with said member, the optical grating comprises a circular array of circumferentially alternate opaque and light transmissive portions, and the illuminated portion and the further portion are diametrically opposed relative to the rotational axis.
The optical system preferably defines an optical path extending from one conjugate coincident with said illuminated portion to another conjugate coincident with the further portion. The system may be so constructed that displacement of said one conjugate along said path produces corresponding movement of said other conjugate in the same direction along said path, axial displacement of the disc producing corresponding displacements of the illuminated and further grating portions in the same direction along the optical path.
In one embodiment, the optical system comprises two convergent lenses of nominally equal focal length spaced along said path by a distance equal to twice said focal length and disposed nominally with the illuminated portion and said further portion each spaced from a respective one of the lenses along said path by a distance equal to said focal length.
To fold the path, and to provide the required apparent movement in contrary directions, the optical system may include an odd number of reflective surfaces.
Preferred embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:
Figure 1 illustrates schematically an optical system employed in a first embodiment of the invention;
Figure 2 illustrates apparatus in accordance with the first embodiment of the invention, for USE in monitoring rotary movement;
Figure 3 illustrates schematically an optical system employed in a second embodiment of the invention;
Figure 4 illustrates apparatus in accordance with the second embodiment of the invention, also for use in monitoring rotary movement, and
Figures 5a and 5b illustrate diagramatically an operational advantage of the apparatus of Figures 2 and 4.
The optical system depicted in Figure 1 comprises two converging lenses 1 and 2 of equal focal length F. These are arranged as shown with
an axial spacing of 2F, and the system has one
pair of conjugate planes as shown, spaced axially
by distances F from the principal planes P 1, P2 of the lenses. The image I is reversed with relation to the object 0, and the magnification is unity. It can
be shown mathematically that if the object is
moved closer to lens 1, i.e. the conjugate distance
at lens 1 is shortened, then the image I moves further from lens 2, i.e. the conjugate distance at
lens 2 is lengthened by a corresponding amount,
while the magnification remains unaltered at
unity.
This optical system is incorporated in the
apparatus of Figure 2 in which a grating disc 4 is fixedly mounted nominally coaxially upon a rotary
shaft 3 of which the rotation is to be monitored.
The disc 4 is formed adjacent its periphery with a
circular grating comprising a multiplicity of radial opaque lines 5 which are spaced circumferentially and uniformly, the width of the lines and of the
intermediate transparent spaces being substantially equal. Although only two diametrically opposite portions 6 and 7 of the grating are illustrated in Figure 2, it will be appreciated that the grating is circumferentially continuous. The grating and the remaining elements of the apparatus depicted in Figure 2 constitute a signal generator for generating an alternating signal of which the instantaneous frequency varies in accordance with variation in the speed of rotation of the shaft 3.This shaft may be, for example, the output shaft of a motor and may carry a reflecting member (not shown) upon which is incident a fixed beam of electromagnetic radiation, the reflected beam rotating about the shaft axis and impinging upon a surface to be scanned.
The elements now to be described are fixed in space relative to the rotational axis of the shaft 3.
A light source 8 is arranged to illuminate the portion 6 of the grating from one side of the grating disc. The two converging lenses 1, 2, together with three plane mirrors 9, 10, and 11 are arranged to project an image of the illuminated portion 6 from one face of the grating disc onto the diametrically opposite portion 7 of the grating via the opposite face of the grating disc. The light projects upwardly from the illuminated portion 6 and along an optical path defined by the lenses 1 and 2 and the mirrors 9, 10, 11 (see arrows 12) and upwardly through the portion 7 to be detected by a photocell 13 which produces an electrical signal of which the magnitude varies in accordance with variation in the intensity of the light which it receives.
The spacing, measured along the optical path, of the coplanar grating portions 6, 7 and lenses 1, 2 is in accord with the Figure 1 arrangement; that is to say, grating portions 6, 7 lie in conjugate planes of the lens system, so that the image of grating portion 6 is in focus at the grating portion 7.The mirrors 9, 10 and 11 fold the optical path to permit the light beam to project upwardly through the thickness of the grating disc onto portion 7 and also ensure that upon rotation of the shaft and grating disc in one rotational sense 14 to produce corresponding movement of the actual grating lines in fixed portion 7, the lines in the image of portion 6 move in the opposite direction:
Since the optical system produces unity magnification, and exact superposition of grating image upon grating proper, the latter acts as a moving light shutter of which the opaque lines effect periodic masking of the bright lines of the former which moves at the same speed in the opposite direction. The intensity of the light entering photocell 11 therefore is modulated at twice the frequency obtainable with a fixed reticule and moving grating as used hitherto.
Furthermore, any spurious axial displacement of the shaft 3 and grating disc 4 will be automatically compensated since any such displacement of the grating disc will cause equal and opposite variations in the optical path spacings of lenses 1 and 2 from the grating portions 6 and 7 respectively. These grating portions will therefore still coincide with pairs of conjugate planes in accordance with the optical features of the lens system as described earlier in connection with Figure 1. The image of grating portion 6 will therefore always be in sharp focus on grating portion 7 resulting in clear and detectable periodic variation in the photocell output.
A further operational advantage of the Figure 2 arrangement will now be explained with reference to Figures 5a and 5b.
Figure 5a illustrates diagramatically in plan view the grating disc 4 when mounted precisely coaxially with the shaft 3. The circle 15 represents (much enlarged) the area in the object plane of the lens system, encompassing the portion 6 of the grating, which is imaged upon a diametrically opposite area, represented by circle 16, encompassing portion 7. If the direction of movement of the actual grating lines within circles 15 and 16 is from A to B then the lines of the superposed image in circle 16 move in the opposite direction from A' to B' in the manner, and with the results, described above.
Figure 3b illustrates diagramatically the situation where the grating disc is mounted eccentrically about its axis 0 upon the shaft 3 with an offset X. The object and image circles 15 and 16 remain fixed relative to the shaft 3 but are accordingly offset by equal, similar distances X, X respectively, in the rotational position of the grating disc depicted, from the grating disc diameter parallel to a line extending through the shaft axis between the centres of the object and image circles.At any rotational position of the grating disc this offset relationship holds true so that, for example, for clockwise rotation of the grating disc in Figure 3b a lag in the grating movement in object circle 1 5 is compensated by an advance in the grating movement in image circle 1 6 so that in the latter circle the effects of the eccentric mounting upon the tangential velocities of the image of grating portion 6 and actual grating portion 7 cancel to produce, for a uniform speed of disc rotation, a constant relative velocity between such image and grating portion 7 and thus a constant frequency of modulation in the light entering photocell 11.Thus, even if the disc is eccentrically mounted on the motor shaft 4, the amplitude peaks in the output signal from photocell 11 define accurately the cumulative increments of disc and accordingly shaft rotation.
In Figures 3 and 4, elements which correspond to elements of the first embodiment are indicated by similar reference numerals to those used in
Figures 1 and 2, but followed by a prime indicium.
In this second embodiment, a grating disc 4' fixedly mounted nominally coaxially upon a rotary shaft 3' carries a peripheral circular grating of alternate radial opaque lines 5' and transparent spaces of equal width.
An optical system, of which the elements are fixed (although adjustably) relative to the rotational axis of the shaft 3', projects an image of an illuminated portion 6' of the grating onto a diametrically opposite portion 7' of the grating, such that the projected image and that opposite grating portion appear to a viewing photoelectric cell 13' to move in circumferentially contrary directions to achieve the advantage of double resolution (i.e. twice the number of cycles in the electrical output from the photocell) over the known stationary reticule and moving grating arrangement.
The optical system depicted schematically in
Figure 3 comprises two conveying lenses 1', 2' of nominally identical focal lengths F, spaced axially by a distance of 4F, and has a pair of conjugate planes as shown, spaced axially by distances 2F from the principal planes P1', P2' of the lenses.
The image I' is not, in this embodiment reversed, and the magnification is again unity.
This system is incorporated in the apparatus of
Figure 4 in which a light source 8' is arranged above the disc 4' to illuminate the portion 6'. The two conveying lenses 1', 2' together with two plane mirrors 9', 10' and a reflecting prism 18 are arranged to project an image of the illuminated portion 6' onto the diametrically opposite portion 7'. The light projects downwardly from the illuminated portion 6' and along an optical path defined by the lenses 1', 2' and the mirrors 9', 10' and prism 18 (see arrows 12') and upwardly through the portion 7' to be detected by a photocell 13', which produces an oscillating output signal in accordance with the disc rotation, as before.
The spacing, measured along the optical path, of the coplanar portions 6', 7' and lenses 1', 2' is in accord with the Figure 3 arrangement. Thus, the grating portions 6', 7' lie in the conjugate planes of the lens system which are spaced from the lenses by distances of 2F to achieve the desired unity magnification and to ensure correct focus of the image of grating portion 6' at portion 7'. the mirrors 9', 10' and the prism 1 8 fold the optical path as required and effect the contrary movement of the projected image of portion 6' and the portion 7'. Again, the moving grating acts as a moving light shutter for the viewing of the contrary moving grating image to produce the doubling of the frequency of modulation of the light entering the photocell 13'.
The optical system of this second embodiment is, in practice somewhat simpler in construction, and more readily adjustable for focus, than that of the first embodiment. To provide this adjustment, the prism 18 and lenses 1', 2' may be moved along the optical axis.
Although the second embodiment does not exhibit the advantage of compensation for axial displacement of the grating disc, it is found, in practice that such displacement is a minor problem compared to that of eccentricity in the mounting of the disc on the shaft. This latter constructional inaccuracy is compensated in the second embodiment in the same way, as described earlier, as in the first embodiment.
It will be appreciated that a linear grating with an oppropriate optical system arranged to project an image of one grating portion upon another grating portion also produces doubling of frequency in the photocell output and, with an optical system according to Figure 1 compensates for movement of, or constructional distortions in the grating producing focal deviations.
Further, the image of the illuminated grating portion may be projected onto another grating having similar line/space domensions to the first and moving therewith.
Claims (8)
1. Apparatus for detecting movement of a member, comprising; a light source; a grating element coupled for movement relative to the light source in accordance with movement of the member and including an optical grating, the light source being arranged to illuminate a portion of the grating; and a photoelectric cell fixed relative to the light source and arranged to view an image of the Iluminated portion, formed by an optical system, through a further grating or portion of the said grating, the optical system being so arranged that the imge and the further grating on portion appear to the photocell to move in contrary directions.
2. Apparatus according to claim 1 for detecting rotary movement of said member about'a rotational axis, said grating element comprising a disc coupled nominally coaxially with said rotational axis for rotation with said member, and said optical grating comprising a circular array of circumferentially alternate opaque and light transmissive portions, and wherein the illuminated portion and the said further portion are diametrically opposed relative to the rotational axis.
3. Apparatus according to claim 2 wherein the optical system defines an optical path extending from one conjugate coincident with said illuminated portion to another conjugate coincident with said illuminated portion to another conjugate coincident with said further portion.
4. Apparatus according to claim 3 wherein the optical system is so constructed that displacement of said one conjugate along said path produces corresponding movement of said other conjugate in the same direction along said path, and is so arranged that axial displacement of the disc produces corresponding displacements of the illuminated and further grating portions in the same direction along said optical path.
5. Apparatus according to claim 4 wherein the optical system comprises two convergent lenses of similar focal length spaced along said path by a distance equal to twice said focal length and disposed nominally with the illuminated portion and said further portion each spaced from a respective one of the lenses along said path by a distance equal to said focal length.
6. Apparatus according to claim 3 or claim 4 wherein the optical system includes an odd number of reflective surfaces to fold said path and to provide said apparent movement in contrary directions.
7. Apparatus according to claim 3 wherein the optical system comprises two convergent lenses of similar focal length spaced along said path by a distance equal to four times said focal length and disposed nominally with the illuminated portion and said further portion each spaced from a respective one of the lenses along said path by a distance equal to twice said focal length.
8. Apparatus for detecting rotary movement of a member, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB7941714A GB2065297A (en) | 1979-12-04 | 1979-12-04 | Apparatus for detecting movement |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB7941714A GB2065297A (en) | 1979-12-04 | 1979-12-04 | Apparatus for detecting movement |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2065297A true GB2065297A (en) | 1981-06-24 |
Family
ID=10509582
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB7941714A Withdrawn GB2065297A (en) | 1979-12-04 | 1979-12-04 | Apparatus for detecting movement |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2065297A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4931636A (en) * | 1988-08-26 | 1990-06-05 | The Boeing Company | Two wavelength optical sensor and sensing system |
-
1979
- 1979-12-04 GB GB7941714A patent/GB2065297A/en not_active Withdrawn
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4931636A (en) * | 1988-08-26 | 1990-06-05 | The Boeing Company | Two wavelength optical sensor and sensing system |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |