WO2008074998A1

WO2008074998A1 - Phase constrast encoding

Info

Publication number: WO2008074998A1
Application number: PCT/GB2007/004801
Authority: WO
Inventors: Marcus Ardron
Original assignee: Renishaw Plc
Priority date: 2006-12-18
Filing date: 2007-12-14
Publication date: 2008-06-26
Also published as: GB0625123D0

Abstract

An encoder for measuring for position of an object, comprising a light source, and a scale (1) and readhead arranged for relative movement therebetween, the scale (1) including an optical feature (2) and the readhead comprising an optical detector (7), wherein in use the light source illuminates a region of the scale (1) and the readhead determines the presence of the feature (2) in that region using a phase contrast technique.

Description

PHASE CONTRAST ENCODING

This invention relates to an encoder, a scale reader for such an encoder, and a method of measuring the position of an object.

An encoder is a device which enables the position of an object to be measured. Often this^' measurement is incremental, but may also be absolute. Typically, a scale is attached to the object, the scale including a series of marks long its length, for example grooves, projections or light / dark lines. The scale may be linear, allowing linear position to be measured, or alternatively attached to a non-linear object, e.g. the circumference of a cylinder, in order to measure circumferential and hence rotational position of the object. A scale reader is provided with a readhead which may detect the marks using a variety of optical techniques. For example, the scale may be illuminated and diffraction means, such as a diffraction grating, is provided for interacting with light reflected from the scale. This produces an interference pattern dependent on the marks on the scale, and the fringes of the interference pattern are in turn detected in the readhead, using a detector that measures the amplitude of the signal received. Electronic means may then be used to interpret the interference pattern detected.

Such apparatus is disclosed in EP-A-0207121 and US-A-4974962 for example.

For relative position measurement, the encoder may count the number of marks detected, which, as the distance between marks is known, indicates the distance travelled by the scale. In order to distinguish the marks from other optical features that may be present on the scale, such as scratches, dust, fingerprints, manufacturing imperfections etc., the marks may comprise a distinct "code word", i.e. a pattern which is unlikely to occur by chance. The scale reader may comprise means to recognise such code words. An example of this is disclosed in EP-A-1646845, which uses a detector array arranged to match the reference mark pattern. Other techniques may use a mask placed near the detector, the mask having a pattern matching that of the reference mark. When the reference mark passes by the readhead, a large peak in the detected signal is obtained, this peak being indicative of the presence of the reference mark. Elsewhere, only a small auxiliary^' signal is obtained. Such an arrangement is known as "auto-correlation".

For absolute position measurement, a more complex series of reference marks is used, wherein each code word or segment of a string is different. The detection of a particular reference mark or segment enables the absolute position of the scale to be determined.

Unless the above-described encoding technique is employed for amplitude reference features (e.g. light / dark / lines), for other features, such as grooves or projections, it is necessary to employ additional diffraction means. This is because such marks, which are typically formed from grooves or projections on a surface, will reflect substantially planar light incident onto that surface in a similar manner from both the marks and their surrounding regions. The amplitude of the reflected signal will be similar in each case. This is also the case for light directed through a transmissive scale surface. Because optical detectors only measure amplitude, the marks do not sufficiently stand out from the surrounding areas to enable robust auto-correlation. In fact, an unwanted optical feature, such as dust, could create more of an amplitude difference than a mark itself. For this reason, a diffraction means, such as a diffraction grating, must be used to create an interference pattern from which the presence of a mark can be inferred. This adds undesirable complexity to the encoder arrangement. However, such non-amplitude marks can be easier to manufacture then amplitude marks. It would be preferable therefore to use an alternative method which would be able to "see" non-amplitude marks without the use of diffraction means.

It is an object of the present invention to overcome this problem, and enable the accurate measurement of position by an encoder without using relatively complex diffraction means. This is achieved by making use of the phase contrast technique to highlight the phase difference created between light reflected or transmitted by optical feature of a scale and light which does not interact which the feature. Such a system provides the ability to read optical features such as surface profile or refractive. index changes, where ■ the features are between about 2λ and lOOλ long, where λ is the wavelength of the incident light used. Suitable wavelengths could lie in the range between about 300nm to about 1500nm, with a typical wavelength of about 850nm.

The phase contrast technique was developed by Fritz Zernike in the 1930s as a method of examining transparent biological samples without the use of dyes, which could kill the specimen. This work earned him the Nobel Prize for Physics in 1953. The mechanism behind the phase contrast technique will be described in more detail below in relation to the present invention.

In accordance with a first aspect of the present invention there is provided an encoder comprising a light source, and a scale and readhead arranged for relative movement therebetween, the scale including an optical feature and the readhead comprising an optical detector, wherein in use the light source illuminates a region of the scale and the readhead determines the presence of the feature in that region using a phase contrast technique.

In accordance with a second aspect of the present invention there is provided an encoder scale reader for detecting an optical feature on a scale, comprising an optical detector and a phase device, the phase device operable to change the phase of a portion of light incident on the phase device.

In accordance with a third aspect of the present invention there is provided a method for measuring the position of an object, comprising the steps of: providing a scale on the object, the scale including an optical feature, illuminating a region of the scale, and using a scale reader to determine the presence of the optical feature in the region of the scale using a phase contrast technique.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 schematically shows a first embodiment of the present invention, using a transmissive scale;

Figure 2a schematically shows a second embodiment of the present invention, using a reflective scale;

Figure 2b shows the apparatus of Fig. 2a from a side view;

Figure 3 a schematically shows a third embodiment of the present invention, for detecting multiple features;

Figure 3b shows the apparatus of Fig. 3 a from a side view;

Figure 4a schematically shows a fourth embodiment of the present invention;

Figure 4b shows the apparatus of Fig. 4a from a side view;

Figure 5 schematically shows a fifth embodiment of the present invention, illustrating a component mounting arrangement;

Figure 6 schematically shows a sixth embodiment of the present invention, illustrating an alternative component mounting arrangement;

Figure 7 schematically shows a seventh embodiment of the present invention, using a beam-splitter;

Figure 8 schematically shows an eighth embodiment of the present invention; and

Figure 9 schematically shows a ninth embodiment of the present invention for use as an absolute encoder. In accordance with the present invention, the phase contrast technique is used to view a surface or refractive index profile using an amplitude detector.

A first embodiment, showing how the phase contrast method is used, is shown schematically in Fig. 1. A light-transmissive scale 1 includes an optical feature, which in this case is formed by a first projection 2 on the surface of the scale. The scale is mounted for movement generally along its length, i.e. between the top and bottom directions as shown in the figure. A lens 3 is positioned parallel to, and in the optical path from the scale. A phase device, in this case a phase plate 4, is positioned parallel to, and in the optical path from the lens 3. The phase plate 4 is translucent, and includes a phase-altering region, which in this case is formed by a second projection 5 on the surface of phase plate 4. The phase plate 4 is positioned so that the projection 5 lies at the focal point of the lens 3, as shown more clearly in the magnified area of Fig. 1. An image plane 6, where a detector (not shown) would be positioned, is located in the optical path from phase plate 4. Collimated light from a source (not shown), e.g. a laser, is directed towards the scale 1 from left to right as shown. The lens 3, phase plate 4 and detector are all components of an encoder readhead.

A plane wave, parallel to the planes of the scale 1, lens 3 and phase plate 4, illuminates the scale 1. The majority of the light incident upon scale 1 passes undiffracted through the plane regions of the scale, as shown by the solid lines. This undiffracted light is focussed by lens 3 at the projection 5. The portion of the incident light which passes through projection 2 is retarded with respect to that portion which passes through the surrounding, thinner region of the scale 1. No interference occurs between the retarded and unretarded light because there is no overlap of their paths. The modified phase front produced by the projection 2 is shown on Fig. 1.

At the boundaries of projection 2, diffraction does occur, so that some light (shown as a dashed line) travels in paths that are no longer parallel to the normal of the plane of the lens. The diffracted light passes through the phase plate 4 at a different region to that which the undiffracted light passes, and so avoids the projection 5. This means that the undiffracted light is retarded relative to the diffracted light by an amount dependent on the thickness of the projection 5 and the difference in diffractive index of the phase plate 4 and the surrounding medium.

Interference occurs between the diffracted and undiffracted light so that an intensity pattern is generated at the image plane, which can thus be detected using a detector.

If the projection 2 on scale 1 is of thickness λ/4 say, where λ is the wavelength of the incident light, then if projection 5 on the phase plate 4 is also of thickness λ/4 (thus retarding the undiffracted light relative to the diffracted light), then a "positive" amplitude image of the scale is created at the image plane. If the projections 2 and 5 were instead to be replaced by grooves (not shown), such that these features were thinner than the surrounding regions and the undiffracted light is thus advanced relative to the diffracted light, then a "negative" image would be created.

A further alternative (not shown), would be to replace projections 2 and / or 5 with regions of different refractive index, for example by doping the scale / phase plate or using insertions of a different material.

The width of the optical feature on scale 1 and the phase-altering region on phase plate 4 must be wide enough to allow for assembly tolerances but narrow enough not to alter the path of too great a proportion of the incident light diffracted by a small angle.

A second embodiment of the present invention, also capable of detecting single feature marks on a scale, is shown in Figs. 2a and 2b. In this embodiment, the scale is reflective, as would more usually be the case.

A light source 8 is mounted on phase plate 4\ arranged to shine light through the phase plate 4' and toward a reflective scale 1\ The light is collimated by lens 3. Reflective scale V includes an optical feature, in this case an indent 2\ The scale is mounted for movement generally in the horizontal direction as shown. Light is reflected from scale V back through lens 3. Undiffracted light, i.e. light which has not illuminated the edges of the indent 2\ is focussed by lens 3 at a phase-altering region on phase plate 4\ In this case, the phase-altering region comprises an indent 5" set into the phase plate 4^Λ on the side further from the scale V. A bi-cell detector 7' is mounted at the image plane, the detector 7' producing two output channels A and B relating to signals received from respective cells in laterally-displaced regions in the image plane.

As for the first embodiment, no diffraction occurs until the light reaches scale V, where the portion of the light which illuminates the step of the indent T is diffracted. The remaining light is undiffracted. Some of this light experiences a phase change due to illuminating indent 2\ this light will be retarded relative to the light which illuminates regions of the scale V away from indent 2\

While the undiffracted light is focussed at the indent 5\ the diffracted light will avoid this area, and so will be retarded relative to the undiffracted light by an amount dependent on the depth of the indent 5\ Interference between the diffracted and undiffracted light causes an interference pattern to be created at the image plane, which pattern can be detected by detector 7' .

A bi-cell detector 7^Λ is used to provide a differential signal by subtracting the output from one cell from the output of the other cell, e.g. Channel A minus Channel B, thereby producing a signal with a steep zero crossing gradient.

As an alternative, the bi-cell could be further split into four cells, where both the left and right adjacent pairs are used to give respective differential signals and the difference between the signals produced by the central two cells and the outer two cells provides a gating function.

As a further, simpler alternative, a single cell detector could be used to give a less robust but cheaper and simpler arrangement. A detector array, such as a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) array could be used, and the output processed in any way required.

The above embodiments relate to a simple case where the scale includes a single optical feature. In practice, such a technique may not be very robust, because dirt may hide or blur the mark, or dirt may appear as a reference feature. Greasy fingerprints are good phase front modifiers and may cause rogue reference marks to appear at the detector. Therefore it will usually be necessary to provide a more complex code word of features to provide a reference mark that is distinct from features caused by dust / fingerprints etc, which allows strong auto-correlation.

An appropriate code word needs to be chosen for the particular geometry proposed and the level of immunity required, but it is advantageous to have the property of minimum signal when the readhead is not aligned with the reference mark compared to the magnitude of the signal when the readhead is aligned. Smaller features lead to a narrower auto-correlation pulse than wider features and hence a sharper reference mark. Feature size approaching the nominal wavelength λ of illumination will give large diffraction angles that require special geometry, the phase contrast technique is not generally suitable for features smaller than about twice the wavelength of the illuminating light.

An embodiment which permits the detection of multiple optical features forming a reference mark is shown in Figs. 3a and 3b. It can be seen that the apparatus used is similar to that of the previous embodiment, in that a light source 8, phase plate 4" with phase-altering region 5" and lens 3 are all similar. However, in this case, reflective scale r' includes a series of multiple optical features formed as a series of indents 2"\ A pair of detectors 7" is positioned at the image plane, each detector being displaced with respect to the other in the direction of scale movement, i.e. left to right as shown. Each detector is fitted with a mask 9 in front of the respective detector, each mask including cut-out sections which correspond to the pattern of indents 2", i.e. the reference mark code. The masks 9 may be positive or negative images of the reference code depending on the relative depths of the scale features and phase plate step, along with the wavelength of the illumination light. The signal outputs of the detectors are respectively termed Channel A and Channel B.

In use, a differential signal is obtained from the two detectors, e.g. Channel A minus Channel B, to cancel common illumination and produce a signal with steep gradient at the zero crossing.

Various alternatives are possible (not shown), for example, a single detector / mask may be used to give a usable but less robust signal.

The detector could be constructed to only be sensitive to light in the appropriate regions without the use of a mask.

A more general detector, such as a CCD or CMOS array, may be used and the autocorrelation performed remotely, i.e. using computing means to process the signal produced by the detector.

The arrangement shown in Figs. 3a and 3b has the image plane, and hence the detector 7", located at approximately twice the focal distance from the lens 3. If the image plane is closer to the lens, the image would be smaller, while if the image plane were further away from the lens, the image would be magnified. In either case, the mask used must be suitably scaled.

A further embodiment which enables the detection of a reference mark is shown in Figs. 4a and 4b. This embodiment is similar to that shown in Figs. 3a and 3b, except that the detector T" used is of different form. Here, there is no need for a mask to be used, instead the detector includes an array of individual detector elements, of which some are connected to produce a signal in Channel A, and others connected to produce a signal in Channel B. It can be seen that the pattern of connected elements matches the code word pattern - as shown the code word includes two relatively narrow indents on the right side of the scale, the distance between them being approximately equal to their width. On the left side of the code word is a relatively wide indent, approximately double the width of the narrow indents, spaced from the narrow indents by a relatively wide distance, again approximately the same width as the wide indent. A differential version of this pattern is matched (but necessarily reversed) at the detector, so that on the left side, alternate A and B connections are made to adjacent detector elements, while on the right, the connections are made to spaced every other element, i.e. so that the connected elements on the right are twice the spacing of those on the left. The differential signal produced by the detector provides a strong auto-correlation peak without requiring laterally offset detectors. Such a system is described more fully in EP-A- 1646845.

Figs. 5 and 6 illustrate two arrangements for mounting the source and detector to provide a system such as shown in Fig. 4a, so that the source is separated from the phase step at the focal plane of the lens.

Fig. 5 shows a two-level board design. Detector 7 is mounted on a printed circuit board (PCB) 11. A flexible PCB 12 is attached to PCB 11, and source 8 is mounted to the flexible PCB 12.

Fig. 6 shows a one-level board design. Here, both detector 7 and source 8 are mounted on a rigid PCB 14. The source 8 feeds into a light guide 13 (e.g. an optical fibre) which is connected at its other end to phase plate 4. This causes an apparent source at the focal plane of the lens, even though the actual source 8 is set back at the same plane as the detector 7.

A further embodiment is shown in Fig. 7, which uses a beam splitter in an alternative mounting arrangement. The scale 1, lens 3 and detector 7 are all as used and previously described in Fig. 4a and 4b. A light source 8 is mounted on a beam splitter 10, on a side orthogonal to the scale 1. A phase-altering region is provided on the side of the beam splitter 10 closest to the detector 7, at an area complementary to source 8. In this case, the phase-altering feature comprises an indent 5.

Light from source 8 is directed by the beam splitter 10 through the lens 3, and onto the scale 1, Undiffracted light reflected from the scale 1 is directed by lens 3 to the indent 5. Detector 7 is positioned in the image plane. It can be seen that the beam splitter here provides the same role as the phase plate 4 used in previous embodiments. Otherwise, the system shown operates in a similar manner.

A further embodiment utilising a beam splitter is shown in Fig. 8. This arrangement is similar to that shown in Fig. 7, except that here the source 8 is mounted on a side of the beam splitter 10 to face the scale 1, while the phase-altering indent 5 and detector 7 are located laterally to the beam splitter 10. With this embodiment, the distance between the scale and the detector is therefore reduced.

Fig. 9 schematically shows an absolute position encoder. Such a device enables the position of a scale to be determined at start-up, i.e. without the need to search for a reference mark. To achieve this, the scale 1 is provided with a relatively complex code pattern along its length. As shown this comprises a series of indents of varying widths and spacing. Since the code being detected varies along the scale's length, a mask or set detector arrangement such as described with reference to Figs. 3a or 4a for example cannot be used. Instead, the detector 7 comprises a multi-element detector such as a CCD or CMOS array.

As the scale 1 passes under the readhead, the detector 7 outputs an electronic "image" of the code that may be used to uniquely identify the position of the scale 1, using remote processing, for example by a computer. The above-described embodiments are exemplary only, and it is apparent that various possibilities will be possible within the scope of the claims. For example, in the embodiments described, the light source is collimated by the imaging lens. In practice various techniques may be used, for example the source collimation and feature imaging may be done by different lenses. A spherical or an aspherical lens may be used, however if linear features are used then the lens or lenses employed may be cylindrical. A combination of lenses may also be used, for example a spherical collimating lens and a cylindrical imaging lens.

The optical features provided on the scale are generally shown as height features, for example a binary height sequence, but a code word for example could be embodied as a smoother height change, or an amplitude or refractive index sequence.

Above-described embodiments illustrate a reference mark formed as a pattern of optical features set into a relatively uniform scale surface. However, as a further possibility this may be reversed, so that the reference mark may be embedded in an incremental encoder scale. In this case, the reference mark may take the form of a pattern of missing features in an otherwise continuous series of features. For example, the continuous series may be formed as a square wave or sinusoidal surface, and the reference mark formed with some of the teeth or peaks missing or gaps / troughs filled, effectively smoothing out the scale at predetermined points. Such patterns of relative smoothness may be identified as a reference in a similar manner to the embodiments described above.

In the above-described embodiments, a readhead uses a phase contrast technique to detect the presence of a feature on a scale. Phase contrast techniques are used in phase contrast microscopes which typically employ a diaphragm with an annular clear region in the illumination path and an annular phase plate in the imaging path. The size and complexity is reduced by the proposed design to better suit the size and cost requirements of an encoder readhead. A source 8 with a small emitter area (such as an LED, laser diode, point source LED or VCSEL) is used, eliminating the need for a diaphragm in the illumination path. The source may typically be less than or equal to 400 microns wide. The phase plate 4 has a simple, small phase step 5 to approximately match the emitter dimensions, in the conjugate position to the light source; the source to diaphragm distance in a typical phase contrast microscope is therefore eliminated. With this design it is possible to use a relatively large proportion of the light from the emitter where a diaphragm would block much of the emitted light, so improving photometric efficiency; this is important in a measuring system where heat input is undesirable. A small emitter

• area may be created by using a relatively large light source in combination with a non annular aperture, for example, a spot, line, square, triangle etc. When used with a directional light source, this arrangement has the advantage of creating a small emitter area without blocking excessive light, as would occur with an annular diaphragm.

In the extreme, this allows a reflex embodiment of the proposed design to consists of only the following parts: emitter 8, single lens 3, phase object (scale) 1, phase plate 4 and detector 7.

Claims

1. An encoder comprising a light source, and a scale and readhead arranged for relative movement therebetween, the scale including an optical feature and the readhead comprising an optical detector, wherein in use the light source illuminates a region of the scale and the readhead determines the presence of the feature in that region using a phase contrast technique.

2. An encoder according to Claim 1, wherein the optical feature is configured such that a portion of light incident upon the feature experiences a different optical path length as compared to the remaining light incident upon a region of scale with no such feature.

3. An encoder according to Claim 2, wherein the optical feature comprises an area of different refractive index to the surrounding scale.

4. An encoder according to Claim 2, wherein the optical feature comprises an area of different thickness to the surrounding scale.

5. An encoder according to any preceding claim, comprising a plurality of optical features.

6. An encoder according to any preceding claim, wherein the feature or features forms or form a reference mark.

7. An encoder according to claim 6, wherein the readhead comprises means for recognising the reference mark.

8. An encoder according to any preceding claim, wherein the readhead comprises a phase device for changing the phase of a portion of light incident on the phase device.

9. An encoder according to claim 8, wherein the phase device comprises a translucent member including a phase-altering region such that light passing through a portion of the member including the region experiences an optical path length different to that of light passing through the member but not the region. • 5

10 An encoder according to claim 9, wherein the phase-altering region comprises a region with a refractive index different to that of the rest of the phase device.

11. An encoder according to either of claims 9 and 10, wherein the phase-altering 10 region comprises a region of different thickness to the surrounding phase device.

12. An encoder according to any of claims 8 to 11, wherein the phase device comprises a phase plate.

15 13. An encoder according to any of claims 8 to 11, wherein the phase device comprises a beam splitter.

14. An encoder according to any of claims 8 to 13, wherein in use the phase device is located in the optical path between the scale and the detector.

20

15. An encoder according to any preceding claim, wherein in use light from the light source is transmitted through the scale.

16. An encoder according to any of claims 1 to 14, wherein in use light from the light 25 source is reflected by the scale toward the detector.

17. An encoder according to claim 16, wherein the light source and detector are mounted on a support.

30 18. An encoder according to claim 17, wherein the support is flexed such that the light source and detector are mounted in different planes.

19. An encoder according to claim 17, wherein the light source and detector are mounted in substantially the same plane, and the light source is connected to a light guide leading away from that plane,

20. An encoder according to any of claims 13 or 14 to 16 when dependent on claim 13, wherein the light source and detector are mounted proximate respective adjacent surfaces of the beam splitter.

21. An encoder according to any preceding claim wherein the light source has a small emitter area.

22. An encoder according to claim 21 wherein the light source has an emitter width of less than or equal to 400 microns.

23. An encoder according to any of claims 21 to 23 wherein the light source comprises one of an LED, laser diode, point source LED or VCSEL.

24. An encoder scale reader for detecting an optical feature on a scale, comprising an optical detector and a phase device, the phase device operable to change the phase of a portion of light incident on the phase device.

25. A scale reader according to claim 24, wherein the phase device comprises a translucent member including a phase-altering region such that light passing through a portion of the member including the region experiences an optical path length different to that of light passing through the member but not the region.

26. A scale reader according to either of claims 24 and 25, wherein the phase-altering region comprises a region with a refractive index different to that of the rest of the phase device.

27. A scale reader according to either of claims 24 and 25, wherein the phase-altering region comprises a region of different thickness to the surrounding phase device.

28. A scale reader according to any of claims 24 to 27, wherein the phase device comprises a phase plate.

29. A scale reader according to any of claims 24 to 27, wherein the phase device comprises a beam splitter.

30. A scale reader according to any of claims 24 to 29, wherein in use the phase device is located in the optical path between the scale and the detector.

31. A method for measuring the position of an object, comprising the steps of: providing a scale on the object, the scale including an optical feature; illuminating a region of the scale; and using a scale reader to determine the presence of the optical feature in the region of the scale using a phase contrast technique.

32. A method according to claim 31, wherein the optical feature is configured such that a portion of light incident upon the feature experiences a different optical path length as compared to the remaining light incident upon a region of scale with no such feature.

33. A method according to claim 32, wherein the feature comprises an area of different refractive index to the surrounding scale.

34. A method according to claim 32, wherein the feature comprises an area of different thickness to the surrounding scale.

35. A method according to any of claims 32 to 34, wherein a plurality of optical features are provided in the scale.

36. A method according to claim 35, wherein the feature or features forms or form a reference mark.

37. A method according to claim 36, including the step of providing a readhead ' comprising means for recognising the reference mark.

38. A method according to claim 37, wherein the recognition means comprises a detector with a plurality of detector elements.

39. A method according to any of claims 31 to 38, including the step of providing a phase device in the optical path from the scale, for changing the phase of a portion of light incident on the phase device.

40. A method according to any of claims 31-39 wherein the light source is provided to illuminate said region of the scale, and wherein the light source has a small emitter area.

41. A method according to claim 40 wherein the light source has an emitter width of less than or equal to 400 microns.

42. A method according to any of claims 40 or 41 wherein the light source comprises one of an LED, laser diode, point source LED or VCSEL.

43. An encoder substantially as herein described with reference to the accompanying drawings.

44. A scale reader substantially as herein described with reference to the accompanying drawings.

45. A method of measuring the position of an object substantially as herein described with reference to the accompanying drawings.