The present invention relates to optical scale reading apparatus comprising a scale and a readhead movable relative to the scale. In particular the invention relates to an incremental scale having embedded reference marks, and a readhead for that scale.
A known form of scale reading apparatus for measuring relative displacement of two members comprises a scale on one of the members having scale marks defining a pattern and a readhead provided on the other member. An incremental scale has scale marks defining a periodic pattern which generates a periodic signal at the readhead when relative movement between the scale and the readhead takes place. These periodic signals produce an up/down count which enables displacement between the scale and the readhead to be determined.
Our earlier European Patent No. 207121 describes an incremental scale reading system which has a filtering effect so that the signal produced at the photodetector array is largely immune to parts of the scale being obscured for example by contamination.
Measurement scales may have reference marks which when detected by the readhead enable the exact position of the readhead to be determined. Reference marks may for example enable the accuracy of the incremental count to be verified.
Reference marks are typically provided on a scale track parallel to the incremental track. This has the disadvantage that it increases the width of the scale.
Furthermore, it has the disadvantage that yawing of the readhead can cause errors in the position of the reference mark relative to the incremental scale as detected by the readhead.
As separate optics are used for detecting the light pattern produced from the incremental scale and the reference mark, thermal longitudinal expansion of the readhead can also cause errors in the relative positions of the reference mark and incremental scale as detected by the readhead.
The present invention provides apparatus for measuring displacement between two members, comprising:
The periodic diffraction means and the imaging means may be provided on a single unit.
The imaging means may comprise a lens, for example a cylindrical lens. The imaging means may comprise a Fresnel zone plate. The imaging means may be at least partially incorporated within the periodic diffraction means. A masking region may be provided between the imaging means and the periodic diffraction means.
The first and second detecting means may be provided at a single detecting unit. The second detecting means may be at least partly incorporated within the first detecting means.
A single light source may be used for the incremental pattern and the reference mark.
The imaging means, periodic diffraction means, first and second detecting means and light source may be disposed in a single plane. The plane may be perpendicular to the direction of the scale.
The invention will now be described with reference to the accompanying drawings in which:
A readhead (not shown) is movable relative to the scale and contains optics which comprise a light source 16, an optical element 18 and a detector unit 20. The light source 16 is directed towards the scale to illuminate the scale and produce a periodic light pattern from the incremental scale. The optical element 18 has an index grating 22 which interacts with the periodic light pattern to form interference fringes at the detector unit 20. The detector unit 20 has a photodetector array 24 which detects these interference fringes to produce a periodic signal. Movement of the readhead relative to the scale causes movement of the interference fringes relative to this detector portion 24 thus producing an up/down count which enables a measurement of displacement.
A scale and readhead of this type are described in our earlier European Patent No. 207121. This arrangement has the advantage that due to a filtering effect of the optics, it is largely immune to the addition or removal of bits of the scale at the reference mark. Thus the readhead can pass over the reference mark with just a slight reduction in the signal to noise ratio. In this type of arrangement, the index grating 22 is positioned between the scale 10 and detector 22. In this arrangement the scale 10 acts as an object which is imaged by the index grating 22. This arrangement allows the reference mark to be embedded in the incremental scale without significant loss of signal. In addition, this arrangement allows the imaging optics 18 to be located between the scale 10 and detector without significant loss of signal. In known alternative arrangements in which the index grating is positioned between the light source and scale, the index grating acts as the object and is imaged by the scale. In this case, placement of a reference mark in the scale or placing imaging optics between the scale and detector has a more significant effect on the signal to noise ratio.
The reference mark may be formed by the removal of lines or the addition of lines. Alternatively it may comprise any pattern which can be differentiated from the incremental scale. For example, the reference mark may comprise an autocorrelator in which the reference mark forms a pattern interlaced with the incremental scale and a matching mask is provided in the readhead, for example as described in International Patent Application WO02/065061.
When the readhead passes over the reference mark 14 a change in the intensity of light reflected by the scale 10 is imaged by a lens 26 in the optical element 18 onto the detector 20. The detector 20 has a split detector 28 onto which light from the reference mark 14 is imaged.
Although in this embodiment a split detector is used to detect the reference mark, other types of detector may be used, for example the outputs from three detector elements could be combined. A multi-role detector element may be used to detect both the incremental and reference mark signals.
The lens 26 illustrated in
The arrangement shown in
Various different arrangements of the optical element and detector are possible, for example the positions of the incremental related portions and the reference mark related portions of the optical element and/or detector may be interchanged.
Various degrees of intermingle of the reference mark with the incremental scale and their associated optics and detectors are possible.
The optical element may combine the incremental and reference mark optics, for example in a diffractive optical element (DOE), multi-level optics or a 3-dimensional optical component, such as a volume holograph etc. The lens may comprise many lens segments, for example a Fresnel lens.
Alternatively, the imaging lens and diffraction grating may be located separately. They are not required to be in the same plane.
This region 31 could be left blank, allowing light to be transmitted without directing it to the reference mark detector.
Unwanted light may fall on the detectors, such as background unmodulated light and cross contamination of modulated light from the incremental/reference mark systems. Baffles can be used in the readhead to reduce this unwanted light. Furthermore cross contamination may be reduced by using different light sources of different wavelengths for the incremental and reference mark systems. Filters may be provided so that the reference mark portions and incremental portions of the detector only detect light of the specific wavelength used in that system.
However, a single light source has the advantage that the incremental and reference mark signals share the same age, temperature coefficient etc. Thus the phase of the incremental and reference mark signals are kept steady, reducing electronics and current consumption.
Furthermore, use of the same light source allows the signals from both systems to be compensated simultaneously using feedback from just one system.
This arrangement of scale and readhead has the advantage that the footprint on the scale of the incremental and reference marks is at a similar location on the scale and the light source, optical element and detector are in the same plane 32 (shown in
If the reference mark detector and incremental detector are on a single detector unit and the lens and index grating are on a single optical element, problems due to longitudinal expansion of the readhead are eliminated.
Although the described embodiment is a linear scale reading system, this invention is also suitable for a rotary system or a two-dimensional system.
Number | Date | Country | Kind |
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0413710.5 | Jun 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2005/002421 | 6/21/2005 | WO | 00 | 12/18/2006 |
Publishing Document | Publishing Date | Country | Kind |
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WO2005/124282 | 12/29/2005 | WO | A |
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Number | Date | Country |
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0 207 121 | Jan 1990 | EP |
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Number | Date | Country | |
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20080013105 A1 | Jan 2008 | US |