Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1989-06-15
1990-09-25
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
25023118, H01J 314
Patent
active
049595423
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to opto-electronic scale-reading apparatus for use in measuring relative displacement of two members. In such known apparatus e.g. British Patent No. 1,504,691 the scale comprises a diffraction grating co-operating with at least one other grating on a read head to produce interference fringes which move relative to the read head during a said displacement of the members, and said measurement is a count of said fringes. It is clear that such a scale has to be of diffraction quality, i.e. the accuracy and reliability of the measurement depends on such parameters as the regularity of the spacing of the scale marks, the sharp definition of the edges of the marks, and the freedom of the scale from scratches and like imperfections. Such a scale can be expensive to produce and protect especially when the scale has to be relatively long.
It is among the objects of this invention to overcome or reduce this difficulty.
It is also known to increase the number of signals obtainable from any two adjacent marks of the scale by phase quadrature interpolation. Known scale-reading apparatus can be subject to phase errors and consequent interpolation errors. It is optionally an object of this invention to overcome or reduce this difficulty.
SUMMARY OF THE INVENTION
According to this invention there is provided apparatus for measuring displacement between two members, comprising a scale defined by marks provided on one of the members, a read head provided on the other member diffraction means for producing interference fringes having movement relative to said read head responsive to a said displacement, and detecting means for detecting said movement, characterized in that there is provided a spatial filter including said diffraction means and mounted on said read head, the filter being tuned to a nominal periodicity lying within a band of periodicities defining the pass band of the filter, the marks of said scale are defined by light sources positioned to illuminate said diffraction means and having a periodicity lying within said pass band, and the light from said sources interacting with said diffraction means to produce said fringes.
It will be seen that in the apparatus according to this invention the diffraction mechanism takes place entirely in the read head. The scale is merely required to provide a pattern of light sources. Thus the scale is not required to be a diffraction grating and the provision of the marks on the scale does not have to be of diffraction quality. The scale may have relatively imperfect markings or the markings of the scale may be capable of being produced with greater economy than in known apparatus.
Further, the read head according to this invention is inherently convolutional, i.e. the fringes constitute a convolution of the scale pattern with a substantially sinusoidal pattern. This makes the read head substantially free from phase quadrature errors.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of apparatus according to this invention, will now be described with reference to the accompanying drawings wherein:
FIG. 1 is a plan view of the apparatus,
FIG. 2 is a perspective view of FIG. 1,
FIG. 3 is an enlarged detail of FIG. 1,
FIG. 4 is a light ray diagram of a first embodiment,
FIG. 5 is a light ray diagram of a second embodiment,
FIG. 6 is a first diagram showing at (a) the response curve of a filter and at (b) a scale periodicity variation,
FIG. 7 is a second diagram showing at (a) the response curve of a filter, at (b) a band of scale periodicities, and at (c) a different position of the latter band,
FIG. 8 is a third diagram showing at (a) the response curve of a filter, at (b) a band of scale periodicities, and at (c) a different position of the latter band,
FIG. 9 shows at (a) an enlarged representation of a scale showing a modulation of the scale marks, and at (b) demonstrating the ramp characteristic of this modulation.
FIG. 10 is a diagram of characteristics of the modulation showing different such characteristics at (a) an
REFERENCES:
patent: 3344700 (1967-10-01), Brake
patent: 3796498 (1974-03-01), Post
patent: 3812352 (1974-05-01), MacGovern
patent: 3833807 (1974-09-01), Takeda
patent: 4051367 (1977-09-01), Sayce et al.
patent: 4079252 (1978-03-01), Brake
patent: 4115008 (1978-09-01), Shepard
Moire'Streifen in der Mebtechnik, Shepard et al., 827 Werstatt & Betrieb, vol. 112 (1979) Nov., pp. 782-786.
Japanese Abstract No. 59-10661, Optical Scale Reader, 1984.
Nelms David C.
Renishaw plc
Shami Khaled
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