Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1988-05-02
1990-12-04
Willis, Davis L.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
250237G, G01B 902
Patent
active
049749625
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to opto-electronic scale-reading apparatus of the kind wherein a read head and a scale are supported for movement one relative to the other for the incremental reading of marks provided on the scale, the scale is illuminated by incident light, the read head is arranged to view the scale in a given direction, and the scale comprises an elongate member having said marks spaced in the direction of the length thereof, spaces being defined between the marks.
GB-932,481 discusses the difficulty of providing good contrast between the marks and spaces at a metallic surface and describes a scale wherein such contrast is sought to be achieved by marks having highly reflective surfaces and spaces having diffusively reflective surfaces. However, the marks are applied to a flat surface of the scale member by a photo-resist process so that both the marks and the spaces have parallel surfaces and act as if they are substantially co-planar. This can still lead to difficulties in providing good contrast especially if the scale surface is contaminated.
GB-1,516,536 discusses the difficulty of contamination of the scale and describes apparatus wherein both the marks and the spaces are illuminated by diffuse light. However, such diffuse illumination necessarily reduces contrast, and contamination can reduce the contrast between the marks and the spaces still further because the marks and spaces would be at parallel substantially co-planar surfaces.
EP-0160811 describes a diffraction grating having regions of different periodicities with a view to forming diffraction fringes in correspondingly different positions. The different fringes define coded reference marks in an incremental opto-electronic scale-reading apparatus. The grating is a phase grating formed by a square or by a sawtooth profile illuminated by coherent light but the grating lines extend in the direction of the length of the incremental scale and do not themselves form the basis for an incremental count of position.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide in an opto-electronic scale-reading apparatus a scale having improved optical contrast between the marks and the spaces especially where the marks and spaces are defined at a metallic surface.
More specifically, an opto-electronic scale-reading apparatus is provided which comprises a read head and a scale supported for displacement relative to each other for the incremental reading of marks provided on the scale. The read head includes a light source for producing light rays incident on the scale, an index grating for diffracting readable rays into fringes in at least one order of diffraction, and an analyzer grating for converting the fringes into light modulations at a rate which is a function of the rate of displacement between the read head and the scale.
The scale comprises an elongate member having marks spaced in the direction of its length and spaces defined between the marks. The marks and spaces are defined by respective first and second surfaces of surface portions of the members, the first surfaces lying at an angle to adjacent second surfaces so that incident rays are reflected by the first surfaces in a first direction to be readable by the read head in the first direction. The incident rays are reflected by the second surfaces in a second direction such that rays reflected in the second direction are substantially not readable in the first direction.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of this invention will now be described with reference to the accompanying drawings wherein:
FIG. 1 is a sectional elevation of a first example of a scale-reading apparatus and shows a first example of a scale.
FIG. 2 is a view on the line II--II in FIG. 1.
FIG. 3 is a sectional elevation of a second example of a scale-reading apparatus also shows a second example of a scale.
FIG. 4 is an enlarged detail of FIG. 3.
FIG. 5 shows the profile of a third example of a scale.
FIG. 6 shows the profile of a fourth exam
REFERENCES:
patent: 2796498 (1974-03-01), Post
patent: 3344700 (1967-10-01), Brake
patent: 4677293 (1987-06-01), Michel
"Diffraction Gratings" M. C. Hutley, Academic Press, pp. 76-79, 1982.
"Interference Generated Moire Fringes in a System of Three Reflection Gratings," Optical Technology, vol. 38, No. 10, Oct. 1971.
Whitefield "Externally Sensed Grating Track Sensing Arrangement", IBM Tech. Discl. Bull, vol. 27, No.-1A, pp. 150-152, 6/84.
Cannings F. E. R.
Stephens W. F. N.
Koren Matthew W.
Renishaw plc
Willis Davis L.
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