Optics: measuring and testing – By light interference – For dimensional measurement
Patent
1999-06-21
2000-11-21
Font, Frank G.
Optics: measuring and testing
By light interference
For dimensional measurement
356356, 356375, G01B 1114
Patent
active
061511285
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical position measuring system for detecting the relative position of two objects which are movable with respect to each other.
2. Description of Related Art
An optical position measuring system in accordance with the species is known from EP 0 223 009 B1 of Dr. Johannes Heidenhain GmbH and will be explained below by means of the schematic representations in FIGS. 1a to 1c. A scale graduation M in the position measuring system is scanned by a scanning unit A, which is arranged parallel with the above mentioned grating M and is displaceable in the x direction. In the process, signals, which have been modulated as a function of the displacement, are detected with the aid of the scanning unit A and are further processed by means of an evaluation unit, not represented.
Besides a scanning plate AP, the scanning unit A contains a optical condenser element K and a support plate T; the individual components of the scanning unit A and their functions will be described in detail in what follows. The scanning plate AP has at least two scanning fields AF1, AF2, which are displaced in respect to each other in the measuring direction x by a fraction of the graduation period TP, i.e. the scanning fields AF1 and AF2 are arranged offset from each other in the measuring direction x by the distance D; both the scanning fields AF1 and AF2 as well as the graduation M have only been indicated schematically. The scanning fields AF1, AF2 on the scanning plate AP are preferably designed as phase gratings, by means of which phase-shifted partial light beams of different orders of diffraction can be generated by each one of the two scanning fields AF1, AF2 by means of a ratio, different from 1:1, between the step width to the groove width. Two of the resultant orders of diffraction are used per scanning field AF1, AF2, preferably the +1st and the -1st orders of diffraction. Therefore a total of four partial signals results, which can be further processed. Here the grating parameters have been selected such that a phase shift of 90.degree. results between the partial signals of the +1st and -1st order of diffraction. For reasons of clarity the representation of the light beam path is omitted.
The various partial light beams are detected via downstream-connected detector elements D11, D12, D21 and D22, which are arranged on the support plate T of the scanning unit A; here, FIG. 1b shows the relative arrangement of the various detector elements D11, D12, D21 and D22 in the detector plane, as well as the light source L arranged on the support plate T.
An optical condenser element K is moreover arranged between the support plate T and the scanning plate AP, which is used for the collimation of the light beams emitted from the light source L as well as for showing the partial light beams reflected in the direction of the detector elements D11, D12, D21 and D22. Therefore the support plate T with the detector elements D11, D12, D21 and D22 and the light source L is arranged in the focal plane of the optical condenser element K within the scanning unit A.
For detecting the four partial signals, it is necessary to separate them spatially. In this case the spatial separation of partial signals phase-shifted by 90.degree. is already made possible by splitting the partial light beams of each scanning field AF1, AF2 into different order of diffraction, i.e. for detection, the +1st and -1st order of diffraction used of each scanning field AF1, AF2 are provided spatially separated. These are two partial signals, shifted by 90.degree., from the scanning field AF1, which are detected by the detector elements D11 and D12; the partial signals, also phase-shifted by 90.degree., generated via the scanning field AF2 are detected by two further detector elements D21, D22. In this case the splitting into different orders of diffraction takes place in the drawing plane of FIG. 1a. The partial signals from the two scanning fields AF1 and AF2 must still be spatially
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Font Frank G.
GmbH Johannes Heidenhain
Natividad Phil
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