Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1996-08-06
1999-01-19
Kim, Robert
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
250237G, G01B 902
Patent
active
058619534
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to opto-electronic scale reading apparatus. Such apparatus nay be used with a scale to determine the magnitude and direction of movement of one member relative to another. Such an apparatus is typically used to measure the relative movement of two members on coordinate positioning machines such as machine tools or coordinate measuring machines.
DESCRIPTION OF PRIOR ART
A known type of such device is described in U.S. Pat. No. 4,959,542. Here, an elongate scale which can be fitted to one of the above members comprises a series of marks, in the form of parallel lines, spaced apart in the longitudinal direction of the scale. A readhead for attachment to the other of the members includes an index grating and an analyser grating. Light reflected from (or transmitted through) the scale interacts with the index grating, and causes the generation of a fringe pattern in the plane of the analyser grating. When the scale and readhead are moved relative to each other, this fringe pattern moves in a corresponding fashion in the plane of the analyser grating, and photodetectors situated behind the analyser grating receive a modulated light signal, from which the distance and direction moved can be determined.
Alternatively, our European Patent Application No. EP 543513 discloses that the analyser grating and the detectors may be combined, and provided on a single semiconductor substrate. This combined grating and detector may be referred to as an electrograting.
The arrangement of U.S. Pat. No. 4,959,542 provides a spatial filtering effect which ensures that the readhead is very tolerant of defects in the scale or dirt which may accumulate on the scale. However, to ensure that fringes with good contrast are produced in the plane of the analyser grating, it is necessary to hold the spacings between the scale, the index grating and the analyser grating substantially in accordance with formulae which are given in the specification of U.S. Pat No. 4,959,542. These formulae relate those spacings to the pitches of the scale, the index grating and the analyser grating.
As a result, it is found in practice that the readhead is fairly sensitive to the offset or "ride height" between the scale and the readhead. This is illustrated in FIG. 1 of the accompanying drawings, which shows a sensitivity curve S illustrating how the amplitude A of the signal output from the readhead varies with the offset or ride height h of the readhead above the scale. It will be seen that a strong, usable signal is obtained only within a relatively narrow tolerance band T. When the scale and readhead is installed on a machine, therefore, it is necessary to ensure that the readhead will remain within this tolerance band T throughout the length of its travel along the scale.
British Patent No. 1,474,049 shows scales and readheads of a type which is different from those discussed above. In one embodiment, light passes from a first grating to a second grating, which reflects it back to the first grating. The second grating is tilted with respect to the first grating so that the return optical path from the second grating to the first is greater than the outward optical path from the first grating to the second. U.S. Pat. No. 4,636,076 shows a development, in which the return optical path differs from the outward optical path by virtue of the light passing through different thicknesses of a material having a refractive index different from the ambient medium. However, these two patents do not address the problems with which the present invention is concerned.
SUMMARY OF THE INVENTION
The present invention provides a scale and readhead apparatus, comprising: longitudinal direction; from the scale to the index grating and analyser; provided for light travelling from the scale to the index grating, and/or from the index grating to the analyser.
With such an arrangement, light from the scale may interact with the index grating and cause a fringe pattern in the vicinity of the analyser. However, by virtue of t
REFERENCES:
patent: 4636076 (1987-01-01), Pettigrew
patent: 4959542 (1990-09-01), Stephens
patent: 4974962 (1990-12-01), Stephens et al.
Kim Robert
Renishaw plc
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