Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1997-07-01
1999-03-23
Le, Que T.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250237G, 356374, G01D 534
Patent
active
058863524
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an opto-electronic rotary encoder which may be employed, for example, to measure angular displacement of one rotatable body relative to another.
BACKGROUND OF THE INVENTION
Such rotary encoders are known per se, and typically include a code wheel, mounted to a first one of the relatively rotatable bodies, having a plurality of radially extending, circumferentially spaced lines, situated at equi-angular intervals about the periphery of the code wheel, thereby to form a scale. An optical readhead, mounted to the other of the relatively rotatable bodies, and in register with the scale, projects light onto the scale which, depending upon the configuration of code wheel, is then either reflected or transmitted. From the reflected or transmitted light, the readhead generates a series of signals which may be used to generate an incremental count indicative of the relative angular displacement of the first and second relatively rotatable bodies.
SUMMARY OF THE INVENTION
A first aspect of the present invention provides opto-electronic encoder apparatus for measuring relative movement having a scale defined by a series of spaced apart lines, and a readhead lying in register with the scale, the scale and readhead being relatively movable in the direction of spacing of the lines; the readhead comprising: a periodic light pattern; generate a resultant fringe pattern; and generating a plurality of phase shifted cyclically modulating signals upon said relative movement; linear pattern, and the other is an angular pattern.
An independent aspect of the present invention provides opto-electronic rotary encoder apparatus for measuring relative angular displacement having a scale defined by a series of spaced apart lines, and a readhead lying in register with the scale, the scale and readhead being relatively movable in the direction of spacing of the lines; the readhead comprising: a periodic light pattern; generate a resultant fringe pattern; and generating a plurality of phase shifted cyclically modulating signals upon said relative movement, comprising: direction of spacing of said resultant fringes, elements of different sets being interleaved in a repeating pattern, and all elements of a given set being electrically connected in common, wherein adjacent element are separated by a distance not equal to the fringe pitch of said resultant fringes.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:
FIG. 1 is a schematic elevation of a rotary encoder in accordance with the present invention;
FIG. 2 is a side view on II--II in FIG. 1;
FIGS. 3a-c are signal diagrams illustrating the generation of resultant fringes;
FIG. 4 is a vector diagram;
FIG. 5 is an elevation of the detector array in the readhead of FIG. 2;
FIGS. 6a-c illustrate the effect of code wheel eccentricity;
FIG. 7 is a graph showing phase error due to eccentricity;
FIG. 8 illustrates the interference fringe pattern generated in the readhead of FIG. 2;
FIG. 9 is an elevation of a part of a modified analyser grating; and
FIG. 10 is an elevation of a detector array used with the analyser of FIG. 9.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIGS. 1 and 2, an angular encoder for measuring relative angular displacement of two members includes a circular code wheel 10, mounted to one of the members in a manner permitting rotation about a central bearing 12, and a pair of opto-electronic readheads 14, mounted to the other of the members. The code wheel 10 has, at its periphery, a rotary scale 20, made up of a series of alternately light-reflecting and non-reflecting lines 22; the lines 22 extend radially and are circumferentially spaced from each other. Alternatively, the lines may be provided by a phase grating (known per se) type structure, etched into the surface of the code wheel 10 to generate a scale.
The readheads 14 each include a diffuse light source 30, which g
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Clark, L et al. "Thin Film Photovoltaic Detector Arrays." International Research and Development Co, Ltd. Conference on Moire Finge Technology, Jun. 19-20, 1972, pp. 1-5.
McFarland Geoff
Wright David A
Le Que T.
Reinshaw plc
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