GB947920A - Improvements in or relating to devices for measuring displacements - Google Patents

Abstract

947,920. Electric selective signalling systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 20, 1960 [April 23, 1959], No.13836/60. Heading G4H. Relates to an arrangement for measuring the magnitude and direction of rectilinear or rotary displacement of a member, e.g. a machine tool member, in which a raster, e.g. a diffraction grating or absorption raster, is attached to the member, said raster having lines substantially at right angles to the direction of displacement, a beam of light is directed onto the raster to form a luminous image consisting of bright and dark moirÚ bands which move substantially at right angles to themselves on displacement of the raster and the luminous image is projected on to a plurality of, e.g. four, photo-electric cells whose output pulses are counted in an electronic counter to give the displacement magnitude, the direction of displacement being indicated by the relative phase relationship of the pulse outputs. According to the present invention the effects of any irregularities in the raster are reduced by arranging that the area of the luminous image covered by the photo-electric cells comprises at least two moirÚ periods, a moirÚ period being the distance between two successive bright moirÚ bands and each period being regarded as divided into a number of zones I, II, III, IV equal to the number of photo-electric cells, the light from each of the first zones I being directed on to a first photo-electric cell, the light from each of the second zones II being directed on to a second photo-electric cell and so on. The arrangement described comprises a fixed raster 1 and a raster 2 movable in the direction of its length and having opaque lines inclined relative to those of the fixed raster 1 to produce moirÚ bands extending parallel to the direction of movement but alternatively the two rasters may have different raster periods or constants in which case the moirÚ bands extend parallel to the raster lines. The invention is also applicable to an arrangement comprising a single raster as described in Specification 906,101. As shown in Fig. 3, light from a source 3 focused by lens 4 on the movable raster 2 and fixed raster 1 is projected by lenses 5, 6 on to a double prism 7 having its axes at right angles to the plane of the paper. In conjunction with a lens 8, light from the zones I, I is simultaneously projected on to the photo-cell 9, and that from zones IV, IV is incident on the cell 12. The outputs from the cells 9. . .12 are fed to an electric counter T which provides an indication of the movement in extent and direction of the raster 2 relative to the fixed raster 1. Specification 760,321 also is referred to.