GB1006698A - Improvements in or relating to an optical system and an apparatus for measuring displacements - Google Patents

Abstract

1,006,698. Electric selective signalling. RAYDOW LABORATORIES Inc. Jan. 1, 1963 [Jan. 19, 1962], No. 148/63. Heading G4H. [Also in Divisions G1 and G2] In apparatus for measuring displacement there are primary and secondary gratings, a mono-chromatic light beam illuminating a predetermined portion of the primary grating, means for superimposing an image of the primary grating on the secondary grating and for displacing the image according to the displacement to be measured. In the form of Fig. 1 the primary grating 23 is illuminated by a lamp 20, the light passing through a narrow band filter 21 and a condenser lens system. The image passes through a collimating lens 24 and is reflected from a mirror 25 through a lens 26 to fall on the secondary grating 27 as shown at 23a. A photo-cell 28 receives light passing through the grating 27. As the mirror 25 rotates through a small angle the image 23a moves over the grating 27 and induces a sinewave signal in the photo-cell 28. In modifications the image is projected through a halfrnirror at 45 degrees or a prism on to the displaceable mirror which returns it to the halfmirror for reflection on to the secondary grating at one side. In a similar form, Fig. 7, the image reflected by the half-mirror 30 is enlarged by a lens 35 and reflected by a mirror 30<SP>1</SP> on to a secondary grating 27<SP>1</SP> which is larger than the primary grating, corresponding to the enlarged image 23a. Rotation of mirror 25 gives photo-cell 28 to provide an output of a corresponding number of cycles. In the form of Fig. 3 an image of the primary grating 31 is projected by lens 24 on to displaceable mirror 25, reflected light being returned through lens 24 on to the secondary grating which is coplanar with the primary grating. As the mirror tilts the image of the primary grating sweeps over the secondary grating and photo-cell 28 generates a sinewave signal as before. For measuring linear displacement the primary grating 23, Fig. 9, is arranged with a halfmirror 30, and mirror 25 to project an image on to the secondary grating 27. The photo-cell 28 may be connected to a meter 45 the reading of which indicates the relative position of the image of grating 23 with respect to grating 27. A pivoted wedge 46 may also be provided to displace the image through the distance of one or more lines. In the form of Fig. 10, the primary grating 23a has lines in both directions at right angles and its image is projected on to X and Y secondary gratings. The displacement in the two directions is indicated by photo-cells 28a, 28b. The sinusoidal output from a photo-cell is applied to a Schmitt trigger to generate a square wave which is is differentiated to obtain positive and negative peaks. The latter are eliminated by rectification and the former pass to a counter for measuring the displacement or the speed of movement.