Improvements in and relating to the measurement of displacements

932,471. Electric selective-signalling systems. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 26, 1959 [June 26, 1958; April 23, 1959), Nos. 20569/58 and 13886/59. Class 40 (1). The displacement of a moving member is measured by comparing a first signal produced electro-optically by relative motion between an optical grating carried by the moving member and an optical grating in constant motion with a second signal having a constant relationship to the movement of the grating in constant motion. The second signal may also be produced electro-optically by relative motion between the constant motion grating and a further grating. Fig. 1 shows the general arrangement in which the moving member 11, moving along a slide 12, carries an optical grating 13 (measurement grating) adjacent to a further optical grating 14 (comparison grating) in constant motion in the direction of arrow 15, the relative motion of the two gratings interrupting light from source 16 to produce in photo-electric cell 19 a succession of pulse-like signal (which may be sinusoidal) the repetition rate (or frequency) of which is a function of the line spacing and relative speed of the two gratings. To produce the second signal a fixed optical grating 21 (reference grating) coacts with a comparison grating 22 in constant motion in the direction of arrow 23 and, in co-operation with light source 24 and photoelectric cell 27, produces a signal having a constant repetition rate which, by appropriate selection of the line spacing of the gratings, is arranged to be the same as that of the signal from photo-electric cell 19 when the member 11 is stationary. The two signals are then fed to a counter 28 in which, when member 11 moves, a signal indicative of the extent and direction of such movement is derived by counting the difference in the number of cycles in the two signals. The comparison grating 22 is preferably the same grating as comparison grating 14 and the two may be formed as an endless loop or, as shown in Fig. 2, as a disc 31. In an alternative construction (Fig. 3), the comparison grating 33 is formed on a rotating drum 32, a direct image thereof being employed for cooperation with reference grating 21b and a reflected image for co-operation with the measurement grating 13b. When the motion of member 11 is rotary instead of linear the measurement grating and comparison grating are formed as rotatable discs 13c and 36 respectively and the reference grating as a fixed radial element 21c