Patent GB-932471-A (GB)
Title: Improvements in and relating to the measurement of displacements
Inventor: BARBER DEREK LESLEY ARTHUR
IPC: H03K5/26
Filed: 1958-06-26
Abstract: 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 (Fig. 4). Instead of producing continuous motion of the comparison grating(s) 14, 22, of Fig. 1, a vibratory motion by means of a suitable electro-mechanical device may be employed (Fig. 5, not shown). In further arrangements (not shown) the comparison grating(s) may be stationary and the reference and measurement signals produced by vibrating either the light sources or photoelectric cells or intervening optical elements. Alternatively, all elements may be stationary and polyphase light sources or polyphase photoelectric cells employed. In the arrangements so far described the position of the moving member may be determined within the limits of the line-spacing of the measurement grating by the count of the difference in the number of cycles of the measurement and reference signals and a finer measurement of any fractional parts of the line-spacings obtained by measuring the phasedifference between the two signals. Instead of this latter step, however, a measurement of fractional parts of the line-spacing may be effected by the use of a further grating, termed an interpolation grating, equivalent to a vernier. This arrangement is shown in Fig. 7, which is similar to Fig. 3, with the addition of an interpolation grating 51 having an effective " pitch " of 0.9 times that of the comparison grating 36F, a double light source 24f (one for each of the latter gratings), an optical system 35f which produces separate images of the two gratings at the beam splitter 34f, a photoelectric cell 52 from which the interpolation signals are obtained and screens 53 to prevent the image of the interpolation grating from falling on the photo-electric cells 19f and 27f. The Specification gives details of arrangements including valve, transistor, decade-type counting tubes and glow tube indicators, for the counting circuits including arrangements for measurement of fractional grating line spacings by either measurement of the phase-difference between the measurement and reference signals or by utilization of the signals produced by the interpolation grating (Figs. 8 to 14, not shown).