GB821224A - Improvements relating to displacement measuring and position control devices - Google Patents

Abstract

821,224. Programming for machine-tools. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 7, 1955 [Dec. 18, 1954], No. 36665/54. Class 83 (3) [Also in Groups XXXVIII and XL (b)] A displacement-measuring device, with which may be associated position-control means, consists of two co-operating optical gratings producing a specific light pattern which, on relative movement of the gratings, undergoes a corresponding and magnified movement, and means for repetitively scanning the light pattern to produce an electric signal phase modulated in dependence on the movement. As shown, one of the gratings 8 is attached to a nut 11 which may be secured, e.g., to a machine-tool worktable. The nut works along a lead-screw 12 driven by servomotor 13. Collimated-light is directed through grating 8 and through a second grating 9 having rulings of the same spacing as those of grating 8, but inclined to the latter at a slight angle. A lens-system is used if necessary to focus the light output in the plane of a rotating shutter 6 having slots 7, and light passing through a slot reaches photo-cell 10. In position-control operation, movement may be controlled by a magnetic tape record 1 having a reference oscillation recorded on one track (2), and a control signal recorded on a second track. The reference signal is derived by head 3 and used to synchronize motor 5, which drives shutter disc 6. The control signal is derived by head 15 and fed to one input of phase-discriminator 14. The output of the photo-cell, which effectively views a vertically-moving horizontal line across the two gratings, is fed to pulseshaper 17 to give a large amplitude-output at the time of maximum light intensity, which output forms the other input of discriminator 14. The output of the discriminator is applied as an error-signal to servo-motor 13, which operates to cause the work-table unit to move in accordance with the recorded control signals. In a modification (Fig. 2), scanning is effected by a cathode-ray tube 20 having a horizontal fanbeam, or a rapid horizontal scan, the vertical scanning waveform being generated at 22 and synchronized with the reference-signals from the tape. The rest of the system functions as in the previous embodiment. The apparatus may be re-arranged to cause one member to follow the movement of another member (Fig. 3, not shown). In this embodiment the generator 22 is not synchronized, and a portion of its output is compared with the shaped photo-cell signals in the discriminator. One grating is attached to the primary member and the other to the following member, which is driven by the servomotor in accordance with the discriminator error signal output. Specifications 669,849, [Group XXXII], and 797, 503 are referred to.