Patent GB-818981-A (GB)
Title: Improvements relating to displacement measuring and position control devices
Inventor: VOLES ROGER
IPC: G01D5/38
Filed: 1954-11-09
Abstract: 818,981. Photo-electric indicating arrangements. ELECTRIC & MUSICAL INDUSTRIES Ltd. Nov. 8, 1955 [Nov. 9, 1954], No. 32334/54. Class 40 (3). [Also in Groups XX and XXXVIII] A device for indicating the displacement of one part relative to another consists of a first optical grating having regular rulings attached to one of the parts and a second parallel ruled grating attached to the other part, this second grating having rulings of similar pitch arranged in groups in such a way that the rulings undergo a phase-shift between one group and the next. Light directed through the gratings gives a light pattern which has sudden intensity changes corresponding to the changes between groups and relative displacement of one part relative to the other in a direction perpendicular to the rulings causes the pattern to undergo a corresponding magnified displacement. The rulings on the gratings may be either prismatic formations or line-type rulings. In one embodiment the second grating may have four groups of rulings A, B, A<SP>1</SP>, B<SP>1</SP>, arranged successively 90 degrees out of phase with the preceding group and light directed through the two gratings is observed by photo-sensitive cells 6, 7, 8, 9 positioned one opposite each group. A slot aperture 5 in a mask 4 limits the light reaching the photo-cells. Electrically the cells are coupled to the junction points of a resistance bridge 10, 11, 12, 13 and any voltage derived from wipers 180 degrees apart amplified at 20 and fed to motor 14 to cause rotation of the wipers until the null position is reached. The wipers track the relative movement of the parts and the movement of the shaft is an indication of that movement. In a second embodiment (Fig. 4), two sets of ruling groups A, B, A<SP>1</SP>, B<SP>1</SP>, are provided on the second grating 3 and two optical commutators 21 and 22 are provided between the slit 5 and two photo-cells 25, 26, one for each set of rulings. Each commutator comprises a hollow opaque cylinder with a helical slot cut round it as shown (see also Fig. 5, not shown), the slots 27, 28 in the two commutators, being cut in opposite directions. The commutators are joined together endwise and driven by motor 29 in accordance with any lack of balance between the output of the photocells. The commutators thus move to establish equal outputs from the cells and their movement is an indication of the relative movement of the gratings. In a modification (Fig. 6, not shown), light passing through the two commutators 21, 22 is directed by mirrors for alternate application to a single photo-cell through a rotating shutter. Gate circuits operated in synchronism with the shutter separate the output for application to a comparison circuit controlling the motor 29. In further modifications, using single photo-cells, light from the two commutators is passed respectively through either red and green filters and a coloured shutter used to pass the beams alternately to the cell or else through filters of different polarization with a corresponding form of shutter. Instead of the comparison of two variable outputs, as in the embodiments above, a single variable output may be compared with a standard source. Wedge or disc-type commutators may also be used. The device may be adapted for displacing a member to which one of the gratings is attached in accordance with instructions recorded on a film as a translucent line of varying lateral position or an area of varying density. Specification 787,641 is referred to.