Patent GB-929626-A (GB)
Title: High precision reading device
IPC: G05D3/14
Filed: 1960-03-14
Abstract: 929,626. Photo-electric measuring systems. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. March 14, 1960 [April 10, 1959], No. 8949/60. Class 40 (3). The invention relates to apparatus, Fig. 1, of the type described in Specification 686,274, for determining the position of a scale 17, in which an image of the scale is formed on a slotted mask 3 and oscillated back and forth by means of a deflector 7, a photo-electric cell 18 located behind the slot 4 in the mask serving to generate a pulse i each time a graduation t of the scale crosses the slot. The deflector 7 is driven by a moving-coil assembly 10-13 supplied from an alternating current mains R and causes the image displacement to vary sinusoidally with time. With graduation t located on the optical axis, the impulses i are equally spaced in time. When the scale is displaced either way, however, the time interval between successive pulses become unequal and provide a measure of the scale displacement. It is the object of the invention to provide a reading device for the apparatus which will furnish an indication of the displacement which is linearly related to the actual displacement. In a first embodiment shown in Fig. 1, there is provided a second similar scale 27 and associated deflector 21, slotted mask 31 and photoelectric cell 32. The deflector 21 is driven in synchronism with deflector 7 from the same alternating current mains, and the impulses n from cell 32 are compared with those, i, from cell 18 in a phase comparator C. Any phase difference results in an error voltage e which drives a motor M to move scale 27 until the error reduces to zero. In this way, scale 27 is caused to assume a position corresponding to that of scale 17. The scale position is shown by a mechanical indicator 40-12 which, by reason of the direct correspondence between the two scales, indicates the original displacement linearly. Magnification is employed in the deflection of deflector 21 and the drive to indicator 40-42. In a modification, Fig. 2 (not shown), a counter coupled to the shaft of motor M replaces indicator 40-42. In a second embodiment, Fig. 3, impulses i from cell 18 are applied to a circuit A which provides a voltage e 1  proporitonal to the difference in timing of successive pulses and hence also the displacement of scale 17. The voltage is applied to a moving-coil assembly 53 which controls the angular displacement of an auxiliary deflector 58 located in the optical path between the main oscillating deflector 50 and mask 3. Deflector 58 isrotated until the image oscillations are symmetrical with respect to slot 4 whereby the impulses i become equally spaced in time and the voltage e 1  tends to zero. The other end of shaft 57 supporting deflector 58 carries a mirror 59 by means of which light from an illuminated slot 63 is projected on to a scale 60 to indicate the angular position of deflector 58. A moving-coil generator 54 furnishes a voltage proportional to the rotational speed of shaft 57 which is applied to circuit A to introduce a stabilizing control. Assuming a linear relationship between image displacement and the angular rotation of deflector 58, the indication furnished on scale 60 is linearly related to the original scale displacement.