Patent GB-937138-A (GB)
Title: Positioning apparatus
IPC: G01B11/27
Filed: 1961-12-06
Abstract: 937,138. Photo-electric position finding. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 6, 1961 [Dec. 30, 1960], No. 43639/61. Class 40 (7). An arrangement for detecting eccentricities in the position of a centrally apertured article with regard to a predetermined axis, comprises a source of light, means for moving a beam from the source of light in an orbit about the predetermined axis to scan along the edge of the aperture in the article, light sensitive means, means for directing the beam of light upon passage through the aperture on to the light sensitive means and means for comparing the times of illumination of the light sensitive means by the beam in opposite sectors of the beam's orbit. The invention is shown applied to the centring of a magnetic disc record 24, Fig. 10, above a revolving drum 33, Fig. 8, housing the light source 52 and a set of read-out heads, Fig. 9 (not shown). A light beam 54 from the source 52 is reflected by mirror 56 and focused by lenses 60 to describe a circular orbit 21. The central hole H in the disc is formed from the intersection of four circles, each smaller as in Fig. 10 or larger as in Fig. 5 (not shown), than the orbit 21, such that the beam is alternately interrupted by the cups D1, D2, D3, D4, and the photo-sensitive element 70, receiving the beam reflected back by the annular mirror 58, gives a pulsed output as in Fig. 2 when the aperture is centrally located or as in Fig. 4 when the aperture is out of centre. Alternatively, Fig. 7 (not shown), the light source may be mounted above the revolving drum and the light beam directed on to the mirrors 56 by two " periscope " mounted mirrors having the lenses 60 between them. In the associated circuit, Fig. 12, the pulses from the photo-sensitive element 70 are used to open gate 92 to let oscillations through from oscillator 90 for counting in counters C1, C2, C3, C4, the number of oscillators counted per pulse being indicative of the width of that pulse. The input to each counter is controlled by gates G1, G2, G3, G4, opened consecutively by pulses from four photo-sensitive elements S1, S2, S3, S4 detecting, in turn, a dark band 96, Fig. 10, or the shaft of the drum. The pulse counts in counters C1 and C3 represent the pulse widths for opposite sectors of the beams orbit and are subtracted in SX to give an output indicative of the records out of centre in the X direction for activating a servomotor MX to readjust the record in the X direction. Counters C2 and C4 deal similarly with the Y direction. Centring of the record is detected by zero detectors ZX and ZY which activate suction devices for clamping the record in position and activate means indicating that the apparatus is ready for use and/or activate the recording heads. Latching gate 91 stops oscillations from 90 reaching gate 92 until the record is placed on its support as detected by photo-sensitive element Sr, whilst photo-sensitive element Sf detects a mark 100, Fig. 10, on the drum shaft to open the latching gate at the start of a cycle. Means, not shown, are also provided for setting the counters and subtractors to zero at the end of each cycle.