1452329 Automatic control of position PHILIPS ELECTRONIC & ASSOCIATED INDUSTRY Ltd 8 Nov 1973 [11 Nov 1972] 51849/73 Heading G3R In apparatus for reading information from a disc shaped carrier the scanning point is controlled radially by a first control signal and tangentially by a second control signal, the first control signal being derived from a position error detector and the second control signal from means providing a phase shift equivalent to 90 degrees of disc rotation. The disc may be magnetic but as shown is optical, a light source 4 being imaged at a and thence reflected by mirrors 9 and 15 on to detector 21. This comprises an information detector 22 and a grating-like position detector 23. The light spot a embraces many (e.g. 50) tracks so that the image received at 23 consists of light and dark bands; their relationship with the grating produces a radial position error signal which is amplified at 26 to control tilting of mirror 9 about pivot 12, thus maintaining constant the radial position of the image on the detector. As shown, the output of amplifier 26 is also applied to a phase shifting network 27, which may comprise an integrator, to control the tilt of mirror 15 about a longitudinal axis, and therefore the tangential position of the image. Alternatively, mirror 9 may be connected to a tachogenerator whose output will be 90 degrees out of phase with displacement, and may thus be used to control mirror 15 directly, or the output of this tachogenerator may be compared with the phase shifted output of another tachogenerator associated with mirror 15, the difference being used as the control signal for 15 Fig. 3 (not shown).
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