1294841 Digital control of position INFORMATION STORAGE SYSTEMS Inc 6 Jan 1970 [21 Jan 1969] 684/70 Headings G3R R69 R7R R9B R4 and R31X [Also in Division G1] In a position servo control system, e.g. for positioning the read/write heads on a magnetic disc store, having a coarse digital and fine analogue control, a position transducer delivers two alternating signals of the same frequency but different phase, successive intersections of which relate to successive increments of position, and the peak amplitudes of the signals are maintained constant by an automatic control circuit. A desired and previous head position are set in registers 14, 15, respectively, and the difference set in a counter 17 which drives via a D/A converter 18 a linear motor 12 positioning the read/write heads. A damping signal from tachogenerator 20 is added at 19. Transducer 13 gives two alternating signals, differing in phase by 180 degrees, successive intersections of which each decrease the counter 17 by 1. Detector 23 senses a zero signal from the D/A converter 18 to change switch 22 to effect fine analogue control and drive the heads to the exact intersection of the alternating signals. A 60 Hz signal is added at 21 to cause the heads to oscillate about the desired track. The transducer comprises a pair of optical gratings 28, 29, one fixed and the other coupled to the heads, and two photo-cells 26, 27 delivering the alternating signals. The sum of the signals is compared with a reference voltage Vr at gate 32 to control a current source 34 supplying the grating lamps 24, 25. During coarse control, switches 35, 41 down, the Va signal is inverted at 36 and gate 32 delivers the difference between the two alternating signals, Va, Vb to current controller 34 to control the ratio of the currents to the two lamps. In the fine control, switches 35, 41 up, the sum of the two alternating signals from gate 33 passes to demodulator 39 which receives a demodulating signal from saturated amplifier 38 fed by Va. The resulting signal, representing the difference between Va and Vb, controls the current ratio as before.