Patent GB-848509-A (GB)
Title: Phase or frequency modulated digital servo system
IPC: G05B19/35
Filed: 1958-05-08
Abstract: 848,509. Programmed electric control systems. BENDIX AVIATION CORPORATION. May 8, 1958 [May 27, 1957], No. 14794/58. Class 38 (6). A system for controlling the displacement of an object, such as a movable part of a machinetool, by means of electric command pulses comprises means for phase-displacing an electric oscillation with respect to a reference electric oscillation in accordance with the number of the command signals and means for converting the phase displacement between the oscillations into an electric signal for controlling the displacement of the object. Pulses of constant amplitude and constant frequency are generated by a pulse generator 10 and the pulses fed into two counters, a command counter 12 and a reference counter 14. The counters 12, 14 are each of the binary type and as described each comprise four stages, each stage of which is in the form of a bi-stable multi-vibrator. Thus each counter will produce an output pulse when sixteen pulses have been fed into it from the pulse generator 10. The output pulses from the command counter 12 are fed directly to a discriminator 16 whilst the output pulses from the reference counter 14 are fed into the discriminator 16 by way of a shaping circuit 18 for converting the square wave pulses into a sinusoidal waveform, an amplifier 20, a magnetic resolver 22 and an amplifier 24. The input to the reference counter 14 is such that the output thereof is phase constant. When no command pulses are being fed into the command counter 12 the output thereof is in phase with that of the counter 14 and in consequence no phase difference exists between the inputs to the discriminator 16 and hence no output signal will appear at its output. Thus no movement of servomotor 26 or the resolver 22 takes place. When, however, command pulses are fed into the command counter 12, the output pulses therefrom are either advanced or retarded in phase relative to the output of the reference counter according to whether the command pulses are positive or negative in polarity, the amount by which the phase is shifted being dependent upon the number of command pulses. In four-stage counters counting sixteen each command pulse will shift the phase of the output 360 of the counter 12 by - degrees. When the 16 inputs to the discriminator 16 from the counters 12, 14 are phase-displaced a D.C. error signal will appear at the output of the discriminator, the polarity of which signal is dependent upon whether the output of the counter 12 leads or lags the output of the reference counter 14 and the magnitude of which signal is dependent upon the amount of phase displacement. This signal will cause the servo-mechanism 26, which in the disclosed embodiment is hydraulically-operated, to drive the rotor of the resolver 22, and hence the movable part of the machine tool, in a direction to annul the difference in phase of the inputs to the discriminator 16. The system may be modified to give co-ordinate movement and the command signals may be derived from a tape. Positional control may be accomplished by suitable choice of the number of command pulses. Specification 829,824 is referred to.