Patent GB-683889-A (GB)
Title: Improvements relating to servo systems
IPC: G05D3/14
Filed: 1949-06-16
Abstract: 683,889. Electric correspondence control systems. FERRANTI, Ltd., and McCALLUM, D. M. Sept. 11, 1950 [June 16, 1949], No. 16014/49. Class 40 (i). In an electric correspondence control system. wherein the positions of input and output members control the frequencies of respective alternating signals applied to a phrase discriminator developing a unidirectional error signal dependent in magnitude and sign on the magnitude and sense of the deviation of the two alternating signals from a predetermined phase relationship (e.g. quadrature) produced by the displacement of the input member from coincidence with the output member, the error signal primarily energizes rapidly acting means for restoring the output frequency to approximate equality with the input frequency, and secondarily energizes slower acting servo-motor means for driving the output member into positional coincidence with the input member and thereby reducing to zero the error signal corresponding to the primary approximate adjustment. Due to the rapidly acting means, the phase difference between the input and output signals is prevented from ever exceeding 90 degres so that the phase discriminator develops a unidirectional error signal instead of a beat-frequency signal as would otherwise occur upon a difference between the input and output frequencies. The input shaft 11 controls the frequency of an oscillator 10, the output from which is amplified and limited in circuit 12, the resultant square waveform being applied to phase discriminator 17. The output shaft 14 is driven by servomotor 27 to control the frequency of multivibrator 13, the output from which is applied, via amplifier 16, to the phase discriminator. Upon departure of the two input signals to the discriminator from phase quadrature, the discriminator, delivers a corresponding d.c. error signal which is applied via smoothing filter 21, to a circuit 23 controlling the frequency of oscillator 13 and via filter 25 and servo-amplifier 26 to the servomotor 27. The circuit 23 comprises a cathode follower which controls the bias voltages applied to multivibrator 13 in accordance with the error signal and thus controls the frequency of the oscillations generated thereby to maintain it in approximate equality with that of oscillator 10. Due to the lack of complete correspondence between the two signals a steady error signal is developed and this drives servomotor 27 and thus a frequency controlling element of oscillator 13 until exact correspondence is achieved between the two signals and the error signal falls to zero. The frequency controlling element may comprise a potentiometer controlling the bias applied to the multivibrator or it may comprise variable capacitive or resistive elements. In a modification, Figs. 3 and 4 (hot shown) means are provided for reducing the oscillatory output (of double the input frequency) from phase discriminator 17. The outputs from oscillators 12 and 13 are combined to provide one input to the phase discriminator in such a manner that the two signals cancel out when the input and output shafts coincide. Under these conditions no output of double the input frequency can be delivered by the phase discriminator 17.