Patent GB-678520-A (GB)
Title: Improvements relating to servo-mechanism
Inventor: STEPHENSON GEOFFREY HUSON
IPC: G05D3/14
Filed: 1949-10-19
Abstract: 678,520. Electric follow-up systems. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Oct. 5, 1950. [Oct. 19, 1949] No. 26785/49. Class 40 (i). [Also in Group XL (c).] An electric follow-up system including a servomotor adapted to effect rapidly and without hunting a rotary displacement # corresponding to an input signal x comprises means for deriving first and second signals dependent respectively upon (x -#) and #<SP>1</SP> and means for applying maximum forward or reverse torque to the servomotor dependent on whether the first signal is greater or less than the second signal, the means for deriving the signals including a non-linear device such that at least one of the derived signals is non- linearly related to its argument. As shown in Fig. 1 an input A.C. signal x is applied via terminals 3 to a subtracting circuit 4 together with an A.C. signal directly proportional to the angular displacement # of the motor shaft 2 produced by the potentiometer b whose slider 5 is coupled to the shaft. The resultant difference signal (x - #) is applied to a non-linear amplifier 8 having a characteristic, Fig. 2 full-line curve, corresponding to the relationship. between the angular velocity #<SP>1</SP> of the motor shaft 2 and its angular displacement # when the motor is under maximum torque. The. output f (x - #) from amplifier 8 is rectified at, 9 and applied to subtracting circuit 10 together with a D.C. signal directly proportional to the angular velocity #<SP>1</SP> of shaft 2 derived from the tacho-generator 11 and the resuitant signal f (x - #) - #<SP>1</SP> is fed via amplifier 12 to drive the motor 1 the gain of the amplifier being sufficiently high that maximum forward or reverse torque is applied to the motor if the resultant signal differs significantly from zero. In a modification, Fig. 4 (not shown), the #<SP>1</SP> signal is applied to a non-linear device having a characteristic which is effectively the inverse of that shown in Fig. 2 and the output is subtracted from a signal. linearly proportional to (x - #) the amplified resultant signal being applied to control the motor. Non-linear amplifier, Fig. 3. The desired full-line characteristic, Fig. 2, of the non-linear amplifier 8 in Fig. 1 is approximated by the dotted-line characteristic of the amplifier shown in Fig. 3 which comprises a screen-grid valve 15 coupled via biassed diode limiters 19, 21 to the input grid of a further screen-grid valve 25 whose anode is back-coupled to the grid via capacitor 27 and a resistor 28 (value R) which is shunted by reversely-connected biassed diodes 29 and 31 in series with resistors 30 and 32 each of value R/10. When the input is less than the abscissa of the point 3b, the output from valve 25 is insufficient to render diodes 29, 31 conductive and the feedback is applied via resistor 28 only producing the dotted characteristic 0-36 but for greater values of input the diodes 29, 31 become conducting on alternate half cycles, and the effective feedback resistance is reduced thereby reducing the gain as shown by the dotted characteristic 36-13 and for inputs greater than the abscissa of point 13 the diode limiters 19, 21 become effective, the output from valve 25 being then constant.