Patent GB-666882-A (GB)
Title: Improvements in or relating to feed-back amplifier systems and servo mechanisms that are adapted to respond to input changes at very low frequencies
IPC: H03F3/36
Filed: 1948-12-01
Abstract: 666,882. Electric control systems. SEDGFIELD, H. B., SUMMERLIN, F. A., and SPERRY GYROSCOPE CO., Ltd. Dec. 1, 1949. [Dec. 1, 19481 No. 31167/48. Class 40 (i). [Also in Group XL (c)] To minimise drift effects in a negative-feed back D.C. amplifier system, Fig. 2, the error signal 4 i.e. the difference derived at C, of the primary input voltage 2 and the fed back portion 3 of the output, is fed to the mainchannel power amplifier A through a compensating device D comprising an amplifier E substantially free from drift errors, the amplified version of the error signal derived from its output 7, being added to the error signal in a combiner F the output 6 of which provides the input to the power amplifier. Alternatively, the input to the compensating amplifier E may be taken from the input 6 to the main-channel amplifier A. The amplifier system may be entirely thermionic or it may be partly electro-mechanieal e.g. the power amplifier A may include an electric servo-motor for driving a load L. The feed-back voltage 3 may be derived in any suitable way from the power amplifier output 1, e.g. in a circuit B or from means driven by the servo-motor. A single compensating amplifier E may be arranged to be switched in successively to a plurality of D.C. amplifier systems which are provided with storage devices which compensate the respective systems for drift during the intervals when the compensating amplifier is disconnected therefrom. The compensating amplifier may be of the "chopper" type i.e. one that operates intermittently to convert input direct voltages or currents to modulated alternating voltages or currents at some fixed carrier frequency and after amplification reconverts the output into corresponding direct quantities. The compensating amplifier may be thermionic or electromechanical. Figure 3 shows an arrangement in which one compensating amplifier E is switched sequentially into two balanced bridge type servo systems. Each servo system comprises a two-phase motor A2 which drives a load to a position corresponding to that of a control knob 9. The control knob 9 turns the wiper 9<SP>1</SP> of a potentiometer 10, forming the bridge ratio arms connected across a D.C. supply 12, 12<SP>1</SP>, to vary the primary input potential 2, this potential altering in polarity with respect to earth as the wiper 9<SP>1</SP> passes through the mid-point of potentiometer 10 by virtue of the earthed centre-tapped potentiometer 13 across the supply. The negative feedback voltage 3 from the main channel amplifier A is derived from a further potentiometer 30, forming, the other two arms of the bridge connected across the D.C. supply 12, 12<SP>1</SP>, the potentiometer wiper 29 being driven through a screw 28 and nut 27 from the servo-motor A2 in the corrective sense, i.e. to balance the bridge, when the primary input voltage 2 and the feed back voltage 3 fed by the respective potentiometer wipers differentially to the potential divider 11, 31, produce earth potential, i.e. zero error voltage at point P. The error voltage at point P is also applied over a resistor 41 and the sequence switch SI, S2, when its wipers 38, 39 inter-connect the contacts of the pairs 33a, 33b, and 36a, 36b to the compensating amplifier E which feeds an amplified but phase inverted version of the error signal through a filter 42 to an input terminal Q. In the combiner amplifier F which consists of a "long-tailed" pair of triodes 14, 15 the inputs at P, Q are combined subtractively, so that the resultant effect is to add the error signal and its amplified version, and applied at point R to the input of the main channel or power electronic amplifier A1 which feeds the variable phase 26 of the motor A2 with A.C. of a magnitude and phase sense relative to the fixed phase 25 in dependence on the magnitude and polarity of the error signal at point P whereby the motor is driven in the corrective sense until balance of the bridge is achieved. The compensating D.C. amplifier E also consists of a "long-tailed" pair of triodes. In the sequence switch position shown the amplifier E is disconnected from the servo-systems : its output R11 is connected to the input P11 and the capacitor 40 is connected between the two inputs P11, Q11. With these connections the amplifier E self degenerates any drift voltages originating within it by developing across the capacitor 40 a correcting voltage to compensate such drift effects. In the next position of the sequence switch, which rotates continuously, the error voltage at terminal P of the first servo system is connected in series with the capacitor 40 to the input P11 of the compensating amplifier. The compensating voltage thus added by the capacitor 40 is of the correct value to compensate for drift effects in the amplifier E so that the output from R11 is an amplified version of the error signal free from any errors due to drift in the compensating amplifier. This. voltage is, as above described, fed to the combiner F through a filter 42 so that only a smoothed signal is passed to input terminal Q and the shunt capacitors in the filter become charged to a voltage equal to this amplified version of the error signal and store this charge to maintain the compensation for drift effects until the compensating amplifier E is again connected in the circuit in question. In the next position of the sequence switch the compensating amplifier is connected in the circuit of a second identical servo system where it performs a similar function.