Patent GB-988213-A (GB)
Title: Servo amplifier with dither
IPC: G05D3/12
Filed: 1963-11-08
Abstract: 988,213. Controlling aircraft. BENDIX CORPORATION. Nov. 8, 1963 [Nov. 9, 1962], No. 44202/69. Heading B7G. A servo-amplifier for controlling operation of an aircraft control surface through a torque motor having a pair of control windings comprises a discriminator including a pair of active elements e.g. transistors, means applying an input control signal to each of said active elements, an oscillator circuit applying a signal of small amplitude oscillations to one of said active elements and means applying the output of said active elements to the respective windings of said torque motor. The invention is embodied in a servo control system for an aircraft control surface 101 controlled by a mem- - ber 7 which may be operated manually or by a gyro, wherein the surface is operated by an actuator 100 having a valve 110 actuated by a secondary actuator 112 controlled by a torque motor 11 and servo valve mechanism 11A. Motor 11 is responsive to windings 10, 10A so connected as to receive a signal from a discriminator 12 through terminals FF<SP>1</SP> and GG<SP>1</SP> of terminal strip 13 and milliammeters 14 and 15. Discriminator 12 receives a control signal from a preamplifier 16 through coupling transformer 17, and a dither generator 18 applies small amplitude oscillations to discriminator 12 through resistor 19 and capacitor 20, the amplitude of dither being controlled by variable resistance 21. Preamplifier 16 receives two signals at an input 22, which comprises the emitter of a transistor 23, each signal being received through a different one of a pair of isolation resistors 24 and 25 from terminals AA<SP>1</SP> and BB<SP>1</SP>. Terminal AA<SP>1</SP> is connected to stator 44 of a synchro 31, the rotor 30 of which is actuated by member 7, and terminal BB<SP>1</SP> receives a signal from the output winding 38 of a transformer 37, the core 114 of which is positioned by secondary actuator 112 to provide feedback. Synchro 31 is associated with a follow-up synchro 34 and rotor 30 of synchro 31, rotor 33 of synchro 34 and primary 36 of transformer 37 are energized respectively through terminals 26, 27 and 28 from a transformer 9 connected to an A.C. source 8. Source 8 is connected also to a power supply 45 which energizes terminal HH<SP>1</SP> and feeds a step-down transformer 51 to energize windings 10, 10A of motor 11 and terminal II<SP>1</SP>, which connects to a common point 53 connecting the bases 54 and 55 of transistors 56 and 57, forming the active elements, in discriminator 12. Terminal HH<SP>1</SP> connects through an isolation capacitor 58 with the emitter 59 of transistor 60 in dither generator 18. The inputs to discriminator 12 from transformer 17 are applied to collectors 61 and 62 of transistors 56 and 57 through coupling resistors 65 and 66 and the output of dither generator 18 through variable resistance 21, previously referred to, is applied across resistor 66 to put the output of the dither generator on the emitter of either transistor 56 or 57 even though the output of preamplifier 16 is applied to both emitters in a push-pull arrangement. A D.C. bias supply 67 is connected through milliammeter 68 and terminal EE<SP>1</SP> to a common point 69 between a pair of emitter bias resistors 70 and 71 connected to emitters 63 and 64. A collector bias supply 72 is connected through milliammeter 73 and terminal DD<SP>1</SP> to transistor 60 of dither generator 18 and also, through the primary 78 of transformer 17 to collector 76 of transistor 23 in preamplifier 16. In operation the control signal generated by control synchro 31 and passed through pre-amplifier 16 and discriminator 12 to operate servo 11, 11A has small amplitude oscillations applied thereto by virtue of the dither signal from generator 18 feeding to resistor 66, whereby minute oscillations are applied to motor 11 which enables it to overcome static friction in servo 11A.