Patent GB-688114-A (GB)
Title: Improvements in or relating to servo control systems
Inventor: WATSON CYRIL DANIEL
IPC: B64C13/00
Filed: 1949-02-02
Abstract: 688,114. Electro-hydraulic aircraft controls. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 1, 1950 [Feb. 2, 1949], No. 2891/49. Class 40 (i) [Also in Group XXXIII] An aircraft control transmission comprises devices e.g. potentiometers associated with the controlling member and the control surface, which devices by their relative adjustment generate electrical signals for controlling a servomotor with follow-up control, and additional devices for automatically modifying the effect of such signals in accordance with variations in the flight condition of the aircraft. The purpose of the invention in the case of a piloted aircraft is to provide the pilot with "feel" and to limit automatically the extent and rate of movement of the control surfaces to safe values. The invention is described as applied to elevators, the control transmission being of the electro-hydraulic type. General arrangement:-The transmission between the pilot's control column 1 and the elevator 2 includes a rigid rod 10 and a hydraulic servomotor 58, a compression spring 3 being mounted between the member 10 and the valve housing 15 of the servomotor, the spring providing the necessary lost motion to permit followup of the valve and imparting feel to the column 1. The servomotor valve 16 is actuated by an electric motor 170 through an impulse-operated power transmission as described in Specification 619,986, [Group XXXIII]. The column 1 and elevator 2 are arranged to move pairs of brushes 24, 25 and 26, 27 over re-entrant potentiometers 28, 29 connected in parallel at a number of points to form a bridge system. A pivoted armature 19 carried by the member 10 is connected by a link 18 to the hydraulic valve-operating mechanism and is operated by pairs of coils 20, 21 and 22, 23 connected to polarized relays 33, 41. The circuits.also include a "phasing potentiometer" 48, 49 mechanically associated with the valve and housing 15, 16, a "speed resistance" 30, 31, "tailload relay" 46, and an "acceleration (or 'g') potentiometer" 55, 57, the last three members being operated automatically under certain flight conditions. The velocity ratios of the mechanical connections between the control column and the elevator; and between the brushes of the stick and elevator potentiometers 28, 29 are unequal. Normal operation of controls.-Upon moving the control column 1 to the right the valve housing 15 and valve 16 move in the same direction, but to different extents owing to the compression of the feel spring 3. thus admitting pressure liquid to the jack 58 to move the elevator. The difference in movement of the elevator with respect to the control column due to the lost motion consequent upon compression of the spring 3 results in a variation of the relative settings of the potentiometers 28, 29 to give a "first error" signal which is transmitted to the relay 33 to close contacts 32, 35 and energize coil 23. The link 18 is moved to the right by the armature 19 thus increasing the displacement of the valve 16 and the supply of liquid to the jack 58. Displacement of the valve 16 also moves the contact 48 over the resistance 49 of the phasing potentiometer, whereby a "second error" signal is transmitted to the relay 41, the contact 47 of which moves to close the circuit of coil 21, which tends to return the armature 19 to its neutral position in opposition to the coil 23. Movement of the elevator 2 and housing 15 result in a reduction of. the first error signal and of the current in the coil 23 so that the supply of pressure liquid is progressively reduced. Also, the value of the second error signal, the chief function of which is to damp any tendency of the elevator to overshoot, is reduced as the valve and housing return to the neutral position. Owing to the inequality of velocity ratios referred to above, the first error signal decreases at a greater rate than the second error signal, and the reattainment of zero voltage occurs when the elevator has moved through a smaller angle than the control column and there is still a second error signal. Liouid continues to be supplied to the jack 58 and the elevator continues to move, with the result that a negative first error signal is generated, closing contacts 32, 34 of relay 33. Coil 20 is thus energized and the link 18 moved to the left, to reduce the relative displacement of the valve 16 and its housing, and thus reduce the supply of liquid to the jack 58. The elevator does not completely follow-up the control column, and comes to rest in such a position that while there is no second error signal there is a negative first error signal causing displacement of the link 18 and compression of the spring 3, which is transmitted as feel to the control column. Effect of tail loading.-The tail load relay 46 is energized by the automatic closing of a 'switch in the event of excessive loads being imposed on the tail surfaces, with the result that the circuit through the brush 55 of the 'g' potentiometer 57 is broken, an alternative circuit being made a second contact 56. A large current is thus applied to the relay 41, and the armature 19 operated to move the valve 16 in such a direction as to supply liquid to the jack 58 to centralize the elevator. In addition the spring 3 is further compressed to give the pilot a stronger feel reaction. Effect of speed.-The speed resistance 30 operates to give the pilot feel proportional to the speed of the aircraft, the brush 31 being adjustable by a pitot or other speed-sensitive device, to 'vary the value of the resistance in circuit and thus alter the value of the first error signal. The effect of higher speed is to reduce the displacement of the elevator and increase the pilot's feel, the resistance 30 being effective, however, only when an error signal exists due to movement of the control column from neutral. Effect of acceleration.-The brush 55 of the 'g' potentiometer 57 is moved by a device near the centre of gravity of the aircraft and sensitive to accelerations normal to the pitching axis and flight path, the device operating when a predetermined acceleration is exceeded. Movement of the brush 55 varies the resistance arms of the bridge formed by the potentiometers 49, 57, to modify the second error signal or generate an equivalent signal if none exists, with the result that the pilot's feel is increased and the elevator moved in the appropriate direction to reduce the acceleration. Effect of excessively rapid movement of control column.-In the event of excessive relative displacement of the member 10 and housing 15 contacts 59, 60 are opened by an abutment 61 to de-energize a cut-out relay 40 and solenoid 64, the effect of which is to lock the link 18 to the member 10 and thus to the control column 1, and to place the valve 16 in direct mechanical connection with the control column in the manner described in Specification 656,712, [Group XXIX]. In addition, "over-ride" springs 167,168, Fig. 2 (not shown) are brought into operation to augment the effect of the feel spring 3. Normal conditions may be restored by operating a resetting button 67 to re-energize the relay 40. A lamp 68 indicates when the relay is cut out. Alternative arrangements.-In one embodiment, Fig. 3 (not shown) the relay 41 is dispensed with, the two corresponding relay coils being connected in series with each other and with the brushes of the potentiometers 49, 57, while in a further arrangement Fig. 4 (not shown) a number of relays of differing sensitivities to control column movement are provided, in order that the valve 16 may be opened or closed in steps of predetermined magnitude.