Patent GB-1435201-A (GB)
Title: Aircraft automatic flight control system
IPC: G05D1/00
Filed: 1973-06-27
Abstract: 1435201 Automatic control of aircraft SPERRY RAND CORP 27 June 1973 [14 July 1972] 30632/73 Heading G3R In an automatic system for controlling the attitude and flight path of an aircraft, under the control of a first switch, and using clutches between respective servomotors and control surface mechanisms, the clutches may be disengaged by operation of a second switch (on the control wheel) so that the surfaces may be controlled by the pilot, the second switch also operating "logic means" whereby the automatic system tracks the motions established by the pilot so that upon resumption of auto the system takes over from the new settings. The Specification describes and illustrates a complete autopilot system having four electric servomotors which respectively adjust the position of ailerons, rudder, elevators and elevator trim tubs in response to signals from vertical and directional gyros, from a manually settable turn potentiometer and a manually rotatable "change pitch" generator, and selectively from a "flight director computer" which may be set to maintain heading, altitude or glide slope, e.g. using a radio beam, and air speed by adjustment of the pitch. Each servomotor has position and rate feedback. When declutched from their associated control surfaces the servos are disconnected from all the demand signal sources except the gyros so that they track the manually made movements of the aircraft. Switching to auto is thus achieved without shock and unless the flight director is engaged the autopilot maintains the aircraft on the manually set attitude, except that if the aircraft is banked at less than 6 degrees when auto is engaged it will automatically be levelled. While a change of heading may be made simply by operation of the turn potentiometer while in auto, if this adjustment is made while flying in manual the actual bank angle is stored in an integrator and is therefore held upon switching to auto; however, this is compared with the bank angle demanded by the computer for the new course and when they become equal logic circuits operate to level the aircraft on the new course. Short term damping is provided by rate circuits and filter networks. The roll signal is cross-fed via a differentiator to the rudder servo for co-ordination in turns and also to the elevators for lift compensation. The system provides for automatic capture of a glide slope radio beam; in this case a 3 degree pitch down bias signal is stored on a capacitor. Roll rate and acceleration limiting. The bank angle demand signal is rate limited, thereby limiting the displacement of the ailerons and hence the rate of roll of the aircraft. In addition this rate limited signal is applied to a lag filter network comprising an integrator and having a time constant of about 1 sec. and this limits the rate of change of the ailerons and hence the acceleration of the aircraft in roll. The overall effect is to provide smooth entry into and exit from turns. A second integrator provides long term roll trim and the gain of the circuit containing this is automatically changed in the manual mode to ensure rapid tracking. Servomotor control. Fig. 3, (not shown). Each servomotor is a D.C. machine energized by pulses (from a switching transistor bridge) whose width normally depends upon the difference between a demand signal and a positional feedback signal from a synchro, the difference being combined with rate feedback from a tachogenerator and with the voltage across the motor (alternately the supply and the back e.m.f.). This is described as voltage control with back e.m.f. damping. The average motor current is also measured, and any amount in excess of a pre-set reference is fed back to provide a current (and hence torque) limiting mode. If the current of any servo exceeds a further reference the auto-pilot is disconnected.