Patent GB-1239936-A (GB)
Title: Aircraft stability and control augmentation system
IPC: G05D1/08
Filed: 1968-07-18
Abstract: 1,239,936. Controlling aircraft. TEXTRON Inc. 18 [July, 1968 18 July, 1967], No. 34360/68. Heading B7G. A control system for aircraft comprises a control loop 4, a stability feedback loop 6 and a pilot controlled loop 8, interconnected at an amplifier 28 in such a way that stability feedback loop gain is maximized whilst avoiding domination of the aircraft by stability feedback. An input to amplifier 28, for example input 26, causes an electro-hydraulic actuator 16 to operate a control system 44 which acts on the attitude controlling means of the aircraft, such as the main rotor lateral or longitudinal control systems or the tail rotor control system in a helicopter. The actuator 16 also operates a feedback transducer 50 through a mechanical link 42, so that the transducer 50 generates an input signal 54 for amplifier 28, the actuator continuing to operate until signals 26 and 54 cancel one another. The actuator also receives a direct mechanical input from the pilot's control stick 10, this input being combined with an electrical input via amplifier 28 derived from control stick 10 via a transducer 32 mechanically connected to the control stick 10 and electrically connected via a compensating network 38 to the amplifier 28. The pilot's control stick movement causes movement of actuator 16 until signal 54 cancels the electrical signal from network 38 at the amplifier 28. The input 26 is part of a stability feedback loop 6, in which loop any disturbances of attitude of airframe 48 caused by gusts of wind and the like are detected by a rate gyro 20 and fed to the control system 44, for cancellation of the disturbances, via compensating network 24, amplifier 28 and actuator 16. The two compensating networks 38 and 24 are used to limit the frequency range of the input signals to the amplifier 28, the upper frequencies being removed by a lag network, effectively a series resistor and capacitor connected between signal and earth, the lower frequencies being removed by a washout, i.e. high pass network, effectively a series resistor and capacitor in the signal line. The lag network prevents sudden disturbances such as weapon recoil from being abruptly corrected, while the washout network renders the stability feedback loop sensitive to rate of change of attitude rather than to a steady attitude so that the pilot's control settings for attitude are not automatically cancelled. In the preferred embodiment the compensating networks 24 and 28 are one and the same, thereby ensuring separation of the pilot's and the stability controls because the input to the amplifier from the rate gyro in response to a pilot's control setting is cancelled by an input from the pilot's control loop 8. In the preferred embodiment of Fig. 10, the pilot's electrical input is derived from a potenibmeter 115 whose slider 114 is mechanically connected to control stick 10; the rate gyro 20 feeds the primary 120 of an input transformer; the transducer 50 is connected via resistors to input 131 of amplifier 132; a balance potentiometer 117 is a fourth input to amplifier 132. The inputs from the gyro transformer 20 and the pilot's input potentiometer 115 are connected as shown to the centre tap 135 of an input transformer 137 of bi-lateral gate 136; the primary of this transformer is shunted by a diode 128, one terminal of the primary being connected to a D.C. source at 140, the other to the collector of transistor 178, which is the output of a pulse generator which is actuated by a 115 volt 400 cycle per second source, transistor 178 giving negative going pulses of 0.0002 seconds duration at a frequency of 400 cycles per second. The bilateral gate is thus opened for a period of 0.0002 seconds by each negative going pulse so that the pilot's input and the gyro input are sampled periodically and stored on capacitor 151, which capacitor controls the lag of theabove mentioned common compensating network. The washout network comprises capacitor 154 and resistor 155, and has a time constant of some 2-3 seconds. The compensated output from transistor 152, and the signals from transducer 50 and balance potentiometer 117 are fed to the input of the three stages comprising transistors 191, 192, 193, the balance potentiometer being set such that when no input signals from controls 10, 20 or 50 are applied the potential from the balance potentiometer cancels the D.C. bias supplied at 165 to the input of amplifier 132. The pulse generator 98 supplies positive going pulses of duration 0.0002 seconds from the collector of transistor 174 via line 176 and diode 177 to the input 131 of amplifier 132, the amplifier 132 thus operating only during 0.0002 second periods during each half cycle of the 400 cycle per second signature at the gate. The emitter of transistor 193 feeds a Darlington pair of transistors 199, 200 through bilateral gate 196, the emitter signal of transistor 193 being sampled and stored on capacitor 197 ; the emitter of transistor 200 is connected via resistor 201 to a torque winding 112 of a motor in the actuator 16. Negative gating pulses similar to those at gate 136 are supplied to gate 196 via a conductor 204 from the collector of transistor 178. Thus during each negative going pulse from transistor 178 a voltage indicative of the gyro 20 or control 115 inputs is stored on capacitor 197 and is compared during each positive pulse at input 131 with a voltage indicative of the signal from transducer 50. The resultant voltage controls the actuator via the Darlington pair of transistors 199, 200. Separate lag and washout networks may be used if desired, though the use of common networks eliminates problems of different phase and amplitude responses.