Patent GB-1249982-A (GB)
Title: Improvements in or relating to automatic control systems
Inventor: HODGKINS JOHN ALFRED
IPC: G05D1/00
Filed: 1967-10-10
Abstract: 1,249,982. Automatic control. SMITHS INDUSTRIES Ltd. Jan.10, 1969 [Oct. 10, 1967], No.46337/67. Heading G3R. A multiplex, e.g. triplex, automatic control for a pair of aircraft elevator control surfaces 1, comprises the three identical sub-channels A, B, and C providing nominally-equivalent drives through common gearing 18 to the surfaces 1, each subchannel including an equalizing arrangement to balance out acceptable differences between the subchannels below a predetermined threshold level, whilst any sustained difference above the threshold level results in the transmission of excessive torque in the incorrect sub-channel which opens a torque switch 16 in the control circuit of an electromagnetic clutch 15 through which the respective sub-channel drives, to disengage the clutch and hence disconnect the drive. Momentary signals above the threshold level do not disengage the clutch, to avoid excessive engagement and disengagement. The torque switch may be of the lost-motion type, or overload release type, of Specification 1, 249, 981 or 1, 249, 983 respectively, in which take-up of the lost-motion, or sustainedoverload release, are effective to open a switch. Each sub-channel comprises a flight instrument set 4 and pitch demand computer 3 controlling through the equalizing arrangement a servomotor 11 driving a tachometer 13 and the clutch 15 to the common gearing 18. The pitch demand signal derived by the computer 3 from the signals supplied by the flight instrument set 4, e.g. for automatic landing, are supplied in the sub-channel to a signal combining circuit 210 where they are opposed by a signal from an equalizer 211 which is dependent on actual rate of pitch sensed by a rate gyro 5. The output signal from the circuit 210 is thus dependent on the difference between the demanded and actual rates of pitch, and is supplied through a further combining circuit 213 and a servo-amplifier 214 to the two phase electric servomotor 11, the signal derived from tacho 13 being fed to circuit 213 as degenerative rate-feedback. The servo-amplifier signal is fed to a pair of comparator-limiter units 219, 220 which compare the signal with corresponding signals from sub-channels B and C respectively, as in the application No.44071/68, feeding difference signals to an adder circuit 223 which provides the basic equilization signal of the subchannel A. Again, a single unit only may be provided, as in Specification 1, 003, 567, supplying a basic signal dependent on the difference from only one of the sub-channels B, C. Short-term datum equalization is provided by feeding the signal from 223 directly to signal combining circuit 213. Long-term datum equalization is provided by feeding the signal from 223 via an integrator 224 which also receives a signal direct from the computer 3, so that its output fed to circuit 213 is a time integral of the two signals and provides additionally long-term monitoring by obriating datum drift. Gain equalization is provided by feeding the signal from 223 to a gain equalizer 225 via a multiplier 226. The equalizer 225 acts to regulate the degree of degenerative rate-feedback from the tacho, and therefore the overall gain of the servo, in accordance with the equalization signal. Gain equalization may also be effected by connecting equalizer 225 in the rate-feedback path between tacho 13 and the circuit 213. Alternatively, the equalizer 225 may be connected in the manner of a variable gain pre amplifier to amplify the output signals from 210, 223 and 225 prior to combination in 213 with the degenerative rate-feedback signal from the tacho 13. Input-gain equalization is provided by feeding the signal from 223 to the equalizer 211 via amultiplier circuit 228. The equalizer 211 acts to provide variable gain between the signal from rate gyro 5 and circuit 210, the gain variation depending on the magnitude and sense of the signal from circuit 228. Alternatively, or additionally, input equalization may be effected by reguation of the signal from computer 3 by connecting an equalizer 211 between the computer 3 and circuit 210, with an associated sign-sensor 229. Contacts 216, 217 in the electromagnetic clutch control circuit remain closed only while the motor 11 is energized, and are controlled respectively by a relay 215 in series with the motor 11, and an injector-detector test unit 218 connected across the servo-amplifier 214 in the motor feed circuit.