Servo Means

1,195,875. Automatic control. SPERRY RAND CORP. Nov.24, 1967 [Dec.1, 1966], No.53468/67. Heading G3R. In a closed loop control system for positioning a movable member, signals produced by misalignment between a position pick-off and the movable member are converted to digital form, stored, and re-converted to analogue form for subtraction from the pick-off signals to eliminate the effect of the misalignment. As shown, the pitch attitude of an aircraft 14 is automatically stabilized by signals from a gyro-vertical (not shown) applied through an amplifier 31 to an electro-hydraulic actuator 32 controlling elevator 16. Follow-up signals dependent on elevator position are derived by applying A. C. signals from a pick-off 13, detecting the position of stabilator 11 relative to the aircraft, and a pickoff 17, detecting the position of elevator 16 relative to the stabilator, through amplifiers 22, 24 to the input of amplifier 31. To eliminate error due to incorrect positioning of the pick-offs, e. g. error caused by signals from pick-off 17 when elevator 16 is in streamline position relative to stabilator 11, the pitch control is disengaged and the output of amplifier 24 applied through a gate 25 to an amplifier 36 the output of which is periodically sampled by a switching transistor 38 and applied to a trigger circuit 37 producing numbers of pulses corresponding to the magnitude of the error above a predetermined minimum value. The phase of the pulses is determined in a comparator 39 so that they are stored separately, according to the sense of error, in a counter 42 the output of which is converted by A. C. analogue form at 43. The output of converter 43 is applied to the input of amplifier 24 in opposition to the pick-off signals so as to eliminate the error, such elimination being independent of variations of supply voltage. When pitch control is engaged, the output of amplifier 24 is still correct by virtue of the error correction stored at 42, and this output, besides being applied as follow-up to amplifier 31, is also applied through a low gain circuit 26 and gate 29 so that, if it is sufficiently large, it operates, through amplifier 36 and trigger 37, a servo 41 to trim stabilator 11 automatically. Phase comparator 39 is rendered inoperative under these conditions, and a resistance-capacity delay network at 40, or elsewhere, may prevent trimming in response to signals of short duration.