Patent GB-574811-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: H03J7/30
Filed: 1943-06-10
Abstract: 574,811. Panoramic receivers; directive radio systems. WHITE, E. L. C., HARKER, M. G., and HOUCHIN, J. P. W. June 10, 1943, No. 9384. [Class 40 (v)] [Also in Group XXXVII] The effect of inertia in prolonging the motion of a body past a desired stopping point is compensated by a control system which provides electrical outputs proportional to the actual and desired stopping positions, the difference in the magnitude of these outputs being utilised to return the body to the desired position. The system is particularly applicable in radio signal searching sets of the kind in which a motor-driven tuner or steerable antenna continuously scans a band of frequencies or portion of the aether until a signal is received, when the drive is stopped. The control system comprises essentially a potentiometer 3 connected across a voltage source 4, the slider 5 of the potentiometer being moved by the body and connected to the lower side of the potentiometer through a condenser 6 and normally closed switch 7. When the body reaches a desired position, switch 7 is opened and there is then zero potential across its terminals 8, 9, but if the body moves beyond the desired position, a potential equal to the difference between the voltage across condenser 6 and that across the effective portion of the potentiometer appears across the switch and may be used for restoring the body to the desired position. Switch 7 is preferably of the electronic kind described in Specification 512,109, and Fig. 2 shows such a switch comprising valves 12, 13 and 20 arranged to control a motor 11 driving a tuning condenser 10 of a signal searching receiver. The armature of the motor is in series with a swamping resistance 24 and its field 25 comprises two parts tending to drive the motor in opposite directions and fed through cathode coupled valves 26, 27. The latter valves are so biassed that the motor tends to run at a fixed speed, determined by the setting of a potentiometer 18 connected in the anode circuit of valve 20, in one direction. A potentiometer 30, controlling the bias on valve 12, provides for zero speed adjustment of the motor when valves 12, 13 and 20 are biassed to cut-off by the application thereto of a negative grid-pulse derived from a multi-vibrator triggered by a received signal. When the valves of the switch are cut-off, the bias for the valve 26 provided by the voltage drop across potentiometer 18 and resistance 19 is cut-off and replaced by the difference in potential appearing between the condenser 6 and the lower part of potentiometer 3, which is also driven by the motor. This potential difference returns the motor to the position at which the signal was received. A switch 33 may then be changed over to connect the grid of valve 26 to an automatic frequency control circuit 37 which holds the set in tune. Motor 11 also drives a generator 32 which provides a bias opposing that provided by potentiometer 18 and resistance 19, the bias provided by this generator being determined by the speed of the motor and falling to zero when movement of condenser 10 has stopped.