Patent GB-1119936-A (GB)
Title: Improvements in protection systems and apparatus
Inventor: EMBLING KENNETH HARRY
IPC: H03B5/34
Filed: 1966-07-01
Abstract: 1,119,936. Proximity detectors. ELECTRONIC MACHINE CONTROL (SALES) Ltd. June 29, 1967 [July 1, 1966], No. 29614/66. Heading G1N. Apparatus for preventing operation of a machine if the operator's hand is within an apertured guard frame comprises an oscillatory circuit into which the frame is connected as a varable capacitor and which is arranged to stop oscillating if the operator's hand is introduced into the frame. The output from the oscillator is used after amplification and rectification to energize a solenoid, the machine being operable only when the solenoid is energized. The oscillatory circuit is combined with a fail-safe device connected to its output by which a capacitance equivalent to that introduced into the circuit by the operator's hand is intermittently connected into the oscillatory circuit to stop it oscillating. A further circuit, sensitive to deviations in the output from the oscillatory circuit from a preset form resulting from this intermittent operation, e. g. deviations produced by drift of the oscillatory circuit from the required sensitivity, prevents the output from the oscillatory circuit being fed to the amplifier and therefore causes the deenergization of the solenoid and stopping of the machine. The output from the oscillatory circuit comprising the guard frame GF, Fig. 2, and valve V1a is connected to a switching valve V1b which is arranged to be conducting when the circuit is oscillating. Under normal conditions, with the switching valve V1b conducting, relay RL3 is energized, closing relay contact RL3/1 thereby connecting capacitor CSC into the oscillatory circuit. This capacitor is adjusted to be equivalent to that introduced into the circuit by the presence of the operator's hand in the guard frame, and which therefore inhibits oscillations of the circuit. The switching valve V1b then becomes non- conducting and the relay is de-energized resulting in the restarting of oscillations and subsequent energization of the relay. This checking cycle may be repeated at a frequency of 150-200 c/s, dependant on the delays of the relay. Alternatively it may be carried out once per machine cycle. The output from the switching valve having a frequency equal to that of the checking cycle is applied both to a trip amplifier constituted by transistors TS1-TS4 and, via relay contact RL2/2, to a main amplifier constituted by transistors TS5-TS8. Should the oscillatory circuit drift to an insensitive condition the resulting reduction in the amplitude of oscillation of the oscillatory circuit will result in a decrease in frequency of energization of the relay RL3 and therefore in the average amplitude of the input to the amplifier. This causes a trip relay RL2 to be de-energized and switches RL2/1 and RL2/2 to be opened, disconnecting the oxcillator from the main amplifier. The circuit is re-energized by operation of a reset switch through which the relay is re-energized and the oscillator reconnected to the main amplifier. The output of the main amplifier is fed via a rectifying bridge D6-D9 to output terminals SOL across which the solenoid is connected. Alternatively the machine may be controlled from a relay RL1 connected across the output from the rectifying bridge.