GB907551A - Improvements in or relating to proximity switches - Google Patents

Abstract

907,551. Locating conductive bodies. BENNIE LIFTS Ltd. Dec. 15, 1958 [Sept. 13, 1957], No. 28856/57. Class 37. A proximity switch comprises an inductor connected in an oscillator circuit so as to prevent oscillation occurring until a conductive body is brought near the inductor when the change in self-inductance permits oscillation. As shown, the anode and grid circuits of a valve Via are coupled by means of a transformer T2 connected so as to avoid phase reversal at normal frequencies. The leakage inductance and interwinding capacitance of transformer T2 are such that the circuit tends to oscillate at a frequency of 1 Mc/s. Oscillation is however prevented by the presence in the cathode circuit of a circuit formed by a ferrite-cored inductor 20 and a variable capacitor C5. When the circuit is not oscillating the anode current is sufficient to energize relay 11. The approach of a conducting body such as a copper or aluminium plate 30 towards the pole pieces of the inductor 20 reduces the impedance of the cathode circuit sufficiently to allow the circuit to oscillate. The grid circuit has a long time constant provided by components R2, C2 so that the oscillation is in the form of short bursts separated by intervals 100 times the duration of each burst. This ensures that the anode current is sufficient low to cause relay 11 to be de-energized when the circuit is oscillating. Reference is made in the Provisional Specification to a blocking oscillator in which the transformer windings are connected normally. Fig. 2 (not shown) relates to the assembly of two circuits of the type shown in Fig. 1 in a metal container having one insulating wall through which the pole pieces of inductor 20 protrude. The container is mounted in a lift car and a series of aluminium plates 30 are mounted adjacent each floor level in the lift shaft.