GB1393177A - Position detector - Google Patents

Abstract

1393177 Photo-electric position detector WESTINGHOUSE ELECTRIC CORP 17 April 1972 [19 May 1971] 17617/72 Heading G1A Apparatus for determining when two objects are in a desired relative position one with respect to another e.g. for use in controlling the operations of a lift before it reaches a particular floor, comprises a pulsed radiation transmitter and a radiation detector which is located so as to detect radiation from the transmitter when the desired position is attained and thereby operate a relay via a transformer coupling circuit. In the lift control application, the transmitter and detector 2, 4 are arranged in angled relationship on support 21 attached to the lift car. At each floor is provided a vane 22 carrying a reflective strip 23 of e.g. chromium plated plastics or metal, polished copper, or gold or silver polyester tape. A shield 30 stops stray light. The transmitter and receiver circuits are detailed in Fig. 4. The transmitter is a light-emitting diode LED emitting within the range 2-10,000Š and in the embodiment described is infrared emissive and can be pulsed in the Megahertz range. In the embodiment, a KH z square wave generator is provided based on a programmable uni-junction transistor PU coupled to a J-K flip-flop F-F. Zener diode ZD1 provides the 30v supply. The flip-flop output 33 is shaped by Zd2 and resistor R5, and amplified by emitter follower transistors Q1, Q2. The receiver comprises a photo-transistor PT associated with an operational amplifier, whose input A is biased to be slightly negative with respect to input B. Incidence of infrared radiation however causes input A to go positive with respect to B producing an output from DA. When this exceeds ZD5 potential, which occurs when transmitter pulses are being received, Q3 is turned on. The output from Q3 is passed by transmission line 9 to the control device, a line driver Q4 being provided, with Zener diode ZD6 to cut out spurious pulses picked up in the line. When Q3 is on, Q4 is turned off, capacitor C2 discharges, Q5 is turned off and Q6 on. Pulses are then transmitted by transformer Tl to the relay circuit. Transformer T is constructed so as to only transmit signals in the kilohertz range. In an alternative arrangement, the transmitter may be on one object and the receiver on the other. Instead of a device which reflects radiation back to the receiver, an arrangement in which a member blocks radiation reaching the receiver may be used.