GB918790A - Improvements in detection and signalling systems - Google Patents

Abstract

918,790. Electric selective signalling systems. GENERAL PRECISION Inc. April 20, 1960 [April 23, 1959], No. 13879/60. Class 40 (1). A passive responder (i.e. a circuit having no power supplies of its own) that emits a response electrical signal upon being influenced by an interrogating electrical signal radiated by an interrogator comprises a number of tuned circuits each resonant at a different frequency and operative to determine a selection of frequencies to be present in the responder signal and at least one of the tuned circuits has one or more environmental condition-sensing (e.g. temperature, humidity, &c.) electrical elements in circuit therewith, each of which elements acts to render operative or to disable the associated tuned circuit according to the value of the condition being sensed by it whereby evidence of that value is provided by the presence or absence in the responder signal of the frequency corresponding to that particular tuned circuit. In the first embodiment described, Fig. 2, a central office 30 transmits a spectrum of frequencies to each of a number of responders, of which one is shown. The spectrum of frequencies is received by each responder in a circuit 38 and passed via tuned circuits 48, 50, 52 to modulate an R.F. oscillator T1, and retransmitted back to the central office, the carrier current of the received spectrum being rectified 40 to provide D.C. power. However, because of the tuned circuits 48, 50, 52, some frequencies of the spectrum are inhibited and are not retransmitted. These represent firstly the identity of the particular responder and secondly whether or not any one of a number of switches 22 has gone open circuit by virtue of the particular variable being sensed having exceeded a certain level. In a second embodiment (Fig. 5, not shown), only one frequency is transmitted by the central office and each responder includes a number of oscillators tuned to different frequencies. In a further embodiment the responders transmit their various frequencies successively. In the preferred embodiments, the condition being sensed is the temperature of axle bearings on railway vehicles and the Specification contains a list of alloys which fuse at fairly low temperatures, the appropriate alloy being chosen so that it fuses before the bearing reaches a dangerous temperature. Specification 917,755 is referred to.