Patent GB-917755-A (GB)
Title: Improvements in or relating to signalling systems
IPC: B61L25/04
Filed: 1959-02-04
Abstract: 917,755. Transistor circuits; induction signalling. GENERAL PRECISION Inc. Feb. 4, 1959 [Feb. 18, 1958; June 4, 1958; July 10, 1958], No. 3963/59. Classes 40 (5) and 40 (6). [Also in Group XXX] In a signalling system comprising an interrogator equipment and a plurality of response equipments, each relatively movable with respect to the interrogator equipment, and wherein a response signal is returned by a response equipment to the interrogator equipment whenever the two equipments move into close proximity, each response equipment includes a plurality of tuned circuits whereby the same interrogator signal gives rise to different response signals from the different tuned circuits in order to identify the response equipment. The outputs of oscillators 1-10 of the interrogator, Fig. 2, are cyclically applied to a radiating coil 210 under the control of a flip-flop 202, driven by 400 cycle/sec, oscillator 201, which steps a counter 227 and opens gate 221 the levels from oscillators 1-10 being individually adjusted by attenuators P1-P10. Received frequencies corresponding with the tuning of circuits 100-102 Fig. 1A, produce during the appropriate time slots a direct voltage which energizes transistor oscillator 114, and the oscillations, when picked up by response coil 218, are applied through AND gate 242 responsive also to a trigger circuit 247, to set a register 244, AND gates G1-G10 of which are also cyclically energized by the counter output. Automatic gain control using a pilot oscillator 205 is provided to produce a level oscillator energizing voltage in the responder whilst moving during interrogation the AGC signal being pulsed alternately with the code oscillator and the returned signal is compared with a reference voltage in a differential amplifier 225 to prevent false operation. Alternate 2500 microsecond bursts of AGC oscillation with 5000 microsecond bursts of code oscillation are transmitted at a repetition rate of 20 c.p.s. The register 244 is reset by a pulse from a monostable multivibrator 250, each time trigger circuit 247 is fired, by the presence of a responder the trigger circuit producing an output pulse of a time interval long enough for registering the responder's identity, e.g. 100,000 microseconds, or until the responder signal strength falls below a requisite minimum. Alternative responder tuned circuit arrangements including series and series parallel arrangements of parallel resonance circuits are described with reference to Figs. 1B and 1C (not shown), and an alternative arrangement in which the oscillator is powered by radiations of optimum frequency for maximum power, and keyed by digit frequencies is described with reference to Fig. 1D (not shown). Series resonant circuits may also be used. The oscillators 1-10 may be replaced by a sweep frequency generator or a stepped oscillator, and provision is made for adjusting the transmission levels at the various frequencies. Radio transmission of registered identities is described with reference to Fig. 6 (not shown), and a potted responder in which the coils of the tuned circuits are wound around ferrite rods and inserted with an epoxy resin compound in a plastic container is described with reference to Fig. 5 (not shown), the ferrite rods being arranged at the corners of an equilateral triangle surrounding the oscillator coil which may be torous shaped for isolation purposes. The invention may be applied to the identification of engines and wagons in a railway yard, the responder being attached when the wagon enters the yard, or of vehicles such as motor buses on a road, aircraft on an airstrip, or of articles such as mailbags on a conveyer or shoot for automatic handling purposes. A responder may be attached to vehicle chassis and information related to movement may be gathered and stored. The response signal may be used for wagon control. The interrogator radiating and pick-up coils may be at a considerable distance from the remainder of the unit, and a number of such coils may be connected via gates to a single interrogator to monitor traffic at junctions. In an alternative embodiment described with reference to Fig. 8 (not shown), a plurality of frequencies are transmitted simultaneously on a carrier and a number of them are selected by a coding network in the responder for return by carrier to the interrogator. The returned frequencies are set up in a register under the control of a multivibrator and the identification is retransmitted possibly with an interrogator identification signal, over a data link system to a remote centre. Suitable responders for suppressing desired ones of the received signals are described with reference to Figs. 9A 9C, 9E and 9G. In a modification of the latter system described with reference to Fig. 10 (not shown), a single frequency transmitted by the interrogator energizes a plurality of oscillators of different frequency, each with a separate radiating coil. A suitable responder comprising three transistor oscillators is described with reference to Fig. 11 (not shown). Examples of suitable frequencies are included. Trigger circuit 247 details of which are shown in Fig. 4 comprises a Schmidt type trigger pair X601, X602, the output of which is applied via a condenser C602 to the junction of a diode D603 with a PNPN diode X603, a rise in voltage of point 607 above the critical value of diode X603 resulting from operation of the trigger causing it to fire and reduce the potential on output terminal 609 substantially to zero. The potential at terminal 606 decays, as capacitor 603 discharges, (in a manner which is linearized by connection R608), until transistor 604 is no longer bottomed, whereupon diode X603 is extinguished and the original potential at the output terminal is rapidly restored by charging C603 through D602. A drop in the input voltage to terminal 246 before the diode has extinguished causes the trigger pair to apply a negative pulse over condenser C601 to the base of transistor X604 whereby it is cut-off, to extinguish the diode. Working frequencies are suggested.