Patent GB-629984-A (GB)
Title: Improvements relating to remote control systems
IPC: B61L7/08
Filed: 1947-07-30
Abstract: 629,984. Electric selective signalling. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. July 30,1947, No. 20657. Convention date, Jan. 16, 1940. [Class 40 (i)] [Also in Group XXXIX] In a remote control system comprising a number of field stations selectively controlled over a common pair of line wires from a central office, wherein the field stations are adapted to signal back selectively to the office, the code transmitter at a field station functions repeatedly to shunt a low resistance across the line wires in order to increase the line current periodically in accordance with the station code, and a line relay at the office, responsive to the line current increases, governs a code receiver selectively responsive to the station code. Transmission from the office is effected by intermittently disconnecting battery from the line. In this way an uninterrupted metallic circuit is maintained at all times over the line so that it can be used for other purposes, e.g. telephony and telegraphy, or for a further code system using different codes, e.g. a telephone selector system. As shown, the invention is applied to the centralized traffic control system described in Specification 415,787, wherein selective control and supervision of a number of series connected field stations is effected over a pair of line wires Y, Z by coded time-spaced D.C. impulses. The line Y, Z is normally energized from battery 80 over contacts of the transmitter relay OT and relays 86, 87, a low-pass filter 9, 10 and the line relays ORC, ORI which are of the biassed polar type and normally held in their deenergized positions, shown. As in Specification 415,787 initiation of an office control code energizes the master control relay OM, which transfers the control of the office coding unit-from relay ORI to ORC, and causes relay ORC to pick up. The transmitter relay OT then operates to deliver a code of impulses and intervals of selected lengths under the control of timing relays OL1, OL2, OLB, OLBP to the line to which the office line relay ORC and all station line relays RC respond. The office coding apparatus responds selectively to operate a selector relay, e.g. 234S at the office, and the selector relay S at the selected station in accordance with a first portion of the code, after which other relays at the selected station controlled by relay S are actuated in accordance with the positions of control levers associated with the office selector relay 234S. During transmission from the office the line circuit YZ is maintained intact by the shunting resistances 54, 55. Field station transmission. This is initiated as in Specification 415,787 by energization of the station start relay ST which picks up relays M and T in turn. In Specification 415,787 the line circuit is broken when relay T picks up to initiate the code to be transmitted. According to the present invention, however, continuity of the line is maintained during operation of the station transmitter relay T by connecting the line wires Y, Z together through the windings of relays RI, RC in the manner of a Wheatstone bridge comprising two low-pass filter branches shunting the lines YZ, the centre or balance points of the branches being connected respectively to the line Y1, Z1 leading to the remoter stations. The current in the remoter line portion Y1, Z1 . . .Y3, Z3 is thereby re-. duced to zero and the current in line YZ increased. The office line relay ORC responds to the increased current and operates a polarised relay PC of the magnetic-stick-type, which reverses the polarity of the line current at contacts 7, 15 and removes the office transmitter relay OT from control of the line. At the transmitting station the line current reversal causes relay R1 to pick up and RC to release, whereupon relay MP energizes and locks. The station coding apparatus does not respond to the initial energization of the transmitting relay T but only when relay RC releases. This gives the station transmitting, precedence over a C.T.C. signal initiated at the office only if it succeeds in reversing the office relay PC. The station indication code is now sent by transmitting relay T periodically increasing.the line current as above described, line relays ORI, R1 at the office and station, respectively, responding to each pick-up of relay T, i.e. the odd numbered code steps. On the final step of the indication code office relay 016 and station relay 16 energize to revert the office and station apparatus to normal. When indication codes are initiated by two or more stations at the same time the station nearest the office retains control of the line as heretofore. The relay T at the nearest station remains picked up on its first energization for a relatively long time interval so that the relays RC at the remoter stations receive a long '' line open " code step, the same as when an office control code is being received whereby the relay E at each of these remoter stations is operated to disable the associated code-transmitter. Telephone selector transmitting apparatus. The selector stations C of known kind are included in the line circuit in turn with the field stations, and are selected at the office by code wheels 89 one for each selector station. A code wheel when wound up and released makes one revolution during which a connecting relay 86 is energized to permit the code transmitter relay 87 to feed D.C. impulses of reverse polarity to the line under the control of contact 97 in accordance with the code-wheel teeth arrangement. A code wheel operation is effective only if the line is available; i.e. if the office relays OM, OL1, OL2 are de-energized to permit energization of relay 86, and during operation preclude initiation of a C.T.C. code. As the line impulses are of reverse polarity the C.T.C. station line relays RC move to their deenergized positions and prevent the associated transmitters from functioning. The office relays ORC, ORI remain.in their de-energized positions so that the C.T.C. system is unresponsive to the selector codes. Each station selector 72 is energized through its condenser 74 by impulses of alternate polarity generated locally in response to the operation of the selector line relay SR which responds to reverse line current only. The selector station line relays are non-responsive to office control codes and respond to indication codes only from those C.T.C. stations more remote from the office. The selectors however are not effectively operated in such cases because of the different codes in the two systems. Telephony. Telephone apparatus 50 at the office is connected through a high-pass filter 51, the line YZ, byepass condensers 35, 52 bridging the line relays at the respective C.T.C. and selector stations, and a repeating coil 11 to the telephone apparatus at the distant end of the line. The low pass filter 9, 10 at the office prevents shunting of the telephone currents through the battery 80 and renders control and selector codes substantially inaudible in the telephones. The coils 31 prevent shunting of the telephone current when the line is bridged at a station and together with the condensers 35 render the indication codes inaudible. Telegraphy. The telephone lines are simplexed, the usual duplex apparatus as used for Morse or printer operation, comprising, at each end of the line, the pole-changing relay PC1, balanced line relay and artificial line AL, connected between earth and the mid-point of balanced resistances 54-55, 12-13. The telegraph currents D in the line wires YZ flow in opposite directions in the line relays and so have no effect upon the C.T.C. or selector systems and the operation of any C.T.C. station transmitter relay T to shunt the line YZ does not affect the telegraph currents. Specification 535,440 also is referred to.