Patent GB-906716-A (GB)
Title: Traffic actuated control system
IPC: G08G1/081
Filed: 1958-09-29
Abstract: 906,716. Road-traffic signals. LABORATORY FOR ELECTRONICS Inc. Sept. 29, 1958 [Oct. 3, 1957], No. 31026/58. Class 118. A traffic control system comprises means at a master station for deriving two sinusoidalwave signals of slightly differing frequencies so that phase coincidence of the two signals occurs repeatedly at intervals corresponding to the period of a desired traffic-control cycle and a plurality of local traffic-control signals each having associated therewith vehicle-operated means which when actuated permit the next occurrence of a predetermined phase relationship between the two sinusoidal-wave signals to initiate a cycle of operation of the corresponding control signals. Thus the present system resembles that described in Specification 843,916; however, in the latter it goes regularly and repeatedly through its cycle without any control from vehicle-operated pads or the like, while in this case the cycle of operation is initiated either by the operation of a vehicle-operated pad or a pedestrian-operated push-button. Likewise, the length of the green interval on the secondary road, while being ultimately determined as to its maximum length by the coincidence of two 400 c/s. waveforms, is also controlled to some extent by the actuation of a pedestrian pushbutton or the vehicle pads. A central master station, Fig. 1 (not shown), generates two 400 c/s. sinusoidal waveforms differing slightly in frequency as determined by the rate at which a phase-changing rotary transformer is driven by a motor and these waveforms are transmitted over lines to a plurality of local controllers, one at each cross-road in the system to be controlled. Fig. 3 illustrates one form of a local controller; Fig. 4 illustrates another more complicated system but essentially performing the same functions. In both systems the coincidences in phase between the two 400 c/s. sinusoidal waveforms FR, FC are used to generate the signals at each of the local controllers to initiate certain of the operations taking place at these controllers. Although a " simultaneous " mode of operation in which all the local controllers operate in phase with the coincidence appearing at the master station is permissible, provision is also made to " offset " the controllers with respect to each other, i.e., to stagger their various cycles of operation with respect to each other to allow for the passage of vehicles from one intersection to another along the roadways. The Specification describes a number of such " offsets," viz. " inbound," " outbound " and " average," these being used at different times of day according to the directions in which there is a greater density of traffic flow. This " offset " requires that phase changes be effected at each local controller to one of the incoming sine-waves from the master station. This is effected in Fig. 3 by deltaconnected phase-changing potentiometers PR20 and PR21. The apparatus is shown in its rest position awaiting the commencement of the cycle of operations, with the wipers W21 to W25 on the first positions of a five-position stepping switch, the wiper W24 being that which operates the various coloured traffic lights. As shown, networks 33, 53 produce pulses when the phase-changed input waves thereto pass through the zero position going from negative to positive, these pulses being applied to a coincidence gate CG1 so that when the two phase-changed waves from the master station are in phase, the coincidence gate gives an output which is applied to relay 62. Relay 62, however, is normally open-circuited by contacts DR1 and BV1 and hence the output of the coincidence gate has no effect. If, however, previous to the coincidence, the vehicle-operated pad VD has been operated, relay DR will also have been operated, this itself holding by virtue of its contacts DR2 and its connection to positions 1, 3 and 5 of switch bank LS25, so that contacts DR1 will be closed and relay 62 is actuated upon a coincidence being applied to the gate CG1. This closes contact 64 energizing motor MM to step the wipers of the stepping switch on to position 2. This causes the green light on the main road to be changed to amber together with any other necessary operations on the minor road not illustrated in the Figure. The ensuing cycle of operations now includes a sequence of fixed time intervals determined by a neon-lamp oscillator. Condenser 74 charges through resistor 81 until neon 75 breaks down, actuating relay 76 which also operates the motor MM to switch the wipers of the stepping switch to position 3 after a predetermined interval determined by the time constant of the resistor 81 and condenser 74. The minor road now shows green and the wipers stay in this position for a length of time determined by resistor 82. They then step to position 4 in which resistor 83 controls the length of time for which the position is maintained, this position being effectively the same as position 3 since green is still shown in the minor road. However, in position 4, the holding circuit of relay DR is opened so that this relay is released, while at the same time relay BV is operated through the fourth contact position of the switch LS25 so that the circuit of relay 62 from the coincidence gate is closed and contacts BV2, connected between the junction of neon 75 and condenser 74 and earth, are also closed. Thus, condenser 74 commences charging in the normal way when this position is reached, but should the vehicle pads VD be closed at any time during this charging interval, relay DR will be operated to close its contacts DR3 and hence discharge the condenser 74, so that the interval determined by the neon tube oscillator will have to start all over again. Given a sufficiently regular actuation of the pad VD, this could continue indefinitely, but the maximum time for which it can continue is determined by further coincidences between the incoming reference and the control frequency signals suitably changed in phase. In this case, phase changers PR20 and PR22 are used. When a coincidence occurs between their outputs applied respectively to networks 33 and 53, the coincidence gate CG1 operates the relay 62 to step the motor magnet on and cause the wipers to move to position 5. Position 5 is amber on the minor road moving from green to red. Again this position is held for a time which is determined by the neon timing circuit, after which the wipers return to position 1 and await a fresh coincidence preceded by actuation of the vehicle pads VD. The stepping switches are shown as occupying only a portion of a complete circle; in practice the five contacts would either make up a complete revolution or preferably a plurality of groups of five contacts would be used to make a complete rotation. In the arrangement shown in Fig. 4, a similar cycle of operations is gone through with the commencement of the cycle initiated by a coincidence preceded by vehicle or predestrian actuation, and the length of the B road at green being extended up to a maximum by pedestrian or vehicle actuation, provision, however, being made for varying the details of the cycle operation if desired. In this arrangement, a sixbank eleven-position stepping switch shown in developed form in Figs. 4a and 4c is used, with certain of the positions being blank, some serving no apparent purpose and others, although normally blank, allowing for slight variation in operation under different conditions. The position of the " split," i.e., the position in the cycle of the ultimate termination of the B-roadgreen condition which is position 8 on the stepping switch, is determined by potentiometer PRB1 under normal operation. However, two other " splits " may be provided; these are provided by putting the appropriate one of the five plugs in the top right-hand corner of Fig. 4a into the corresponding " split " socket to its left. Doing this operates either relay S2 or relay S3, which operation causes the potentiometer PRB1 to be switched out of circuit and either potentiometer PRB2 or PRB3 to be substituted in its place. Similarly, the timing of the commencement of the cycle is normally determined by potentiometer PR8. This provides the " simultaneous " mode of operation in which the relay SIM is operated over a line from the master station and the local controller operates in phase with the waveforms generated at the master station. When " offset " is required, relays IR or OR and OR<SP>1</SP> are actuated, according as to whether inbound or outbound offset is required, by respective actuation over lines OC2 or OC1 from the master. Average offset is achieved by actuating both OR and IR at the same time. Doing this causes the potentiometer PR8 to be switched out of circuit and corresponding ones of the potentiometers PR4 and PR5 and PR6 to be substituted in its place. There is also so-called " free " operation in which no control is sent from the master station and the whole of the timing is effected by timing circuits operated in the local controller itself, the coincidences determined by the two input sine-waves being replaced by the charging of condenser C11, Fig. 4c, which, when it charges sufficiently, causes the circuits shown in Fig. 4 to operate in the same way as if the coincidences had occurred. Even, however, if the operator at the master station intends the local controller to operate in the " free " condition, this can be negatived at the local controller by turning switch CS, Fig. 4a, to its second position; the system then operates in " average offset." Position 3 of switch CS ensures that there is always a " pedestrian-walk " interval in the minor road B during the cycle of operations, whether or not the pedestrian push-button is actuated. Position 4 ensures that at the end of the cycle of operations, a vehicle call is simulated whether or not a vehicle actually passes to operate contacts VD or not, so that the cycle of oper