999,906. Measuring velocity acoustically. GENERAL SIGNAL CORPORATIQN. April 4, 1962 [May 1, 1961], No, 12937/62. Heading H4D. In an arrangement for measuring vehicle velocity the entry of the vehicle into the measurement zone initiates the application of a predetermined potential to a normally discharged capacitor for a period determined by the length of the vehicle, after which the capacitor is caused to discharge until the vehicle leaves the measurement zone at which instant the discharge is stopped and the vehicle velocity determined by measuring the remaining charge. The invention is described in connection with a system in which the reception of pulses of ultrasonic frequency energy reflected from a vehicle immediately following the reception of such pulses by reflection from the roadway is utilized to initiate the charging of the capacitor, the discharge of which is terminated when vehicle reflected pulses are replaced by road reflected pulses. The total duration of the voltage " wave " (charge and discharge) of the capacitor is thus proportional to the length of the vehicle and by making the charge period constant (assuming that the length of each vehicle is aqual to the average length of all vehicles) the average value of the capacitor charge is proportional to vehicle flow velocity. When, however, the vehicles include a significant number which are of greater than average length an inaccurate measure of flow velocity is obtained and to correct for this the presence of such vehicles is detected separately and employed to increase the period during which the predetermined voltage is supplied to the capacitor. Figs. 1A and 3 show the arrangement for detecting the presence of vehicles and comprises ultrasonic transmitting and receiving transducers T, R, respectively positioned over each lane of a roadway, the transducer T being energized by pulses of ultrasonic energy of about one millisecond duration under the control of a free-running multivibrator (Fig. 2, not shown) and the received reflected pulses from transducer R supplied via an amplifier to a rectifier and filter 16 (Fig. 3). The output from the latter is fed to three gated amplifiers 24, 25, 26 which are " opened " under the control of signals from respective generators 23, 27, 28 controlled by the free-running multivibrator (Fig. 2, not shown) such that amplifier 26 is " opened " during the period when any pulses reflected from the roadway would arrive at transducer R, whilst amplifiers 24 and 25 are " opened " at respective moments when reflected pulses from high vehicles (arrived, also, to be long vehicles) and from low vehicles (assumed to be average length vehicles) are expected to arrive at the receiving transducer R. Output signals from either amplifiers 24, 25 are utilized to trip a flip-flop 19 to one condition whilst an output from amplifier 26 trips it to the other condition. The " condition " of the flip-flop is detected and employed to actuate a relay VR, Fig. 4 (also in Fig. 2, not shown) operation of which is indicative of the entry of vehicles into the measurement zone. Additionally, when a predetermined number of pulses is received from amplifier 24, a relay TD (see also Fig. 4) is actuated and operation of the latter is indicative of the presence of a vehicle of longer than average length. For deriving a measure of vehicle velocity operation of VR is utilized via contact 31 to trip a multivibrator 32 (Fig. 4) to the condition in which V2 conducts for a short period, determined by the circuit constants, during which a relay SP1 operates via contact 40 to supply the predetermined potential (B+) to an averaging circuit 41, 42, 43 and a capacitor 44. At the end of such period relay SP1 releases and capacitor 44 discharges via contacts 40, 45 and 22 to ground. When the vehicle leaves the detection zone VR releases and the opening of contact 22 leaves capacitor 44 with a potential which is proportional to vehicle speed and this is then indicated on a calibrated meter M. If, however, the vehicle is of greater than average length relay TD will have been actuated and this operates via contact 48 and a self-holding relay HM such that when relay SP1 releases the closing of contact 53 operates via a relay HT and contact 55 to trip a multivibrator 60 to the condition in which VS is conductive and via contact 49 to open the holding circuit for relay HM. V5 remains conductive for a period determined by the constants of the circuit and the operation of relay SP2 and contact 45 results in the almost immediate re-application of the predetermined potential (B+) to the circuit 41 . . . 44 when relay SP1 releases contact 40. When SP2 releases (as shown in the drawing) the new charge on capacitor 44 leaks away to earth via contact 22 until the latter opens on the release of relay VR when the vehicle leaves the detection zone.