Patent GB-985575-A (GB)
Title: Vehicle detection and speed measuring apparatus
IPC: G01S13/92
Filed: 1962-03-13
Abstract: 985,575. Sonic Doppler speed measurement. GENERAL SIGNAL CORPORATION. March 13, 1962 [April 12, 1961], No. 9583/62. Heading H4D. Vehicle speed is measured by apparatus operating on the Doppler principle and incorporating means for discriminating between vehicular movement in opposite directions. The invention is described in connection with a system utilizing sound energy (20 kc./s.), but the circuits employed for analysing the Doppler signals to derive a measure of vehicle speed are stated to be also operable in response to Doppler signals produced by systems utilizing radio-frequency waves. In one embodiment (Fig. 2) a transmitting acoustic transducer T1 energized by an oscillator 10 operating at frequency F1 (20 kc./s.) directs a steady beam of sound energy along a roadway (see Figs. 1 and 1A). Energy reflected by vehicles such as V1, V2, moving in opposite directions, and due to the Doppler effect, having respective frequencies F2, F3 is received by a receiving acoustic transducer R1 together with energy of frequency F1 by a direct leakage path from T1 and supplied via a coupling device 11 to a notch filter 12 (see also Fig. 2A) which removes the frequency F1 and supplies the frequencies F2, F3 via a band-pass amplifier 13 which passes the range of frequencies likely to be encountered in the frequencies F2 due to approaching vehicles (e.g. 20 to 23 kc./s.) to a mixing and limiter circuit 14. In the latter circuit an optimum level of frequency F1 is mixed with the F2 frequency (which results in the Doppler beat signal having maximum amplitude and is preferable to utilizing the leakage F1 between T1 and R1) and the combined signals comprising the frequencies F1 F2 together with their sum and difference fed via a cathode follower 15 to a low-pass filter 16 which selects the frequency F2-F1 (i.e. the Doppler beat frequency due to vehicles approaching the transducer system) and supplies it via a transformer to output terminals 17, 18. If instead, the speed of vehicles moving in the opposite direction is required the band-pass amplifier 13 is made to pass the range F1-F3. Due to the removal of the frequency F1 by the notch filter 12, the arrangement of Fig. 2 does not provide accurate speed information for slowly moving vehicles, since, in this case, F2 is very close to F1 and tends to be rejected by the notch filter and to overcome this the latter may be replaced by a band-pass amplifier which passes the frequency F1 and the desired frequency F2 or F3 (Fig. 3, not shown). When it is merely desired to provide information as to the presence of vehicles in the detection zone the signals F2, F3 are separately derived from the received signals F1, F2, F3 and utilized to operate indicating devices (Fig. 4, not shown). Also, if information as to the passage of a vehicle in a particular direction is required without speed measurement an envelope detector responsive to the Doppler beat frequency derived by either of the methods described above and the output of which controls an indicating device may be employed (Fig. 5, not shown). In the case of speed measurement two methods are described for analyzing the Doppler beat frequency signals to produce an actual speed indication. Each method consists of charging a condenser by equal area pulses (one pulse per cycle of Doppler beat frequency), and for periodically sampling the charge to produce a control signal for a calibrated speed indicator. The first method is characterized by effecting the sampling when the amplitude of the Doppler beat frequency exceeds a predetermined value and the second by effecting the sampling after the reception of a predetermined number of Doppler beat frequency cycles. In both methods the analyzing apparatus is positioned remotely from the measurement zone and is supplied with the Doppler beat frequency over a transmission line. Fig. 6 shows the arrangement for the first method and comprises a balanced transformer, to which the transmission line is connected, supplying an amplifier V1, level setting (limiter) diode V2, Schmitt trigger V3, V4 and a one-shot multivibrator V6, V7. The latter generates equal area pulses, one for each positive half-cycle of the Doppler beat frequency at the input of V3 (when after a few cycles the level of the received beat signals reaches a level sufficient to operate the trigger V3, V4) which after amplification in V8 are applied as positively going pulses to an integrating capacitor 77, the charge on which is directly dependent on the repetition rate of the pulses. This charge controls the conduction of cathode-follower V9 and thus determines the potential of the plate of diode V10 which potential is transferred from the cathode to a capacitor 78 via a switch 85, which closes for 50 millisecond periods in successive cycles of about 1 second. The switch is operated by a relay coil R10 controlled by a one-shot multivibrator V14, V15 which is itself controlled by the change of state of a Schmitt trigger V12, V13 when the charge on capacitor 82, supplied via an envelope detector D1, D2 from the anode of V1, reaches a predetermined level. At the termination of the 50 millisecond charging period V15 becomes conductive, R10 is energized and contact 85 opens. The drop in anode potential of V15, when it becomes conductive is effective via capacitor 83 to bias off a normally conducting valve V16, thus de-energizing relay coil R11 and causing contact 94 to close and supply the charge on capacitor 78 in the cathode of V11  to a calibrated speed indicating meter 95 an also to a valve V11a which, by virtue of a long time-constant circuit including capacitor 96, supplies to meter 97 a measure of the " flowvelocity " rather than individual vehicle speed. Additionally, switch 94 supplies the charge on capacitor 78 back to the transmission line via resistor 98 so that speed observation may also be made at the location of the measurement zone by means of meter 19 (Fig. 2). The period for which contact 94 is closed (about one second) constitutes the sampling period and during this period the potential on capacitor 78 is free to follow that of the cathode of diode V10 via contact 86 operated jointly with contact 94. During the 50 millisecond charging period a discharge path is provided via resistor 87 and contact 86 if it should happen that the potential on capacitor 78 is greater than the (new) potential of the cathode of diode 86. If a vehicle detection indication is also required the contact 90 operated by relay R10 may be employed. Due to the obliquity of the transmitted sound energy beam with respect to the direction of vehicle movement (" cosine factor ") the Doppler beat frequency decreases slightly, for vehicles travelling at constant speed, as such vehicles come closer to the position of the transmitting and receiving transducers. This causes a slight decrease in the charge on capacitor 77 and the accuracy of speed measurement is thus dependent on the sampling for each vehicle occurring when it is of approximately at the same position in the measurement zone. In the embodiment just described the sampling period is initiated when the charge on capacitor 82 reaches a value sufficient to operate the trigger V12, V13, i.e. when the amplitude of the received beat signals exceeds a predetermined level. In a second embodiment the " cosine factor " is compensated by establishing a gating interval, of the same duration for each vehicle, during which constant area pulsed (one for each beat frequencycycle) are fed to an integrating capacitor (normally held in a discharged state but not free to discharge between successive pulse in the gating interval) and the sampling of the charge on this capacitor is then initiated at an instant determined by the reception of a predetermined number of beat frequency cycles. The general arrangement is shown in Fig. 8 and circuit details of the various " blocks " are described with reference to Figs. 9A and 9B (not shown). Referring to Fig. 8, the Doppler beat frequency is supplied via terminals 100, 101 and cathode-follower 102 to a first branch including a diode-limiter 103, Schmitt trigger 120, gate 121, one-shot multivibrator 122 (producing the constant area pulses) and a charging diode 125 which, when gate 121 is open and contact 127 closed supplies the constant area pulses (one for each cycle of beat frequency) to capacitor 126. The cathodefollower 102 also supplies a second branch including a threshold adjuster 104, Schmitt trigger 105 and a pair of diodes 106, 107 so arranged that after a few (three) cycles of the beat frequency (pulses from trigger 105) a gate 108 passes such pulses to a capacitor 11 (previously fully discharged) which on reaching a predetermined potential trips a further Schmitt trigger 109 which supplies a positive voltage over line 110 to abruptly raise the D.C. level of the signals set by the threshold adjuster 104. This ensures continued operation of trigger 105 even though the received beat signals are momentarily diminished in amplitude and since capacitor 11 is fully discharged at the commencement of the charging cycle the instant at which trigger 109 is operated represents, for each vehicle, the same position in the measurement zone. The output of trigger 109 controls a flip-flop 112 which, via an amplifier 113, relay FS and contact 115, causes a positive potential to be applied to a normally discharged capacitor 114. The latter controls a modified multivibrator 116 to operate via relay CD, a contact 117 such that during a predetermined interval capacitor 126 is discharged to earth after which circuit 116 reverts to its first state and the resulting operation of contact 117 triggers a one-shot multivibrator 118. Operation of the latter is then effective via a amplifier 123 to open gate 121 for a predetermined interval during which pulses from trigger 120 control the supply of constant area pulses to the charging