GB1289208A - - Google Patents

Abstract

1289208 Road traffic detectors TOKYO SHIBAURA ELECTRIC CO Ltd 6 Oct 1970 [6 Oct 1969 7 Nov 1969] 47439/70 Heading G4Q Apparatus for detecting road traffic 13 includes a coil 11 embedded in the road, the effect of a vehicle on the coil causing an increase in the frequency of the output of an oscillator 10. The oscillator output is applied to a frequency-tocurrent converting circuit 15 ... 17, the output of this circuit being applied to an operational amplifier 18 having a feedback circuit 20, 21 whereby the voltage output of the amplifier is proportional to the output frequency of the oscillator. The amplifier output operates a relay 22 to close a contact 221 and operate a vehicle counter 27. The amplifier output is also applied to one terminal of an amplifier 24 through a delay circuit 23, 25, the other input of the amplifier 24 being connected to earth. The amplifier 24 compares its two input voltages and generates an output which is applied to the input of amplifier 18 to reduce the input to amplifier 18 to zero. Thus the feedback path through the amplifier 24 constantly produces, after a short delay, an output which reduces the output of amplifier 18 to zero. However, the delay is made sufficiently long to ensure that a vehicle may be detected by the coil 11 and operate the relay 22. On the other hand, slow variations in the oscillator frequency due to seepage of water through the road bed or due to changes in the oscillator itself are continuously cancelled out. The relay 22 may also close a contact 222 to connect in circuit a capacitor 28 and further increase the time delay e.g. to five minutes. This causes the relay 22 to remain energized for a maximum period of five minutes in the event of a traffic jam in which a vehicle remains on the detector coil 11 for a period of time, the actual time period being registered by a recorder 29. A telemetering device 30 serves to transmit the traffic information.