Abstract: The detector system comprises a CW transmitter which is coupled to a receiver in a manner such that an approach of a vehicle produces an amplitude increasing excursion in the envelope of the received signal. At the receiver, the envelope of the received signal is sampled to derive the quantity .alpha.t/V.sub.p which is proportional to the speed of the vehicle, where V.sub.p is equivalent to the envelope voltage of the received signal at an instant I.sub.p during the leading edge portion of each excursion at which the first-occuring marked decrease of the slope of the leading edge occurs less the envelope voltage prior to the excursion and .alpha.t is an electrical quantity proportional to the maximum slope of the leading edge of each excursion during the leading edge portion preceding the instant I.sub.p.

Abstract: Disclosed is a vehicle detection system comprising a transmitter coupled to a receiver via a sensor arranged to produce a change in the envelope of the received signal upon passage of a vehicle. In order to distinguish between changes in the received signal caused by variations in the environmental conditions and the approach of a vehicle, the system is provided with an identification circuit which periodically samples the received signal. The sampled voltages, which are representative of the envelope of the received signal, are then stored and supplied to a comparator which compares the envelope with the sampled voltage and provides an output signal indicating approach of a vehicle when the difference between the envelope level and the sampled voltage exceeds a predetermined value.

Abstract: In the disclosed system the speed of a vehicle passing over the sensing coils of the detection system is determined by sampling the envelope voltages occurring during the leading edge portion of the disturbance in the envelope of the received signal produced by an approaching vehicle. This is accomplished by sampling the envelope voltage at a first instant at which the slope of the leading edge is at a maximum and at a second instant occurring at a fixed time interval subsequent to the first instant. The voltage sampled at the first instant is then subtracted from the voltage sampled at the second instant and the difference divided by a voltage sampled at a third instant at which the slope of the leading edge first starts to decrease to produce a signal indicative of the vehicle speed.