Abstract: In a vehicle presence detection system in which an inductive sensor loop is the frequency-determining element of the tuned circuit of a loop oscillator, discrimination against variations in inductance due to environmental changes is achieved by comparing detect and tracking reference numbers with a sample number that is a function of the oscillator frequency and updating each reference number repeatedly at real time intervals independent of loop oscillator frequency. The current sample number is compared with either the tracking reference or the detect reference according to whether or not the system is indicating the presence of a vehicle.

Abstract: In a vehicle detection apparatus having a plurality of channels, each arranged for detecting vehicles according to their influence upon the frequency of a respective loop oscillator the oscillation frequency of which is dependent on the inductance of a respective vehicle-sensing inductive loop, a common computer performs a calculation for each channel in turn whereby changes in the frequency of the respective loop oscillator are monitored and analysed. In one embodiment, the computer calculates an appropriate operational number, the time taken for that particular number of loop cycles to occur is measured and stored as a period count and the period count is compared with an environmental reference number to determine whether or not a vehicle detected flag should be raised or lowered. Neither the operational number calculated by the computer nor the reference number is fixed but provision is made for updating both.

Abstract: An electronic vehicle detection installation of the type using a detector loop laid in the roadway senses changes in the inductive time constant of the loop. The changes are sensed by determining the phase difference between a voltage impressed on the loop and the loop current or by making the time constant the predominant frequency determining parameter of an astable oscillator. A digital processing circuit is provided to sense vehicle-indicative changes in the oscillator frequency. Clock pulses are counted over a selected number of oscillator cycles, the scan cycle, and the number counted over one scan cycle is compared with that stored from the preceding cycle. Provision is made to adjust the stored value for ambient changes and for the continued presence of a vehicle after a selected presence period.