Environmental tracking in inductance loop vehicle detection systems

Abstract

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. In the case of the tracking reference, if the difference between it and the sample is no greater than a threshold value, each reference is updated by repeatedly incrementing or decrementing it by a predetermined amount for a number of times determined by the time that has elapsed since the last comparison cycle, but if the difference is greater than the threshold, indicating a vehicle movement, the tracking reference is immediately made substantially equal to the sample in one step. Both the threshold values and the predetermined amounts of increment or decrement may be different for different directions of change. In a permanent vehicle presence detecting system, the detect reference is updated in a similar manner except that there is no change of the detect reference if the difference threshold is exceeded. In a system that only detects a continuously present vehicle for a finite presence time and then `forgets` it, the detect reference is only incremented stepwise for change in one direction, changes in the opposite direction being accomplished by making the detect reference equal to the sample in one step.

Inventors

• 👤 Richard K. Duley

Bibliographic Details