I UsingSignaHo - Noise Ratio as Evidence to Detect PresenceAbsence of Vehicles

There are three different loop detectors used to detect present and absent vehicles.’ One of them is known as self-tuning detector. This type of detector utilizes both pavement loops as part of a parallel resonant tank circuit and a closed-loop feedback circuit. It automatically adjusts a voltage-tuned oscillator to a predetermined frequency relative to the resonant frequency of the tank.’ A change in the loop inductance shifts the resonant frequency of the tank, which results in a change in the feedback voltage. It is used to provide an indication of vehicle presence through the relay and relay-driver circuits. The second type is bridge-balance loop detector that has found acceptance in traffic-control applications. A vehicle crossing the loop unbalances the bridge circuit. This produces a change in the signal amplitude that is used to indicate vehicle presence. The third type is phase-shift detector and it is the most widely used. It is energized from an oscillator circuit. The loop is tuned to resonance with respect to the oscillator operating frequency by a variable tuning capacitor connected in parallel with the loop. Thus when a