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A vehicle detection system provides an input for traffic-actuated signal control, traffic control, roadway surveillance, and data collection systems.
The inductive loop detector has become the most popular form of detection system. Inductance is a circuit element, typically a conducting coil, in which electrical current is generated by electro-magnetic flux through the circuit. The principal components of an inductive loop detector system include one or more turn of loop wire wound in a shallow slot sawed in the pavement, a lead-in cable to a gate operator cabinet, and an electronic detector unit housed in the cabinet.
When a vehicle passes over the loop or is stopped within the loop, it decreases the inductance of the loop. This decrease in inductance then actuate the detector output relay (or circuit) which, in turn, sends an impulse to the controller unit signifying that it has detected the passage or presence of a vehicle.
Contrary to popular belief, inductive loop detectors sense metal surfaces, not necessarily metal mass. Actually, the heavy metal engine in the loop area increases the inductance (because of the phenomenon known as the ferromagnetic effect). The peripheral metal - the large, flat horizontal plane of the vehicle - has the opposite effect and more than offsets the increase from the ferrous mass of the engine, and the net effect is an overall reduction in inductance.
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