Traffic light induction loops work by detecting changes in inductance caused by the presence of a vehicle over or near the loop embedded in the road surface. These changes trigger the traffic light controller to adjust signal timing.
Here's a breakdown of the process:
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The Loop as a Tuned Circuit: The induction loop, consisting of a wire coil buried beneath the pavement, acts as an inductor in a tuned electrical circuit. This circuit includes the loop wire itself and a lead-in cable connecting it to the traffic signal controller.
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Vehicle Detection and Eddy Currents: When a vehicle (containing metal) passes over or stops within the loop, it disrupts the electromagnetic field. This disruption induces eddy currents within the metal components of the vehicle.
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Inductance Decrease: The eddy currents circulating within the vehicle draw energy from the loop's electromagnetic field. This effectively decreases the inductance of the loop circuit.
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Signal to the Controller: The traffic signal controller continuously monitors the inductance of the loop circuit. When the inductance drops below a predetermined threshold (due to the presence of a vehicle), the controller registers the vehicle's presence.
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Traffic Light Adjustment: Based on the vehicle detection and pre-programmed logic, the traffic signal controller adjusts the timing of the traffic lights. This could involve extending the green light for the lane with the detected vehicle, shortening the red light, or initiating a signal change.
In essence, the metal body of a vehicle changes the magnetic properties of the inductive loop, providing the signal that a car is waiting. It is this change in inductance, and not the weight of the car, that triggers the signal.
