301UM0031_01
TD136 Enhanced Vehicle Detector User Manual
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4.
PRINCIPAL OF OPERATION
The inductive loop vehicle detector senses the presence of a vehicle over an area defined by a loop of
two or more turns of wire laid under the road or pavement surface. This loop of wire is connected to
the detector by a twisted pair of wires called a loop feeder.
A vehicle passing over a sensing loop causes a small reduction in the inductance of the loop, which is
sensed by the detector. The sensitivity of the detector is adjustable to accommodate a wide range of
vehicle types as well as different loop and feeder combinations.
Upon detection of a vehicle passing over the loop the detector operates its output relays which may be
used to indicate controls associated with the installation.
4.1 Detector Tuning
Tuning of the detector is fully automatic. When power is applied to the detector upon installation of the
system, or when a reset is initiated, the detector will automatically tune itself to the loop to which it is
connected. The detector will tune to any loop to inductance range 20 to 1500 microhenries.
This wide range ensures that all loop sizes and feeder combinations will be accommodated in the
tuning range of the detector.
Once tuned, any slow environmental change in loop inductance is fed to a compensating circuit within
the detector, which keeps the detector correctly tuned.