Operation Manual

301UM0030_01 PD130 Enhanced Vehicle Detector User Manual Page: 12 of 29
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 with an inductance in the range 20 to 1500 micro-henries
(µH).
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.
4.2 Detector Sensitivity
Sensitivity of the detection system is dependent on factors such as loop size, number of turns in the
loop, feeder length and the presence of metal reinforcing beneath the loop.
The nature of the application determines the required sensitivity, which may be adjusted by means of
the sensitivity switches on the front of the enclosure.
Sensitivity levels of the PD130 Enhanced Vehicle Detector have been carefully optimised for parking
and vehicle access control applications. The detection of small unwanted objects such as bicycles and
trolleys can be eliminated by selecting lower sensitivity levels whilst high-bed vehicles and
vehicle/trailer combinations will not lose detection by using A
utomatic Sensitivity Boost (ASB) option.