307UM0100_01 May 2016
PD17x User Manual
Page 11 of 31
4.
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.
Figure 4. Magnetic field from Inductive loops interacting with a vehicle
4.1
Detector Tuning
Tuning of the detector is fully automatic. The detector will re-tune if any of the following events occur:
-
When power is applied to the detector and Powerfail is OFF
-
A channel reset is initiated via the push button
-
A detect of greater than 16%
L/L occurs.
-
A fault has occurred and is self-healing.
The detector will automatically tune the channel to its connected loop. 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
DIP switches as indicated on the detector.
Multiple sensitivity levels have been provided in the PD17x Series Detector, making it one of the more sensitive
detectors available. The detection of small unwanted objects such as bicycles and trolleys may be eliminated by selecting
lower sensitivity levels while high-bed vehicles and vehicle/trailer combinations will not lose detection by using
Automatic Sensitivity Boost (ASB) option.
ASB operates as follows: When ASB is disabled, the un-detect level is dependent on the sensitivity setting of the
detector. Hence as the detector is made less sensitive, the un-detect level will reduce accordingly. When ASB is enabled
the un-detect level is fixed irrespective of the sensitivity setting and will be equivalent to the un-detect level when the
sensitivity is on maximum setting.
