Nortech PD27 Series - 4 Opera�On; Detector Tuning; Detector Sensi�Vity

307UM0200_01 May 2016 PD27x User Manual Page 11 of 30
4. Operation
The inducve loop vehicle detector senses the presence of a vehicle over an area dened 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 reducon 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 combinaons.
Upon detecon of a vehicle passing over the loop the detector operates its output relays, which may be used to indicate
controls associated with the installaon.
Figure 4. Magnetic field from inductive loop interacng with a vehicle
4.1 Detector Tuning
Tuning of the detector is fully automac. 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 iniated via the push buon
- 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 combinaons will be accommodated in the tuning range of the
detector.
Once tuned, any slow environmental change in loop inductance is fed to a compensang circuit within the detector,
which keeps the detector correctly tuned.
4.2 Detector sensitivity
Sensitivity of the detecon 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 applicaon determines the required sensivity, which may be adjusted by means of the Sensivity
DIP switches as indicated on the detector.
Mulple sensivity levels have been provided in the PD27x Series Detector, making it one of the more sensitive
detectors available. The detecon of small unwanted objects such as bicycles and trolleys may be eliminated by selecng
lower sensitivity levels while high-bed vehicles and vehicle/trailer combinaons will not lose detecon by using
Automac Sensivity Boost (ASB) opon.