Operation Instructions
MBE35_50 OPERATION MANUAL 2005_10
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5.2.2 Vehicle detection
When a vehicle enters the loop, the body and frame provide a conductive path for the
magnetic field; causing a loading effect, which in turn causes the loop inductance to
decrease. The decreased inductance causes the resonant frequency to increase from
the nominal value. If the frequency change exceeds the threshold set by the
sensitivity setting, the detector module will output a detect signal.
There has been a misconception that an inductive loop requires a mass of metal for
detection. Placing a single wire around the perimeter of the loop and shorting the
ends together will quickly disprove the misconception. The single wire forming a
shorted turn provides a current path for the magnetic field; thus causing a loading
effect similar to that of a vehicle. The shorted turn effect of the single wire coil in the
proximity of the loop acts much like a shorted turn secondary of a transformer.
5.2.3 Wire turns required for loops
The Inductive loop detectors will tune from 70
µ
H to 500
µ
H (
µ
H = micro Henry). It is
preferable that the loop and lead-in have a minimum of approximately 70
µ
H for
stability. The loop inductance should be equal to or greater than the lead-in
inductance.
If the inductance of the loop exceeds the requirements above, a proper functioning of
the detector cannot be guaranteed.
The loop inductance also influences the loop sensitivity. The best results are between
100 and 300
µ
H.
