ATS Handheld Inductive Loop Tester HILT 9000 - Manual (Page 11)

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Overview of Traffic Loop Detection Systems
The inductive loop represents the most commonly used method
to detect vehicles. The inductive loop is simply a coil of wires
embedded into the pavement and can be characterized by several
parameters, L (inductance),
Q (quality factor), R (active resis-
tance, measured using AC signal), and R
DC
(DC resistance).
These parameters are affected by the type of pavement, number
of turns in the loop, type of wire, length and type of the lead-in
cable, shape and dimension of the loop, and presence of any
objects near the loop.
A detector is connected to the loop and is used to measure the
AC characteristics of the loop and change of those. A part of the
circuitry in the detector and the loop creates an oscillating
circuit. The oscillating frequency of this circuit depends on the
parameters of the loop and the parameters of the detector. When
a vehicle or other metallic mass is located above the loop almost
all of the characteristics (except DC resistance R
DC
) are altered.
This change causes the oscillating frequency to drift which in
turn is detected by the detector.
Unfortunately, the deterioration of the loop begins virtually from
the moment of installation. There appear to be two basic mecha-
nisms for loop degradation, mechanical and chemical. Under
daily and seasonal temperature variations, the pavement is
constantly flexing. To this may be added the pounding of heavy
vehicles, particularly at the approaches to signalized intersec-
tions where loops are often located. These mechanical factors
cause flexure of the wires comprising the loop, especially at the
juncture between pavement and berm, leading initially to fine
cracks in the insulation, and perhaps ultimately to actual
breakage of the wires. After insulation failure occurs (fine cracks
or breakage), there is a path for water intrusion. While water
itself is a poor conductor, its conductivity is greatly increased by
the highly ionic de-icing salts which saturate the berm and all
cracks and fissures of the pavement. Aside from the ground
leakage which results, these highly active solutions can actually
erode the copper conductors by displacement, aided by electroly-
sis of even the small potentials of the wire to ground. Lead-in
wires are also vulnerable, especially on long pulls through
conduit, because of the high probability of minor insulation
failures due to chafing and stretching during installation.