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Loop detectors detect the frequency changes from zero to different level, the
inductance changes are computed by change in frequency.
The change in inductance due to the presence of vehicle is recorded at a small
time interval.
The waveform obtained by plotting the sampled inductance changes is referred to
as the vehicle inductive waveform or inductance signature.This waveform depends on number of vehicle parameters such as vehicle length,
speed, and metal surface of the vehicle.
Fig. 1 shows an inductive waveform of a typical
passenger car.Horizontal axis records data points at 10 milliseconds interval.
This is the common shape of inductance waveform that has one peak in the middle
with monotonic decrease in both sides.
Vehicle signatures are functions of vehicle speed and vehicle type, so many
features can be derived from the vehicle signatures directly or indirectly.
Volume and occupancy are directly derived from processing raw vehicle signatures
whereas speed is estimated based on the vehicle signature feature vectors.
Vehicle length is obtained based on vehicle speed.
By combining vehicle length with existing vehicle signature features, vehicle
classification can be measured.
It is easy to observe signature differences arising from the vehicle speed.
Duration or occupancy has an inverse proportional relationship with speed while
slew rate shows a proportional correspondence with speed.
Figure 1:
Inductive waveform of a typical passenger car, source
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A series of vehicle signature acquired by the Inductive Loop Detectors located
at upstream and downstream of a freeway and different distance measures to find
the re identification accuracy level.
Double-axle trucks produce a double picked vehicle signature when the resolution
of detector is adequate.
Thus, it can be easily used for vehicle-type identification purposes.
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