8
Loop Length and Sensitivity
Loop length and sensitivity is associated with the length of the active area of the loop in the direction of
travel and the strength of the electromagnetic field and its inherent interaction with the electromagnetic
of iron. . The active area is size of the loop electromagnetic field in which a vehicle will change the
inductance of the loop above the detection threshold. The size of the active area is determined by the
sensitivity of the loop (Q), the depth of the loop, and the sensitivity of the detector.
For loops installed according to the specifications given in XL Series loop detector manual ,
the active area of the loop should be close to the physical loop size. For existing loops with
low Q the active area may be diminished.
A low loop ‘Q’ will effect the performance of the detector both in respect to detecting smaller
vehicle and the accuracy achievable by the vehicle detector in measuring the vehicle length.
The following example indicates how low ‘Q’ effects vehicle length measurement and how
adjustments in the detctor may overcome the measurement error.
The error in the loop length directly affects the error in the vehicle length. For example if there
is an error in the loop length of -0.4m then vehicles will be reported as being 0.4m shorter
than they actually are. This error can be minimised by following these steps:
1. Set the detectors to the most sensitive setting possible without crosstalk (crosstalk
can only occur between cards)
2. Using the debug mode in the console, observe the lengths of 5 or more Holden or
Ford sedans. These cars have a typical length of 4.9m
3. Average the results and subtract 4.9m.
4. The result is the amount by which the loop length value should be modified.
5. For example: 5 vehicles are measured at 4.1m, 4.3m, 4.0m, 4.0m 4.1m. The average
is 4.1m. Subtracting 4.9 gives – 0.8m. Therefore the loop length should be decreased
by 0.8m.
