7
Addendum
Note on Insulation Resistance measurement
The Insulation Resistance measurement is an indicative measurement of the nominal state of the loop.
The objective of this test function is to provide an objective TEST OK or FAIL status for the loop and
feeder cable. Variations in battery level capacity and rapid repetitive testing producing accumulated
mutual capacitance resulting from moisture ingress in the cable and field joints will effect readings.
Note on the nature of detecting the position of a loop winding
The most sensitive area of a loop is above the loop wire itself as the lines of force are at RIGHT
ANGLES to the eddy current circuit producing the largest opposing force. The ‘right angle’ force
however when measured with the probe field at right angles produces a NULL field. The NULL field
is detected by the circuitry and thus the position of the wire is determined. Trawling the probe over
the loop other than in a completely horizontal orientation may induce erroneous results as the angle of
incidence between the loop field and the device probe field will induce a reading.
Note on the validity of cross-talk compensation by simple adjustment of capacitance
Resonant frequency change is the basis of inductive vehicle detection
ω
is the frequency in radians/sec
For a resonant circuit:
ω
= 1 / SQRT(LC)
therefore
ω
is proportional to 1/SQRT(L) or L^-0.5
Therefore any variation in L of X% translates to a much smaller variation of
ω
. However, this
variation is not affected by C. C only affects the absolute value of
ω
.
What does all this mean? Changing the resonant frequency point for a fixed inductor (a loop)
requires changing the capacitance C. However . . .
Since C is not related to the change in
ω
when L changes - it can be stated that doing so is a pointless
exercise unless the frequency is be shifted by at least 5Khz and the device must easily accommodate
site variations while incurring preset incremental capacitance steps.
Note on ‘Q’ calculation (effective performance of the loop)
*’Q’ for normal vehicle detection may be as low as 6 however for vehicle classification applications
and accurate speed detection ‘Q’ should be greater than 15.
Q or the performance of the loop is effectively the relationship between inductance of the loop
and resistance / impedance in the feeder cable hence longer feeders (higher
impedance/resistance) need more inductance in the loop for compensation.
The following formula and reference gives it a scientific presence:
Q (at resonance) = Frequency (radians) X[Multiplied by ] inductance (henries)
__________________________________________ [divided]
DC Resistance (ohms)
** Where Radians = 2 (multiplied) pi (multiplied) hz
Reference: Electronics A Top-down approach to computer-aided circuit design by Hamley 1994
P904-908
Loop length is the length of the active area of the loop in the direction of travel. 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.
