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Frequency modification switches at the front panel allow the oscillator frequency to be modified
when multiple loops and multiple sensor units are operated close to each other. This prevents
interference or crosstalk from occurring between loops connected to different sensor units. A
loop transformer within the oscillator circuitry isolates the road loop from the internal circuitry
of the sensor improving its resistance to external surges induced in the road loop and also
allowing the sensor to operate with loops that are shorted to ground.
The sinusoidal oscillations are fed through a squaring circuit to produce a square wave which is
suitable for the digital frequency measurement circuitry. The loop oscillator frequency is
measured by counting the number of oscillations from a stable crystal oscillator source which
occur over a predetermined number of loop oscillator cycles. This is known as period
measurement and defines the frequency of the loop oscillator to a very fine resolution. A
combination of the loop frequency and the selected sensitivity is used to determine the number of
cycles over which to perform the period measurement.
Once a stable oscillator condition has been established the sample is stored as a reference against
which subsequent samples will be compared. Small, slow changes in the oscillator frequency due
to environmental drift are programmed out allowing the direct comparison of fast changes due to
the entry of a vehicle in the detection zone. Once a period shift exceeding the detection threshold
set by the sensitivity level occurs, the output and detect indicator for that channel are activated.
The Operational mode that was set on the front panel switches will determine whether a single
125 millisecond output pulse is issued (pulse mode) or whether the output signal remains until
the oscillator frequency returns below the detection threshold indicating the exit of the vehicle
from the detection zone (Presence mode).
If the loop oscillator frequency exceeds the operating range of the sensor unit, as might happen
when the road loop connection is open or short circuited then a fault condition is signaled by the
detect output entering the fail-safe condition, the fault led lighting solid yellow and the detect led
indicators flashing. If the oscillator frequency should then return to within the operating range,
the detect output and led indicator will return to their normal operating state while the yellow
Event indicator will continue to flash indicating that a historical fault has occurred. The flash
sequence indicates the type of fault that was sensed. A single flash followed by a pause indicates
an open circuit loop or. A double flash followed by a pause indicates a shorted loop or too small
a loop inductance. A triple flash followed by a pause indicates a greater than 25% change in
loop inductance.
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