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Model U-1400 Operations Manual Rev Apr 2020 Page
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Section 7
Theory of Operation
The Reno A&E Model U-1400 detector digitally measures changes in the resonant frequency of four independent
loop circuits to determine if a vehicle has entered the detection zone. The Model U-1400 detector applies an
excitation voltage to each loop circuit resulting in the loops oscillating at their resonant frequencies. The current
flow in the loop wire creates magnetic fields around the loop wire. When a vehicle passes over the loop area, the
conductive metal of the vehicle causes a loading of the loop’s magnetic fields. The loading decreases the loop
inductance, which causes the resonant frequency to increase. By continuously sampling the loop’s resonant
frequency, the magnitude and rate of change can be determined. If the frequency change exceeds a selectable
threshold (set by the sensitivity settings), the detector will deactivate a Vital output if one is mapped. If the rate of
change is slow, typical of environmental drift, the detector will continuously track and compensate for the change.
The Model U-1400 detector also monitors the loop frequency for out of range conditions such as an open or
shorted loop circuit.
The Model U-1400 detector is a scanning detector. The scanning method sequentially turns on and off each
channel’s loop oscillator. Each channel’s oscillator circuit supplies the excitation voltage that is coupled to the
loop circuit by a loop isolation transformer. The transformer provides high common mode isolation between the
loop and detector electronics, which allows the detector to operate on poor quality loops including a single short to
ground. The transformer also limits the amount of static energy (lightning) that can transfer to the detector
electronics. A spark gap transient suppression device is connected across the loop inputs connected to the isolation
transformer. This device dissipates static charges prior to the transformer. A network of four capacitors is
connected to the detector side of the isolation transformer. Three of the capacitors can be switched in or out of the
oscillator circuit to shift the frequency of the loop oscillator circuit thus providing frequency separation between
adjacent loops. The three switchable capacitors are electronically switched using FETs and are selected when
programming parameter values with the front panel pushbutton switches.
The outputs from the four loop oscillators are tied together and fed into a common squaring circuit. This is
possible since the detector is a scanning detector allowing only a single loop oscillator to be operating at any given
time. The sine wave from each loop oscillator circuit is squared to provide a precise zero crossing signal for the
input to the microcontroller. This signal is called the loop sample. The loop sample is an integral number of
complete oscillations from the loop oscillator circuit. The number of loop oscillations counted is a function of the
selected sensitivity setting for the detector channel. The required number of loop oscillations needed for a loop
sample increases as the sensitivity setting is increased. The microcontroller uses the period of the loop sample for
accumulating high-speed (32 MHz) crystal clock pulses generated by the microcontroller’s internal high-speed
crystal oscillator. The number of crystal clock pulses accumulated during consecutive loop samples is compared to
the internal reference number of crystal clock pulses stored in the microcontroller’s memory.
When a vehicle enters the loop zone the loop inductance decreases. This decrease in loop inductance causes an
increase in the loop oscillator frequency. In turn, an increase in loop oscillator frequency results in a decrease of
the time period for the loop sample. Hence, when a vehicle enters the loop zone, the number of crystal clock
pulses accumulated during a loop sample period decreases. By comparing the new count with the reference count,
a percentage change can be calculated that indirectly relates to the inductance change. If the magnitude of the
change exceeds a selectable threshold (sensitivity setting), the detector deactivates a Vital output if one is mapped.
The rate of change is also monitored. Slow rates of change caused by environmental fluctuations are tracked and
automatically compensated for. This process is conducted independently for each of the four loop oscillator
circuits.
The microcontroller uses the high-speed crystal clock count to calculate the loop inductance, frequency and
percentage of change. If selected, the values are displayed on the seven-segment LCD. The microcontroller also
processes the pushbutton switch selections for the LCD display and stores the operating parameters in non-volatile
memory. Stored parameters are changed with the front panel switches or via the front panel RS-232
communications port and are unaffected by loss of power or detector reset. The microcontroller continuously
processes the loop samples and detector operation is not affected during the operation of the switches or the LCD.
NOTE: When either the sensitivity or frequency of a loop input channel is changed, the loop input channel is reset.
In addition, the microcontroller conditions the outputs based on Loop Status Inputs, Vital Inputs, Health Status
Inputs, and the programmed settings of the various timers (Primary Vital Output Delay, Primary Vital Output
Extension, and Loop Check) and options (Option 1, Option 2, Option 3, Option 4, Option 5, Option 6 and Option
7).
