DSP222H_Manual_A
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the sensitivity the same over the wide spectrum of loop frequencies, the sensor unit first
measures the loop frequency and calculates the number of complete cycles needed for
a sample. This value is then used for all subsequent loop measurements for this
channel.
The sensor unit checks the completed sample to confirm that it is still within acceptable
operating limits and then determines whether the sample has changed sufficiently with
respect to the stored reference to indicate the presence of a vehicle. It then controls the
output and indicators appropriately. Minor changes in period occurring over a relatively
long time are due entirely to environmental fluctuations. By altering the stored reference
slowly, these environmental changes are ignored. In this fashion, the sensor unit can
compensate for temperature changes and other long-term effects such as water on the
pavement.
5. Detailed Description of Circuit Operation
The following description is valid for both channels of the DSP-222. Reference
designators are shown for channel 1 with those related to channel 2 in parentheses.
Refer to section 8 for block diagram and schematic.
The loop oscillators consist of two PNP transistors in a basic Franklin type circuit. Q1
(Q4) is coupled to Q2 (Q5) to form the active oscillator. The loop is connected to the
oscillator circuit with the isolation transformer T1 (T2). The reflected roadway loop
inductance is resonated with the capacitor C5 (C11). Frequency modification capacitors
C3 (C9) and C4 (C10) are switched in via SW1 (SW2) and are used to change the
operating frequency of the loop circuit. Neon surge arrestor I1 (I2) provides high voltage
spike protection. When loops are connected to the oscillators and the unit is under
power, the oscillators will resonate at their natural frequency according to the
inductance of the roadway loop and the capacitance described above.
Surge protective diode D1 (D2) is used to limit voltage surges that may appear on the
circuit side of the loop transformer. The output signal from the oscillator is present on
the collector of Q1 (Q4) and is a sine wave at the oscillator frequency.
The loop frequency signals from the oscillators are routed to the squaring circuits
formed by transistor Q3 (Q6). The outputs of the squaring circuits are passed through a
low-pass filter network consisting of resistor R9 (R18) and capacitor C6 (C12). From
here, the square wave loop frequency is presented to the capture register input of the
microprocessor.
The Microchip PIC 16F73 U1 is an advanced RISC microcontroller using Harvard
architecture. The processor is being clocked with a 20 MHz crystal. This speed insures
fast instruction processing as well as a high-speed time base for the two independent
capture registers used in the period measurement of the loop frequencies.
