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Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II

APPENDIX I. DIGITAL RATIOED PERIOD-SHIFT ELECTRONICS UNIT ANALYSIS

ABSTRACT

Appendix I describes the derivation of the digital ratioed period-shift electronics unit sensitivity, which is shown to be equal to the threshold count Npt divided by 2. The response time is identical to the response time of the digital period-shift electronics unit. Thus, the response time is proportional to the threshold count divided by the product of the sensitivity and the frequency of the crystal clock oscillator.

ANALYSIS

The ratioed period- shift electronics unit utilizes the change in inductance caused by a vehicle passing over a loop to measure the altered electronics unit oscillator frequency using a counter to count the number of pulses that occur during the electronics unit oscillator period and a variable threshold count to make the sensitivity independent of the oscillator frequency.

A simplified block diagram of a single channel, digital ratioed period-shift electronics unit is illustrated in Figure I-1. This electronics unit is similar to the digital period shift electronics unit except for the variable threshold count, which allows the sensitivity to be independent of the electronics unit oscillator frequency.

Figure I-1 shows that the inductance at the detector terminals L sub D feeds into the detector oscillator, which in turn feeds into the detector oscillator counter. The detector oscillator counter is also adjusted by the sensitivity control. multiplier. The period counter counts the counts the time for a fixed number of loop cycles. Memory update logic takes the value required to indicate that a vehicle is present and uses it to adjust the threshold sensitivity capital S sub T. A comparison unit compares the time for the count with no vehicle present with the actual time. This delta difference is then compared to the threshold sensitivity and if it exceeds the threshold, a call is output.

Figure I-1. Digital ratioed period-shift electronics unit block diagram.

The ratioed period-shift electronics unit sensitivity SDpis given by

Equation I-1. Capital S subscript Capital D superscript P equals negative 2 times Capital N subscript P T divided by Capital N subscript P C. (I-1)

where the threshold count Npt is a constant and the count established by the period counter Npc is inversely proportional to the electronics unit oscillator frequency. The oscillator frequency is a function of loop inductance.

A variable threshold Npt is selected by multiplying the count established by the period counter by a preset sensitivity factor. Thus,

Equation I-2. Capital N subscript P T equals Capital S subscript T times Capital N subscript P C. (I-2)

or

Equation I-3. Capital N subscript P T equals negative 2 times Capital S subscript T times Capital N subscript P C divided by Capital N subscript P C which in turn equals negative 2 times Capital S subscript T. (I-3)

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