Improving the Performance of Loop-Sensor-Based Traffic-Information System by (M+PLL) Circuit

Abstract

Conventionally a phase-shift detection method (PSDM) and a frequency-shift detection method (FSDM) have been used in loop detectors. The PSDM has a fast response time and is very effective in detecting vehicles traveling at normal speeds. However, it is well known that the detection results are erroneous for vehicles traveling at low speeds in heavy traf®c conditions. On the other hand, the FSDM greatly improves the detector performance for heavy traf®c conditions. However, this method is not effective in fast and normal traf®c conditions. Thus, in order to collect accurate traf®c data for all traf®c conditions, this paper proposes combining two methods using the digital OR logic. In the developed circuit, a phase-locked loop (PLL) circuit is used for measuring the phase change. This paper also develops a new loop detector instrumentation method using the so-called M circuit for detecting frequency change. The developed method has been tested for various traf®c conditions. Experimental results show that the new combined M and PLL detection method greatly improves the accuracy in all traf®c conditions, reducing the error rate in measuring traf®c¯ow by more than 83%, when compared to the PSDM.