Application of Digital Polarity Correlators in a Sonar Ranging System

Abstract

In robotics applications, ultrasonic transducers are frequently used as rangefinders, thanks to their low cost and small size. The pulse compression techniques were adopted to locate multiple objects at the same time and to eliminate frequent misreadings caused by crosstalk or external ultrasound sources. However, a few problems become apparent when implementing the pulse compression techniques. First, each transducer must be equipped with a processing system for the implementation of correlation functions. This may significantly increase the complexity of the total system, cutting down one of the advantages of ultrasonic systems. Second, the majority of transducers for use in air are based on piezoelectric design, which usually used over a narrow bandwidth. Among digital correlators, significant reductions of the processing cost can be achieved with a polarity correlator. However, this polarity correlation adds quantisation noise to the signals and leads to a degradation of the output signal to noise ratio. This paper presents an analysis of the polarity correlator applying to a sonar ranging system, consisting of piezoelectric transducers and a chirp filter. A numerical evaluation of the output signal shows that the signal to noise ratio degrades only a small amount comparable with that obtained by direct digital correlators.