A method for short or long range time-of-flight measurements using phase-detection with an analog circuit

This paper describes a novel method to determine accurately the distance between a transmitting transducer and a receiving transducer in an ultrasonic ranging system (URS). The accuracy of the method is similar to that achieved by phase mea surement systems, which exhibit better accuracy than the conventional ranging systems that use magnitude thresholding to measure time-of-flight (TOF). Significant contributions of the method described here are that the maximum measurement range is not limited to one wavelength (or one period) of the acoustic wave used, and that the circuitry is very simple. A one-wavelength range limit is typical for phase-based ranging systems. The maximum range is limited only by the method used to measure TOF, which in our case was the maximum count of a 16-bit counter at 1 MHz clock rate. The method described uses simple analog circuitry to extract a digital pulse with a duration equivalent to the TOF. The prototype developed is very stable and robust, with an accuracy of /spl plusmn/0.295 mm. for a range of 0.5842 in (0.05%). This accuracy can be further improved by compensating for variations in the speed of sound due to temperature changes.