Low cost but accurate ultrasonic distance measurement using combined method of threshold-correlation

Ultrasonic measurement of distance larger than one wavelength is generally based on the measurement of time of flight (TOF), the time that a transmitted ultrasonic pulse takes to reach the receiver. The correlation method is widely used for estimating TOF, combined with the phase shift method to reduce the measurement errors. Although producing high accuracy, the combined method is expensive due to heavy computation and complex, not flexible, and very expensive hardware needed. In this paper the proposed method uses the threshold method, for the rough TOF estimation only, but then enhances the accuracy by measuring the phase shift using the correlation method with limited data samples of one wavelength. In this way it enables lighter computation, and therefore, limited software and hardware resources can still be used. Experimental results using a pair of transducers operating at 40 kHz and the sampling rate of 500 kHz show an accuracy better than 0.5 mm with the average error of about 2%.