Multiple-frequency ultrasonic distance measurement using direct digital frequency synthesizers

In this article, an improved algorithm for multiple-frequency continuous wave (MFCW) ultrasonic distance measurement is presented. Currently, MFCW method can evaluate target distance with high resolution. This method is based on phase difference detection. It therefore avoids the traditional acoustic attenuation problems that have limited time-of-flight (TOF) methods. Previous studies used divider algorithms to generate the required multiple frequencies for the MFCW method. However. divider algorithms are limited by low frequency resolution. This limitation results in the MFCW method's restricted measurement range and frequency shift issues. This study proposes the direct digital frequency synthesizers (DDFSs) algorithm to accurately generate multiple frequencies for MFCW distance measurement. The advantages of the DDFS include fast switching times, smooth frequency transitions, and fine frequency steps. In addition, the DDFS algorithm is easily programmed into complex programmable logic devices (CPLDs) and other microprocessors. These devices are commonly used in ultrasonic distance measurements. Experimental results indicate that when measuring the range from 50 mm to 3000 mm, with temperature and humidity compensation, the combined standard uncertainty of the distance measurement is approximately 0.26 mm. Furthermore, the algorithm is simple to use and can easily be adapted for use with other microprocessors. The main advantages of this distance measurement are high accuracy, increased range and ease of implementation. (C) 2008 Elsevier B.V. All rights reserved.