Fast measurement of low air velocity based on a new approach to the phase modulated ultrasonic method

Abstract

This article presents new possibilities for using ultrasonic waves with phase modulation for air velocity measurement, which are obtained by using a field-programmable gate array (FPGA) to generate, acquire, and process ultrasonic signals. The designed measurement system allows for measuring the velocity of air or other gases from zero velocities with an expanded uncertainty of 0.02 m/s plus 1 % of the measured value. The main source of measurement uncertainty is the uncertainty of the calibration stand. The static characteristics are verified by moving the entire measurement system mounted on a trolley along a closed measurement chamber, in which the air movement is negligible. Due to limitations on the maximum speed of the trolley movement, the system was tested only up to 1 m/s, even though the measurement range of the system is much larger. The use of a programmable logic system enables the measurement cycle to be performed and the velocity component value to be obtained with a frequency of 200 Hz, much faster than would be possible using microprocessor systems. Thanks to the high time resolution of 5 ns, when measuring the duration of the ultrasonic signal periods, it was possible to precisely capture changes in the response of the receiving transducer and changes in the phase of the ultrasonic wave generated by the transmitting transducer.