Central Frequency Detecting Type FM Ultrasonic Flowmeter Using Regenerative Modulation Elimination Method

This paper reports on a new measuring principle and measurement accuracy of FM ultrasonic flowmeter which utilizes the elimination method of FM components to detect the phase difference between the transmitter carrier signal and the FM eliminated received signal. FM ultrasonic signal is used for the field stabilization, and examination experiments for the stabilization of the FM sound fields in a measuring pipe are also reported. The stabilization method is the one similar to the so-called warbletone that is used in the indoor measurement of the sound pressure. The proposed measuring method is higher in sensitivity than the method to detect the modulated signal component because the flow rate is measured by the central frequency (say, 183kHz) component of FM ultrasonic. To eliminate FM components with simple electronic circuits that can eliminate FM components in accordance with change of modulation has such advantage that there is no restriction on the modulation index. Next, it is specified through an experiment that the FM ultrasonic gives the sound wave stabilized for the measurement in a pipe because of decreasing in standing wave ratio. The resultant accuracy was good (error: about±1%) for a small flow rate. Therefore, it can be concluded that the present FM ultrasonic flowmeter is with sufficient stability and high sensitivity.