Research And Design Of Dual-frequency Ultrasonic Doppler Flowmeter

Ultrasonic Doppler flowmeter is mainly applied to measure flux of sewage or other liquid with solid grain. It is of high resolving power, and can respond quickly with the variety of velocity, moreover, it is insensitive to the pressure and temperature of liquid. But its accuracy and stabilization must be improved in further. Dual-frequency Doppler technology can solve it. Therefore research of dual-frequency ultrasonic Doppler flowmetere is significative.On the basis of measurement principle of single-channel Ultrasonic Doppler flowmeter , referring to an amount of literatures, this paper puts forward theory of measurement of Dual-frequency Doppler flowmeter, and experiments qualitatively proved that this method is practicable. Dual-frequency Doppler measurement system is designed with a TMS320F240 DSP as the core. The system designed displays not only the velocity and flux but also the dynamic spectrum of Doppler, which is advantageous during its installation, besides a standard current (4~20mA) is available.Unlike to the conventional Doppler flowmeter, Dual-frequency Ultrasonic Doppler flowmeter generates two independent frequencies. The two frequencies are used to automatically identify signal and eliminate noise, the Doppler information is squared .This makes the Doppler information easier to identify and track.In the system, digital signal processing is introduced to analyze Doppler signal. First the data collected from each transmission contains both Doppler and noise information. FFT is required to get the power spectrum of the data. Second one set of collected data is then shifted by a ratio of the transmission frequencies. This process aligns the Doppler information in the same frequency domain, while noise is misaligned. Third once the Doppler information is aligned, the two sets of data are multiplied. As a result, the Doppler information is squared and noise falls away. This makes the Doppler information easier to identify and track. At last The algorithm of approaching peak value applies in the power spectrum. Thus Doppler frequency shift is obtained. According to Doppler effect, velocity and flux can be calculated.Experimental results show that Dual-frequency Doppler technology makes flwometer more accurate and stable.