Signal Modeling And Processing Of Electromagnetic Flowmeter For Slurry-flow Measurement

Electromagnetic flowmeter is based on electromagnetic induction principle, and is often used to measure water flow and two-phase flow. When the electromagnetic flowmeter with low-frequency square-wave excitation measures the water flow, its output signal is steady and measurement accuracy is high. But when it measures the slurry flow, its output signal fluctuates largely and the measurement accuracy is low because of effect of slurry noise. The electromagnetic flowmeter made by China cannot measure the slurry-flow accurately at present, and the effective method to deal with slurry-flow is not proposed in China.In order to study the characteristics of slurry-flow signal, an electromagnetic flowmeter is excited by 25Hz square-wave, and both the water-flow and slurry-flow experiments are performed. The output signal of electromagnetic flow sensor is analyzed, including amplitude demodulated, calculation of amplitude probability density and fitting of function. The results show that the signal amplitude probability density of pure water is consistent with Gaussian distribution and that of slurry is close to Gaussian distribution.According to two types of signal amplitude probability density models and the random inherent characteristics of slurry interference, the demodulated results of slurry flow signal is proposed with 38 half-cycle sliding filtering, and more accurate amplitude is obtained. And then a slurry-signal processing method based on model is proposed for electromagnetic flowmeter. The method is as follows: demodulate the amplitude of slurry signal, calculate the amplitude basis; reconstruct flow signal through amplitude basis; use comb filter to filter reconstructed signal, demodulate the filtered signal; perform 3-second sliding filter for demodulated amplitude; output the amplitude.The model based slurry-signal processing method of electromagnetic flowmeter is realized by Matlab language, and is applied to different signals, such as the constructed signal, actual water-flow signal and actual slurry-flow signal. The results show that the method studied in this paper can reduce the interference of slurry noise effectively, follow the changing flow and output flow rate steadily. On this basis, a DSP-based slurry-flow signal processing system is developed, and the experiments are conducted. The experimental results show that the steady-state output volatility of the system is less than 5% when the slurry-flow is constant; the response time of the system is less than 4 seconds when the slurry-flow changes.