Study And Implementation Of Digital Signal Processing Software System For Electromagnetic Flowmeter

The electromagnetic flowmeter (EMF) has the advantage that the result ofmeasurement is free from the influence of density of fluid, temperature, pressure, etal. So it is increasingly used to measure the flow of conductive liquid in industry.Some well-known foreign flowmeter corporation’s production have very highwater-flow measurement accuracy and can be used to measure two-phase fluid suchas slurry. In addition, they also have complete functions in diagnose and protection.Therefore, foreign products are now the market leader in domestic market. Whilemost domestic products adopt low frequency square wave exciting mode and MCUbased low pass filter to process the output signal. This limits the precision ofmeasurement to be further improved.To overcome these limitations, digital signal processing method has beenapplied, and system software has been developed based on DSP to process theoutput signal. Spectrum analysis which has been applied to output signal showingthat the spectrum of the signal generated by single frequency square waveexcitation is combination of fundamental signal and odd harmonics. Comb-shapedband-pass filter algorithm is proposed to deal with the output signal for suppressingnoises effectively. Simulations of the algorithm are carried out by MATLAB toensure its validity. For the saturation problem of the ADC caused by the randombias of the output signal in the real measurement, a bias adjustment approach isproposed. To avoid this method affect the comb-shaped band-pass filter algorithm,the algorithm was improved. An additional motivation method is adopted to detectwhether the sensor’s tube is empty by calculate the peak to peak values of the500Hz empty tube detection pulse.The system software of EMF is developed base on TI TMS320F28335DSP toimplement the comb-shaped band-pass filter algorithm, the bias adjustmentapproach and the empty tube detection method. A series of tests and water flowcalibrations are performed. The calibration result of water flow shows that themeasurement accuracy is better than0.3%in a flow range of2.5m3/h to22.5m3/h,which satisfies the requirements of industrial measurement. The comb-shaped band-pass filter algorithm is now applied in our cooperative corporation’s productsand achieves better effect.