| Electromagnetic flowmeter(EMF)is widely used in the measurement of water flow and solid-liquid two-phase flow,due to the absence of obstructions and moving parts in its measuring tube.When measuring solid-liquid two-phase flow,solid particles collide with the electrode to produce a non-stationary 1/f slurry noise,which seriously affects the measurement accuracy.In order to suppress the slurry noise,the excitation frequency needs to be increased.Most electromagnetic flowmeters use rectangular wave excitation,which measure the flow in the steady-state section of the signal,so it is difficult to improve the excitation frequency.The sine wave excitation EMF does not need a stable section during excitation,and has the characteristics of high excitation frequency and fast measurement response speed,so it is suitable for measuring thick slurry flow.However,the sensor coil of EMF is inductive load,and the use of high-frequency sine wave excitation will increase impedance of excitation coil and decrease the excitation current.At the same time,the differential interference generated by high-frequency sine wave excitation are strong,which will lead to the increase of zero-flow output and measurement error.In order to solve the above-mentioned difficulties in the measurement of the highfrequency sine wave EMF,this paper have developed a high-frequency resonance EMF.By forming a resonant circuit with a series capacitor in the excitation circuit,when the sine wave excitation frequency is the natural resonance frequency,the excitation circuit impedance is minimum and the excitation current is maximum.Aiming at the difficulty for resonant circuit to accurately calculate the natural resonant frequency,a resonant frequency identification method based on linear interpolation is proposed,according to the characteristic that phase difference between voltage and current of excitation circuit the resonant state is zero.Regarding the problems of strong differential interference and large zero-flow output during resonance sine wave excitation,a water flow signal processing method is proposed to suppress differential interference and reduce system zero-flow output,which based on phase adjustment.During actual flow measurement,the sensor output signal offset that randomly changes results in ADC sampling saturation,an offset adjustment method based on threshold judgment is used.An offset signal processing method based on step correction is proposed,which eliminates the step interference caused by offset adjustment and avoids the influence of offset adjustment on the algorithm.Based on the hardware system of the resonant EMF transmitter developed by our team,the transmitter software measurement system was developed,to realize the proposed water flow signal processing method and offset adjustment method in real-time.Finally,combining the developed system with a DN40 electromagnetic flow sensor,a series of water flow calibration experiments were carried out.Then water flow experiment results show that when excitation frequency is 75 Hz or 125 Hz and flow velocity is 0.5m/s?5m/s,the measurement error of this system is within ±0.5%,and the repeatability is not higher than 0.1,which meets the measurement accuracy of 0.5.When excitation frequency is 243 Hz and flow velocity is 1m/s?5m/s,the measurement error of this system is within ±0.5%,and the repeatability is not higher than 0.1.When flow velocity is 0.5m/s,the measurement error is within ±2% and the repeatability is not higher than 0.6. |
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