| Electromagnetic flowmeter, which is based on Faraday's induction law, is a velocity-type flowmeter. Currently electromagnetic flowmeters generally adopt low-frequency square-waveform exciting mode, which can not only erase polarization effect of electrodes generated by dc exciting, but also overcome orthogonal interference produced by line-frequency sinusoidal waveform exciting, the low-frequency square waveform exciting mode is widely used. But under the square waveform exciting mode, the change rate of magnetic intensity in the coils is too high because of alternating suddenly of polarity, which produce intensive differential interference and in-phase interference, while the amplitudes of above two interferences are independent of flow rate. So at the tiny flow rate stage, these two interferences decrease signal-to-noise ratio obviously, zero-point stability and measurement accuracy.Trapezoidal waveform exciting mode can decreases dB/dt obviously, and minimizes effect of differential interference and in-phase interference. And three-value exciting mode can further improve zero-point stability and measurement accuracy by dynamically adjusting zero-point offset when exciting voltage is zero.Based on the conceptions mentioned above, electromagnetic flowmeter prototype based on three-value low-frequency trapezoidal waveform exciting mode was developed. In the thesis followed works are mentioned:(1) The basic theory, development and status of electromagnetic flowmeter were introduced in detail;(2) The electromagnetic flowmeter prototype of three-value low-frequency trapezoidal waveform exciting mode, based on MSP430 MCU was developed, and the prototype can modify exciting mode, exciting frequency and sample method as required;(3) Based on the flowmeter prototype, different exciting modes were experimented. Experiment results proved the advantage of the low-frequency three-value trapezoidal waveform exciting mode.
|
PDF Full Text Request