Multi-frequency Excitation And Its Application For Coriolis Flowmeter

Coriolis flowmeters have an excellent performance in mass flow measurement.They are widely used in industrial flow measurement,which can provide multi-parameter measurements such as density,temperature and volume flow.Coriolis flowmeters rely on oscillating tubes to sense fluid flows.The characteristics of the measuring tube directly affect the flow calibration factor.However,potential erosion or corrosion can reduce the tube wall thickness and affect their measurement performance,and severe thickness reduction may even result in structural integrity issues.As the tube thickness directly affects their stiffness,a diagnostic parameter related to stiffness will aid the diagnostics of potential measurement and structural integrity issues.Coriolis flowmeters are usually used for single phase fluids,that is liquids or gases.If the gas is entrained in the liquid,the measurement accuracy may be affected by the two-phase flow.In the two-phase flow condition,the damping ratio of the measuring tube is constantly changing,and the significantly increased compressibility caused by the gas entrainment also brings difficulties to the field application of the Coriolis flowmeters.In this thesis,the following research work is carried out from two aspects: flow calibration factor and measurement error of gas-liquid two-phase flow:(1)A single degree of freedom vibration model of the measuring tube was constructed.Based on the frequency response function of the model,the stiffness calculation formula is derived.By adding an additional frequency signal to the excitation signal,and then combining the quadrature demodulation algorithm,filter design,phase-locked loop,PID control,control loop design,and finite element analysis,stiffness calculation has been achieved.(2)Under the condition of gas-liquid two-phase flow,the force analysis of the bubbles in the measuring tube was carried out.The compensation method is proposed based on the vibration difference of gas and liquid.A resonator model for measuring bubble compression in the tube was constructed.Finite element simulation and Bode diagram analysis were performed on the vibration mode of the measuring tube.The combined excitation signal of the two frequencies is used to cause the measuring tube to vibrate simultaneously in the first mode and the third mode.Vibration characteristics are used to compensate for measurement errors.(3)According to the simulation model,the control algorithm,stiffness algorithm,bubble compensation algorithm and dual-mode compensation algorithm have been implemented on a commercially available DSP hardware platform.A PC program was also developed to realize real-time monitoring and parameter modification.The stiffness algorithm and the compensation algorithm were tested experimentally.The experimental results show that the stiffness algorithm can track the change of the stiffness of the measuring tube,and the compensation algorithm can significantly improve the accuracy of the Coriolis flowmeters when measuring the gas-liquid two-phase flow.