Research On Converter Of Coriolis Mass Flowmeter In Two-phase Flows

Coriolis mass flowmeters is widely used in many fields for their excellent performance in single-phase flows and direct mass flow measurement.The difficulty of Coriolis mass flowmeters is the application for multiphase flows,such as gas-liquid two-phase flows.In two-phase flows,with the rapid changing of fluid mixture,the resonance frequency of the measuring tube fluctuates significantly,and the damping ratio also increases sharply compared with single-phase flows.For a traditional analog drive control method,it relies on the analog feedback loop to drive the measuring tube.Due to significant frequency fluctuation and reduced signal to noise ratio in the sensor signals,it is difficult for the analog feedback loop to respond to two-phase flows in a timely manner.It may result in significant and non-repeatable errors in the measurement and sometimes the analog drive control may even lose its control and the measuring tube stops vibrating.Various digital signal processing methods have been used to calculate phase difference and achieved high accuracy for single-phase flows.However,for two-phase flows,the frequency and amplitude of the sensor signal fluctuate sharply,and different digital signal processing methods are affected to different degrees.Aimed at improving the measurement of Coriolis mass flowmeters for two-phase flows,where the damping of measuring tubes increases sharply,and the resonance frequency and amplitude of vibration signals fluctuate violently,an all-digital Coriolis flow converter is developed in this paper.The converter system uses all-digital drive and realizes two digital signal processing methods for phase difference calculation.The specific research work is as follows:(1)Aimed at preventing the loss of operating points and stop of measuring tube vibration caused by traditional analog drive methods in two-phase flows,an all-digital drive method is proposed in this paper.Firstly,a single degree of freedom(SDOF)model is used to simulate the vibration system,and its equation of motion is analyzed.Following this,a control strategy is proposed where the principle of digital PLL(Phase Locked Loop)is adopted and the frequency and amplitude of the driving signals are determined by a PI controller.(2)Aimed at reducing the effects of frequency and amplitude fluctuation of sensor signals on the precision of digital signal processing methods in two-phase flows,three methods for phase difference calculation are evaluated.These include the QD(Quadrature Demodulation)method,the Hilbert method and the SDTFT(Sliding Discrete-Time Fourier Transform)method.In this paper,the principles of the three methods are firstly described in detail,and then specific improvements are applied to each method.The sensor signals under the conditions of single-phase flow and two-phase flow are established by using a random walk model.The performances of the three methods for tracking the phase difference are evaluated in the simulation.(3)Based on a latest DSP from Analog Devices,ADSP-BF706,the converter program is implemented in its development board,which completes three major functions: full digital drive,digital signal processing,USB communication.Among them,the digital signal processing module includes two methods for calculating the phase difference: the QD method and the Hilbert method.The converter program is tested under both single-phase flow and two-phase flow conditions.