Study Of Signal Processing And Driving Methods Of Coriolis Mass Flowmeter

The Coriolis Mass Flowmeter (CMF) is widely used in industries, because it has many advantages - high measurement accuracy, being able to measure fluid mass flow directly and measure various parameters of fluid, etc. Originally analog signal processing methods are used for CMF, but some companies and institutes devote themselves to digital signal processing methods because they have many advantages over analog methods. Moreover, the driving system is an important part of CMF. It supplies the driving force for flow tubes, and makes flow tubes oscillate at their natural frequency and the fixed amplitude. At the same time the oscillation frequency of the driving system can change with the natural frequency of flow tubes. On the basis of our lab's previous achievements, the author has studied three kinds of digital signal processing methods - the method based on a digital phase locked loop, the method based on quadrature demodulation, and the method based on Adaptive Funnel Filter (AFF) and Sliding Goertzel Algorithm (SGA). The author has also studied the CMF driving principle, various analog driving circuits, driving waves, and digital driving methods.According to the features of the signal of CMF, i.e. the signal frequency varies in a small range and the signal is subjected to the harmonics, a digital method based on a digital phase locked loop is used to process the signals of CMF, tracks the change of the signal frequency and calculates the phase difference. The technique proposed by the U.S. patent has been improved and made the simulation. The simulation results show the method can attain our aim under harmonic condition. Then the calculation errors have been analyzed.The method based on quadrature demodulation is studied and the equations used to calculate the frequency, magnitude and phase (difference) of signals are given. The author has designed two kinds of low pass filters (LPFs) and has studied two kinds of frequency-tracking methods. The theoretical analysis and simulation results show that the method is only affected by noise whose spectrum close to that of the signal. The accuracy will be improved if better LPFs are designed.A new kind of method is proposed for the signal processing of CMF in this paper. The method, combining the AFF with SGA, can accurately track the signal frequency and calculate the phase difference between two signals. Compared to other methods, the proposed method has the following advantages. Firstly, it can attain high steady-state estimation performance and good tracking properties at the same time. Secondly, the real time performance is very good. Thirdly, it is less prone to numerical overflow problems in the fixed-point arithmetic implementation. Lastly, it is quite suitable for the time varying sinusoidal signal estimation.Now the driving system of CMF works in the analog mode. The operation principle of the driving system is analyzed, various driving signals are compared from power consume,feasibility, and driving frequency. Then several kinds of driving circuits are studied in this paper.Digital driving methods are also discussed in the article, which are the method based on the nonlinear model, the method based on a MDAC, the method based on a DAC and a MDAC and the method based on synthesized wave. According to the problems of the method based on synthesized wave, the author proposes a digital driving algorithm, which can make the flow tubes start to vibrate and track automatically the change of the natural frequency.