| Mass flow rate is frequently to be measured in production, treating and trading of energy such as oil and natgas. Being an instrument to directly measure the true mass flow rate, Coriolis Mass Flowmeter (CMF) has been attracting more and more market demand in nearly thirty years, for its high accuracy and high rangebility. Especially, the market demand of CMF grows explosively in China along with the rapid development of the petrochemical industry. However, CMF is so expensive that most customers can’t afford it and have to take volume flowmeters instead, for the international market of CMF has been monopolized by few foreign companies. Such an awkward situation is even more severe in China ascribing to the obvious deficiency in stability and reliability of homebred CMFs when compared to similar foreign products. The main cause of deficiency in stability and reliability of CMF is zero drift, that is, the reading of CMF will float with changing of environment and time, which severely restricts the low flow rate accuracy of CMFs. To solve this problem, high performance materials and high-level techniques are adopted in existing products, which results in high price of CMFs at the same time. Unfortunately, zero drift has become a bottleneck of homebred CMFs for the domestic processing craft can not meet requirement.This thesis has conducted a systematic study on zero drift of CMFs, aiming to revise zero of CMF with relatively low requirement of processing craft and materials by modeling electromechanical systems. Thus, low-cost CMFs can also acquire stable zero performance with ordinary materials and conventional processing craft. Firstly, an analytical model of measuring tube of CMF has been built, which takes the material and configuration of tube as the model parameters, mass flow rate in the tube as independent variable and deduces the time difference between detection signals derived from the upstream and downstream of measuring tube. Secondly, basing on the above analytical model, further research on the mechanism and physical nature of zero drift of CMFs has been carried out, which analyses the influence of each parameter on zero drift quantitatively. Thirdly, a CMF zero drift computation model is built on the basis of others’ work of lumped mass models. This model takes the primary instrument of CMF as a black box regardless of the specific physical causes of zero drift, and calculates and compensates zero drift online without turning off measured fluid, which is achieved just by monitoring the vibration performance of primary instrument and parameters identification. At last, this thesis studied the practical work mechanism of zero drift compensation, including driving force calculation of driving electromagnet and fuzzy PID control of vibration of measuring tube. Driving force calculation enables online calculation of vibration damping of measuring tube which decides the on-off of parameter identification and online compensation. Fuzzy PID control can regain the stability of measuring tube vibration quickly and rebuilt the regular measurement of CMF quickly.Main works of this thesis are as below:1. Research on modeling method of measuring tube. The quasi static time difference method presented in others’ work can help to understand the operating principles of CMF easily and intuitively. However, it can’t be applied to precise modeling of CMF measuring tube for key parameters such as torque and torsional rigidity can’t be derived from this method. Basing on principle of virtual work and unit load method, this thesis developed the quasi static time difference method to analytically calculate the torsional deformation and time difference between signals derived from upstream and downstream of measuring tube. In the developed model, material and configuration parameters of measuring tube are the only required variables and calculations of torque and torsional rigidity are avoided which endows the model with good practical applicability and lays the foundation for research on influences of physical quantities on zero drift of measuring tube.2. Research on the mechanism of zero drift of CMFs. Being the most important component of the primary instrument, measuring tube is also the main source of zero drift. In this chapter, causes and varying patterns of zero and zero drift of CMFs are discussed, and influences of various physical parameters on the zero drift of measuring tube are calculated quantitatively basing on the analytical model built in the former chapter. Meanwhile, how the other components of the primary instrument influence zero drift is also analyzed qualitatively. This chapter offers an understanding of the physical nature of CMF zero drift and owns a reference value for design and engineering application of CMF. Furthermore, it lays foundation for zero drift compensation.3. Research on zero drift compensation method of CMFs. A lumped parameter model of CMF primary instrument is built in this chapter, which takes all components of primary instrument such as the measuring tube, driving electromagnet and detection electromagnets of upstream and downstream into consideration. In addition, calculation and compensation models of zero drift of CMFs are built on that basis and parameters of zero drift compensation model are identified by free attenuation method which is verified by experiments. Basing on this compensation model, parameters identification and online compensation of zero drift can be accomplished without cutting off the measured fluid in pipe or dismantling CMF from pipe.4. Research on practical work mechanism of CMF zero drift compensation method. When applying the zero drift compensation model built in the former chapter to practical situation, the identification of model parameters will obstruct the regular measurement of CMF. Moreover, accuracy of CMF will decrease sharply when identify parameters of measuring tube with free attenuation method. Therefore, this chapter devotes to minimize the interference with regular measurement of CMFs which is derived from zero drift compensation. First of all, an online calculation method of driving force is proposed with which the vibration damping is obtained online to decide the on-off of zero drift compensation. This method deduces the analytical algorithm of driving force generated by driving electromagnet from the space magnetic field where the multi-coils are placed. And the space magnetic field is rebuilt from the measured boundary magnetic field. In the next place, a fuzzy PID control method for measuring tube vibration has been come up with in this chapter, so CMF measuring tube can start oscillation and regain stability quickly after zero drift compensation which contributes to the quick recovery of regular measurement. Both parts of work introduced above have immensely improved the practicability of zero drift compensation method. |
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