Electromagnetic flowmeter product characteristics and development trend

In recent years, with the rapid development of modern information and digital technology, as well as the upgrade of human electronic manufacturing technology, the product quality, reliability and functionality of various electronic instruments have been greatly improved. This article The upgrading of the electromagnetic flowmeter products also witnessed such a process. Electromagnetic flowmeter is widely used in various petrochemical enterprises and urban water supply, sewage treatment and other projects. It has the advantages of simple structure, wide range and corrosion resistance. The accuracy of electromagnetic flowmeter is a key indicator that reflects the quality of electromagnetic flowmeter products and is closely related to the accuracy of measurement results. This article starts from the development history of electromagnetic flowmeters and makes a more in-depth analysis of its four development directions. These four directions are: the structure of electromagnetic flowmeters, the excitation method of electromagnetic flowmeters, and the signal processing of electromagnetic flowmeters. method, intelligent technology of electromagnetic flowmeter. This article covers the current technological development status of these four aspects and the future development direction, as well as the future development trend of electromagnetic flowmeters. Today, as intelligent technology continues to develop and improve, future electromagnetic flowmeters will still focus on excitation optimization and signal processing technology. At the same time, the structure of the electromagnetic flowmeter will continue to change to adapt to increasingly complex measurement environments and meet individual measurement requirements. trend. For more information about the selection and product description of electromagnetic flowmeter products, please click 

1. Introduction Flow meters are realized using physical principles An instrument for measuring fluid flow over a period of time. Electromagnetic flowmeter has the advantages of wide range, corrosion resistance, simple structure, etc., and is one of the most popular types of flowmeters at present. The theory of electromagnetic flowmeters was produced in the 1920s [[21O Most of contemporary electromagnetic flowmeters are based on computer technology, and their functions are enhanced with the enhancement of computer information processing capabilities, storage capabilities, computing capabilities and computer control functions. Four directions of electromagnetic flowmeter technological innovation deserve attention: the structure of the electromagnetic flowmeter, the excitation method of the electromagnetic flowmeter, the signal processing technology of the electromagnetic flowmeter, and the intelligence of the electromagnetic flowmeter. This article takes this as a clue to summarize the development history of electromagnetic flowmeters and analyze its development trends. 2. Structure of electromagnetic flowmeter Electromagnetic flowmeter uses electrodes and fluid to form a loop to measure the electrical parameters generated in the loop. The measurement principle of traditional electromagnetic flowmeter is shown in Figure 1. An electromagnetic coil generates a magnetic field with a magnetic field strength B in a pipe of diameter d and cross-sectional area A. When fluid passes by, it will cut the magnetic field lines and generate an induced electromotive force U, and the measuring electrode receives the electromotive force signal. The flow rate can be calculated according to the formula Q=(1/k)*(UA/Bd). In the formula: Q is the flow rate; k is the correction coefficient. Traditional electromagnetic flowmeter measurement chart Since traditional electromagnetic flowmeter cannot measure fluids with low conductivity and is sensitive to friction and adhesion effectsSensing, it can only measure the fullness of the fluid pipe, etc., so its structure needs to be changed so that it can adapt to more complex environments. The main method to change the structure of the electromagnetic flowmeter is to change the number and position of the electrodes, thereby forming a capacitive electromagnetic flowmeter, a non-full tube electromagnetic flowmeter, etc. 1.1. Capacitive electromagnetic flowmeter Capacitive electromagnetic flowmeter fundamentally solves the problems of electrode surface adhesion, corrosion, friction and other problems. The electrode and the measured fluid are separated by an insulating lining, or an insulated measuring tube is directly used.

The electrode is placed outside the measuring tube or embedded inside the measuring tube. The structures of the embedded electromagnetic flowmeter and the external electromagnetic flowmeter are shown in Figure 2. Structural diagram of two electromagnetic flowmeters. The electrode and the measured fluid pass through the insulating tube to form a detection capacitor, and the flow signal is coupled through this capacitor. Its main structural form can be divided into two types according to the installation position of the electrode: the electrode is embedded inside the insulating lining of the measuring tube (embedded type), and the electrode is attached to the outside of the measuring tube (external type). The embedded structure is similar to the structure of ordinary electromagnetic flowmeters, while the external type mostly uses ceramic surface metallization technology to attach the electrode to the outside of the measuring tube. 1.2 Non-full pipe electromagnetic flowmeter Ordinary electromagnetic flowmeter can only measure the flow of the full pipe flow. In many cases, due to the high flow rate, sometimes the pipe is not full, the ordinary electromagnetic flowmeter cannot be applied, so it is hoped that the electromagnetic flowmeter can Measurement of partial pipe flow. Currently, the common non-full tube electromagnetic flowmeters on the market include the following types. ① Multi-electrode non-full tube electromagnetic flowmeter. There is a pair of signal injection electrodes at the bottom, multiple pairs of measurement electrodes in the middle, and a full tube electrode at the top. When the pipe is full, the flowmeter has the same function as an ordinary electromagnetic flowmeter. When the pipe is full, the cross-sectional area of ​​the fluid is fixed. At this time, calculating the flow value only requires measuring the flow rate of the fluid. When the pipe is not full of fluid, the full pipe electrode detects that the pipe is not full, and uses an algorithm to correct the measurement value. At this time, the measurement method of the flow meter is changed to measuring the fluid flow rate and liquid level height. The signal injection electrode works together with three pairs of measuring electrodes at different positions to measure the height of the liquid level and the velocity of the fluid. The structural diagram of the multi-electrode non-full tube electromagnetic flowmeter is shown in Figure 3. Structural diagram of multi-electrode non-full tube electromagnetic flowmeter ②Capacitive type non-full tube electromagnetic flowmeter. The structural diagram of the capacitive non-full tube electromagnetic flowmeter is shown in Figure 4. Structural diagram of capacitive non-full tube electromagnetic flowmeter. Capacitive non-full tube electromagnetic flowmeter uses changes in liquid level to change the pole distance of the capacitance. It can be measured by measuring the capacitance coupling value between the sending electrode and the detection electrode. flow value. ③ Non-full tube electromagnetic flowmeter developed using impedance or signal attenuation. The non-full-tube electromagnetic flowmeter with this structure is one of the current domestic research directions. Its structure is that a pair of signal transmitting electrodes are attached to the bottom of the flow tube, and a signal receiving electrode is attached to the middle of the flow tube. Since the signal propagates in the fluid, it will attenuate, and the longer the propagation time, the greater the attenuation. Therefore, the liquid level height can be known by the signal attenuation received by the signal receiving electrode; at the same time, the electrode can also measure the magnetic field lines cut by the fluid. The electromotive force generated is used to measure the flow rate of the non-full pipe. impedanceThe structural diagram of the non-full tube electromagnetic flowmeter with signal attenuation or signal attenuation is shown in Figure 5. Structural diagram of impedance or signal attenuation non-full tube electromagnetic flowmeter ④ Intelligent non-full tube electromagnetic flowmeter. This kind of flowmeter is one of the directions for the intelligent development of electromagnetic flowmeters. Two excitation coils with different connection methods are used, and the principle related to the weight function and the geometric position is applied to establish the functional relationship of the liquid level, and then the liquid level is obtained through online calculation. Jiang Yulin and Ding Wenbin improved the calculation method of weight function and induced potential.

For a non-full pipe flowmeter, since its fluid distribution is different from that of an ordinary electromagnetic flowmeter, its weight function is also different. Wei Kaixia and Li Bin conducted a finite element numerical analysis of its weight function in the case of a non-full pipe. Obtain the weight function under different liquid levels. In addition, there are electromagnetic flowmeters with other functions, such as binary electromagnetic flowmeters that change the information transmission channel and string together signal lines and power lines, submersible electromagnetic flowmeters used to measure channels, in order to reduce power consumption and improve excitation. Different-diameter electromagnetic flowmeters designed for efficiency and sensitivity, split-flow electromagnetic flowmeters for oil-water two-phase flow measurement, and other electromagnetic flowmeters. 3. Optimization of excitation method The choice of excitation method affects the accuracy, energy consumption and other parameters of the entire flow meter system. Therefore, after the structure of the electromagnetic flowmeter is determined, the selection of the excitation method is particularly important. The excitation method can be divided into two basic forms, that is, the form of using alternating magnetic field (including sine wave excitation, rectangular wave excitation, three-valued wave excitation and dual-frequency rectangular wave excitation) and the form of using constant magnetic field (including DC power excitation and Permanent magnet excitation. 2.1 Alternating magnetic field excitation. Power frequency sine wave is the earliest excitation method used in electromagnetic flowmeters. It has fast measurement speed and is less affected by electrochemical reactions. However, due to its high frequency, it is easy to produce in-phase noise and differential due to eddy currents. Noise compensation is difficult and the zero point is easy to drift. Low-frequency rectangular wave excitation has the advantages of simple implementation, stable zero point, and resistance to power frequency interference, and has become the main excitation method used by flowmeter manufacturers for measuring fluid speed and slurry measurement in actual production applications. With the improvement of accuracy requirements, low-frequency excitation can no longer meet the requirements, so high-frequency square wave excitation and dual-frequency rectangular wave excitation are proposed abroad. Although high-frequency square wave excitation or dual-frequency rectangular wave excitation can effectively overcome interference such as slurry noise and flow noise, Improve the measurement speed, but the core technology of the high-frequency excitation part has not been disclosed. There is no domestic manufacturer that can provide products with independent property rights, and there are few relevant documents. Although the dual-frequency rectangular wave excitation has both high-frequency measurement speed and high speed. It has the advantage of good low-frequency stability and is not sensitive to flow noise, but it will increase power consumption due to the need to execute complex algorithms. Liu Tiejun and Gong Tongsheng improved it based on dual-frequency excitation research and proposed a time-division method.

Dual-frequency excitation method. This method not only takes into account the advantages of low frequency and high frequency, but also can achieve high-precision measurement of flow within a wide measurement range. 2.2 Constant magnetic field excitation Compared with the alternating magnetic field excitation method, constant magnetic field excitation is used. The excitation method is simpler to implement and is less affected by power frequency interference, and the use of constant magnetic field excitation can simplifychemical sensor structure. The key problem with constant magnetic field excitation is that electrochemistry and other factors will produce severe polarization on the measuring electrode of the electromagnetic flowmeter, causing polarization voltage to be generated at both ends of the measuring electrode. If the polarization voltage is too large, the induced electromotive force generated by the measurement signal will be submerged. Alternating magnetic field excitation can eliminate electrode surface polarization by continuously changing the direction of excitation. Therefore, most electromagnetic flowmeters at home and abroad currently use alternating magnetic field excitation. The constant magnetic field excitation method is used in flow measurement of liquid metal with high conductivity, extremely small fluid internal resistance, and no polarization effect. In order to overcome the polarization phenomenon on the electrode surface, the currently used methods can be divided into the following two types. ① Starting from the principle of polarization voltage, analyze the correlation of the polarization voltage on the two electrodes and fundamentally eliminate the influence of the polarization voltage, such as the differential contrast elimination polarization voltage method. However, because there are many factors affecting the polarization voltage, and its randomness is much greater than the induced electromotive force that reflects the flow signal, its effect of eliminating polarization is not ideal. ②The other is to avoid the principle of polarization voltage and try to control the polarization voltage at a stable value without affecting the measurement of fluid induction signals, such as the relay capacitance feedback suppression polarization method. Zhejiang University proposed a new method that uses a rapidly changing alternating electric field on the electrode to suppress the polarization voltage, and this alternating electric field is only excited during the non-sampling time period. Shanghai University has proposed another feedback method, that is, a method of dynamic tracking and feedback of equal amounts of energy on the measuring electrode to eliminate the electrode polarization problem of the magnetic steel excitation electromagnetic flowmeter. Currently, this method is the focus of current research on constant magnetic field excitation methods. 4. Improvement of signal processing method The electromagnetic flowmeter achieves the purpose of measuring flow by collecting electrical signals over a period of time. In this way, various interference signals will inevitably be mixed during the measurement process, so the signal detection and processing method is improved. becomes particularly important. 3.1 Ordinary electromagnetic flowmeter signal processing The detection and processing of signals is actually amplification, collection and interference suppression of signals. Research on signals mainly focuses on interference suppression. The interference of electromagnetic flowmeter mainly includes polarization voltage interference, power frequency interference, electrochemical interference, fluid collision interference, differential interference, zero point drift, etc. In addition, some studies have found asymmetric flow of fluids. The asymmetry of the electrodes and excitation coils will also produce corresponding measurement errors. Many domestic institutions have done a lot of research in these aspects. For example, Shanghai University proposed a feedback signal amplification processing method, which uses rectangular wave excitation to overcome the interference caused by polarization voltage and power frequency, and increases the excitation frequency or changes the excitation method to overcome electrochemical interference and interference caused by fluid collision with pipes. Zhou Zhen, Wang Qiang and others separated the interference signal and the flow signal by modeling the inter-electrode signal of the flow meter, determined the IA value in advance to perform bias adjustment to suppress the interference caused by low-frequency drift, and used the digital-analog hybrid optimal filtering method to eliminate the differential interference. For the constant magnet excitation method, interference mainly comes from polarization voltage interference and zero-point drift interference. The method to eliminate zero-point drift interference is the capacitive isolation method., feedback signal processing method one and three sampling zero drift elimination methods, etc. Shi Bingxin and Li Jingyun announced an electromagnetic flowmeter that uses photoelectric transmission signals, which can effectively reduce interference during the transmission process. 3.2 Capacitive electromagnetic flowmeter signal processing The electrode part of the ordinary electromagnetic flowmeter is in contact with the measured liquid with a metal conductor, and the fluid will generate collision noise on the electrode when it flows. The capacitive electromagnetic flowmeter developed later does not have the electrode part in direct contact with the fluid being measured, but induces the induced electromotive force of the fluid through the pipe wall, which fundamentally solves the problem of stray noise. However, since the capacitive reactance of the coupling capacitor is the main signal internal resistance of the capacitive electromagnetic flowmeter, its coupling capacitance value is very small and the internal resistance is large, so the measured signal signal-to-noise ratio will be very small. In order to obtain a higher signal-to-noise ratio, a high input impedance preamplifier and a high common-mode rejection ratio differential amplifier must be used to perform impedance conversion and amplification of the signal. Currently, there are two signal detection methods: direct detection of induced voltage and detection of current through "virtual ground".

The voltage detection method technology is mature, but it is greatly affected by fluid factors. The detection current method obtains high potential through "virtual ground" and appropriate resistance value, calculates the capacitance through port = CE, and finally obtains the current value through differentiation. This method can fundamentally eliminate the influence of capacitor leakage current, but this method is greatly affected by changes in coupling capacitance value, and the circuit is complex, so it is generally rarely used. Lu Guofeng, Wang Baoliang and others introduced the cross-correlation detection method. The cross-correlation detection method is based on the property that the cross-correlation function is related at the same frequency and uncorrelated at different frequencies. Through the cross-correlation operation, the effect of filtering out the noise is achieved. Once the frequency of the transmitted signal is known, a reference signal of the same frequency can be sent out at the receiving end, and the weak measurement signal can be extracted by correlating it with the chaotic signal. In subsequent data processing, they used a rotating capacitor filter based on the correlation detection principle. This circuit has strong anti-interference ability and a high signal-to-noise ratio. Due to the emergence of smart electromagnetic flowmeters, more and more signal processing technologies are no longer pure circuit filtering, but more software filtering is used. For example, Matlab can be used to process signals online to effectively reduce interference, or Use wavelet transform to process signals to suppress interference, etc. 5. Intelligentization of flowmeters With the development of microprocessors, electromagnetic flowmeters are also developing in the direction of intelligence. Its intelligent direction can be divided into signal processing intelligence and control intelligence, both of which work together to form an intelligent electromagnetic flowmeter. Its main technologies include software technology, self-diagnosis function, program-controlled amplifier technology, microprocessor anti-interference technology, etc. Software technology is a symbol of intelligent signal processing, that is, the entire working process of the electromagnetic flowmeter is controlled through software. Digital filtering, nonlinear fitting, and zero-point self-correction are common technologies. Digital filtering can complete filtering functions that analog filtering cannot complete, such as: pulse interference elimination, digital circuit burr interference elimination, A/D converter resistance to power frequency, and ensuring the reliability of digital input to the microprocessor.Reliability. In addition, online data analysis and data reconstruction are also one of its research directions, such as using wavelet transform to separate flow signals and slurry signals in slurry fluids, and signal processing methods using notch filter banks. The electromagnetic flowmeter is a non-interference measurement, and its measuring electrode is prone to wear, corrosion, scaling and other phenomena after contact with the fluid. These phenomena will greatly affect the measurement accuracy of the electromagnetic flowmeter. In order to facilitate disassembly and maintenance, the electromagnetic flowmeter has a self-diagnostic function.

Its functions are increasing, including signal linearity, excitation circuit integrity and accuracy (including excitation coil resistance and excitation current), monitoring and diagnostic processes, and changes in environmental conditions (such as whether the liquid conductivity changes, the fluid The content of air bubbles and solid particles in the electromagnetic flowmeter then emerged. A self-diagnostic technology that can detect excitation current abnormalities without changing the structure of the electromagnetic flowmeter can realize automatic conversion of the electromagnetic flow metering range, and the gain control method can effectively weaken it. The problem of differential interference peak overloading the amplifier facilitates flow signal potential processing and improves the ability to resist differential interference. Previous anti-interference technology solved various interference problems between input and output. However, when electromagnetic flow meters are introduced into intelligent systems, Various interferences from the microprocessor will also affect the accuracy of the measurement results, and even cause the entire flow measurement system to run out or collapse. Currently, a combination of software and hardware is often used at home and abroad to improve the anti-interference ability of the microprocessor. The best software anti-interference methods include: software instruction redundancy measures, software trap anti-interference methods, software "watchdog" technology, etc. Pure software anti-interference will waste a lot of CPU power, so use hardware to eliminate most interference first. Commonly used hardware anti-interference include: photoelectric isolator, grounding technology, power-down protection technology, etc. 6. Conclusion In recent years, electromagnetic flowmeters have continued to develop with the increase in demand. Among many electromagnetic flowmeter technology developments, the author believes that in the future. The development of electromagnetic flowmeters still focuses on excitation optimization and signal processing technology. At the same time, electromagnetic flowmeters will continue to add various intelligent functions to cope with more and more complex measurement environments.