Electromagnetic interference of electromagnetic flowmeter
是指根据法拉第电磁感应定律，用来测量导电性液体体积流量的仪表。 The so-called electromagnetic flowmeter refers to the instrument used to measure the volumetric flow of conductive liquids according to Faraday's law of electromagnetic induction. It can measure fluid flow in laminar, turbulent, pulsating flow and streamline vibration. The flow tube is smooth, the pressure loss is small, the measurement range is wide, the accuracy is high, and the response is sensitive. It can obtain a signal proportional to the flow rate. In addition, the measurement tube is straight through without dead angles, which is convenient for cleaning and disinfection of fire bacteria. It has been widely used in medicine and food industry for special hygienic requirements. The potential value induced in the liquid is very small, so a high-amplifier amplifier must be introduced. However, this makes the electromagnetic flowmeter particularly vulnerable. The disturbance of the external electromagnetic field, even if a very weak signal is amplified at a high magnification, will have a huge impact on the result. This will inevitably greatly affect the accuracy of the instrument, and it will also have a great impact on the stability and reliability of the control system. Big hidden danger. Therefore, it is particularly necessary to study the electromagnetic interference and corresponding countermeasures of electromagnetic flowmeters. Here are a few mature ideas to discuss with you
1 electromagnetic flowmeter measurement principle
According to Faraday's law of electromagnetic induction, when a conductor moves in a magnetic field and cuts the magnetic field lines, an induced electromotive force is generated at both ends of the conductor. Set in a uniform magnetic field, there is a pipe with a diameter D perpendicular to the direction of the magnetic field. The pipeline is made of small magnetically permeable material, and the inner surface is lined with insulation. When a conductive liquid flows in a pipeline, the conductive liquid will cut off the magnetic field lines, and an induced electromotive force will be generated in a direction perpendicular to the magnetic field and the direction of flow. If a pair of electrodes are installed on both ends of the pipe section perpendicular to the diameter of the magnetic field, call one to prove that as long as the flow velocity in the pipe
v is an axisymmetric distribution, and an induced electromotive force is generated between the two stages:
E = BDv (}
The volume flow of the available pipeline is:
Q =} rD * D / 4y⑦
Comprehensive ① and Japanese won:
E = 4BQ / two'1 = KQ③
K is a meter constant, and K is a constant when the pipe diameter D has been determined and the magnetic induction B is maintained constant. At this time, the induced electromotive force has a linear relationship with the volume flow.
2 excitation method
的上作原理可知，为使式③成立，第一个必须满足的条件就是要有一个均匀恒定的磁场，才能保证感应电动势E与体积流量Q具有线性关系，这就需要选择合适的励磁方式，也就是产生磁场的方式，一般有三种励磁方式可供选择，即直流励磁、交流励磁、低频方波励磁 It can be known from the principle of the electromagnetic flowmeter that in order for formula (3) to be true, the first condition that must be met is to have a uniform and constant magnetic field to ensure that the induced electromotive force E has a linear relationship with the volume flow rate Q. This requires the selection of a suitable Excitation method, that is, the method of generating a magnetic field, there are generally three types of excitation methods to choose from, namely DC excitation, AC excitation, and low-frequency square wave excitation.
3 suppression and elimination of ten disturbances
This system needs to be used in an inferior on-site environment. It has high requirements for resistance to interference and reliability. In the process of system development and design, the system will improve resistance to interference and reliability. As an important content. According to the characteristics of general electromagnetic flowmeter systems, it is necessary to discuss the hardware optimization aspects, such as electromagnetic coupling, and electrostatic induction is an important source of noise from electromagnetic flowmeters. In the electromagnetic flow transmission, the two-electrode leads are in an alternating magnetic field. When the transmission is powered on, the induced electromotive force generated in the closed loop of the leads is superimposed on the measurement signal, which affects the operation of the system. The generation of various excitation methods will cause the same problem of electromagnetic interference. The DC excitation method is prone to polarized interference, and the AC excitation method is prone to quadrature interference ((90 degrees ten power, in-phase ten interference). The following is to explain the ten disturbance problem and its suppression under AC excitation.
Application of electromagnetic flowmeter in petrochemical industry With the rapid development of computer technology, the petrochemical instrument automatic control system is gradually developing towards digitalization, networking, modeling, and intelligence. Based on the development of existing information systems, petrochemical enterprises continue to deepen their comprehensive automation systems, strengthen the application of security control systems, and improve their basic automation and advanced control levels to enhance their market competitiveness. Application of new automatic detection and analyzer: The development of domestic and foreign instrumentation systems towards digitalization, intelligence, networking, and miniaturization has enabled petrochemical enterprises to improve the application level of automatic detection instruments. In order to meet the requirements of fieldbus control systems, fieldbus transmitters have developed rapidly. This transmitter is all digital, with simple structure, higher resolution and stability than general intelligent transmitters. As fieldbus digital instrument products become more and more complete, and have the characteristics of high reliability, interoperability (that is, the integration of different brands of products), etc., they will be more applied in the field of petrochemical process control. The development of commercial trade at home and abroad requires the improvement of measurement accuracy of commodity delivery. The accuracy of mass flowmeters used in petrochemical factory metering applications is ± 0.1% or higher. Petrochemical companies have also promoted the application of online analytical instruments to strengthen product quality management. It will directly affect the product quality and advanced control application level of petrochemical enterprises, so it has been actively promoted by petrochemical systems, mainly including online oil quality analyzers, online gas or liquid chromatographs and other physical property analyzers. New The on-line multi-channel near-infrared (NIR) spectrum analyzer has been applied to the refinery blending system of petrochemical enterprises and achieved good results. The new-generation laboratory low-cost gasoline quality indicator rapid tester has been successfully used in Sinopec Hangzhou Refinery and other units, and has been well received by users. Soft-sensing technology is also developing rapidly, and it is mainly used to solve some analysis and detection problems of petrochemical enterprises. Due to the increasing requirements for environmental protection, the application of environmental protection instruments has also increased, such as online flue gas analysis, comprehensive water quality analyzer, online CY-2C zirconia analyzer, DO analyzer and pH analyzer. At present, there is a great demand for online analytical instruments in petrochemical companies. Compared with foreign products, there is a certain gap between domestic online analytical instruments. Most of the online analytical instruments in use today are imported products.
在满足现场显示的同时，还可以输出4～20mA电流信号供记录、调节和控制用，现已广泛地应用于化工、环保、冶金、医药、造纸、给排水等工业技术和管理部门。 While satisfying the on-site display, the electromagnetic flowmeter can also output 4-20mA current signals for recording, adjustment and control. It has been widely used in chemical industry, environmental protection, metallurgy, medicine, papermaking, water supply and drainage and other industrial technology and management departments . Petrochemical is paying more and more attention to the maintenance work of the factory, and especially interested in predicting maintenance issues. Some companies require on-line and real-time sensing systems for indicators that monitor the efficiency, vibration, and corrosion of heat exchangers and furnaces, and assess "health" conditions. The use of instruments and systems with diagnostic and predictive maintenance capabilities can increase the production potential of existing equipment by 1% to 3%, while reducing unplanned maintenance costs by 10% to 30%. The electromagnetic flowmeter is a high-precision, high-reliability and long-life flow meter. Therefore, we have paid great attention to each link in the process of designing the product structure, selecting materials, formulating processes, manufacturing and assembly, and factory testing. China's most advanced production equipment and flow real-time calibration device dedicated to electromagnetic flowmeters can ensure the long-term high quality of the product in terms of software and hardware. The electromagnetic flowmeter is specially designed with a Chinese LCD display with a wide backlight temperature range. It is complete and practical, intuitive to display, and easy to use, which can reduce the inconvenience caused by the English menus of other electromagnetic flowmeters. In addition, our exclusive design of 4-6 multi-electrode structure further guarantees measurement accuracy and does not require a grounding ring at any time, reducing the volume of the instrument and the trouble of installation and maintenance. An electromagnetic flowmeter that uses the principle of electromagnetic induction to measure the flow rate of a medium fluid. It applies a magnetic field on both sides of the pipe, and the measured medium flows through the pipe to cut the magnetic field lines, which generates induced potentials on the two detection electrodes, the size of which is proportional to the speed of the fluid. The electromagnetic flowmeter is characterized by no moving parts and parts protruding from the fluid. The flow rate of various suspended matters, vapors and viscous substances such as rubber latex, synthetic fiber pulp and photosensitive latex. The electromagnetic flowmeter has good sealing performance and can also be used in tap water and underground water channel systems. And the measurement process is not in contact with the fluid, suitable for the pharmaceutical, biochemical and food industries. This flow meter also measures blood flow. Its range ratio is about 100: 1, the accuracy is generally 1%, and the high-precision electromagnetic flowmeter can reach 0.1% (mean square error). Because this sensor must maintain a certain proportional relationship between the resistance in the pipeline and the impedance of the measurement circuit, it is difficult to manufacture. Difficulties begin to occur when the conductivity of the measured medium is about 10 ohm · cm, and principle difficulties arise when the conductivity is lower. When the electrical conductivity is 10 ohm · cm, the "demarcation line" between the conductive medium and the dielectric is reached, and the thermal noise level increases significantly as the internal resistance increases.