The Physical Mechanism, Characteristics and Countermeasures of Interference Noise of Electromagnetic Flow Meters

by:Kaidi Sensors     2022-08-05
Abstract: The physical mechanism, characteristics and countermeasures of electromagnetic flow meter interference noise are provided by excellent flow meter and flow meter manufacturers and quotation manufacturers. In order to discuss the anti-interference technology of electromagnetic flowmeter, the physical mechanism and characteristics of electromagnetic flowmeter interference noise must be analyzed and studied first, so as to adopt corresponding anti-interference countermeasures according to the characteristics of various interference noises to improve electromagnetic flow. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following are the details of the physical mechanism, characteristics and countermeasures of electromagnetic flowmeter interference noise. In order to discuss the anti-interference technology of electromagnetic flowmeter, it is necessary to analyze and study the physical mechanism and characteristics of electromagnetic flowmeter interference noise first, so as to adopt corresponding anti-interference countermeasures according to the characteristics of various interference noises, so as to improve the anti-interference of electromagnetic flowmeter. ability to interfere. 1 Power frequency interference noise Power frequency interference noise is the electromagnetic coupling between the excitation winding of the electromagnetic flow sensor and the input loop of fluid, electrode and amplifier. In addition, the power frequency common mode interference at the working site of the electromagnetic flowmeter, the power frequency string introduced by the three power supplies Mode interference, etc., the physical mechanism of its generation is the principle of electromagnetic induction. First of all, the power frequency interference generated by the electromagnetic coupling between the excitation winding of the electromagnetic flow sensor and the input loop of the fluid, electrode and amplifier has the greatest impact on the operation of the electromagnetic flowmeter, and its appearance and characteristics are different under different excitation technologies. Therefore, anti-interference is adopted. The measures are also different, as shown in Figure 1, the characteristics of this power frequency interference noise under various excitation techniques. Under the power frequency sine wave excitation magnetic field, the electromagnetic coupling power frequency interference noise is expressed as orthogonal interference, also known as the transformer potential. The signal potential is 900, and the magnitude is several orders of magnitude larger than the flow signal potential. Under the condition of low-frequency rectangular wave excitation, three-value low-frequency rectangular wave excitation and dual-frequency rectangular wave excitation, the electromagnetic coupling power frequency interference noise is manifested as differential interference, and its waveform is a pulse waveform, in which the amplitude is proportional to the rate of change of the magnetic flux , and attenuates exponentially. Generally speaking, its amplitude is much larger than the quadrature interference under sinusoidal excitation conditions. In addition, this differential interference is only generated when the excitation magnetic flux changes, and when the magnetic flux is constant, the next magnetic There will be no differential interference before the change of the channel, and it is time-periodic. For the quadrature interference noise under power frequency sine wave excitation, a complex automatic quadrature suppression system is used to reduce the influence of the quadrature interference noise, but since the quadrature interference noise is several orders of magnitude larger than the flow signal potential Any imperfection will cause a part of the quadrature interference to be converted into in-phase interference, which will make the zero point drift of the power frequency sine wave excitation electromagnetic flowmeter, and it is difficult to improve the flow measurement accuracy. Using low-frequency rectangular wave excitation, three-value low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation, the quadrature interference noise evolves into differential interference. Since the differential interference has a time period, the synchronous sampling technique is used in the constant period of the magnetic field, that is, after the differential interference decays to zero, the wide pulse synchronous sampling (even times the power frequency period) is used to avoid the power frequency interference in the flow signal potential. Impact. Secondly, the method of controlling the rate of change of the excitation current (excitation magnetic flux) is used to reduce the amplitude of the differential interference, but reduce the time interval of the flow signal sampling; the gain of the differential interference period can also be made Odb by using the programmable gain technology, while the constant magnetic flux The period gain is 100db to reduce the influence of the magnitude of the differential disturbance. Power frequency common mode interference and power frequency series mode interference are common interferences, mainly due to electromagnetic shielding defects, distributed capacitive coupling, and poor grounding of electromagnetic flowmeters. Input protection technology, high input impedance, and high common mode rejection are used. Compared with the bootstrap preamplifier technology, repeated grounding technology, and power frequency wide pulse synchronous sampling technology, it improves the ability to resist power frequency interference. 2 Electrochemical interference noise generated by fluid medium characteristics Electrochemical polarization potential interference is due to the different polarities of the electrode-induced electromotive force at the two poles, resulting in the polarization of the electrolyte on the electrode surface. Although the use of positive and negative alternating excitation magnetic fields can significantly reduce the polarization potential by an order of magnitude, it cannot fundamentally completely eliminate the polarization potential interference. Its characteristics are related to the properties of the fluid medium, the properties of the electrode materials, and the shape and size of the electrodes. It has the characteristics of slow change and small order of magnitude. As shown in Figure 2, the fluid electrochemical potential interference and its solution. Therefore, selecting the appropriate electrode material (such as tungsten carbide) and designing the best electrode shape and size is one of the effective methods to reduce the polarization potential; in addition, the square wave excitation technology with alternating positive and negative polarities is used to cooperate with the microprocessor synchronization width. Pulse sampling technology, to use the microprocessor operation function to subtract the two sampling values ​​before and after to eliminate the polarization potential interference in the flow signal potential.
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