In actual operation of electromagnetic flowmeter, orthogonal interference and in-phase interference are caused by sudden changes in the excitation magnetic field. The necessary interference of an electromagnetic flowmeter with alternating excitation.
If the magnetic field does not change during measurement, these two interferences will be zero. Common mode interference and series mode interference are mainly caused by electromagnetic interference and electrostatic interference near the electromagnetic flowmeter. They can be suppressed through electromagnetic shielding and good grounding, and followed by a high decibel common mode rejection ratio (CMRR). The differential amplifier is ground-canceled. In addition, electromagnetic flowmeters are instruments used to measure various types of fluids. It will inevitably be used in industrial detection and control production.
At this time, the flow meter is surrounded by power frequency interference signals generated by itself or radiated by other industrial equipment, so that the final flow signal will be superimposed with power frequency signals. For power frequency interference, choose the excitation period (signal period) to be an integer multiple of the power frequency signal, then there must be two points in each periodic signal that are subject to similar power frequency interference. At this time, subtracting the signal amplitudes of the two points can eliminate the power frequency serial mode interference. After determining that the excitation period is an integer multiple of the power frequency period, the signal processing of the plug-in electromagnetic flowmeter will need to solve the following two aspects: modern intelligent instruments pursue high dynamic response speed, which requires the excitation period to be small enough (minimum is the power frequency cycle). However, an excessively high excitation frequency will make the zero point drift unstable and increase the difficulty of signal processing.
Therefore, in the signal processing of electromagnetic flowmeter, a comprehensive consideration must be given between response speed and signal stability method. In actual flow signals, the difference between microvolt level and eo (maximum of several hundred millivolts) is very large, almost more than a thousand times. At this time, if an amplifier is used to directly amplify the signal, the amplifier output will be saturated due to the presence of %, and the value cannot be measured accurately. Therefore, in signal processing, it is necessary to effectively amplify the value while eliminating the influence of eo as much as possible. At present, the signal processing methods of E+H electromagnetic flowmeters generally include capacitive isolation method, zero-point drift feedback method and three-sampling method. However, the capacitive isolation method cannot be used in high-precision measurement situations because the signal will be distorted during processing. The response time of the zero-point drift feedback method is too long and cannot be used in environments that require fast response, while the three-dimensional sampling method is based on the assumption that the zero-point drift is uniform. Therefore, these three methods may not be able to meet the requirements well in environments that require high precision and high response speed, and another signal processing method needs to be designed. The principle of electromagnetic flowmeter is Faraday's law of electromagnetic induction and is used to measure the flow rate in conductive liquids and slurries in closed pipes. The main components of the sensor are: measuring tube, electrode, excitation coil, iron core and yoke shell. Anyone who understands electromagnetic flowmeters knows that low-frequency rectangular excitation has the advantage of being able to overcome the large polarization voltage in DC excitation, and also has the advantage of avoiding orthogonal interference and in-phase interference caused by electromagnetic induction interference in AC excitation. It is an advantage that takes into account both DC excitation and AC.
An excitation method that combines the advantages of both. In theory, it makes power frequency interference, excitation phase interference, electrode polarization, zero point drift and other interferences can be overcome. However, in practice, due to reasons such as electromagnetic induction, electrostatic effects, and electrochemical reactions, the voltage output by the electrode is not only the induced electromotive force proportional to the fluid flow rate, but also contains various interference components. Therefore, it must be eliminated in the subsequent signal amplification processing part.
Hangzhou Economic Development Area，Hangzhou 310018，China
