In electromagnetic flowmeter, flow and interference signals are present simultaneously and mutually. Therefore, in the testing process, we need to remove the interference signal to extract a more accurate flow signal and to carry out more accurate electromagnetic flow calculation.
We found that: after a lot of repeated tests, electromagnetic flowmeter in the work process will be subject to interference from multiple parties, mainly three kinds:
(1) Flow electrochemistry generated by the interference noise.
(2) Electromagnetic coupling process generated by the electrostatic induction.
(3) Power generated by the interference noise.
Various interference components fill the flow signal corners, so the voltage signal measured by the sensor contains more noise.
1. Various interference generation mechanisms
1.1 Quadrature interference
In Figure 1, we can observe that a closed loop is formed between the two electrodes, the excitation coil, and the internal resistance of the converter, which is called the primary winding.
Theoretically, the magnetic lines of force B should be parallel to the magnetic induction lines generated by the excitation coil. Still, in practice, this is not possible due to process differences. However, in practice, the ideal situation cannot be achieved due to differences in the process, and the magnetic lines of force may cross the magnetic induction lines of the excitation coil. It results in a corresponding induced electromotive point, referred to as the transformer effect.
When the current passes through the excitation coil, its steady state will change in stages. It is because electromagnetic induction pervades all processes in the operation of electromagnetic flow meters. Therefore, the constant state of the current flowing through the excitation coil undergoes a long process when it is transformed.
1.2. In-phase interference
Electric and magnetic fields can be converted in time by electromagnetic induction. As the fluid flows in the magnetic field, it will continuously cut the magnetic induction lines. It results in a closed orthogonal interference eddy current. In addition, a secondary magnetic flux will be formed, including another fast quadrature interference eddy current inside the fluid.
1.3. Serial mode interference
Before the 1960s, the sensor in an electromagnetic flowmeter usually used a single-ended signal transmission method. It uses two measuring electrodes for signal transmission from one electrode to the other.
This signal transmission method can result in the superimposition of several interfering signals during the transmission process. In this case, there are two situations: one is that the superposition of multiple signals leads to the work of the preamplifier to saturate the normal process, and the other is that more time is needed for interference removal in the process of signal extraction.
Contemporary electromagnetic flowmeter in the sensor replaced the signal transmission method, the use of a differential way for signal transmission, and the measured fluid as a signal transmission end. And the two electrodes as another signal transmission end. This transmission method can effectively suppress the electromagnetic circuit generated in the case of series mode interference.
1.4. Common-mode interference
Common mode interference is caused by the same interference in the amplifier at the front end of the converter. Under normal circumstances, common mode interference does not significantly affect the measurement results.
However, if the parameters in the amplifier at the front of the converter do not match, the common mode interference will change to series mode interference. In this case, the measurement results will be affected to a large extent.
The leading cause of common-mode interference is electrostatic interference. Therefore, we can effectively reduce the impact of common mode interference on electromagnetic sensors by shielding them from electrostatic interference.
1.5. DC interference
DC interference mainly comes from the electrochemical noise when the electromagnetic flowmeter is working. When the electrolyte and the contact parts once, the electrolyte in the positive and negative ions will immediately appear in directional movement. Even if there is no electricity, the electrolyte in the positive and negative ions will produce a directional sign. At this point, a significant potential difference is formed in the electrolyte, and this phenomenon is known as polarization.
2. The impact of excitation technology on accuracy
The excitation current will produce a corresponding magnetic field during operation, generating the induced electric potential. To be able to calculate the electromagnetic flowmeter, a more suitable excitation method needs to be chosen. Excitation technology has long existed, including DC excitation, low-frequency rectangular wave excitation, dual-frequency rectangular wave excitation, etc.
2.1 DC excitation
DC excitation is mainly generated by the stable magnetic field produced by DC excitation and the constant magnetic field produced by permanent magnets. It can be directly observed from the diagram that this type of excitation is more stable and subject to less interference.
2.2 DC excitation
In reality, the DC excitation method contains many interfering signals, the more common of which are in three forms.
(1) The polarisation voltage signal is superimposed on the interference due to the DC signal causing the fluid in the measuring tube to polarise.
(2) The polarisation voltage is susceptible to temperature effects causing it to follow changes.
(3) The need to utilize a larger DC amplifier will make signal amplification more difficult.
2.3 AC excitation
AC excitation is a form of excitation that is mainly generated in AC currents, usually in the presence of a 60Hz sinusoidal current. The figure shows the waveform of AC excitation. The most significant advantage of this type of excitation is that it effectively reduces the effect of the wind on the electrodes.
In addition, it is possible to use AC excitation to induce a higher electric potential, which can increase magnetic induction. It is also possible to amplify the signal directly without needing a front-end amplifier.
While the advantages of AC excitation contributed to its worldwide popularity in the 1920s and 1950s, there were also several apparent disadvantages to the AC excitation method of operation.
These include significant quadrature and in-phase interference, hysteresis, and eddy current losses. However, in the case of slurry and pulsating streams, using AC excitation can still be a quick and easy way to detect them.
2.4. Low-frequency rectangular wave excitation
The low-frequency rectangular wave excitation method effectively reduces the interference caused by electromagnetic induction and is less prone to polarization.
The graph shows that t/s in U/V occur very regularly, ensuring that the flow signal reaches a smooth state within a specific conversion time. In contrast, the waveform in the graph has a long stabilization time, thus ensuring a stable signal sampling process.
It is important to note that if the measurement is carried out using a slurry hysteresis device, it generates a certain amount of noise, thus forming a new interference, which can impact the detection results.