Suppression and Elimination of Interference in Electromagnetic Flow Meters

According to the characteristics of the general electromagnetic flow meter system, this paper mainly discusses from the aspect of hardware optimization that electromagnetic coupling and electrostatic induction are important sources of interference noise generated by electromagnetic flow meters. In an electromagnetic flow transmission, since the leads of the two electrodes are in an alternating magnetic field, when the transmission is powered on, an induced electromotive force is generated in the closed circuit of the leads. This kind of interference signal is superimposed on the measurement signal, affecting the operation of the system. Various excitation methods will bring about different electromagnetic interference problems. The DC excitation mode is prone to polarization interference, while the AC excitation mode is prone to orthogonal interference (90-degree interference), in-phase interference (i.e., power frequency interference), etc.

In-phase interference, power frequency interference or common-mode interference of electromagnetic flow meters refers to the interference signals that appear on the two electrodes of the transmitter at the same moment and have the same amplitude and phase. When the flow rate is zero, that is, when the liquid being measured is stationary, the co-phase signal measured is the co-phase interference signal. There are many methods to suppress in-phase interference in electromagnetic flow meters. In terms of the transmitter, the electrodes and excitation coils are made balanced and symmetrical in terms of geometric shape, size and performance parameters, and are strictly shielded respectively to reduce the influence of distributed capacitance between the electrodes and the excitation coils.

To reduce the in-phase interference caused by ground current, when installing the grounding wire, the flange plates of the pipes at both ends of the transmitter and the housing of the converter should be connected to the same point to minimize the in-phase interference, but it cannot completely eliminate it. Therefore, a differential amplifier circuit with a constant current source is usually adopted in the preamplifier stage of the converter. By taking advantage of the high common-mode rejection ratio of the differential amplifier, the in-phase interference signals entering the input end of the converter cancel each other out and are suppressed, which can achieve a very good effect. Meanwhile, to avoid interfering signals, the signal between the transmitter and the converter must be transmitted by shielded wires.

Orthogonal interference refers to the interference that differs from the flow signal by 90 degrees in phase. When an electromagnetic flow transmitter adopts the AC excitation method, an alternating magnetic field is generated. The closed loop composed of electrodes, lead wires, the measured medium and the input circuit of the converter is in the interference alternating magnetic field. The closed loop cannot be completely parallel to the magnetic field lines produced by the alternating magnetic field of the transmitter. There will always be some alternating magnetic field lines passing through this closed loop. This generates an interfering electromotive force within the circuit. In electromagnetic flow meters, measures are taken from both the transmitter and the converter to eliminate or suppress the 90° interference.