, as a kind of relying on the electromagnetic principle of flow meter, in the process of using various will be affected by external interference, the user during the installation process to pay special attention to avoid bad measurement because of the external environment for the impact of instrument, this article is aimed at the electromagnetic flow meter in use process is the main type of interference to do a simple description.
Now in general use in the industrial production of electromagnetic flowmeter, generally provide direct power supply, with 50 hz power output after a and the flow rate is proportional to the current signal (
10 ma, or 4
Traffic signals generated by electromagnetic flow meter for mV level ac signal, usually set fluid average speed of 1 m/s, traffic signal for mV 1 or 0.
Actual measurement of the electrical level signal in addition to the traffic signals is a mixture of in-phase, orthogonal and common-mode interference signals, especially when interfering voltage ac excitation amplitude is higher, can reach tens of mV or larger, will seriously affect the measurement.
Therefore, inhibition and elimination of interference has always is the key to the electromagnetic flow.
Orthogonal interference signals and suppress the cause and effect orthogonal interference signal refers to its phase and side flow signal by 90 degrees, and does not change with flow, there are two key reasons causing the jamming signal.
One is on the electrode lead, on the input impedance and measured medium composed of input circuit, as a result of the action of alternating electromagnetic field, create an additional induction electric potential, the electric potential has nothing to do with the average flow velocity of fluid, and potential traffic signal phase into 90 degrees.
By a type of the induction electric potential E and B are of the same phase sinusoidal magnetic field, and the sinusoidal alternating magnetic field and enter a loop chain interference potential en for added in addition to the above reasons, another reason for orthogonal interference is alternating magnetic flux produce eddy current in the conductive fluid being measured.
If the magnetic field on both sides of the electrode of vortex asymmetry, so also can produce additional orthogonal interference between two electrodes potential difference.
Avoid the interference from the orthogonal method are many, the main methods, one is to use signal out line automatic compensation, the other one is for 90 degrees in andadministrative level interference signal is the depth of the negative feedback, and then USES the phase-sensitive circuit, make the orthogonal interference is greatly weakened.
Usually at the same time the two measures.
Methods about signal automatically eliminate lead - as shown in figure 4
Figure electrode in A two wires and potentiometer R, concatenated.
The lead electrode B level through the next link preamplifier input resistance (
Is the electrode lead to signal the load resistance of RL)
RL is closely associated with the sliding contact of potentiometer, which constitute the basis of the two perpendicular to magnetic field lines loop.
Adjust the position of the sliding contact, make the two loop current is equal, in the fluid between the two electrodes, without orthogonal flow induced current, most so that it can eliminate the orthogonal interference.
The rest after the main amplifier, an orthogonal interference by phase-sensitive rectification of 90 degrees of jamming signal, driven by the heater thermistor unbalanced bridge of communication, then drew unbalanced output to the input end of the main amplifier constitute a negative feedback, which can effectively inhibit the 90 degrees of residual interference signal.
In-phase interference signals and suppress the cause and effect in phase interference refers to the two electrodes, respectively, of one the same jamming signal amplitude and phase, the interference in phase of there are two main reasons.
One is caused by field winding and electrode electrostatic induction between in-phase interference, as shown in figure 4
29, in the field winding and electrode A, B insulation resistance between the RM and distributed capacitance Cf, excitation voltage by RM and Cf and the resistance between the two electrodes, on the ground in phase interference of the electric potential et.
The second is caused by the earth current in phase interference electric potential.
If in the vicinity of a flow meter is of high power electrical equipment, especially due to poor insulation leakage occurs, the current will lead to different ground potential, as shown in figure 4
30, detector electrodes and contact measured medium, the converter is grounded.
Because of the measured medium and converter circuit is different, so that by the two different ground potential difference caused by the ground current was fetching converter, constitute a potential interference et.
In order to eliminate the interference of electromagnetic flowmeter, for electrostatic effect, strict screening measures should be taken to make the distributed capacitance decrease greatly, at the same time reduce the excitation voltage.
For the interference caused by the ground current, should as far as possible away from large electrical appliances equipment, make the converter, measured medium, pipeline one-point earthing, also can take converter & other;
Floating empty & throughout;
Converter of preamplifier stage adopts differential input circuit.
Power supply voltage and frequency fluctuations affect compensation voltage change will make the magnetic field strength, it will directly affect the traffic signal, the value of E.
Iron loss caused by power frequency changes and excitation coil impedance fluctuations, thus will cause fluctuations in traffic signal E.
To compensate the measuring error of the parameter fluctuations, converter application of hall element composition of the multiplier for the depth of the negative feedback, make the output current and flow rate is proportional to the directly, and the fluctuation of the power supply.
By above knowable, USES the low frequency square wave excitation not only all kinds of interference voltage amplitude declined significantly, and restrain the way is also greatly simplified.
In this paper by embellish instrument technology co. , LTD. Arrange release.
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