Electromagnetic
flowmeter due to its high reliability, strong corrosion resistance, high measuring accuracy, easy to change the measurement range, is currently in petroleum, chemical metallurgy, paper and other industries has been widely used;
Especially in today's energy shortage, the development of electromagnetic flowmeter for resource scientific management provides an important basis of technical support.
However, the rapid development of science and technology puts forward higher and newer requirements on electromagnetic flowmeter, especially in the area of the * degree and stability of the flow measurement.
Electromagnetic flowmeter in the jamming signal and traffic signal mixed together, not only complicated composition and some order of magnitude of interference signal and traffic signal, moreover may cause noise into the microprocessor system run is unusual, in this case, how to suppress interference becomes a key technology research and use of electromagnetic flowmeter.
1.
The mechanism and countermeasures of noise in the electromagnetic
flow meter working principle of the electromagnetic flowmeter based on Faraday's law of electromagnetic induction, by measuring the conductive fluid in the magnetic field induction electromotive force to calculate fluid flow.
Excitation magnetic field, the induced electromotive force acquisition electrode, signal amplifying circuit and AD converter are vulnerable to job site of coupled electric and magnetic radiation, so the electromagnetic coupling is a major way of interference in the flow measurement ZUI;
Second measured liquid of electrochemical reaction and mud interference is another noise source of the flow measurement.
1.
1 differential interference mechanism and countermeasures in the current design of electromagnetic flowmeter, low-frequency rectangular wave excitation method combines the advantages of both dc excitation and ac excitation, become one of the main excitation mode.
Under low frequency excitation method, main show is: the interference produced by exciting current mutation differential interference signal.
Ideal for low-frequency rectangular wave excitation magnetic field signal, in fact, as the excitation current changes,
dI / dt)
On the magnetic field to produce differential interference signal, as the current stability, interference signals disappear, so synchronous sampling technology can effectively restrain differential interference signal.
In practical design, module converter using AD company 24 Sigma Delta AD7714 modulus converter.
AD7714 can adopt the way of hardware and software sampling start time control, so as to realize synchronous sampling.
Using the software, when FSYNC bit is logic 1 control register, the AD converter in the reset state;
To the writing logic 0, devices began to sampling the input signal.
In the application of low frequency excitation signal is produced by MSP430 single chip microcomputer internal timer, produce sampling signals at the same time.
As shown in figure 1, sampling start time lag excitation signal cycles.
1.
2 series mode power frequency interference mechanism and countermeasures on the electromagnetic flowmeter work site there are a lot of power frequency signal, the overlay on the excitation circuit, electrode, power frequency interference of front-end amplifier noise greatly effects on the flow of accurate measurement.
The radiation coupled to the excitation circuit of radiation power frequency magnetic field (
Including its harmonic)
Impact excitation magnetic field, the flow measurement.
When the fluid velocity is small, power frequency interference signals and the effective flow in the same order of magnitude, seriously affect the measurement result.
Coupling power frequency noise known fundamental wave frequency 50 hz, so can use either analog or digital filter to filter bandwidth limitations within 50 hz to suppress noise.
Used in the practical application of Sigma Deltad AD converter AD7714 internal contain digital filtering module, digital filter relatively analog filter in addition to has the characteristics of high flexibility, easy parameters setting, can also reduce the noise introduced during A/D conversion.
AD7714 digital filter for (
sinr / x)
°
Low-pass filter, the frequency or the transfer function is: type: fs for sampling frequency;
AD7714 sampling frequency for 19.
2kHz;
Sampling number N for filtering;
For the digital filter frequency response.
By setting the filter sampling number can change digital filter cutoff frequency and frequency of a trap.
In this design, considering the modulus conversion speed and remove noise performance, set the value of N filter cutoff frequency is 50 hz filter frequency response as shown in figure 2, the workers in the filter frequency 50 hz and even order harmonic had good inhibitory effect.
In addition to the use of digital filter, in the A/D input setting RC analog low-pass filter can also bring other role.
Observed in the actual measurement of flow signal exists in the spike pulse noise, may make the AD converter saturation, lead to failure, digital filter using analog filter can eliminate these signals in advance.
1.
Three common mode interference, crosstalk noise mechanism and countermeasures of common mode interference is mainly due to the electromagnetic shielding defects, bad earth, return to the current imbalance by stray capacitance, etc.
Common-mode interference may lead to circuit certain reference potential changes, is one of the causes of the electromagnetic flowmeter zero drift.
At the same time high radiation of common mode signal electric field worsen electromagnetic compatibility of the circuit.
Crosstalk is caused by a lack of printed circuit board design of electromagnetic compatibility considered cause the loss of signal quality, especially the expressway line and analog electric Louis affected.
By reference potential changes caused by common-mode interference signals, voltage application flow using differential form, through twisted-pair cable into the amplifier, the front-end amplifier selects high common mode rejection ratio, low drift, high input impedance of the op-amp, can effectively restrain common mode interference.
In addition, the electromagnetic flowmeter circuit board design conform to the requirements of the electromagnetic compatibility to reduce crosstalk effects on signal: use proper rate as low as possible to meet the requirement of function of logic devices.
Choose in the process of logical state transform input current consumption of the smaller components.
As far as possible choose surface encapsulation of components.
Reasonable arrangement of components layout, analog and digital parts segregation, prevent influence the analog signal digital signal.
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