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Common small signal processing methods for electromagnetic flowmeters

Common small signal processing methods for electromagnetic flowmeters
Due to the improvement of the automation and intelligence of the current industrial production process, coupled with the requirements of energy saving and cost accounting management, the flow meter has become more and more important in the whole meter. Although the electromagnetic flowmeter can accurately measure the fluid flow, how to solve the "small signal" problem occurring in the measurement is also an important factor affecting the measurement accuracy. Kaifeng Liyuan Flowmeter Co., Ltd. (formerly Kaifeng Flowmeter Factory) provides the following electromagnetic flowmeters with information on small signal processing technology:

Traffic accumulation small and medium signal phenomenon

2.1 Small signal generation mechanism

Electromagnetic flowmeters have errors and are susceptible to interference in actual measurement sites and process flows, such as the "small signal" phenomenon. According to the working principle of the electromagnetic flowmeter, the interference problem of the electromagnetic flowmeter is generated by various excitation methods [2].

The following problems exist in many factories:

(1) When the pump starts, the liquid cuts the magnetic lines through the flowmeter to generate flow signals. These liquids are finally used by the subsequent links. The flow measurement value is valid and should be accumulated; when the pump stops, the pipeline will also There is a small amount of liquid reflux, reverse cutting magnetic lines of force, and also produces small signals, but these liquids are not used by actual production, and the flow meter will still send signals to the PLC for sampling accumulation, which will cause large errors.

(2) Due to external vibration or large magnetic field interference, when there is no medium flow, external vibration and other signals act on the sensor, introducing a “false flow” signal, which is also accumulated when the signal is higher than the metering lower limit of the meter.

(3) The surface of the electrode adheres to a layer of insulation, is disturbed by the outside world, or the milliamp signal sent by the electrode exhibits a zero drift during amplification and conversion, causing the signal to deviate from the normal value, thereby causing an error.

All of these phenomena are small signal phenomena.

2.2 Small signal processing measures

The small signal problem caused by the interference signal cannot be solved from the process design, and only the solution can be found from the aspect of process control. It can be handled in both hardware and software.

(1) The hardware mode can be composed of comparison circuit and electronic switch. This method is mainly used to filter out interference signals, but it will increase the production cost of the enterprise. Because there will be many flow meters in one station, it is necessary to replace many stations accordingly. This kind of instrument [3].

(2) The software mode can be realized by, for example, PLC software, and the small signal problem can be solved by the following logic logic block diagram.

The DCS control subroutine flow is shown in Figure 2.

Common small signal processing methods for electromagnetic flowmeters

3 Electromagnetic flowmeter installation

3.1 Installation site

Generally, the electromagnetic flow sensor housing is protected to IP65 (dust-proof and dust-proof according to GB4208). All the following requirements are applied to the installation site:

(1) When measuring the mixed phase fluid, select the place that will not cause phase separation; when measuring the two-component liquid, avoid the place where the mixing is not uniform; when measuring the chemical reaction pipeline, it should be installed downstream of the fully completed reaction section [ 4].

(2) Try to avoid the negative pressure inside the measuring tube.

(3) Choose a place with small vibration, especially for integrated instruments.

(4) Avoid large motors and large transformers nearby to avoid electromagnetic field interference.

(5) It is easy to realize the place where the sensor is grounded separately.

(6) Avoid environments with high concentrations of corrosive gases around.

(7) The ambient temperature is in the range of -25 ∼ -10 ° C and 50 ∼ 600 ° C. The integral structure temperature is also subject to electronic components, and the range is narrower.

(8) The installation location should be easy to access and maintain.

3.2 Straight pipe length requirements

In order to obtain normal measurement accuracy, the electromagnetic flow sensor must also have a length of straight pipe upstream, but its length is lower than most other flow meters. 90° elbows, T-tubes, concentric reducers, and fully open gate valves are generally considered to require only a straight pipe section [5] of 5 times diameter (5D) from the center line of the electrode (not the connection end of the sensor inlet), different opening degrees The valve requires 10D, and the downstream straight pipe section is (2~3)D or no requirement, but the butterfly valve disc is prevented from protruding into the sensor measuring tube.

4 Conclusion

Through the introduction of small signal resection scheme in the control process, the flow rate of conductive fluid is measured more accurately, which meets the requirements of energy saving and cost accounting management of enterprises. This scheme provides a good help for the efficiency of each section of the factory.

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