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Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption
In the 1950s and 1960s, with the development of electronic technology and microelectronics, the electromagnetic flowmeter market was gradually improved, and with the advent of low-voltage micro-power large-scale integrated circuits, the application of electromagnetic flowmeters Bring another broad prospect. Subsequently, the Kaifeng flowmeter manufacturers gradually increased and became the base of China's flowmeters. Kaifeng Liyuan Flowmeter Co., Ltd. (formerly Kaifeng Flowmeter Factory) is an older meter manufacturer.

This paper analyzes and studies the measurement principle of a low-voltage micro-power battery-powered electromagnetic flowmeter abroad, which mainly has the following aspects:

a. Theoretical basis of low voltage micro-power electromagnetic flowmeter;

b. Transmitter structure design of low voltage micro power electromagnetic flowmeter;

c. Low-voltage micro-power electromagnetic flowmeter calibration analysis.

1 Working principle

According to Faraday's law of electromagnetic induction, when the conductor moves in the magnetic field to cut the magnetic lines of force, the induced potential e is generated at both ends of the conductor, if B, L, V are perpendicular to each other;

Then: e = BLV; (B, L, V are the magnetic field magnetic induction, the length of the conductor and the speed of movement, respectively)

Applied to electromagnetic flowmeters are: (D is the pipe diameter) then the volume flow Qv can be expressed as:

It can be seen from the above formula that the volume flow rate Qv is linear with the induced electromotive force e and the inner diameter D of the measuring tube, and inversely proportional to the magnetic induction intensity B of the magnetic field, regardless of other physical parameters. This is the principle of electromagnetic flowmeters measuring the volumetric flow of conductive liquids (see Figure 1).

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption

The main features of the electromagnetic flowmeter: the transmitter has a simple structure and no moving parts; it is not affected by the temperature, viscosity, density and conductivity (within a certain range) of the measured medium during the measurement process; the range is extremely wide. It is only proportional to the average flow velocity of the measured medium, and is independent of the flow state (laminar flow, turbulent flow) under the axisymmetric distribution, and is sensitive and linear.

It can be seen that it is suitable to further develop the electromagnetic flowmeter into a low voltage micro power electromagnetic flowmeter. However, in the low-voltage micro-power state, battery power is generally used, so that the power consumption of the excitation circuit system needs to be solved. On the one hand, it is required to reduce the power consumption of the excitation circuit system, and on the other hand, it is required to obtain a sufficient flow signal (induced electromotive force). e). To meet these requirements, the transmitter structure of the excitation circuit system must be addressed in the design.

2 Excitation circuit system transmitter structure analysis

The electromagnetic flowmeter transmitter is mainly composed of a measuring duct, an excitation system, an electrode and an interference adjusting mechanism. In order to make the sensor work stably and reliably, and accurately sense the flow signal, the structure must be carefully considered.

In order to reduce the excitation coil to consume too much electric energy, a special structure is made according to the magnetic field principle of the excitation coil. High-quality ultra-high magnetic permeability ferrite is selected as the excitation coil core (ultra-high magnetic permeability nickel-zinc NiZn ferrite μi>12000) (Fig. 2).

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption

3 excitation signal processing method

The magnetic field of the electromagnetic flowmeter is obtained by the excitation coil. At present, a three-valued low-frequency square wave excitation form is used (see Figure 3).

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption

The low-voltage micro-power electromagnetic flowmeter adopts a high-precision double-integral analog-to-digital conversion, which has a good elimination effect on various spikes and AC power frequency interference. In particular, in the field of excitation, a three-valued low-frequency square wave with good zero-point stability and strong resistance to power-frequency interference is used, which can well reduce the mutual interference problem between positive and negative periods, and the flowmeter is designed to reduce power consumption. The excitation field strength is enhanced by the magnetizing inrush current to achieve the performance and effect of the three-value low-frequency square wave excitation.

4 flow signal processing method

1) The flowmeter adopts the 6B68-0031 low-voltage micro-power large-scale integrated circuit MPU (MicroProcessorUnit) microprocessor produced by Hitachi, Japan. As the central controller design scheme, the CPU in the chip controls the operation of the entire instrument, and 74HC02A and SL130 combination, complete the operation and storage of the flow signal and control the excitation signal function; the output has the functions of instrument analog signal (current signal) output and frequency output.

2) The internal CPU of the computer processes the data signal, controls the software support and performs calculation and control on the flow data. The induced electromotive force e received by the electrode in the measuring tube segment is first amplified by the variable gain preamplifier, and then amplified by the first stage signal, and the amplified signal is converted into the CPU micro by A/D conversion. The processor simultaneously sends the processed flow data result to the display for display value, and in the intelligent design, the CPU microprocessor performs remote data transmission with the pulse signal and the digital signal (data stream) in the external I/O interface circuit. .

5 electromagnetic flowmeter calibration analysis

According to the technical parameter declaration of the product (DN100), refer to the water meter and electromagnetic flowmeter verification procedures, and verify it in the laboratory and the use site after the product is installed.

In the laboratory, the volumetric water flow standard device (standard metal measuring instrument accuracy is 0. 2) is used for calibration. At the use site, the imported portable ultrasonic flowmeter of class 1. 5 is used (calibrated by the national water flow detection station). For comparison, the experimental data is shown in Table 1. The indication error satisfies the allowable error of the specification declaration, as shown in Fig. 4.

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption

Electromagnetic flowmeter uses the measurement principle of low voltage and micro power consumption
6 Conclusion

It can be seen from the analysis of the flow meter that most of the electromagnetic flow meters in the past were powered by 220V AC. With the development of industrial production, environmental protection and energy conservation, battery-powered electromagnetic flowmeters have their own unique advantages in many flow measurement instruments. Hardware and software design are different from AC-powered electromagnetic flowmeters. It is the successful combination of fluid mechanics theory and electronic technology, which makes the design of the instrument more reasonable, the performance is superior, and the measurement is more accurate. The future will surely attract more attention.

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