Development of electromagnetic flowmeter using ARM920T core

　　With the development of flow detection technology, higher application requirements are put forward for flow measurement instruments. Traditional flow detection instruments generally complete the measurement work through simple information analysis and processing based on their own measurement mechanisms. Therefore, there are limitations in processing power, measurement accuracy, error correction, function expansion, etc. The new generation of flow detection instruments will replace them with better performance. At present, high-speed, high-precision, and large-capacity embedded processors are becoming more and more common in the fields of control and measurement.

　　1 Basic principles of electromagnetic flowmeter

　　The electromagnetic flow meter is a measuring device that measures the flow rate of fluid in a pipe based on Faraday's law of electromagnetic induction. The principle of the electromagnetic flow meter sensor is now explained, as shown in Figure 1.

Figure 1 Schematic diagram of electromagnetic flow meter sensor

　　When the fluid flows through a transverse magnetic field B in the pipeline, it is equivalent to a conductor with a certain conductivity cutting the magnetic lines of force, forming a motional electromotive force and an induced current. The electromotive force E can be drawn through the two radial electrodes of the pipeline, and its magnitude is proportional to the magnetic field B, flow velocity V and pipe diameter D, that is:

　　E = B·V·D （1.1）

　　The volume flow rate Q of the fluid is proportional to the flow velocity V and the cross-section of the pipe. The flow rate Q can be determined by measuring the electromotive force E between the two electrodes.

　　Q = V·πD2/4 =πD·E/4B （1.2）

　　When the excitation current, pipe size and fluid density ρ are determined, the mass M of the fluid depends only on the detection of the induced potential E between the two electrodes. The mathematical model of the electromagnetic flow meter is:

　　M = Coe·ρ （E-E0）·x （1.3）

　　Where: Coe is the instrument coefficient; E0 is the instrument zero correction; x is the multi-segment nonlinear correction.

　　2 System structure

　　The electromagnetic flow meter consists of two parts: the measuring device and the circuit. The circuit part mainly consists of the detection input module, the excitation output module, the flow output module, the graphic display module, the keyboard module, the communication and debugging interface, the power module, and the most important core board based on the ARM9 embedded system. Figure 2 shows the system block diagram of the embedded electromagnetic flow meter.

　　Figure 2 Block diagram of electromagnetic flow meter system

　　After the system is initialized, the core board outputs a digital excitation signal to the excitation module, which drives the sensor's excitation coil to generate a magnetic field of a certain strength after D/A conversion and current amplification. The flow rate sensing electrode of the sensor sends a weak induction signal, which is amplified and filtered by the input module, converted into a digital quantity by A/D and input into the ARM9 processor for further digital analysis and processing. The instantaneous flow, cumulative flow and dynamic flow graph are directly displayed through the display module. In addition, the flow output module outputs a standard 4-20 mA instantaneous flow signal of the intelligent instrument.

　　2.1 Detection input and A/D conversion circuit

　　1. A/D signal conversion mechanism

　　A/D converter is a circuit that converts analog signals into digital signals. Analog quantity can be voltage or current signal. For non-electrical signal physical quantities such as sound, light, pressure, temperature, humidity, etc. that change continuously with time and state, they can be converted into electrical signals through suitable non-electrical signal physical quantity sensors (such as liquid level sensors, pressure sensors, temperature sensors, photoelectric sensors). Analog quantity can only be displayed by LED digital and automatically controlled after being converted into digital quantity. Or it can be collected, analyzed and calculated by computer. At present, there are many types of A/D conversion, which can be divided into successive approximation type and double integral type according to the conversion principle. The effective number of bits of common A/D converters is 4, 6, 8, 10, 12, 14, 16 and so on. The A/D conversion process includes four steps: sampling, holding, quantization and encoding. Generally, the first two steps are completed once in the sampling and holding circuit, and the last two steps are completed once in the A/D conversion circuit.

　　2. Detect input module

　　The detection input module includes a differential measurement amplifier, low-pass and high-pass filters, a gain amplifier, and an A/D conversion circuit, as shown in FIG3 .

　　Figure 3 Input and A/D conversion circuit block diagram

　　Since the electrode output signal of the electromagnetic flow meter is very weak, generally only 0-10mV, and the industrial environment interference is very large. Therefore, in order to ensure the measurement accuracy, the input signal sent to the A/D conversion should reach the range of -215~+215V, and the voltage gain of its analog part should be above 60dB. Among them, the preamplifier uses the instrument amplifier AD620 with differential input, and the high-pass filter and low-pass filter use second-order active filters to form a bandpass filter to filter out power frequency interference and clutter. The amplifier is completed by the operational amplifier CA3240A. The A/D conversion unit uses MAX1297AEEG to realize 12-bit parallel analog-to-digital conversion, which is directly connected to the I/O line of the core board.

　　2.2 Excitation output circuit

　　The task of the excitation circuit of the electromagnetic flowmeter is to provide a stable driving current to the excitation coil. The current waveform is in the form of square wave, three-value square wave and trapezoidal wave. The purpose of the waveform change is to combine the signal processing circuit to analyze the accuracy, zero point stability and anti-interference ability of the electromagnetic flowmeter under different excitation modes. Exploratory research is conducted to develop a high-precision electromagnetic flowmeter. The circuit outputs digital quantity from the SPI2 port of the core board, forms an analog signal through D/A conversion, and outputs through V/I conversion excitation and current amplifier with current negative feedback, which is suitable for various changes in excitation waveforms. The block diagram is shown in Figure 4. The D/A conversion circuit uses the AD7243 chip to realize 12-bit SPI synchronous serial input and -5~+5V bipolar output. It is connected to the SPI2 port of the ARM9 core board, as shown in Figure 4.

　　Figure 4 Excitation circuit block diagram

　　The excitation amplifier uses a CA3240A operational amplifier, which has the characteristics of high power supply voltage and can obtain a large output dynamic range. Current amplification is achieved using two pairs of composite tubes, and the tubes are required to be matched as much as possible. After connecting to the excitation coil, the current negative feedback of the large loop is introduced to stabilize the output excitation current.

　　2.3 Flow output module

　　When the electromagnetic flowmeter is measuring, analyzing and processing, in addition to displaying the instantaneous flow and cumulative flow on site, it usually outputs a standard 4-20 mA current signal. Therefore, the flow output circuit uses the AD421 conversion circuit to realize the flow output function. The AD421 chip is a low-voltage, serial input D/A conversion circuit with 4-20 mA loop current output and supports the HART communication protocol.

　　The voltage reference REFIN of D/A conversion uses REF OUT2 (2.15 V) provided by the chip. A 0.01μF capacitor is connected between LV and VCC in the flow output circuit, which determines that the +24V loop power supply LOOP Power generates a 3.13 V power supply. The +24V loop power supply LOOP POW-ER returns from LOOP RTN through the internal control current to form a 4-20mA current loop.

　　3 System software design

　　The embedded processor ARM9 core in the software system of the electromagnetic flow meter mainly considers the initialization settings of the core board and each hardware module. After the system is started, it completes the command control and data transmission between the core board and each hardware module by calling the underlying driver, and establishes the corresponding interrupt service subroutine and interrupt vector table. The system program is established using a modular structure. The electromagnetic flow meter application system is mainly managed by the timer interrupt. The output and conversion retention of the excitation signal, the multiple data collection of the induction signal, the display and external output of the flow are all completed by the timer interrupt service.

　　The software platform of this system is mainly based on embedded Linux as the operating system to establish the development environment. MiniGUI is the graphical user interface support system, on which the function is developed. SQLite is the database engine, and the database design of the flow measurement system is carried out.

　　The control process of the system completes the functions of parameter setting, flow signal detection and control, alarm, etc. After the system is started, the current status is displayed in the interface and the user's input settings are received. At the same time, another thread is generated to realize flow detection and control.

　　4 Conclusion

　　After careful research, development, design and experiment, the electromagnetic flow meter based on ARM920T core developed in this paper effectively solves the problem of accurate measurement and control of liquid flow, improves the measurement accuracy of liquid flow, and realizes remote monitoring. The system can be widely used in petrochemical, industrial and mining enterprises. The embedded microprocessor S3C2410A selected by the intelligent instrument can realize multiple excitation modes, USB storage of data, Ethernet communication, color screen display and other functions. At the same time, in the design process of the electromagnetic flow meter, advanced hardware and software collaborative design solutions are adopted. In addition, S3C2410A is used as a high-end application research in the electromagnetic flow meter system, and a modular design method is adopted in hardware, which improves the application and research level of the electromagnetic flow meter and reduces the design difficulty.

　　Through system testing, the intelligent instrument can be connected with the automatic sensors in the industry to form a flow measurement control system. It can be widely used in the flow measurement and control of various industrial liquids, such as the flow measurement of chemical plant reagents, the flow measurement of oil, the flow measurement of gasoline and kerosene, etc. It has a good cost-effectiveness and a good prospect for promotion and application.

　　Innovation of the author of this article: This article applies the ARM9 core to the electromagnetic flow meter, so that the electromagnetic flow meter can be applied in digital filtering of input signals, preservation of historical data, changes in output of multiple excitation signals, special processing of measurement information, dynamic graphic display of measurement results, and humanized management and control.

　　The electromagnetic flow meter can be connected with the automation sensors in the industry to form a flow measurement control system, and can be widely used in the flow measurement and control of various industrial liquids, such as the flow measurement of chemical plant reagents, the flow measurement of oil, the flow measurement of gasoline and kerosene, etc. It has a good cost performance and a good prospect for promotion and application.