Abstract: This paper addresses user demand for high-end electromagnetic flow meters and proposes a hardware and software design scheme for applying the ARM920T core to electromagnetic flow measurement instruments. The system uses the S3C2410A microprocessor as the hardware core and designs the hardware system around it; an embedded Linux operating system is used to establish the development environment, and application programs are developed based on this environment. Keywords: Survey Control; ARM920T; Linux [b][align=center]Development of Electromagnetic Flowmeter Instrument Based on ARM920T Zhi Li-ping Liu Wen-hua[/align][/b] Abstract: According to the requirements of complicated intelligent instruments, this paper presents a design method for the hardware and software of an intelligent electromagnetic flowmeter instrument based on the ARM920T. The system adopts the S3C2410A as its hardware core and focuses on its hardware design; it adopts Linux as its embedded operating system and sets up the development environment, and based on which the application software is developed. Key words: Survey Control; ARM920T; Linux 0 Introduction With the technological development of flow measurement instruments, higher application requirements have been placed on flow measurement instruments. Traditional flow measurement instruments generally rely on their respective measurement mechanisms and complete measurement work through simple information analysis and processing. Therefore, they have limitations in terms of processing power, measurement accuracy, error correction, and functional expansion. A new generation of flow measurement instruments will replace them with superior performance. Currently, high-speed, high-precision, and high-capacity embedded processors are increasingly widely used in the fields of control and measurement. 1 Basic Principle of Electromagnetic Flow Meters An 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 explained below, as shown in Figure 1. [align=center] Figure 1 Schematic diagram of electromagnetic flow meter sensor[/align] When the fluid flows through a transverse magnetic field B in the pipe, 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 extracted through the two electrodes in the radial direction of the pipe. Its magnitude is proportional to the magnetic field B, the flow velocity V, and the pipe diameter D, that is: E = B·V·D (1.1) The volumetric flow rate Q of the fluid is proportional to the flow velocity V and the cross-sectional area of ​​the pipe. As long as the electromotive force E between the two electrodes is measured, the flow rate Q can be determined. Q = V·πD²/4 =πD·E/4B （1.2） When the excitation current, pipe size, and fluid density ρ are determined, the fluid mass M depends only on the detection of the induced electromotive force 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-point correction; x is the multi-segment nonlinear correction. 2 System Composition Structure The electromagnetic flow meter consists of two parts: a measuring device and a circuit. The circuit part mainly consists of a detection input module, an excitation output module, a flow output module, a graphic display module, a keyboard module, a communication and debugging interface, a power supply module, and most importantly, a core board based on an ARM9 embedded system. Figure 2 shows the system block diagram of the embedded electromagnetic flow meter. [align=center]Figure 2 Block Diagram of Electromagnetic Flow Meter System[/align] After initialization, the core board outputs a digital excitation signal to the excitation module. After D/A conversion and current amplification, it drives the excitation coil of the sensor to generate a magnetic field of a certain intensity. The flow velocity sensing electrode of the sensor sends out a weak sensing signal, which is amplified and filtered by the input module, converted into a digital signal by A/D conversion, and input to the ARM9 processor for further digital analysis and processing. The instantaneous flow rate, cumulative flow rate, and dynamic flow rate graph are directly displayed through the display module. In addition, the flow output module outputs a standard 4-20 mA instantaneous flow rate signal of the intelligent instrument. 2.1 Detection Input and A/D Conversion Circuit 1. Conversion Mechanism of A/D Signal An A/D converter is a circuit that converts analog signals into digital signals. Analog signals can be voltage or current signals. For physical quantities such as sound, light, pressure, temperature, and humidity, which are non-electrical signals that change continuously with time and state, they can be converted into electrical signals by suitable non-electrical physical quantity sensors (such as liquid level sensors, pressure sensors, temperature sensors, and photoelectric sensors). Analog signals can only be displayed and automatically controlled by LED digital displays after being converted into digital signals, or collected, analyzed, and calculated by computers. Currently, there are many types of A/D converters, which can be divided into successive approximation type and dual-slope type according to the conversion principle. Common A/D converters have effective bit widths of 4, 6, 8, 10, 12, 14, and 16 bits. The A/D conversion process includes four steps: sampling, holding, quantization, and encoding. Generally, the first two steps are completed in one step in the sample-and-hold circuit, and the last two steps are completed in one step in the A/D conversion circuit. 2. Detection 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 Figure 3. [align=center] Figure 3 Input and A/D Conversion Circuit Block Diagram[/align] Because the electrode output signal of the electromagnetic flowmeter is very weak, generally only on the order of 0-10mV, and the industrial environment is subject to a lot of interference. Therefore, to ensure measurement accuracy, the input signal sent to the A/D converter should be in the range of -215V to +215V, and its analog section voltage gain should be above 60dB. The preamplifier uses the AD620 differential input instrumentation amplifier, and the high-pass and low-pass filters use second-order active filters to form a band-pass filter to filter out power frequency interference and noise. The amplifier is implemented using an operational amplifier CA3240A. The A/D conversion unit uses a MAX1297AEEG to implement 12-bit parallel analog-to-digital conversion, directly connected to the I/O lines of the core board. 2.2 Excitation Output Circuit The excitation circuit of the electromagnetic flowmeter is tasked with providing a stable drive current to the excitation coil. The current waveform can be in the form of square wave, three-value square wave, and trapezoidal wave. The purpose of waveform variation is to analyze, in conjunction with the signal processing circuit, multiple indicators such as the accuracy, zero-point stability, and anti-interference capability of the electromagnetic flowmeter under different excitation modes. This is an exploratory study for the development of a high-precision electromagnetic flowmeter. This circuit outputs a digital signal from the SPI2 port of the core board, which is converted into an analog signal through D/A conversion, then excited by V/I conversion and output by a current amplifier with current negative feedback, suitable for various excitation waveform variations. The block diagram is shown in Figure 4. The D/A conversion circuit uses the AD7243 chip to achieve 12-bit SPI synchronous serial input and -5 to +5 V bipolar output. It interfaces with the SPI2 port of the ARM9 core board, as shown in Figure 4. [align=center] Figure 4 Excitation Circuit Block Diagram[/align] The excitation amplifier uses the CA3240A operational amplifier, which is characterized by high power supply voltage and a large output dynamic range. Current amplification is achieved using two pairs of composite transistors, requiring the transistors to be matched as much as possible. After connecting to the excitation coil, a large-loop current negative feedback is introduced to stabilize the output excitation current. 2.3 Flow Output Module When performing measurement, analysis and processing, electromagnetic flow meters, in addition to displaying instantaneous flow and cumulative flow on-site, usually output a standard 4-20 mA current signal. Therefore, the flow output circuit utilizes the AD421 conversion circuit to achieve the flow output function. The AD421 chip is a low-voltage, serial-input D/A conversion circuit with a 4-20 mA loop current output and supports the HART communication protocol. The voltage reference REFIN for the D/A conversion is the REF OUT2 (215 V) provided by the chip. A 0.1 μF capacitor is connected between LV and VCC in the flow output circuit, which determines that the +24V loop power supply LOOP POWER generates a 313 V power supply. The +24V loop power supply LOOP POWER returns via the LOOP RTN through internal control current, forming a 4-20mA current loop. 3 System Software Design The embedded ARM9 processor core in the software system of the electromagnetic flow meter mainly considers the initialization settings of the core board and various hardware modules. After the system starts, it calls the underlying driver to complete the command control and data transmission between the core board and various hardware modules, and establishes the corresponding interrupt service subroutines and interrupt vector tables. The system program adopts a modular structure. The electromagnetic flowmeter application system is mainly managed by timer interrupts. The output, conversion, and holding of the excitation signal, multiple data acquisitions of the induction signal, flow display, and external output are all completed by the timer interrupt service. The software platform of this system mainly uses embedded Linux as the operating system to establish the development environment. MiniGUI is used as the graphical user interface support system, and functions are developed on this basis. SQLite is used as the database engine for the database design of the flow measurement system. The system control flow completes functions such as parameter setting, flow signal detection and control, and alarm. After the system starts, the current status is displayed on the interface and user input settings are received. Simultaneously, another thread is generated to implement flow detection and control. 4. Conclusion Through careful research, development, design, and experimentation, the electromagnetic flowmeter based on the 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 enables remote monitoring. This system can be widely used in petrochemical, mining, and other enterprises. The intelligent instrument utilizes the S3C2410A embedded microprocessor, enabling functions such as multiple excitation modes, USB data storage, Ethernet communication, and color screen display. Furthermore, an advanced hardware and software co-design scheme was adopted in the design of this electromagnetic flow meter. In addition, the S3C2410A, as a high-end application in electromagnetic flow meter systems, employs a modular design approach in hardware, improving the application and research level of electromagnetic flow meters and reducing design complexity. System testing shows that this intelligent instrument can connect with automated sensors in industry to form a flow measurement and control system. It can be widely used for flow measurement and control of various industrial liquids, such as reagents in chemical plants, petroleum, gasoline, and kerosene, offering excellent cost-effectiveness and promising prospects for widespread application. The author's innovation lies in applying the ARM9 core to the electromagnetic flow meter, enabling its application in areas such as digital filtering of input signals, historical data storage, variation of various excitation signals, special processing of measurement information, dynamic graphical display of measurement results, and user-friendly management and control. This electromagnetic flow meter can be connected to automated sensors in industry to form a flow measurement and control system. It can be widely used for flow measurement and control of various industrial liquids, such as flow measurement of reagents in chemical plants, flow measurement of petroleum, and flow measurement of gasoline and kerosene. 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