Development of electromagnetic flow meter based on ARM920T core

Development of electromagnetic flow meter based on ARM920T core time: 15:27:22 Source: China transmission network Author: Zhi Liping Liu Wenhua

0 Introduction

　
With the development of the Traffic Measuring Instrument Technology, it puts forward higher Application Requirements for the flow measuring instrument. Traditional traffic detection instruments generally use simple information analysis and processing based on their respective measurement mechanisms to complete the measurement.
. Therefore, there are limitations in terms of processing capabilities, measurement accuracy, error correction, and function expansion. The new generation of traffic detection instruments will be replaced with better performance. Currently, high-speed, high-precision, large-capacity
Embedded processors are widely used in the field of control and measurement.

1 Basic Principle of electromagnetic Flow Meter

The electromagnetic flow meter is a measuring device for measuring the fluid flow in the tube according to the law of electromagnetic induction. The principle of the electromagnetic flow meter sensor is described in 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 line to form an inactive potential force and a Inductive Current, the radial two electrodes in the pipeline can lead to E. Its size is proportional to the magnetic field B, the velocity V, and the diameter D, that is:

E = B · V · D (1.1)

The volume flow Q of the fluid is directly proportional to the flow rate V and the cross section in the pipeline. As long as the emf e between the two electrodes is measured, the flow Q can be determined.

Q = V · π D2/4 = π D · E/4B (1.2)

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

M = Coe · P (E-E0) · x (1.3)

Coe indicates the instrument coefficient, E0 indicates the instrument zero point correction, and x indicates multi-segment nonlinear correction.

2 system composition

The electromagnetic flow meter consists of a measuring device and a circuit, the circuit consists of the detection input module, excitation output module, traffic output module, graphic display module, keyboard module, communication and debugging interface, power module, and the most important core board composition. Figure 2 shows the system diagram of the embedded electromagnetic flow meter.

Figure 2 Structure of the electromagnetic Flow Meter System

　
After the system is initialized, the core board outputs A digital excitation signal to the excitation module.
Conversion and current amplification, the excitation coil of the driving sensor produces a magnetic field of a certain intensity. The sensor's Flow Rate Sensor electrode sends A weak sensing signal, which is amplified and filtered by the input module.
Convert it into a digital data input arm9-processor for further digital analysis. The display module displays the instantaneous traffic, accumulated traffic, and dynamic traffic graphs. Output by the traffic output module
4 ~ 20 mA standard smart meter instantaneous flow signal.

2.1 check input and A/D Conversion Circuit

1. Conversion Mechanism of A/D signal

　
A/D converter is A circuit that converts analog signals into digital signals. Analog values can be voltage or current signals. For non-electrical signals that change continuously with time and state, such as sound, light, pressure, temperature, and humidity
Processing, which can be converted into electrical signals by appropriate physical sensors (such as level sensors, pressure sensors, temperature sensors, and Photoelectric Sensors. Analog data can only be converted to digital data
Display and automatic control. Or collected, analyzed, and computed by a computer. Currently, there are many types of A/D conversions. The conversion principles can be divided into successive approximation and dual-integral conversion. Valid digits of common A/D converters
There are 4, 6, 8, 10, 12, 14, 16, and more.
The A/D conversion process consists of four steps: sampling, holding, quantization, and encoding. Generally, the first two steps are completed once in the sampling and holding circuit, the next two steps are completed once in the/D conversion circuit.

2. Detection input module

The detection input module consists of A differential measuring amplifier, A low-pass and high-pass filter, A gain amplifier, and A/D conversion circuit, as shown in 3.

Figure 3 input and A/D conversion circuit diagram

　
The electrode output signal of the electromagnetic flow meter is very weak, generally only 0-10mV orders of magnitude, and the industrial environment interference is very large. Therefore, to ensure the measurement accuracy
The conversion input signal must reach-215 ~ + 215 V
The voltage gain of the analog part should be above 60 dB. The amplifier AD620 is used for the front amplifier with differential input, and the high-pass filter and low-pass filter form the band-pass using the second-order active filter.
The filter filters out power frequency interference and clutter, and the amplifier uses the op amp CA3240A. A/D conversion unit
MAX1297AEEG implements 12-bit Parallel modulus conversion and is directly connected to the I/O line of the core board.

2.2 excitation output circuit

Excitation of electromagnetic Flow Meter
The task of a magnetic circuit is to provide a stable driving current to the excitation coil. The current waveform is in the form of a square wave, a three-value square wave, and a ladder wave. The waveform change aims to combine the signal processing circuit to analyze the electromagnetic
The accuracy, zero-point stability, and anti-interference capabilities of traffic meters. This is an exploratory research on the development of high-precision electromagnetic flowmeter. This circuit outputs the number at the SPI2 port of the core board, passing through
Analog Signal is formed by conversion, and the current amplifier output with negative current feedback is stimulated by V/I conversion, which is suitable for various excitation waveform changes. The following figure shows the structure. D/A conversion circuit
The AD7243 chip enables 12-bit SPI synchronous serial input ~ + 5 V bipolar output. And ARM9.
The SPI2 port of the core board is interconnected, as shown in figure 4.

Figure 4 Excitation Circuit Diagram

The excitation amplifier uses the CA3240A op amp, which features a high power supply voltage and a large output dynamic range. Current amplification is achieved by using two pairs of composite pipes, and the tubes must be paired as much as possible. After the excitation coil is connected, the negative current feedback of the Large Loop is introduced to stably output the excitation current.

2.3 traffic output module

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

The D/A conversion voltage reference REFIN selects the REF OUT2 (215
V ). The indirect 00.1 μF capacitance between LV and VCC In the flow output circuit determines that the loop power of + 24 V generates 313
V power supply, + 24 v loop power supply LOOP POW-ER by internal control current returned by loop rtn, forming 4 ~ The current loop of 20mA.

3 System Software Design

　
In the software system of the electromagnetic traffic meter, the embedded processor arm9-kernel mainly considers the core board and initialization settings of each hardware module. After the system is started, it calls the underlying driver to complete the core board.
Command Control and data transmission between each hardware module, and corresponding interrupt service subroutine and interrupt vector table are established. The modular structure is used to establish the system program. The Electromagnetic Flowmeter application system is mainly interrupted by the timer.
Management, excitation signal output and conversion maintenance, multiple data acquisition of induction signals, traffic display, and external output are all completed by the timer interrupt service.

The software platform of this system is mainly based on embedded Linux to establish the development environment. MiniGUI is a graphic user interface support system, on which features are developed. SQLite is a database engine designed for the traffic measurement system.

The control process of the system includes parameters, traffic signal detection and control, and alarms. After the system starts, the system displays the current status on the interface and receives user input settings. At the same time, another thread is generated to implement traffic detection and control.

4. Summary

　
After careful research, development, design and experiment, the electromagnetic flow measuring instrument developed based on ARM920T kernel effectively solves the problem of Precise Measurement and Control of liquid flow, and can improve the liquid flow
Measurement accuracy, and remote monitoring. This system can be widely used in Petrochemical, industrial and mining enterprises. This intelligent meter uses the embedded microprocessor S3C2410A to implement multiple excitation methods.
Type, data USB
Storage, Ethernet communication, and color screen display. At the same time, in the design process of the electromagnetic flow meter, the advanced hardware and software collaborative design scheme is adopted. In addition, S3C2410A is in the electromagnetic flow meter system.
As a high-end application research, the system adopts modular design method in hardware, which improves the Application and Research Level of electromagnetic flow meter and reduces the design difficulty.

Through system testing, this intelligent meter can be connected with industrial automation sensors to form a flow measurement control system, which can be widely used for the Flow Measurement and Control of various industrial liquids, for example, chemical plant reagent flow measurement, petroleum flow measurement, gasoline kerosene flow measurement, and other aspects, and has a very good cost-effectiveness, the promotion and application prospects are good.

Innovation point of this article: This article applies the arm9-kernel to the electromagnetic traffic meter, enables the electromagnetic flow meter to filter input signals, store historical data, output changes of various excitation signals, handle special measurement information, Display Dynamic measurements, and humanized management and control. application.

The electromagnetic flow meter can be connected with Automatic sensors in the industry to form a flow measurement control system and can be widely used for the Flow Measurement and Control of various industrial liquids, for example, chemical plant reagent flow measurement, petroleum flow measurement, gasoline kerosene flow measurement, and other aspects, and has a very good cost-effectiveness, the promotion and application prospects are good.