introduction
Electromagnetic flowmeter is a kind of measuring instrument for measuring the volume flow of conductive medium. It has the characteristics of high measurement accuracy, good stability and high reliability. In addition to measuring the flow rate of general conductive liquid, electromagnetic flowmeter can also measure the volume flow of liquid-solid two-phase liquid, high-viscosity liquid and salt, strong acid and strong alkali liquid, which can be widely used in cement, chemical industry, textile, metallurgy. Industrial, technical departments such as mining, paper, medicine, water supply and drainage, food and beverage, environmental protection, etc., the performance, quality and reliability of their products have an important impact on the economic benefits of the above-mentioned enterprises.
The Embedded System is application-centric and computer-based, and the hardware and software can be cut down to meet the strict requirements of the application system for functions, reliability, cost, size, power consumption and other indicators. Dedicated computer system. It is a user-oriented, product-oriented, application-oriented dedicated system. Compared with general-purpose computer systems, it has the following characteristics: embedded CPUs usually have the advantages of low power consumption, small size, high integration, low cost, etc. Many functions in the CPU are integrated into the chip; the development and application of the embedded system is not easy to form a monopoly in the market; once the embedded system products enter the market, they generally have a long life cycle; the hardware of the embedded system Both software and software must be designed efficiently; embedded systems must have a set of cross-development tools and environments to develop.
The instrumentation is an important part of the parameter detection process, and generally has three parts: a transformation, a measuring device and a display device. Aiming at the requirements of the instrumentation, the software and hardware are reasonably tailored to achieve high integration and miniaturization, so that the instrument not only has the functions of sensing measurement, compensation calculation and display, but also has more powerful information processing, comparative reasoning, fault diagnosis and self-control. Learn about intelligent functions such as self-adaptation. The network access technology of the embedded system can also enable the instrument to perform functions such as remote maintenance, remote control, fault alarm, information release and sharing that were previously impossible. The high development of embedded system technology has brought unprecedented development space and opportunities for detection and instrumentation technology, paving the way for the intelligent, network and miniaturization of instrumentation, and has brought a broad development for the instrument industry. prospect.
The traditional electromagnetic flowmeter generally adopts 8-bit or 16-bit single-chip microcomputer. Due to the weak performance of the single-chip microcomputer and the few peripheral interfaces, it is impossible to complete complex algorithms or multiple parallel tasks. In the display mode, LED or segment LCD is generally used, and only digital numbers can be displayed. , letters, Chinese characters and some rough patterns; generally use small-capacity EPROM memory in data storage, traffic data storage is generally stored in binary mode, not universal; in communication mode, generally adopt RS 232 or RS 485, The openness is not high and the networking distance is short. In order to improve performance and function, some electromagnetic flowmeters even use a multi-chip microcomputer to work together, which increases the complexity of the design and implementation of the electromagnetic flowmeter. Therefore, it is necessary to design a magnetic flowmeter using a high-performance 32-bit processor.
Based on 32-bit ARM processor and embedded Linux operating system to build embedded system development platform, and apply it to the design and development of electromagnetic flowmeter, it will help to improve the quality of high-end electromagnetic flowmeter products and better meet users. Demand, but also has important practical significance for the development of China's electromagnetic flowmeter detection technology. Due to the complexity and difficulty of the design and development of embedded systems, the high-performance 32-bit embedded system development platform built in this paper can also be applied to the design of other high-end instrumentation, shortening the design and development cycle of new products. Enhance the company's ability to respond to customer needs and product replacement capabilities.
1 overall design
1.1 ARM processor selection Combined with the application background of electromagnetic flowmeter, choose 32-bit embedded ARM processor. According to the application selection of the ARM processor, and considering the ARM processor price, performance, integration and function, design support and development tool support, and the application requirements for Ethernet and USB interface functions, Atmel was selected. The company's AT91RM9200 industrial grade chip.
1.2 Choice of Embedded Operating System For an embedded operating system, there are usually some evaluation criteria: practicality, number of resources to be consumed (including SDRAM, FLASH MEMORY, etc.), richness of support software, reliability, and Its performance and so on. Linux has been ported to many platforms, including ARM, MIPS, SH3, Alpha, X86, PowerPC, etc., applications, protocol stacks, and rich device drivers developed by thousands of enthusiasts; more importantly, these drivers Many are released in the form of Open Source, which benefits everyone.
1.3 Overall Design Framework Through the selection of electromagnetic flowmeters and the development of intelligent, integrated and networked electromagnetic flowmeter products at home and abroad, the overall structure of the system is shown in Figure 1. The system is mainly composed of a hardware subsystem and a software subsystem.
The hardware subsystem of the electromagnetic flowmeter mainly comprises a power module, a signal input module, a CPU core module, an excitation output module, a human-computer interaction module, a communication interface module and a debugging interface module. The function of each module is described as follows:
Power Module: Provides a stable and reliable power supply for the entire hardware subsystem.
CPU core module: The minimum system consists of ARM processor, SDRAM, FLASH and auxiliary circuits. It is also the basic hardware platform for running embedded Linux.
Signal input module: Amplifies, filters, and converts small voltage or current signals output by the sensor into digital quantities for software processing.
Excitation output module: The low-frequency three-value rectangular square wave is output through the D/A converter, and is output to the sensor through power amplification.
Human-computer interaction module: It consists of keyboard interface and TFT liquid crystal display interface. System parameters are set through keyboard and menu interface, and instantaneous flow, cumulative flow and flow curve are displayed by LCD screen in real time.
Communication interface module: including Ethernet interface, USB interface and RS232 interface, complete data communication and traffic transmission.
Debug interface module: consists of JTAG debug interface and DBCOM debug serial port.
The software subsystem consists of a boot loader (Boot Loader), an embedded Linux operating system, and application software. The startup of embedded Linux requires a bootloader to assist the completion. Linux and kernel are mainly composed of process management, memory management, file system management, device control and network protocol, which provide the operating environment for the application software. The hardware device control in the figure provides related device drivers in the hardware platform, realizes information interaction and control between the operating system and the hardware, and the system function call provides an interface for information interaction between the Linux kernel and the application software.
2 hardware system design
The hardware structure diagram of the electromagnetic flowmeter based on the low frequency three-value rectangular wave excitation is shown in Fig. 2. Among them, the CPU core module is the running basis of the system software, and the FLASH memory is used to store the binary image file of the executable program, which is loaded into the SDRAM memory space when the system starts up; due to the complexity of the hardware system, the debugging module strengthens the hardware system. Debug function; the flow sensor consists of a pair of excitation coils and a pair of symmetrically distributed detection electrodes.
The ARM9 processor generates a low-frequency three-value rectangular wave through the D/A chip connected thereto, and the excitation coil receives the excitation signal of the low-frequency three-value rectangular wave through the power amplification to generate an induced magnetic field, which is generated on the electrode by the fluid cutting magnetic line. The induced electromotive force (ie, the measurement signal) is sent to the signal input module for processing, and the signal input module amplifies, filters, and A/D samples the signal input by the flow sensor and inputs it into the ARM9 processor. The system provides a user-friendly menu display operation interface by TFT LCD screen, and combines the keyboard for human-computer interaction. At the same time, Ethernet, USB and RS 232 communication interfaces can output instantaneous flow, accumulated flow and system parameters.
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