| Programmable logic controllers, also called programmable controllers or PLCs, are special-purpose computers used for industry control. They are easy to configure and program, foreseeable when running, and can keep working well even in tough industry environment. So PLCs are widely applied in industry control. A small general-purpose controller based on dsPIC30F4013is designed in this paper, as a substitute for PLCs in some small control systems. This controller can be configured and program in a more flexible way, and reduces the cost largely on the premise of good reliability.In respect of hardware design, the system in this paper designed plenty of peripheral equipment based on dsPIC30F4013according to the architecture of PLCs, realizes most of the necessary modules of PLCs, including CPU(Central Process Unit), digital I/O,HSC(high-speed pulse), analog I/O, serial port communication, human-machine-interface, and so on. Special process circuits are designed for I/O device, which enhance the system’s ability for anti-interference and drive. As to software, according to the characteristic of MODBUS information frame, the system achieves the function of capturing, analyzing and sending information data by the program compiled in C function programming language. The application adopts CRC checksum method to ensure the accuracy of communication data. The design of this embedded controller based on dsPIC30F4013complies with the trend of the development of industry control, and provides a more flexible way to develop control system, and reduces the cost largely, but possessed high practical value.This controller is a set of remote data acquisition, measurement and automatic control in one PLC. The major market position is in the valve chambers of long gas pipeline. Valve chambers, generally only have valves and pipes, and no measurement of pressure control equipment, so the number of I/O points is relatively small and the process is relatively simple. In the past, the users adopt the imported PLC controllers to data collection and monitoring for the valve chambers, to some extent it is a waste of resources for the PLC system, and it is difficult for users to accept the high cost. In view of this situation, we develop a small and medium-sized PLC systems based on MODBUS protocol to meet the market demand.The system uses the integrated structure and the integration of AI, AO, DI, DO, and HSC five types of signals which are input and output channels. The channels of AI can capture the standard4-20mA signal of field instrumentation, and they can acquire the signal of two-wire or three-wire instrumentation through board jumper settings. The channels of DI provide switch data acquisition channels, and the type of the input signal is dry contact input. The channels of AO have two selectable output signal types, namely,4-20mA or1-5V signals. The channels of DO provide switch data output channels, and the type of the output signal is dry contact output, besides, we can select normally opened contact output or normally closed contact output by the panel toggle switch. The channels of HSC can detect the frequency which is no more than15KHz signal.Field bus is one of the hotspot in automation development, and which is regarded as a computer LAN in automation area. As a foundation of industrial communication network, field bus links up the field equipments of production process. MODBUS field bus is a technology which efficiently supports distributed real time with a very high level of security. It attracts attention with a very high level of capability and a very high level of security and a very better of anti-jamming and a very fast of communication speed and a very low of maintenance cost and a particular design. It is regarded as one of field bus with expansibility. Many industrial equipments, such as PLC, DCS and intelligent instruments use MODBUS as their communication standards. It is realistic meaning to research and develop instrument based on field bus.In this dissertation, the most popular MODBUS techniques currently used are reviewed briefly, a systematic analysis on the MODBUS based field bus control system is presented. A General-Purpose Controller based on MODBUS is designed while its feasibility and potential applications is demonstrated. The main work of this dissertation is listed below:First, the format and then definition of the system structure of MODBUS protocol are introduced briefly. This part in detail described the different parts of the bus protocol, which include the transmission method, the frame structure, error verification, function code, and so on.Second, the design theory and system structure based on the MODBUS protocol in the field bus control system are investigated. The General-Purpose Controller is constructed according to this protocol. This system basically is a control and manage network, which has high real-time, high reliability, data integrality, and usability. The data exchange and transfer between this system and the company information system are realized by the special interfaces and equipment.Third, a successful application of the General-Purpose Controller is demonstrated in the area of the industrial engineering, by text, data and figures. The General-Purpose Controller is proved by this application. The potential system applications and market forecast are introduced as well in this part.Through the application in the real world, comparing with the traditional control system, the system proposed in this paper has been found that the MODBUS control system shows great advantages in terms of reliability, error-handling, data exchange, system management, as well as anti-interruption for environmental. Therefore, it will have a great future and market in instrument control area. |
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