Traditional analog control system using electric instrument, common DDZ-and DDZ-type ⅱ–ⅲ instrument. The characteristics of simple structure, low price, great size, great power, installation complexity, generality and flexibility, accuracy and stability. In addition, its control functions are simple, complex process control cannot be achieved. With the development of technology and new process control computers emerging and popular industrial computer (IPC), programmable logic controller (PSC), distributed control systems (DCS).
1. PLC and PSC
Programmable controller (PSC) are ddz-ⅲ instrument based on microprocessor technology to develop fourth–generation instrument. Its power, flexibility, reliability, accuracy, digital communications capabilities are unmatched by conventional electric meter. PSC industrial devices and PLC are intelligent, have their own characteristics. PLC dominated by switching control, analog control; and the PSC is dominated by closed–loop analog control, switch control, and can display, alarm and manual operation. Therefore, the PSC used in analog control system is more suitable for a variety of process control requirements. PLC reliability, flexibility, strong ability to switch control and communications networking capabilities, also featured in analog control. Special switching, analog hybrid control system shows its unique advantages.
2. PLC and DCS
Distributed control system (DCS) was published in 1975, it was 3C (computer, and communications, and control) technology of the product, it will control analog equipment, data acquisition devices, process control instrumentation, process monitoring device combined together, meet the requirements of various DCS. PLC analog control function of strengthening today, most are equipped with a variety of intelligent modules, with a PID regulation, composition and composition network control function, also realizes the functions of DCS can fulfill. So far, development of PLC and DCS are getting closer. On the development trend and advantages of PLC and DCS control system synthesis and combine both, forming a new kind of distributed computer control system.
3. PLC and IPC
Industrial computer (IPC) was developed by the computer application, its high degree of standardization of the hardware structure and bus, compatibility, software is rich in resources, in particular, a real-time operating system support, require real-time, complex system models dominate the field. Poor standardization of PLC, products are not compatible, so development is difficult. PLC ladder diagram programming was welcomed by the unfamiliar with computer electrical technician and PLC for industrial environment design, reliability is very high, being considered will not damage the equipment, and the IPC on the reliability or not enough good.