In the 1960 of the 20th century United States introduction of programmable logic controllers (Programmable Logic Controller,PLC) instead of the traditional relay control device since the PLC have been developing rapidly, has been widely used all over the world. Meanwhile, PLC functionality has been improved. With computer technology, signal processing, control network technology and user demand continues to improve, PLC in switch processing based on the analog processing and motor control functions. PLC today is no longer limited to logical control, motion control, process control and other fields also plays a very important role.
As the discrete control product of choice for business, PLC has been rapid development in in the 1980 of the 20th century to 90, PLC annual growth rate of 20%~30% around the world. As the degree of plant automation improvement and PLC continues to expand the market base, and PLC in industrial growth slowed in developed countries in recent years. However, PLC in developing countries such as China is growing very rapidly. Comprehensive information, in 2004 the global PLC for about $ 10 billion of sales revenue, occupied a very important place in the field of automation.
PLC is troubled by imitating the original relay control theory developed in the 1970 of the 20th century only switch logic control PLC, first apply the automobile manufacturing industry. Its stores perform logical operations, sequence control, clock, counting and arithmetic operation instructions and through digital input and output operations to control various types of machinery or production processes. User prepared controls expression of the technological requirements of the production process, in the user program memory and advance into the PLC. Article-by-article implementation of the stored procedure returns the running in order to complete the process the requested operation. PLC program indicates that the program step to store addresses in the CPU counters, while the program is running in the process every step of the implementation of the counter automatically plus 1, from the initial step in the process (step ordinal of zero) are performed to the final step (usually the END directive), and return to the starting–step cycle operation. PLC each the time required to complete a cycle operation called a scanning cycle. Different types of PLC, circular scanning cycle between 1 microsecond to dozens of microseconds. PLC ladder diagram programming, in terms of solving logic, showed rapid advantage of microsecond magnitude less than 1 millisecond resolution 1K logic programs. Switch to handle all inputs as 16-bit (32-bit) for an analog. Use another CPU to complete large PLC analog operation. Controller for PLC results.
Same number I/O systems with PLC than with DCS, its cost is lower (around 40%, about). PLC do not have dedicated stations, which is common with software and hardware, so maintenance costs are much lower than DCS. A PLC controller, you can receive thousands of I/O points (up to a maximum of more than 8,000 more I/O). If control is rarely equipment chain, loop, PLC is appropriate. PLC as a result of monitoring software, in terms of enterprise management information system design, easier.
Past 10 years, with the price of PLC falling and consumer demand continues to expand, more and more small device controlled by PLC, PLC application in China is growing very rapidly. With China‘s rapid economic development and continuous improvement of basic automation, future PLC for a period in China will maintain the momentum of rapid growth.
General PLC applied to special equipment can be considered when it is an embedded controller, but General embedded controller and PLC with high reliability and good stability. Practical work, met some of the user‘s original uses an embedded controller, universal PLC or custom PLC is now gradually replacing embedded controllers.