Since the United States introduced Programmable logic controller (PLC) to replace traditional relay control devices in the 1960s, PLC has developed rapidly and been widely used around the world. At the same time, the functions of PLC are constantly improving. With the continuous development of computer technology, signal processing technology, control technology, and network technology, as well as the increasing demand of users, PLC has added functions such as analog processing and motion control on the basis of switch processing.
Today’s PLC is no longer limited to logical control, but also plays a very important role in fields such as motion control and process control. As the preferred product for discrete control systems, PLC developed rapidly from the 1980s to the 1990s, with an annual growth rate of 20% to 30% worldwide. With the continuous improvement of factory automation and the expansion of PLC market capacity base, the growth rate of PLC in industrialized countries has slowed down in recent years. However, the growth of PLC in developing countries such as China is very rapid. Based on relevant information, the global sales revenue of PLC in 2004 was around 10 billion US dollars, occupying a very important position in the field of automation. PLC was developed by imitating the original relay control principle. In the 1970s, PLC only had switch logic control, and was first applied in the automotive manufacturing industry.
It stores instructions for performing logical operations, sequential control, timing, counting, and operations; And control various machinery or production processes through digital input and output operations. The control program developed by the user expresses the process requirements of the production process and is stored in the user program memory of the PLC in advance. The operation shall be executed one by one according to the contents of the Stored procedure to complete the operations required by the process flow. There is a program counter in the CPU of the PLC that indicates the storage address of the program step. During program operation, this counter automatically increments by 1 for each step executed. The program is executed sequentially from the starting step (step number is zero) to the final step (usually the END instruction), and then returns to the starting step for cyclic operation. The time required for PLC to complete each cycle operation is called a scanning cycle.
Different models of PLCs have cyclic scanning cycles ranging from 1 microsecond to several tens of microseconds. PLC programming with ladder diagram shows the advantage of being fast in solving logic, with less than 1 millisecond in solving 1K logic programs at the microsecond level. It processes all inputs as switching variables, with 16 bits (also 32 bits) being an analog variable. Large PLCs use another CPU to perform analog operations. Send the calculation results to the PLC controller. For systems with the same I/O points, the cost of using PLC is lower than using DCS (approximately 40% savings). PLC does not have a dedicated operation station, and the software and hardware used are universal, so the maintenance cost is much lower than DCS. A PLC controller can receive thousands of I/O points (up to over 8000 I/Os). If the controlled object is mainly equipment interlocking with few circuits, using PLC is more suitable. Due to the use of universal monitoring software, PLC is easier to design management information systems for enterprises. In the past decade, with the continuous reduction of PLC prices and the expansion of user demand, more and more small and medium-sized equipment have started to use PLC for control.
The application of PLC in China has grown very rapidly. With the rapid development of China’s economy and the continuous improvement of basic automation level, PLC will continue to maintain a high-speed growth momentum in China for a period of time in the future.
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