As the first choice for discrete control, PLCs have been rapidly developed in the 1980s and 1990s. The worldwide growth rate of PLC controllers is maintained at 20% to 30%. With the continuous improvement of the degree of factory automation and the continuous expansion of the PLC controller market capacity base, the growth rate of PLC in industrial developed countries has slowed down in recent years. However, PLCs in developing countries such as China are growing rapidly. According to relevant data, the sales revenue of global PLC in 2004 was about US$10 billion, which occupies a very important position in the automation field.
The PLC controller is developed by mimicking the principle of the original relay control. In the 1970s, the PLC controller had only the switching logic control. The first application was the automobile manufacturing industry. It stores instructions for operations such as logic operations, sequential control, timing, counting, and operations, and controls various types of machinery or production processes through digital input and output operations. The user-programmed control program expresses the process requirements of the production process and is stored in advance in the user program memory of the PLC controller. Run one by one according to the contents of the stored program to complete the operation required by the process. There is a program counter in the CPU of the PLC controller indicating the storage address of the program step. During the running of the program, the counter is automatically incremented by one step. The program executes from the starting step (step number is zero) to the final step (usually For the END instruction, then return to the initial step of the loop operation. The time required for the PLC controller to complete one cycle of operation is called a scan cycle. Different types of PLC controllers, cycle scan cycle between 1 microseconds to tens of microseconds. The PLC is programmed with a ladder diagram and shows fast advantages in solving logic. In the microsecond range, the 1K logic program is solved in less than 1 millisecond. It treats all input as a switch, and 16 bits (also 32 bits) are analog. The large PLC controller uses another CPU to perform the analog calculations. Send the calculation result to the PLC controller.