The working principle of the PLC control system is mainly divided into the following stages:
Input sampling stage: PLC reads all input states and data in sequence in a scanning manner and stores them in the corresponding units in the I/O image area. At this stage, PLC will collect and process the input signals and convert them into digital signals for subsequent program execution. Input signals can come from various sensors, switches and other devices, such as temperature sensors, pressure sensors, limit switches, etc. These signals enter the PLC through the input module, and the input module will filter, amplify, isolate and other processes on the signals to improve the quality and reliability of the signals.
User program execution stage: The core of the PLC is its built-in microprocessor, which performs logical operations, mathematical operations and other processing on the signals in the input register according to the program written by the user. User programs are written in programming languages such as ladder diagrams, instruction lists or structured texts, which describe the control logic and operation steps that the PLC needs to perform. During the execution of the program, the PLC will execute each instruction in the order of the program, process the input signal, and update the value of the internal register according to the processing results.
Output refresh stage: After program processing, the PLC sends the output signal to the output module. The output module converts the digital signal into the corresponding switch quantity or analog quantity signal to control the action of the field equipment, such as controlling the start and stop of the motor, adjusting the opening of the valve, etc. At this stage, the PLC will update the value in the output register to the output module, and the output module will drive the corresponding external device according to these values, thereby realizing the control of the production process.
Cyclic scanning: The working process of the PLC is cyclic scanning. In each scanning cycle, the PLC completes the three stages of input sampling, user program execution and output refresh in sequence. The scanning cycle is usually at the millisecond level, so the PLC has high real-time performance. After a scanning cycle is completed, the PLC will immediately start the next scanning cycle and repeat the above process continuously to realize the continuous monitoring and control of the production process.
Fault diagnosis: The PLC has a fault diagnosis function and can monitor its own working status and the status of the field equipment in real time. When an abnormality is detected, the PLC will issue an alarm signal and take corresponding protection measures to ensure the safe and stable operation of the system. For example, when an input signal exceeds the normal range, an output module fails, or communication is interrupted, the PLC will detect these problems in a timely manner and remind the operator to handle them through indicator lights, alarms, etc.
