PLC (Programmable Logic Controller) is a control device widely used in industrial automation. It processes and evaluates various input signals based on user-programmed control logic, then outputs corresponding control signals to achieve automated production process control. This article provides a detailed introduction to the basic working principles and scanning operation mode of PLCs.
I. Basic Working Principles of PLC
The working principle of PLC primarily involves the following steps:
1. Input Signal Acquisition: The PLC receives various signals from the field through input modules, such as digital signals and analog signals. These signals undergo processing like opto-isolation and filtering before being converted into digital signals recognizable by the PLC's internal system.
2. Program Processing: The PLC processes and evaluates input signals based on the user-programmed control logic. Program processing primarily involves functions like logical operations, arithmetic calculations, and data conversion. During program processing, the PLC generates corresponding control commands based on different conditions.
3. Output Signal Control: The PLC sends the processed control commands to the output module. The output module converts the digital signals into the required signal formats for the field (such as digital signals, analog signals, etc.) and outputs them to the actuators (such as relays, servo motors, etc.), thereby achieving control over the production process.
4. Communication and Monitoring: PLCs possess communication capabilities, enabling data exchange with other devices (such as host computers, touchscreens, etc.) via communication interfaces to achieve remote monitoring and control. Simultaneously, PLCs can display real-time operational status and fault information through their own displays.
II. Characteristics of PLC Scan Operation Mode
The characteristics of PLC scanning operation are primarily reflected in the following aspects: First, PLCs employ a "sequential scanning, continuous looping" operational mode. Each scanning cycle, also known as the scan period or working cycle, begins with the CPU executing the first instruction. It then sequentially executes the user program line by line until completion, after which it returns to the first instruction to initiate a new scanning cycle. Second, PLC program execution follows a periodic scanning pattern adhering to a left-to-right, top-to-bottom principle.
Additionally, each scan cycle comprises three steps: input sampling, program execution, and output refresh. PLC operation thus cycles continuously in this manner. Finally, due to centralized sampling and centralized output, input/output lag occurs, resulting in response delays. These constitute the primary characteristics of PLC scanning operation.
III. PLC Scanning Operation Mode
The PLC scanning operation mode is primarily divided into the following stages:
The PLC scanning operation mode consists of three main stages: the input sampling stage, the user program execution stage, and the output refresh stage.
1. Input Sampling Phase
During the input sampling phase, the PLC sequentially scans and reads all input states and data, storing them in corresponding units within the I/O image area. Upon completion of input sampling, the system transitions to the user program execution and output refresh phase. During these two phases, even if input states or data change, the corresponding units in the I/O image area remain unchanged. Therefore, if the input is a pulse signal, the pulse width must exceed one scan cycle to ensure the input is captured under all conditions.
2. User Program Execution Phase
During the user program execution phase, the PLC always scans the user program (ladder diagram) sequentially from top to bottom. When scanning each ladder diagram, it first scans the control circuit formed by the contacts on the left side of the ladder diagram. It then performs logical operations on the control circuit formed by the contacts in the order of left to right and top to bottom. Based on the results of these logical operations, it updates the status of the corresponding bit in the system RAM storage area for the logical coil, or updates the status of the corresponding bit in the I/O image area for the output coil, or determines whether to execute the special function instruction specified by the ladder diagram. That is, during user program execution, only the status and data of input points within the I/O image area remain unchanged. The status and data of other output points and soft devices within either the I/O image area or system RAM may change. Moreover, the execution results of ladder diagrams positioned higher in the stack affect subsequent ladder diagrams that utilize these coils or data. Conversely, the refreshed logic coil status or data from lower-level ladder diagrams only affects upper-level ladder diagrams during the next scan cycle.
3. Output Refresh Phase
Upon completion of the user program scan, the PLC enters the output refresh phase. During this period, the CPU refreshes all output latches based on the corresponding status and data in the I/O image area, then drives the corresponding peripherals through the output circuitry. This constitutes the PLC's actual output.
In summary, the fundamental operating principle of a PLC involves collecting field signals through input modules, processing and evaluating them according to user-programmed control logic, and then controlling actuators via output modules to achieve automated production process control. The PLC's scanning operation comprises phases including system self-test, input scan, program processing, output scan, communication, and monitoring. These phases form a scan cycle, which the PLC continuously repeats to realize automated production process control.