In the industrial automation control system, PLC (Programmable Logic Controller) is an indispensable core component, PLC reads the status of various input signals (such as buttons, sensors, etc.), and after internal logic operation, controls the action of the actuator (such as motors, valves, etc.). Among these input signals, the emergency stop button receives a lot of attention because of its importance.
In PLC programming, the design logic of the emergency stop button is often different from that of conventional buttons. Generally, the physical contact of the emergency stop button is connected to a normally closed state, while in the PLC program, it is written as a normally open state. There are unique considerations behind this design.
First of all, from the point of view of hardware connection, the emergency stop button is connected to the DI (digital input) point of PLC in normally closed state. This means that under normal conditions, the line of the emergency stop button is connected, and the DI point of the PLC will receive a high level signal, indicating that the system is in normal operation. And when the emergency stop button is pressed, its normally closed contact is disconnected and the PLC's DI point receives a low level signal, indicating that the system needs to stop running immediately.
Next, from the PLC program's point of view, we represent the state of the emergency stop button as normally open. This is because under normal conditions, the emergency stop button's line is on, but we don't use this high level signal directly in the program to trigger any action. Instead, we focus on the low-level signal generated when the emergency stop button's normally closed contacts break when it is pressed. This low-level signal triggers the relevant logic in the PLC program to bring the system to a stop.
The advantage of this design is that it enables monitoring of the disconnected state of the line where the emergency stop button is located. If the line of the emergency stop button is disconnected due to some reason (e.g., aging of the line, poor contact, etc.), then the DI point of the PLC will not be able to receive a high level signal, and the program will immediately assume that the emergency stop button has been pressed, thus triggering the stop logic. This design greatly improves the safety and reliability of the system.
In addition, designing the emergency stop button as a normally closed state also helps to achieve program simplicity and readability. In the PLC program, we only need to focus on the low level signal of the emergency stop button to determine whether the system needs to stop running. This design makes the program logic clearer and reduces the possibility of errors.
In addition to the emergency stop button, there are a number of contacts with protection functions (such as thermal relays, thermostats, etc.) also use a similar design logic. These contacts are usually connected to the PLC's DI points in a normally closed state and are represented as normally open in the program. This design also improves the safety and reliability of the system and simplifies the program logic.
In summary, the design logic of the emergency stop button in PLC programming takes into full consideration the safety and reliability requirements of the system. By connecting the physical contact of the emergency stop button to a normally closed state and writing it as a normally open state in the program, we are able to monitor the disconnected state of the line where the emergency stop button is located and trigger the stopping logic when necessary to protect equipment and personnel safety.