Complete Guide to Emergency Shutdown in PLC Systems

Today, let’s talk about a classic project in Siemens PLC programming—the alarm interlock emergency shutdown system. This is a very practical system frequently encountered in factories, but if not handled correctly, the machines may suddenly stop working at any time. So, let’s directly discuss the confusing aspects of this project and how to make the system more reliable.

Basic Concept of the Project

The alarm interlock emergency shutdown system mainly monitors the operational status of factory equipment. In simple terms, when a device is malfunctioning, the alarm will sound first, and in urgent situations, the system will forcibly shut down to prevent accidents. There are several key points:

• • Equipment Status Monitoring: Collect operational parameters of the equipment, such as temperature, pressure, and current.

• • Alarm Notification: Trigger an alarm immediately when parameters exceed the predefined range.

• • Interlock Shutdown: If the situation is urgent, shut down directly to prevent accidents.

Preparation Before Development

Everyone should fully understand the process flow of the equipment and clarify the normal range of each parameter before writing the program, as it’s easy to report errors otherwise. Siemens’ Step 7 or TIA Portal are good tools to use flexibly.

Program Structure Planning

The system will be divided into three major parts:

1. 1. Signal Acquisition Part: Obtain parameters such as temperature and pressure from sensors.

2. 2. Alarm Judgment Part: Determine whether these parameters exceed the established safety range.

3. 3. Interlock and Shutdown Part: Decide whether to execute shutdown operations based on the judgment results.

Common Code Errors and Optimizations

Many beginners get confused when writing this system due to too many variables. Variable naming should not be too random; it’s better to add prefixes to indicate which module they belong to. Having too many layers of parameter settings can complicate later modifications. It’s recommended to extract commonly used parameters, group them, and make reasonable use of FB and FC function blocks.

Without Further Ado, Let’s Look at the Code

Let’s assume the device has three parameters to monitor: temperature (Temp), pressure (Pressure), and current (Current).

// Input Acquisition
Temp := IN_Temperature_Sensor;
Pressure := IN_Pressure_Sensor;
Current := IN_Current_Sensor;

// Alarm Detection
IF Temp > MAX_TEMP THEN
    Alarm_Temp := TRUE;
END_IF;

IF Pressure > MAX_PRESSURE THEN
    Alarm_Pressure := TRUE;
END_IF;

IF Current > MAX_CURRENT THEN
    Alarm_Current := TRUE;
END_IF;

// Interlock Control and Shutdown Judgment
IF Alarm_Temp OR Alarm_Pressure OR Alarm_Current THEN
    // Execute shutdown operation here
    Stop_Machine := TRUE;
    // Send signal to alarm system
    Trigger_Alarm := TRUE;
END_IF;

// Reset signal under normal conditions
IF NOT (Alarm_Temp OR Alarm_Pressure OR Alarm_Current) THEN
    Reset_Alarm := TRUE;
    Stop_Machine := FALSE;
    Trigger_Alarm := FALSE;
END_IF;

To reiterate, after writing the code, do not forget one thing: find a simulator or actual equipment for testing. Discovering issues only during on-site operations can be quite stressful.

Additional Optimization Tips

During coding, you may encounter situations where the alarm frequency is too high, which can be counterproductive. Here, consider implementing an alarm delay; for instance, a temporary spike in temperature does not necessarily indicate a real problem with the equipment. You can set a delay for judgment before rushing to shut down. This operation is also relatively simple: just add a timer.

Additionally, do not forget to have contingency measures for the emergency shutdown part to prevent safety issues caused by PLC failures. How to do this? Configure a hardware watchdog for the PLC, so that in case of problems, a backup relay can automatically shut down the equipment.

Conclusion

In summary, the alarm interlock emergency shutdown system is very important, especially in high-risk industrial environments. Do not be lazy and just write a “sequence control” and call it a day; consider all possible situations thoroughly. After writing the code, it’s best to have an experienced engineer review it to optimize any issues in advance, rather than having to patch things up after an accident occurs.

Still unclear about something? No problem! Feel free to leave a message, and we can tackle it together; learning never stops!