PLC Programming for Special-Purpose Machine Construction: Best Practices and Tips

PLC programming (programmable logic controller programming) is at the heart of every modern special-purpose machine. Especially in special-purpose machine construction, where no two systems are alike, flexible, clean, and future-proof programming is crucial for functionality, efficiency, and maintainability. In this blog post, we provide you with tried-and-tested tips and proven methods to take your control programs to the next level.

1. Structuring the program

A well-structured PLC program not only makes commissioning easier, but above all facilitates troubleshooting and subsequent extensions. Therefore, use:

• Clearly named program blocks (FCs, FBs, OBs)
• Modular programming: e.g., dedicated blocks for axes, sensors, HMI, etc.
• Global constants and symbol tables for managing values centrally

2. Commenting and documentation

• Documentation is often underestimated, but saves time and frustration in the long run:
• Comments directly in the code: short but clear
• Functional description for each block

Overview of interfaces (inputs and outputs, variables)

This is especially essential for special-purpose machines that will later be maintained or expanded.

3. Using libraries and standards

Use your own or manufacturer-provided libraries, e.g., for:

• Drive technology (Siemens DriveLib, SEW function blocks)
• Safety functions
• Communication (e.g., OPC UA, Modbus)

This saves time, increases quality, and ensures reusability.

4. Simulation and testing in advance prevent errors

Use simulation functions (e.g., PLCSIM from Siemens) to test programs before loading them onto real hardware. This saves time during commissioning and helps eliminate major errors in advance.

5. Flexible error handling and diagnostics

Implement clean error management from the outset:

• Collective fault messages
• Acknowledgment logic
• Error history (FIFO memory, ring buffer)
• Plain-text error messages for the HMI

Make everyday work easier for operators and service technicians!

6. Practical tip: Plan the HMI and PLC hand in hand

Many programmers treat the HMI (human-machine interface) separately from PLC programming. However, both sides benefit from:

• Shared symbol names and structures
  
• Central text lists for error messages
• Status overviews for machine modes, cycles, etc.

An integrated approach leads to a consistent user experience.

Conclusion

PLC programming in special-purpose machine construction is challenging, but manageable with the right approach. Structure, reusability, simulation, and clean error handling are the keys to a maintenance-friendly and robust control program.

If you want to implement your PLC projects professionally or need support with control cabinet construction, please feel free to contact us!