Considering PLC project design

PLCs ARE an important feature of modern production facilities - they communicate, monitor and control simple to complex automated processes, ranging from conveyor and robot cells to defect classification using Machine Learning algorithms in real-time. They also act as a physical interface between devices on the production floor and SCADA/HMI systems. Designing and commissioning a PLC can seem complex, but often these projects are remarkably straightforward. 

It's all about best practice and working smartly with the PLC’s software. For example, the code written to control the PLC can seem daunting at first, but by following a clear structure and using supporting tools, engineers can make it far more manageable. Using a disciplined workflow from the beginning can also give them the confidence that everything is under control.

Modularising code

When working on a PLC project, breaking the overall control system into smaller functional blocks, classes or modules - each responsible for a specific task - can help simplify things. In turn, this makes the code easier to read and navigate, not just for the engineer who is writing it but also for support technicians or other individuals who need to access it. Modularising also means that engineers can assign different tasks to different parts of a team. For example, on a packaging line, one engineer may be responsible for the conveyor control module, while another focuses on product detection and rejection. 

Following international standards like IEC 61131-3, which supports multiple programming styles including ladder logic, structured text, function block diagrams and sequential function charts, engineers can ensure their modules are readable, consistent, and maintainable across projects

Using a modular approach can also make future updates simpler, as individual units are easier to maintain than a whole system. If new functionality is needed, changes can be made to the relevant module without disrupting the larger base of code, and other parts of the program. For example, if a customer requested an additional feature on a machine that’s already installed, we could help write and apply that update to just the relevant module while securing the rest of the system and leaving it unaffected. Being able to add new functionalities easily can also improve system scalability.

Using a modular design, engineers can reuse code and save time across their PLC projects. Many projects share similar logic, and by building flexibility into a module from the beginning, it can be adapted and reapplied rather than needing to be rewritten for each process. One example is sensor handling: some sensors are normally closed (NC) while others are normally open (NO). Instead of writing separate code for each type, engineers can design the module with an option to invert the response to the sensor. If it’s NC, engineers will be looking for a false result, which tells them the sensor is active. However, if it’s NO, they will look for a true result showing the sensor is active.

The importance of documentation

Clear documentation and consistent commenting are equally important. Without archived information, fault-finding and handovers are often time-consuming. When working on well-documented projects, engineers can return to their work months later and quickly understand the logic, or another member of the team should step in and be able to pick up tasks. Clear documentation and notes also enable engineers to log in to the system, follow the code and troubleshoot with little difficulty. 

In our experience, good documentation benefits the end customer too. Many expect not just a working machine but also an accompanying codebase and documentation. If comments and standards are adhered to throughout development, the documentation package for delivery - including source code explanation and supporting reports - becomes more straightforward. 

To support the documentation process, Beckhoff launched the TE101 TwinCAT 3 Documentation Generation, a tool that automatically generates reports from PLC code using special comments and markups. Developers can embed explanations, create lists and tables and insert images, and export as an interactive PDF, HTML or MSHC file.

Source control

Source control is an important - but often underused - element of PLC projects. It involves using tools to track code changes, allowing engineers to systematically view and download updates. When working on complex projects, or those that involve upgrading software or reverting to previous versions, source control can help simplify things. Without it, engineers could be left relying on ad hoc methods, like zipping up files and naming them “Revision 1” or “R2”. This can put the files at risk and make them more time-consuming and cumbersome to share.

We use a more modern approach, with TwinCAT 3 to integrate with Visual Studio, which supports source control. Since TwinCAT 3 PLC projects are text-based, tools like Git and Team Foundation Server can help store code in a repository. Engineers can perform updates, track changes with visual cues highlighting additions or deletions and create branches for new features, for example. This makes it easier to manage updates, visualise changes, test new features or functionality and add comments, improving collaboration and reducing risks.

Some companies have recognised the benefits of source control, but many PLC users still rely on manual zip files. Transitioning to a more modern source control workflow can enhance the project’s flexibility, scalability and traceability, making them future-proof.

As well as source control, engineers can use performance optimisation tools like Beckhoff’s TwinCAT 3 PLC Profiler to measure runtime performance, highlight time-consuming or inefficient code and detect potential runtime issues early in development, ensuring the system operates smoothly once deployed.

PLCs are a cornerstone of modern industrial automation facilities - which is why it’s important to use them in a structured, effective way. The key to any PLC project is ease and accessibility, which means modularising and simplifying complex code where possible and ensuring there is an audit trail and strong documentation. 

Beth Ragdale is software business manager at Beckhoff UK

For more information: 

www.beckhoff.com

Tel: +44 1491 4105-39