What are some of the challenges of PLC system design and testing?

One of the most important points is certainly doing a requirements specification. This is more common in "regular" software development and unfortunately still somewhat unknown in PLC programming. At least for safety-relevant topics it is even required though. One might refer to the well-known standard ISO 13849-1, which in its new 2023 edition will make proper specifications mandatory for fulfilling the standard. Another helpful standard to consider here is IEC 62061. This one in particular has more content on the management of functional safety topics.

Requirements analysis is a crucial first step in designing a PLC system. One of the main challenges is managing the complexity of the requirements. Industrial processes can be highly intricate, involving numerous interdependent variables and conditions. Accurately capturing all these details in the system's requirements is essential but can be daunting. Additionally, requirements are often subject to change due to evolving business needs or technological advancements, which can complicate the analysis phase. Misunderstanding or omitting critical requirements can lead to costly redesigns or system failures.

Challenges in doing requirement analysis become critical while there is a slight changes made after project framing. Integrating new PLC systems with older legacy systems, requiring additional interfaces or modifications.

The logic design phase involves creating the control algorithms and logic that will drive the PLC system. A key challenge in this phase is optimization. The logic must be efficient to ensure quick response times and minimal resource usage while still achieving all functional requirements. This often involves balancing complex conditions and actions, which requires a deep understanding of both the system being controlled and the capabilities of the PLC itself. Error handling is another critical challenge in logic design.

I would suggest defining rules for naming conventions as part of Control System Design basis / Philosophy including; - Naming of soft signals, Function Blocks, Programs - Detailed comments for each type - Structure of Program just like HMI design - Calculation Blocks

Selecting the right hardware for a PLC system is a crucial step that can significantly impact the system's performance and reliability. One of the challenges in this phase is ensuring compatibility between the chosen hardware components and the existing infrastructure. The hardware must meet all functional requirements while also integrating seamlessly with other systems and components already in place. This often involves thorough research and comparison of different vendors and products to find the best fit. Balancing cost and performance is another challenge during hardware selection. While it is important to choose reliable and high-performance components, budget constraints often limit options.

I would highlight power supply issues which would greatly impact operation and maintenance. Choose power supply based on ambient conditions, hazardous area classification if any, grounding, reputed brands, fault signal, heat dissipation, redundancy with redundancy module etc.

Consider : - Install hardware at best possible location inside panel (heat dissipation) - Have proper distance from all sides as per OEM guidelines - Choose proper cable lugs - Double check earthing connections - Separate AC powered components from DC ones - Ensure cables are pealed properly - Ensure Breakers/Relays/Fusibles blocks are out before FAT/commissioning

The wiring and installation phase involves the physical setup of the PLC system, presenting several logistical challenges. One of the main challenges is dealing with physical constraints, such as space limitations or existing infrastructure. Installing new systems in environments where space is at a premium or where legacy equipment must remain operational can require creative solutions and precise planning to ensure everything fits and functions correctly.

Many people only begin testing their programs once they are running on the machine. This approach is far too late. To ensure quality, it's essential to establish a strong test pipeline early in the development process. Implementing a good unit test framework allows for automatic testing sequences throughout development, ensuring issues are identified and resolved promptly.

System testing is a critical phase that validates the PLC system's functionality and reliability. One challenge in this phase is developing comprehensive test cases that cover all possible scenarios, including normal operations, edge cases, and potential failure modes. This requires a deep understanding of the system's requirements and potential operating conditions, as well as the ability to anticipate unusual situations that could arise during real-world use. Real-time testing presents additional challenges. Simulating real-world conditions to verify system performance can be complex and resource-intensive, requiring specialized equipment and expertise.

Good enough for testing but I would say things to do after testing : - remove jumpers / forces and document if cannot be removed - put labels/tags to components if they must need to be powered up or powered off during testing or after testing - lock the PLC cabinet if not handed over to end user after testing.

Consider Local support and supply of spares as part of hardware selection else you will be surprised later: - High Lead Time - High Prices - Little to no Local Support - Upgradation in less than 5 years

Beyond the core design and testing phases, several additional considerations can impact the success of a PLC system. Creating clear and thorough documentation is essential for assisting with future maintenance and updates. Documentation should provide detailed explanations of system operation, including logic flowcharts, hardware configurations, and troubleshooting guides, to support operators and technicians.

Simulating real world conditions in a controlled environment may not reveal all potential issues. Doing a validation test and be careful while implementing in the real world like being cautious will save a lot of money. HIL(Hardware in the Loop) testing with real hardware can be challenging. Addressing these challenges requires careful planning, thorough testing, and colloboration between engineers involved in system design, implementation and maintenance.