Maintainable Control Software Design using Design Patterns in IEC 61499

Control software design in cyber-physical systems plays a vital role in ensuring maintainability and adaptability in response to evolving process requirements. Variability-aware designs facilitate the efficient integration of new functionalities and depend on the selected design elements and on the way these elements interact. Designers can enhance flexibility by applying design patterns and adopting alternative design elements when an existing design requires substantial modification effort. In IEC 61499, using discrete interfaces as a design element in function block (FB) implementations often incurs high effort due to numerous discrete connection instances and limited encapsulation. Complexity is further increased by implicit one-to-many connections among subsystems, which negatively affect system understandability and maintainability. This article analyzes two baseline implementations that follow pure service-oriented architecture (SOA) interaction patterns: choreography and orchestration, implemented in IEC 61499 using FBs with discrete interfaces. We then restructure these baseline implementations using design patterns and adapter interfaces, resulting in two new implementations with adapter interfaces and a hybrid interaction structure. We evaluate modification effort across three variability cases to quantify the changes required in the original and redesigned implementations. The results demonstrate a measurable reduction in modification effort and improvements in control software maintainability. Based on these findings, we derive guidelines to support practitioners in selecting suitable designs and interface strategies for a specific variability context in control software design.