Control Systems: Phenomena and Structuring Functional Requirement Documents

Influenced by the Parnas and Madey´s four-variable model and the concept of phenomena in problem frames, we desire to provide guidelines to facilitate refinement-based formal modelling. These guidelines are based on monitored, controlled, mode and commanded (MCMC) phenomena of a control system. Commanded phenomena reflect the role that an operator plays in system control. The mode phenomenon captures the states of the controller. Requirements of several case studies have been formally modelled using the MCMC phenomena. This helped to identify some of the ambiguities and advantages of the guidelines. In particular, we realised that the concept of commanded phenomena and its difference with monitored phenomena can cause confusion. Also, it was noticed that the mode is a special phenomenon, as it can be modified by operator requests or internally by the control system. In this paper we clarify the concept of commanded phenomena and differentiate between monitored and commanded phenomena clearly. The concept of mode phenomenon is also introduced in details. As practical examples, the phenomena of two case studies, namely a cruise control system and a lane centering controller (LCC), are identified. The MCMC phenomena are also used to structure the requirement document (RD) of a control system. This can help with the transition from an informal RD to a formal model. This approach is used to structure the RD of the LCC case study which is supported by our industrial partner.