Control systems: phenomena and structuring functional requirement documents

Influenced by the Parnas and Madey’s fourvariable 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

From requirement document to formal modelling and decomposition of control systems

Formal modelling of control systems can help with identifying missing requirements and design flaws before implementing them. However, modelling using formal languages can be challenging and time consuming. Therefore intermediate steps may be required to simplify the transition from informal requirements to a formal model.In this work we firstly provide a four-stage approach for structuring and formalising requirements of a control system. This approach is based on monitored, controlled, mode and commanded (MCMC) phenomena. In this approach, requirements are partitioned into MCMC sub-problems, which then will be formalised as independent sub-models. The formal language used in this thesis is Event-B, although the MCMC approach can be applied to other formal languages. We also provide guidelines and patterns which can be used to facilitate the process of modelling in the Event-B language.The second contribution of this work is to extend the structure of machines in Event-B language and provide an approach for composing the formal MCMC sub-models in order to obtain the overall specification. The composition deals with phenomena that are shared amongst the formal sub-models. In our third contribution, patterns and guidelines are provided to refine the overall formal specification further in order to define design details. In addition, we discuss the decomposition of a formal model of a controlsystem.As practical examples, the MCMC approach is applied to the requirements of three automotive control systems, namely a cruise control system, a lane departure warning system, and a lane centering controller

Evaluation of a Guideline by Formal Modelling of Cruise Control System in Event-B

Recently a set of guidelines, or cookbook, has been developed for modelling and refinement of control problems in Event-B. The Event-B formal method is used for system-level modelling by defining states of a system and events which act on these states. It also supports refinement of models. This cookbook is intended to systematize the process of modelling and refining a control problem system by distinguishing environment, controller and command phenomena. Our main objective in this paper is to investigate and evaluate the usefulness and effectiveness of this cookbook by following it throughout the formal modelling of cruise control system found in cars. The outcomes are identifying the benefits of the cookbook and also giving guidance to its future users

Structuring functional requirements of control systems to facilitate refinement-based formalisation

Good requirements structure can greatly facilitate the construction of formal models of systems. This paper describes an approach to requirements structuring for control systems that aims to facilitate refinement-based formalisation. In addition to the well-known monitored and controlled phenomena used to analyse control systems, we also identify commanded phenomenon reflecting the special role that an operator plays in system control. These system phenomena guide the structure of the requirements analysis and documentation as well as the structure of the formal models.We model systems using the Event-B formalism, making use of refinement to support layering of requirements. The structuring provided by the system phenomena and by the refinement layers supports clear traceability and validation between requirements and formal models. As a worked example, we structured the requirements of an automotive lane departure warning system using this approach. We found missing requirements through this process and we evolved the requirement document through domain experts’ feedback and formal modelling