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Problem decomposition and sub-model reconciliation of control systems in Event-B

To break the complexity of the formalisation process, we propose to model a functional requirement document of a control system as composeable monitored, controlled, mode and commanded sub-models. Influenced by the problem frame approach and the decomposition of the four-variable model, we suggest decomposing requirements of a control system into monitored, controlled, mode and commanded sub-problems. Each sub-problem can be formalised in a step-wise manner as a separate sub-model. To introduce the phenomena shared amongst the subproblems, the sub-models are reconciled. We propose a reconciliation process in the Event-B formal language based on the shared-variable and the shared-event styles which were originally developed for a model decomposition. The advantages and disadvantages of shared-variable and the shared-event reconciliation steps are also discussed. The requirements of an automotive cruise control system are decomposed and formalised as sub-models. These sub-models are also reconciled to introduce shared phenomena.