Semi-formal Requirements Specification. The purpose of the semi-formal requirements specification is to describe the informal requirements of the cruise control system in a semi-formal way such that they can be easily modelled with Event-B. During pilot deployment we decided to use the Problem Frames approach [Jac95, Jac01] and several extensions we developed for semi-formal require- ments engineering because it allows to structure the problem into require- ments, machine, and the physical world. Furthermore, the problem frames approach and our extensions allows us to decompose a given problem, e.g., the development of a cruise control system, into different subproblems which can be handled separately and later recombined in a subsequent phase. The development task in problem frames is to design a machine by building soft- ware that is then executed on a general-purpose computer, specialising the computer to serve a particular purpose [Jac95]. That purpose is to meet a recognised need, which is called requirement. Satisfying the requirement involves transforming the physical world around us. In Problem Frames, the part of the world to be transformed is called the environment. Xxxxxxx states that the parts of any systems engineering problem are the machine, the problem world (environment), and the requirement. Figure 4.1 shows a generalized Problem Frame diagram and its application to the cruise control system. In our application of Problem Frames to the cruise control system, the machine we want to build is the cruise control software running on the en- gine controller. The physical world around us, i.e., the problem world the cruise control software is interacting with, consists of the human machine interface, i.e., the pedals, the lever, and the ignition, as well as the vehicle including engine, engine control, and wheels which need to be controlled by the cruise control. The requirements relate phenomena controlled by the hu- man machine interface, e.g., control signals with phenomena controlled by
Semi-formal Requirements Specification. 19 Machine Problem World software/ hardware Requirement what the problem is about “stakeholder” needs “If the driver presses the resume button, the vehicle shall resume and maintain the previously stored target speed”
