Modelling Hybrid Systems in Event-B and Hybrid Event-B: A Comparison of Water Tanks

Hybrid and cyberphysical systems pose significant challenges for a formal development formalism based on pure discrete events. This paper compares the capabilities of (conventional) Event-B for modelling such systems with the corresponding capabilities of the Hybrid Event-B formalism, whose design was intended expressly for such systems. We do the comparison in the context of a simple water tank example, in which filling and emptying take place at different rates, necessitating a control strategy to ensure that the safety invariants are maintained. The comparative case study is followed by a general discussion of issues in which the two approaches reveal different strengths and weaknesses. It is seen that restricting to Event-B means handling many more things at the meta level, i.e. by the user, than is the case with its Hybrid counterpart.