Deriving specifications of control programs for cyber physical systems

Cyber Physical Systems (CPS) exist in a physical environment andcomprise both physical components and a control program.Physical components are inherentlyliable to failure and yet an overall CPS is required to operate safely, reliably and costeffectively. This paper proposes a framework for deriving the specificationof the software control component of a CPS from an understanding of the behaviour requiredof the overall system in its physical environment.The two key elements of thisframework are (i) an extension to the use of rely/guarantee conditions toallow specifications to be obtained systematicallyfrom requirements (as expressed in terms of the required behaviour in the environment)and nested assumptions (about the physical components of the CPS); and (ii) the use oftime bands to record the temporal properties required of the CPS at a number of differentgranularities.The key contribution is in combining these ideas;using time bands overcomes a significant drawback in earlier work.The paper also addresses the means by which the reliability ofa CPS can be addressed by challenging each rely condition in the derived specificationand, where appropriate, improve robustness and/or define weaker guarantees thatcan be delivered with respect to the corresponding weaker rely conditions.