Constraint-Behavior Contracts: A Formalism for Specifying Physical Systems

Contract-based design (CBD) is a system development methodology that addresses the ever-increasing complexity and heterogeneity of cyber-physical system design problems. In CBD, systems and subsystems are represented by assume-guarantee contracts, formal specifications that make a distinction between supported environments and guarantees offered by a component. While the assume-guarantee formalism offers compact encoding of specifications, the port directions, i.e., the classifications of ports as input and output ports, implied by its semantics make it difficult to express properties of physical systems and subsystems where the relation between inputs and outputs is implicit. In this paper, we propose constraint-behavior contracts to specify physical components without the need to classify ports. The operations and relations between constraint-behavior contracts are defined to facilitate system reasoning without port directions. The capability of constraint-behavior contracts to integrate with assume-guarantee contracts gives the user the choice of a formalism to use at different abstraction layers. A case study based on an Unmanned Aerial Vehicle design problem shows that the proposed constraint-behavior contracts can facilitate system verification by expressing physical components, reducing the number of contracts, and providing an intuitive encoding of contracts.