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Compositional Design of Cyber-Physical Systems Using Contracts


The realization of large and complex cyber-physical systems (such as "smart" transportation, energy, security, and health-care systems) is creating design and verification challenges which will soon become insurmountable with the current engineering practices. These highly heterogeneous systems, tightly combining physical processes with computation, communication, and control elements, would substantially benefit from hierarchical and compositional methodologies to make their design possible let alone optimal. Several languages and tools have been proposed over the years to enable model-based development of complex systems. However, an all-encompassing design framework that helps interconnect different tools, possibly operating on different system representations, is still missing.

In this dissertation, we introduce a design methodology that addresses the complexity and heterogeneity of cyber-physical systems by using assume-guarantee contracts to formalize the design process and enable the realization of system architectures and control algorithms in a hierarchical and compositional way. In our methodology, components are specified by contracts, and systems by compositions of contracts. Contracts explicitly define the assumptions of a component on its environment and the guarantees of the component under these assumptions. Contract operations and relations, such as composition, conjunction and refinement allow proving that: (i) an aggregation of components are compatible, i.e. there exists a legal environment in which they can operate; (ii) a set of specifications are consistent, i.e. there exists an implementation satisfying all of them; (iii) an aggregation of components refines a specification, i.e. it implements the specification contract and is able to operate in any environment admitted by it. While horizontal contracts are used to specify components and aggregations of components at the same level of abstraction, we introduce the notion of vertical contracts to reason about richer refinement relations and mappings between different abstraction levels, possibly described by heterogeneous architectures and behavior formalisms. Moreover, we further investigate the problem of compatibility for systems with uncontrolled inputs and controlled outputs, by establishing a link between the theory of contracts and the one of interfaces, which rely on different mathematical formalisms, while sharing the same objectives. From this link, we derive a new projection operator on contracts that enables the preservation of the semantics of interface composition and compatibility.

Resting on the above contract framework, the design is carried out as a sequence of refinement steps from a high-level specification to an implementation built out of a library of components at the lower level. To allow for requirement analysis and early detection of inconsistencies, top-level system requirements are captured as contracts, by leveraging a front-end pattern-based specification language and a set of back-end formal languages, including mixed integer-linear constraints and temporal logic. Top-level contracts are then refined to achieve independent development of system architectures and control algorithms, by combining synthesis from requirements and optimization methods.

To enable efficient architecture selection under safety and reliability constraints, we explore two optimization-based methods that use an approximate reliability analysis technique to overcome the exponential complexity of exact computations. The Integer-Linear Programming with Approximate Reliability (ILP-AR) method generates larger, monolithic optimization problems using approximate but efficient reliability computations with an explicit theoretical bound on the error. Conversely, the Integer-Linear Programming Modulo Reliability (ILP-MR) method breaks the complex architecture selection task into a sequence of smaller optimization tasks without reliability constraints, interleaved with exact reliability checks. By relying on efficient mechanisms to prune out candidate architectures that are inconsistent with the reliability constraints, ILP-MR can run faster than ILP-AR on large problem instances.

We further explore two methods to systematically design control strategies for a given architecture. The reactive synthesis-based optimal control mapping (RS-OCM) method generates controllers by combining reactive synthesis from linear temporal logic contracts with optimization techniques based on simulation and monitoring of signal temporal logic contracts. Different design concerns are then addressed by leveraging the most appropriate abstraction levels, using contracts from the pre-characterized library to accelerate verification tasks. The programming-based optimal control mapping (P-OCM) method uses, instead, a discrete-time representation of the system and a formalization of the design requirements in terms of arithmetic constraints over real numbers to cast the control problem as an optimization problem over a finite time horizon. The optimization problem is then solved with a receding horizon approach and scales better than monolithic reactive synthesis from linear temporal logic.

We demonstrate, for the first time, the effectiveness of a contract-based design flow on real-life examples of industrial relevance, namely, the design of aircraft electric power distribution and environment control systems. In our framework, optimal selection of large, industrial-scale power system architectures can be performed in a few minutes. Design validation of power system controllers based on linear temporal logic contracts shows up to two orders of magnitude improvement in terms of execution time with respect to conventional techniques. Finally, our optimization-based load management scheme allows better resource utilization than a conventional one.

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