Reachability analysis of continuous and discrete time systems is a hard problem that has seen much progress in the last decades. In many cases the problem has been reduced to bisimulations with a number of limitations in the nature of the dynamics, soundness, or time horizon. In this article we focus on sound safety verification of Unbounded-Time Linear Time-Invariant (LTI) systems with inputs using reachability analysis. We achieve this by using Abstract Acceleration, which over-approximates the reach tube of a system over unbounded time by using abstraction . The technique is applied to a number of models and the results show good performance when compared to state-of-the-art tools.
I. IntroductionLinear loops are an ubiquitous programming pattern. Linear loops iterate over continuous variables, which are updated with a linear transformation. Linear loops may be guarded, i.e., terminate if a given linear condition holds. Inputs from the environment can be modelled by means of non-deterministic choices within the loop. These features make linear loops expressive enough to capture the dynamics of many hybrid dynamical models. The usage of such models in safety-critical embedded systems makes linear loops a fundamental target for formal methods.Many high-level requirements for embedded control systems can be modelled as safety properties, i.e. deciding reachability of certain bad states, in which the system exhibits unsafe behaviour. Bad states may, in linear loops, be encompassed by guard assertions.Reachability in linear programs, however, is a formidable challenge for automatic analysers: the problem is undecidable despite the restriction to linear transformations (i.e., linear dynamics) and linear guards.The goal of this article is to push the frontiers of unbounded-time reachability analysis: we aim at devising a method that is able to reason soundly about unbounded trajectories. We present a new approach for performing abstract acceleration. Abstract acceleration [26], [27], [34] approximates the effect of an arbitrary number of loop iterations (up to infinity) with a single, non-iterative transfer function that is applied to the entry state of the loop (i.e., to the set of initial conditions of the linear dynamics). This article extends the work in [34] to systems with non-deterministic inputs elaborating the details omitted in [39].The key contributions of this article are: 1) We present a new technique to include inputs (non-determinism) in the abstract acceleration of general linear loops. 2) We introduce the use of support functions in complex spaces, in order to increase the precision of previous abstract acceleration methods.