Global Almost-Sure Reachability in Stochastic Constant-Rate Multi-Mode Systems

A constant-rate multi-mode system is a hybrid system that can switch freely among a finite set of modes, and whose dynamics is specified by a finite number of real-valued variables with mode-dependent constant rates. We introduce and study a stochastic extension of a constant-rate multi-mode system where the dynamics is specified by mode-dependent compactly supported probability distributions over a set of constant rate vectors. The almost-sure reachability problem for stochastic multi-mode systems is to decide whether for all ε > 0 and for all pairs of start and target states in a path-connected and bounded safety set there exists a control strategy that almost-surely steers the system from the start state to the ε-neighborhood of the target state without leaving the safety set. We prove a necessary and sufficient condition to decide almost-sure reachability and, using this condition, we show that almost-sure reachability can be decided in polynomial time. Our algorithm can be used as a path-following algorithm in combination with off-the-shelf path-planning algorithms to make a robot with noisy low-level controllers follow a path with arbitrary precision.