Undirected colored Petri net for modelling and supervisory control of AGV systems

This paper presents closed AGV systems with bidirectional guide path networks, zone control for avoiding collisions, and dynamic route planning. An AGV system is represented as a colored Petri net with undirected arcs and directed tokens, which substantially reduces the number of net components and simplifies the insight into the model. We study the problem of marking liveness and associate this property with the permanent ability of the vehicles to attain any edge in the network. The requirement is a weak form of marking liveness, as it does not require that each transition be live with respect to each of its colors. For the analysis of the net dynamics, we introduce the notion of a partially directed graph, that is, a graph which can have both directed and undirected edges. The results allow us to determine uniquely the character (live or not-live) of states in the system, which can thus be applied in the design of the supervisory control for AGV systems.