Constraint-based model for the cyclic multi-hoists scheduling problem

This article presents a method for the resolution of a material handling sc heduling problem. The case studied is a real industrial problem. It consist s of finding a cyclic schedule for hoist movements in a treatment surface s hop. In this kind of facility, several hoists are used for all the handling operations and they have to share common zones. Then it is necessary to co ntrol that there is no collision. The mathematical formulation of the probl em is based on a combination of disjunctive constraints. The constraints de scribe either movement schedule or collision avoidance. The resolution proc edure presented identifies all the collision configurations and then uses a branch and bound-like algorithm to find the optimal solution of a given pr oblem. The language chosen for our implementation is the constraint logic p rogramming language: Prolog IV, which is able to solve constraints with rat ional variables. It actively uses the constraint propagation mechanism that can be found in several languages.