Hierarchical approach to GRAFCET using forcing order

In 1987, the function chart "GRAFCET" was endowed with two new concepts intended to supplement the IEC 848 Standard : the macro-step and the forcing order. In this paper, our field of interest is the forcing order and more particularly the hierarchy between partial grafcets which is engendered. Initially, we describe the basic principles of the forcing order and of the hierarchical structure of a global grafcet. A method, which uses graph theory, is then developed. The proposed algorithm is intended to prove the coherence of forcing orders. If a hierarchical incoherence is detected, forcing orders involved are identified.