ABSTRACT
A variety of im portant Petri net (PN) based methods of preventing deadlocks arising in flexible m anufacturing systems (FMS) are to add a monitor and related arcs for each possibly emptiable siphon such th a t no siphon can be emptied. Since the number of siphons grows in general expo nentially with respect to a PN size, their disadvantages lie in th a t they have to add so many additional monitors to the net th a t the resulting net super visor is too complex compared with the original one. This chapter explores the ways to minimize the number of monitors to be added when liveness enforcing PN supervisors are designed. Siphons in a PN are distinguished by elementary and redundant ones. This chapter proposes conditions under which a redundant siphon can be always marked by making its elementary siphons invariant-controlled. Based on the theory of elementary siphons, two novel deadlock prevention policies are developed. FMS examples are used to illustrate the proposed concepts and policies and show the signifi cant advantages over the previous methods.
