Abstract: This paper develops a deadlock prevention policy for a class of Petri nets, which can model a wide class of
flexible manufacturing systems, where deadlocks are caused by unmarked siphons. We distinguish emptiable siphons in a plant
Petri net model by elementary and dependent ones. By adding a control place(monitor) for each elementary siphon, all
dependent siphons can be controlled by adjusting control depth variables of elementary siphons. A prior condition is
developed under which a dependent siphon can be implicitly controlled. Furthermore, by structural analysis and optimization
design of a supervisor, a method is proposed to decide whether an additional control place is redundant. By removing
control-redundant control places, a simplified liveness-enforcing supervisor with more permissive behavior can be obtained.

Key words: elementary siphon, Petri net, deadlock prevention, flexible manufacturing system