Theory and applications of resource control Petri nets for automated manufacturing systems

Design methods based on Petri net synthesis theory for discrete-event concurrent systems such as automated manufacturing are explored. These systems exhibit very complex behaviors due to the interactions among the concurrent sub-systems. To handle this complexity, the methods developed in this thesis adopt the concept of modular composition, which uses basic modules for modeling sub-systems and then merges the modules to represent the synchronization. A synthesis approach using Strongly Connected State Machines (SCSMs) as basic modules is presented. An SCSM models a process that controls a resource type in the system. The interactions among sub-systems are modeled by the merging of common transitions and common transition sub-nets of the SCSMs. For verifying qualitative properties, we have proven that a net constructed by this approach is conservative and thus bounded. In addition, theorems are developed to calculate P-invariants for checking the liveness of the net. This P-invariant calculation is more straightforward than the traditional technique of solving the system equations of the net. Furthermore, a restricted class of merged nets is proven to be live and reversible with no posterior analysis required. It is also shown that this class of nets is more general than the class of nets considered in a paths-based method by Krogh and Beck. The difficulty of checking liveness using P-invariants is solved by placing a marking restriction on SCSM modules and two restrictions on module interactions. These types of restricted SCSMs are defined as Resources Control Nets (RCNs). A net synthesized by merging RCNs is proven to be conservative and thus bounded. For verifying the liveness of the net, an algorithm is developed, which only examines the structure of the net and appears to be more efficient than state enumeration techniques such as the reachability graph method. The algorithm can check two sufficient conditions for structural liveness of the net and obtain a set of initial markings which guarantee liveness if the net is proven to be structurally live. Some resources such as parts that can be disassembled or assembled are difficult to describe by using the method based on RCNs. This is resolved by extending RCNs and by modifying the restrictions on module interactions. These modified RCNs are defined as Extended Resource Control Nets (ERCNs). We have proven that a net synthesized by this method is conservative and thus bounded. Moreover, two sufficient conditions for structural liveness of the net can be determined by using an algorithm that is modified from the liveness-checking algorithm based on RCNs. Similar to the previous algorithm,this only examines the net structure. Applications of the synthesis methods developed in this thesis are demonstrated by using several automated manufacturing examples, ranging from simple automated systems to complex flexible manufacturing.