Synthesis Of Live Resource-Shared Assembly Systems Using Petri Nets

In the entire automatic manufacturing process,assembly operations are the ending stages,which have a direct impact on production costs and production efficiency.Therefore,it is very necessary to study the automated manufacturing systems with assembly operations.Nevertheless,there is little research on the automated manufacturing systems with assembly operations so far.The existence of assembly operations brings even greater difficulties and challenges to the establishment of an automated manufacturing system model fulfilling liveness requirements.Throughout a production process,some resources are shared to reduce costs and improve system flexibility.This further increases the necessity to investigate liveness implementation in automated manufacturing systems with assembly operations sharing resources.The main contributions of this thesis can be concluded as follows:(1)It proposes an assembly resource control net(ARCN).The focus is to solve the problem that the synthesized systems may not fulfill the liveness requirement.By observing the subsumed subprocesses of the subnet,our research shows that there is a type of improper process advancement sequence after synthesis,which may lead to emptied siphons in the system,resulting in deadlocks.In response to this cause of deadlock,this thesis uses the general mutual exclusion constraint(GMEC)method to design the controller in a subnet to avoid deadlocks so that the synthesized systems in terms of ARCN can fulfill the liveness requirement.(2)It proposes a simple marked graph with resources(SMGR).An entire net is decomposed into several subnets using shared synthesis inverse operation.With regard to each subnet,a liveness-enforcing monitor is developed for the complementary set of each siphon.As far as not all the resources in the system flow into the complementary set of each siphon,the system achieves its liveness property.This method does not require to supervise the entire system.Instead,one only needs to monitor the local behavior for the sake of the liveness fulfillment of the whole system.(3)It proposes a set of linear programming formulations so as to verify the liveness property for systems allowing assembly operations,further validating the effectiveness of our developed method in the thesis.This linear programming oriented liveness verification method can elegantly avoid the enumeration for either all siphons or all reachable states.To sum up,the main objectives of this paper are to analyze and control the subnets during the synethesis process so that the final synthesized system can fulfill the liveness requirement.An obvious advantage of this method is to avoid the highly complicated liveness analysis and deadlock avoidance problems,which are degraded to much simpler sub-problems,reducing the computational complexity in both time and space.Another significant benefit of this method is to make in the modeling process more flexible.