Supervisory Controller Design For The Petri Nets Based On Transition Priorities

As a type of automatic manufacturing systems,a flexible manufacturing system(FMS)is made up of a computer numerical control(CNC)machine tool and a material transport system.An FMS is also a computer control system which can produce a small and medium batch of products efficiently.However,due to the resource allocation problem in the system,it may lead to deadlock.Deadlock will pause the whole or part of the system,in some cases,it can produce great economic losses and disastrous.As a result,deadlock problem must be considered and solved when one designs a controller for an FMS.Most of the FMSs have the characteristics of discrete event dynamic systems,and Petri nets are very suitable to modeling and analysizing the systems.Researchers have proposed many methods to solve the deadlock problem in FMSs.Deadlock prevention,which establishes a relatively fixed sequence of resource usage through offline methods,adds corresponding constraints during the system design phase,makes the system design and planning phase to overcome the deadlock problem,and avoid deadlock due to processing during operation.And reduce production efficiency.(1)In this thesis,based on transition priority,a deadlock prevention policy for a class of Petri nets,named S~3PR,is developed.First,for a Petri net,its reachability graph is computed where reachable markings can be categorized into good,dangerous,bad,deadlock,and local isolated loop(LIL)ones.Good and dangerous markings constitute the maximum permissive behavior of the net.On the contrary,others are illegal markings(including bad,deadlock,and LIL markings).Then,to prevent the system from reaching illegal markings,the subclass structures,which are related to illegal markings of the reachability graph,are proposed and classified according to different situations.Transition priorities are designed in each subclass structures.As a result,legal branches are left.(2)This paper presents a new extended model based on Petri nets,which assigns different levels of priority to Petri net transitions:a transition with a high priority will trigger preferentially than a transition with a low priority.The Petri net model with the transition priority vector is different from the general net model in that priority is placed on the transition.For some transitions with the same input bank and enable,the input arc on the transition has inconsistent priority.The level of power,the order of triggering will change.In this paper,by changing the priority method,the system reaches different states and then achieves the control goal.(3)Finally,the transition priority vector of the whole net could be obtained by the control algorithm,which is divided into two methods.One is the iterative algorithm.A transition priority vector could be obtained once eliminating one illegal marking.If the controlled net is not live after the vector is applied into the net,the above method is repeated in the new net iteratively.The liveness of the net is not implemented until all the illegal markings are eliminated.The one is the direct control algorithm.The final transition priority vector could be obtained by eliminating all the illegal markings of the whole net once.The liveness of the controlled net under the constraints of the obtained transition priority vector is performed.The work utilizes the direct control algorithm.Traditional deadlock control policies need to add monitors to the original net,which makes the net structure more complex.In this work,one only needs to set up the proper priorities of transitions,which can achieve a live net.The presented policy is straightforward,generally applicable,and easy to use.The resulting net has a simple structure.