Deadlock Avoidance In Automated Manufacturing Systems With Uncontrollable/Unobservable Events Based On Petri Nets

In recent years,the deadlock problem in automated manufacturing systems has been widely concerned.A lot of research for it based on Petri net has been done by many scholars.It is well known that the existence of uncontrollable or unobservable transitions will greatly increase the possibility of the system's deadlock.Therefore,it is especially important to find a method to solve the problem in systems with uncontrollable or unobservable events.There are some shortcomings in the existing research methods for the control of deadlock in automated manufacturing systems with unobservable and uncontrollable events,such as offline computing controller controls the system,global information that needs to know,and the problem of the maximal permissiveness.Aiming at these shortcomings,we do the following research in this thesis.First,we analyze and implement the deadlock-free control method by using the global reachability graph.Second,the deadlock-free control method of Petri nets with uncontrollable and unobservable transitions is studied by using local reachability graph information obtained through online real-time step size prediction.Third,based on above analysis,analysis method of using real-time prediction of optimal step size is obtained to realize deadlock-free control of Petri nets with uncontrollable and unobservable transitions.Compared with the existing methods,the advantages of this method is reflected in:(1)it is predicting and controlling in real time online;(2)the local information of the system rather than the global information is obtained by step size,which greatly reduces the complexity of calculations and analysis;(3)the method achieves maximal permissiveness for the system with the uncontrollable and unobservable events.The main contributions of our work are as follows: 1.Analysis and implementation of deadlock-free control of fully controllable and observable automated manufacturing system based on global reachability graph 2.The influence of uncontrollable or unobservable transitions on the deadlock-free control of the system is analyzed,and the idea of optimal step size is briefly introduced.3.For the automated manufacturing system with uncontrollable events,the algorithm for its deadlock-free control is introduced in detail.The main idea is to use the heuristic method to find a feasible step that can be predicted online and can achieve the deadlock-free control.Due to the redundancy of the local information predicted by the feasible step size,the complexity of calculation and analysis is increased.For this issue,use the idea of dichotomy to obtain an optimal step size,and then predicted online in real time according to the optimal step size.Based on the predicted local reachability graph information avoid system deadlock is realized.4.For the automated manufacturing system with unobservable events,the strategy of deadlock avoidance based on optimal step size is introduced and analyzed in detail.5.The second development is carried out on the SIMIPN software platform developed by the laboratory.The idea of optimal step size is used to implement the deadlock-free control method for the automated manufacturing system with uncontrollable events.The correctness of the method proposed in this thesis is proved.