Robust Liveness Enforcement Of Petri Nets With Uncontrollable And Unobservable Transitions Based On Structural Analysis

Automated manufacturing systems(AMSs)are the collections of manufacturing activities implemented by resources.For the deadlocks caused by unreliable resources,scholars pro-pose many related robust control strategies.However,these robust control strategies are proposed under the assumption that all the events of an AMS are controllable and observ-able.Generally,uncontrollable and unobservable events are common in the actual systems,which may make the existing robust control strategies ineffective.This thesis concentrates on robust deadlock control problems of AMSs with uncontrollable,unobservable events and unreliable resources,which are modeled by Petri nets.This thesis studies the two subclasses of Petri nets,namely,a system of simple sequential processes with resources(S~3PR)and a system of sequential systems with shared resources(S~4R),respectively.The work of this thesis is summarized as follows:1.For an S~3PR with uncontrollable,unobservable transitions and unreliable resources,a robust deadlock control strategy based on the structure of Petri nets is proposed.A recovery subnet is added to the holder of each unreliable resource to model resource failure and re-covery.In order to prevent a strict minimal siphon in a Petri net model from being emptied,an extended constraint set is constructed based on the complementary set of a strict minimal siphon to obtain a robust liveness constraint.Due to the uncontrollability and unobservability of transitions,an AMS cannot enforce inadmissible robust liveness constraints.It is neces-sary to decide the admissibility of robust liveness constraints and transform the inadmissible constraints into admissible ones.To this end,we design a monitor for each admissible robust liveness constraint such that it can be enforced.Finally,an iterative algorithm is developed to perform the above steps and a robust liveness controlled system is obtained.2.For an S~4R with uncontrollable,unobservable transitions and unreliable resources,a ro-bust deadlock control strategy based on the structure of Petri nets is developed.Since there are multiple parallel processing processes in an S~4R,this thesis redefines the model of a recovery subnet and adds it to the holder of each unreliable resource.A robust liveness con-straint is constructed based on the max~′-controlled condition of siphons.The treatment of uncontrollable and unobservable transitions in an S~4R is consistent with that in an S~3PR.We need to decide the admissibility of robust liveness constraints,and transform inadmissible constraints into admissible ones.A monitor is calculated by enforcing an admissible robust liveness constraint.The above steps are performed through an iterative algorithm.A robust liveness supervisor is obtained,which makes an S~4R remain live regardless of whether the unreliable resources fail or not.Finally,the feasibility of the control strategies proposed in this thesis is verified by several examples.The results show that the control strategies proposed in this thesis ensure the live-ness of AMSs with uncontrollable,unobservable events and unreliable resources regardless of whether the unreliable resources fail or not.