Resource failures may happen in automated manufacturing systems (AMSs) because of different reasons in the real world, making most existing deadlock control policies unapplicable. This paper develops methods for the robust deadlock control of AMSs with unreliable resources based on Petri nets. The considered AMSs are modeled with generalized systems of simple sequential processes with resources (GS³PR). First, a method based on reachability graph partition technique is provided to analyze the robust legal markings and the forbidden ones in an unreliable GS³3PR (U-GS³PR), in which resource failures and recovery procedures are modeled with recovery subnets. Then, the control problem for such a system is converted into a problem for controlling the forbidden states in a U-GS³PR and control places can be designed by solving the maximal number of forbidden markings problems. Since the robust legal reachability spaces computed may be nonconvex and such a system cannot be optimally controlled by the conjunctions of linear constraints, we propose an interval-inhibitor-arc-based robust deadlock control policy for a system with nonconvex legal reachability spaces by solving the maximal number of tq-critical marking/transition separation instances problems (MNTMPs(tq)). Finally, examples are presented to demonstrate the proposed methods.

In an automated manufacturing system (AMS), resources are, in general, subject to unpredictable failures, which invalidate many existing deadlock control strategies. In this article, we propose an adaptive deadlock control policy for an AMS with multiple types of unreliable resources. The considered AMS is modeled with a system of simple sequential processes with resources. First, based on an elementary siphon control method, monitors are added for elementary siphons and some particular dependent siphons to ensure the liveness of a system if there are no resource failures. By considering the fact that an unreliable resource may fail in a system, recovery subnets are added to describe the resource failures and recoveries. Since a monitor added for a siphon may not be able to guarantee that the corresponding siphon is always marked if the failure of a resource in the siphon occurs, the concept of switch controllers is presented so as to make the siphon always remarked if it is emptied by resource failures. It is verified that the adaptive controller proposed in this article can guarantee the liveness of the controlled system no matter whether unreliable resources break down or not. More importantly, if there is no resource failure, the system can maintain predefined production without degrading planned system performance. Finally, examples are presented to illustrate the validity of the proposed method.

This paper proposes an adaptive supervisory control policy for an automated manufacturing system (AMS) with multiple types of unreliable resources. The considered AMS is a generalized system of simple sequential process with resources (GS $^{3}$ PR). In order to model resource failures and recoveries, modified recovery subnets are developed, which model more complex resource failure scenarios compared with traditional recovery subnets. For the purpose of guaranteeing the execution of a system’s production successfully, from the perspective of the structural analysis of the system, a siphon control-based technique is introduced and a type of controller is developed. It is verified that the proposed adaptive control policy assures the deadlock-freeness of the controlled system no matter if there exist resource failures, and retains all behavior of the original system. Finally, examples are presented to demonstrate the proposed method. Note to Practitioners —Resources failures are ubiquitous in an automated manufacturing system (AMS). Their occurrences can lead partial or global system stagnation, which causes unnecessary downtime and even catastrophic results. Most of the existing robust deadlock control strategies for AMSs with unreliable resources usually restrict system evolution in an unnecessarily conservative way such that part of permissive behavior is excluded if no resource failure occurs, which affects the utilization of resources and reduces the productivity of the system. In order to address the issue, this work considers a type of AMSs where each processing stage may require an arbitrary number of units, but all from a single resource type. For such an AMS, we propose an adaptive deadlock control policy that makes the controlled system deadlock-free, keeping all the system behavior as in the case that unreliable resources do not break down. The proposed method is realized by dynamically adjusting the working modes of a system to adapt to resource failures and recoveries, which is readily accessible to practitioners from a technical viewpoint.

This paper addresses the verification of strong cur-rent-state opacity with respect to real-time observations gener-ated from a discrete-event system that is modeled with time labeled Petri nets. The standard current-state opacity cannot completely characterize higher-level security. To ensure the higher-level security requirements of a time-dependent system, we propose a strong version of opacity known as strong current-state opacity. For any path (state-event sequence with time information) π derived from a real-time observation that ends at a secret state, the strong current-state opacity of the real-time observation signifies that there is a non-secret path with the same real-time observation as π. We propose general and non-secret state class graphs, which characterize the general and non-secret states of time-dependent systems, respectively. To capture the observable behavior of non-secret states, a non-secret observer is proposed. Finally, we develop a structure called a real-time concurrent verifier to verify the strong current-state opacity of time labeled Petri nets. This approach is efficient since the real-time concurrent ver-ifier can be constructed by solving a certain number of linear programming problems.

It has been a hot race to design optimal controllers to be maximally permissive with fewest monitors in the shortest amount of time for flexible manufacturing systems modelled by Petri nets. Recent maximally permissive deadlock prevention controls for systems of simple sequential processes with resources reduce the computation burden by considering only a small portion of all forbidding markings and employ much fewer monitors by a linear integer programming method. Maximal permissiveness is ensured by not forbidding any live state. However, it still requires costly reachability analysis. Our previous work avoids reachability analysis by classifying siphons and adding monitors to critical siphons only. However, some live states may get lost and the number of monitors required is as many as that of critical siphons. This paper proposes a method to merge several monitors into a single one while not losing the live states. It achieves the same best results in the existing literature while avoiding the time-consuming reachability analysis which does not scale well with the large size of the nets.