Strict Minimal Siphon-Based Colored Petri Net Supervisor Synthesis for Automated Manufacturing Systems With Unreliable Resources

Various deadlock control policies for automated manufacturing systems with reliable and shared resources have been developed, based on Petri nets. In practical applications, a resource may be unreliable. Thus, the deadlock control policies proposed in previous studies are not applicable to such applications. This paper proposes a two-step robust deadlock control strategy for systems with unreliable and shared resources. In the first step, a live (deadlock-free) controlled system that does not consider the failure of resources is derived by using strict minimal siphon control. The second step deals with deadlock control issues caused by the failures of the resources. Considering all resource failures, a common recovery subnet based on colored Petri nets is proposed for all resource failures in the Petri net model. The recovery subnet is added to the derived system at the first step to make the system reliable. The proposed method has been tested using an automated manufacturing system deployed at King Saud University.publishedVersio

Intelligent Colored Token Petri Nets for Modeling, Control, and Validation of Dynamic Changes in Reconfigurable Manufacturing Systems

The invention of reconfigurable manufacturing systems (RMSs) has created a challenging problem: how to quickly and effectively modify an RMS to address dynamic changes in a manufacturing system, such as processing failures and rework, machine breakdowns, addition of new machines, addition of new products, removal of old machines, and changes in processing routes induced by the competitive global market. This paper proposes a new model, the intelligent colored token Petri net (ICTPN), to simulate dynamic changes or reconfigurations of a system. The main idea is that intelligent colored tokens denote part types that represent real-time knowledge about changes and status of a system. Thus, dynamic configurations of a system can be effectively modeled. The developed ICTPN can model dynamic changes of a system in a modular manner, resulting in the development of a very compact model. In addition, when configurations appear, only the changed colored token of the part type from the current model has to be modified. Based on the resultant ICTPN model, deadlock-free, conservative, and reversible behavioral properties, among others, are guaranteed. The developed ICTPN model was tested and validated using the GPenSIM tool and compared with existing methods from the literature.publishedVersio

Deadlock prevention and deadlock avoidance in flexible manufacturing systems using petri net models

Deadlocks constitute an important issue to be addressed in the design and operation of FMSs. It is shown that prevention and avoidance of FMS deadlocks can be implemented using Petri net models. For deadlock prevention, the reachability graph of a Petri net model of the given FMS is used, whereas for deadlock avoidance, a Petri-net-based online controller is proposed. The modeling of the General Electric FMS at Erie, PA, is discussed. For such real-world systems, deadlock prevention using the reachability graph is not feasible. A generic, Petri-net-based online controller for implementing deadlock avoidance in such real-world FMSs is developed

Comparison and Evaluation of Deadlock Prevention Methods for Different Size Automated Manufacturing Systems

In automated manufacturing systems (AMSs), deadlocks problems can arise due to limited shared resources. Petri nets are an effective tool to prevent deadlocks in AMSs. In this paper, a simulation based on existing deadlock prevention policies and different Petri net models are considered to explore whether a permissive liveness-enforcing Petri net supervisor can provide better time performance. The work of simulation is implemented as follows. (1) Assign the time to the controlled Petri net models, which leads to timed Petri nets. (2) Build the Petri net model using MATLAB software. (3) Run and simulate the model, and simulation results are analyzed to determine which existing policies are suitable for different systems. Siphons and iterative methods are used for deadlocks prevention. Finally, the computational results show that the selected deadlock policies may not imply high resource utilization and plant productivity, which have been shown theoretically in previous publications. However, for all selected AMSs, the iterative methods always lead to structurally and computationally complex liveness-enforcing net supervisors compared to the siphons methods. Moreover, they can provide better behavioral permissiveness than siphons methods for small systems. For large systems, a strict minimal siphon method leads to better behavioral permissiveness than the other methods

Flexible high-density puzzle storage systems

In a traditional storage system, when one of the mechanical or electrical parts or the software fails to operate correctly, it may lead to the failure of the entire system imposing huge costs to the system (Gue et al., 2014). However, in a grid storage network, if a grid fails to operate, the system continues to operate. Therefore, failure of a system component will not jeopardize the whole system functionality. As such, reliability is one of the key advantages of grid storage networks. Another benefit of grid storage systems is flexibility. Various systems such as GridFlow, GridPick, GridStore, are developed that consist of multiple autonomous conveyors to store and move items in different directions. A unit module is capable to work independently. To retrieve an item, empty modules open a temporary aisle to move the item to the retrieval point. This study developed a tote retrieval algorithm for high-density storage systems (HDSS). The algorithm is capable of retrieving totes from all sides of a storage puzzle. This is novel since methodologies of prior studies were confined to retrieving items from a single grid or a single side of a puzzle. The algorithm was decentralized and agent-based where each grid acts as an agent. The study undertook developing algorithms that could minimize the number of movements in the network for tote retrievals and to prevent deadlocks. Deadlock prevention algorithms are capable to resolve a diverse range of situations that cause network deadlocks. An object-oriented software program was developed that implemented the algorithms. The software tool simulated puzzles of various sizes with different number of escorts available for retrieving requested totes. Thousands of iterations of the puzzle configurations were resolved for the analysis. It was found that incremental increase in the number of escorts in the puzzle reduces the number of movements for retrieving totes. However, depending on the puzzle size, there were increments that additional escorts had minimal impact. It was also found that average retrieval movements for 3 totes increase logarithmically with the number of cells in a puzzle. To validate the methodology and its software implementation, results from the simulation were compared with the results from other studies with mathematical solutions. The program replicated the optimum number of movements for a puzzle that contained one escort and retrieved one tote

Petri Nets at Modelling and Control of Discrete-Event Systems with Nondeterminism - Part 2

Discrete-Event Systems (DES) are discrete in nature. Petri Nets (PN) are one of the most widespread tools for DES modelling, analyzing and control. Different kinds of PN can be used for such purposes. Some of them were described in [3], being the first part of this paper. Here, the applicability of Labelled PN (LbPN) and Interpreted PN (IPN) for modelling and control of nondeterministic DES, especially with uncontrollable and/or unobservable transitions in the models, will be pointed out. Moreover, another kinds of nondeterminism in DES (errors, failures) will be modelled, and the possibilities of the error recovery of failed system will be presented

Study for the design of a management system for AGV networks

Automated Guided Vehicles are a vital part of the future intelligent manufacturing processes. In order to make the better profit, it is important to study if deadlocks can occur and how to tackle them. In this project we demonstrate how Petri Net models, which are perfect for representing deadlocks, can be mapped in the simulation software FlexSim. Eventually, we are using this software in order to evaluate different study cases with deadlocks