Scheduling and discrete event control of flexible manufacturing systems based on Petri nets

A flexible manufacturing system (FMS) is a computerized production system that can simultaneously manufacture multiple types of products using various resources such as robots and multi-purpose machines. The central problems associated with design of flexible manufacturing systems are related to process planning, scheduling, coordination control, and monitoring. Many methods exist for scheduling and control of flexible manufacturing systems, although very few methods have addressed the complexity of whole FMS operations. This thesis presents a Petri net based method for deadlock-free scheduling and discrete event control of flexible manufacturing systems. A significant advantage of Petri net based methods is their powerful modeling capability. Petri nets can explicitly and concisely model the concurrent and asynchronous activities, multi-layer resource sharing, routing flexibility, limited buffers and precedence constraints in FMSs. Petri nets can also provide an explicit way for considering deadlock situations in FMSs, and thus facilitate significantly the design of a deadlock-free scheduling and control system. The contributions of this work are multifold. First, it develops a methodology for discrete event controller synthesis for flexible manufacturing systems in a timed Petri net framework. The resulting Petri nets have the desired qualitative properties of liveness, boundedness (safeness), and reversibility, which imply freedom from deadlock, no capacity overflow, and cyclic behavior, respectively. This precludes the costly mathematical analysis for these properties and reduces on-line computation overhead to avoid deadlocks. The performance and sensitivity of resulting Petri nets, thus corresponding control systems, are evaluated. Second, it introduces a hybrid heuristic search algorithm based on Petri nets for deadlock-free scheduling of flexible manufacturing systems. The issues such as deadlock, routing flexibility, multiple lot size, limited buffer size and material handling (loading/unloading) are explored. Third, it proposes a way to employ fuzzy dispatching rules in a Petri net framework for multi-criterion scheduling. Finally, it shows the effectiveness of the developed methods through several manufacturing system examples compared with benchmark dispatching rules, integer programming and Lagrangian relaxation approaches

An agile and adaptive holonic architecture for manufacturing control

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. 2004. Faculdade de Engenharia. Universidade do Port

On The Computational Complexity Of The Manufacturing Job Shop And Reentrant Flow Line

This paper presents a comparison study of the computational complexity of the general job shop protocol and the flow line protocol in a flexible manufacturing system. It is shown that a certain representative problem of finding resource invariants is NP-complete in the case of the job shop, while in the flow line case it admits a closed-form solution. The importance of correctly selecting part flow and job routing protocols in flexible manufacturing systems to reduce complexity is thereby conclusively demonstrated. 1 Introduction In a general flexible manufacturing system (FMS) where resources are shared, a key role in part routing, job selection, and resource assignment is played by the FMS controller. Given the same resources of machines, robots, fixtures, tooling, and so on, different structures result under different routing/assignment strategies by the controller. Unstructured strategies are generally classified as the so-called job shop organization, while structured protocols ..

Scheduling of flexible manufacturing systems with automated guided vehicles using petri net models

In this thesis, Petri net models for Flexible Manufacturing Systems (EMS) are constructed. A firing sequence of the Petri net from the initial marking to the final marking can be seen as a schedule of the modeled FMS. By using the branch-and-bound algorithm, an optimal schedule of the FMS can be obtained. Automated Guided Vehicle Systems (AGVS) are increasingly used for material handling in factories and warehouses. An AGVS can reduce labor costs and is ready to be integrated into an automated factory. This thesis presents two AGVS models (shared and duty) which integrate the control of AGVS with the control of part processing facilities. Both types of AGVS are modeled by Petri nets. We want to compare the two AGVS in terms of systems performance and discuss which application is more suitable for each AGVS type. We also want to consider and solve AGV jam problems. The objective of the AGV jam-free control module is to guarantee a jam-free condition among AGVs in an EMS. Results have been obtained and analyzed

Module-based architecture for a periodic job-shop scheduling problem

AbstractThis paper addresses the Petri net (PN) based design and modeling approach for a periodic job-shop scheduling problem. Asynchronous synthesis for net-modules of the jobs is suggested in this paper for optimal allocation of shared resources to different operations. To make sure the completion of all the jobs in a single iteration of a production cycle and the correct calculation of a makespan, the synchronization problem among jobs is tackled by introducing the special synchronizing transition in the model. A timed-place PN is adopted for the purpose of finding the feasible schedule in terms of the firing sequence of the transitions of the PN model by using the heuristic search method. Further, the characterization of the PN model is performed and it is shown that the PN model for a periodic job-shop scheduling problem is equivalent to a class of PN known as parallel process net with resources (PPNRs). The modeling approach is demonstrated with a practical example and a makespan is calculated for the example

An Iterative Approach for Collision Feee Routing and Scheduling in Multirobot Stations

This work is inspired by the problem of planning sequences of operations, as welding, in car manufacturing stations where multiple industrial robots cooperate. The goal is to minimize the station cycle time, \emph{i.e.} the time it takes for the last robot to finish its cycle. This is done by dispatching the tasks among the robots, and by routing and scheduling the robots in a collision-free way, such that they perform all predefined tasks. We propose an iterative and decoupled approach in order to cope with the high complexity of the problem. First, collisions among robots are neglected, leading to a min-max Multiple Generalized Traveling Salesman Problem (MGTSP). Then, when the sets of robot loads have been obtained and fixed, we sequence and schedule their tasks, with the aim to avoid conflicts. The first problem (min-max MGTSP) is solved by an exact branch and bound method, where different lower bounds are presented by combining the solutions of a min-max set partitioning problem and of a Generalized Traveling Salesman Problem (GTSP). The second problem is approached by assuming that robots move synchronously: a novel transformation of this synchronous problem into a GTSP is presented. Eventually, in order to provide complete robot solutions, we include path planning functionalities, allowing the robots to avoid collisions with the static environment and among themselves. These steps are iterated until a satisfying solution is obtained. Experimental results are shown for both problems and for their combination. We even show the results of the iterative method, applied to an industrial test case adapted from a stud welding station in a car manufacturing line