237 research outputs found
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 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
Synthesis of Liveness-Enforcing Petri Net Supervisors Based on a Think-Globally-Act-Locally Approach and a Structurally Minimal Method for Flexible Manufacturing Systems
This paper proposes a deadlock prevention policy for flexible manufacturing systems (FMSs) based on a think-globally-act-locally approach and a structurally minimal method. First, by using the think-globally-act-locally approach, a global idle place is temporarily added to a Petri net model with deadlocks. Then, at each iteration, an integer linear programming problem is formulated to design a minimal number of maximally permissive control places. Therefore, a supervisor with a low structural complexity is obtained since the number of control places is greatly compressed. Finally, by adding the designed supervisor, the resulting net model is optimally or near-optimally controlled. Three examples from the literature are used to illustrate the proposed method
A Petri-Net-Based Scheduling Strategy for Dual-Arm Cluster Tools With Wafer Revisiting
International audienceThere are wafer fabrication processes in cluster tools that require wafer revisiting. The adoption of a swap strategy for such tools forms a 3-wafer cyclic (3-WC) period with three wafers completed in each period. It has been shown that, by such a scheduling strategy, the minimal cycle time cannot be reached for some cases. This raises a question of whether there is a scheduling method such that the performance can be improved. To answer this question, a dual-arm cluster tool with wafer revisiting is modeled by a Petri net. Based on the model, the dynamical behavior of the process is analyzed. Then, a 2-wafer cyclic (2-WC) scheduling strategy is revealed for the first time. Cycle time analysis is conducted for the proposed strategy to evaluate its performance. It shows that, for some cases, the performance obtained by a 2-WC schedule is better than that obtained by any existing 3-WC ones. Thus, they can be used to complement each other in scheduling dual-arm cluster tools with wafer revisiting. Illustrative examples are given
An agile and adaptive holonic architecture for manufacturing control
Tese de doutoramento. Engenharia Electrotécnica e de Computadores. 2004. Faculdade de Engenharia. Universidade do Port
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