NaiQi Wu

Avoiding deadlock and reducing starvation and blocking in automated manufacturing systems

Deadlock-free operations of automated manufacturing systems (AMS) are essential for high machine utilization and productivity. Based on the resource-oriented Petri net models of AMS and our previous work (2000) on a necessary and sufficient condition for deadlock-free operation, this paper proposes a new control policy such that it can avoid deadlock completely, and reduce starvation and blocking situations significantly. It attempts to release an appropriate number of jobs into the system and control the order of resource usage based on the state information in the net model. The theoretical results for the correctness of this policy are presented. An AMS allowing routing flexibility and varying operation times is used to demonstrate the potential of the proposed policy.

Necessary and sufficient conditions for deadlock-free operation in flexible manufacturing systems using a colored Petri net model

Concurrent competition for finite resources by multiple parts in flexible manufacturing systems (FMS) results in deadlock. This is an important issue to be addressed in the operation of the system. A Petri net model, called colored resource-oriented Petri net (CROPN), is developed in this paper. The concurrent resource contention and the important characteristics of the production processes necessary for deadlock control are well modeled by this model. Based on the developed model, necessary and sufficient conditions and an efficient control law are presented for deadlock-free operation in FMS. This control law is a policy of dynamic resource allocation. It determines when a resource can be allocated to which job to avoid deadlock. This control law allows as many active parts as possible to be in the system, while deadlock is totally avoided. This control law is easy to implement and can be embedded into the real-time scheduler. A simple example is used to illustrate the application of the approach.

Resource-Oriented Petri Net for Deadlock Avoidance in Flexible Assembly Systems

In many flexible assembly systems, base components are transported with pallets; parts to be mounted onto the base ones are transported by trays with no pallets. When an assembly operation is performed by using some parts in a tray but not all, the tray with the remaining parts still occupies a buffer space. In this way, an assembly/disassembly material flow is formed. In such a material flow, deadlock can occur both in the base component and part flow. Furthermore, the assembly operations can also result in a deadlock. Thus, it is a great challenge to tackle deadlocks in such processes. This paper models them using resource-oriented Petri nets. Based on the models, a deadlock control policy is proposed and proved to be computationally efficient and less conservative than the existing policies in the literature. An industrial case study is used to show the results.

Deadlock Control of Automated Manufacturing Systems Based on Petri Nets—A Literature Review

Deadlocks are a rather undesirable situation in a highly automated flexible manufacturing system. Their occurrences often deteriorate the utilization of resources and may lead to catastrophic results in safety-critical systems. Graph theory, automata, and Petri nets are three important mathematical tools to handle deadlock problems in resource allocation systems. Particularly, Petri nets are considered as a popular formalism because of their inherent characteristics. They received much attention over the past decades to deal with deadlock problems, leading to a variety of deadlock-control policies. This study surveys the state-of-the-art deadlock-control strategies for automated manufacturing systems by reviewing the principles and techniques that are involved in preventing, avoiding, and detecting deadlocks. The focus is deadlock prevention due to its large and continuing stream of efforts. A control strategy is evaluated in terms of computational complexity, behavioral permissiveness, and structural complexity of its deadlock-free supervisor. This study provides readers with a conglomeration of the updated results in this area and facilitates engineers in finding a suitable approach for their industrial scenarios. Future research directions are finally discussed.

A Petri Net Method for Schedulability and Scheduling Problems in Single-Arm Cluster Tools With Wafer Residency Time Constraints

With wafer residency time constraints for some wafer fabrication processes, such as low pressure chemical-vapor deposition, the schedulability and scheduling problems are still open. This paper aims to solve both problems. A Petri net (PN) model is developed for the system. This model describes when the robot should wait and a robot wait is modeled as an event in an explicit way. Thus, to schedule a single-arm cluster tool with wafer residency time constraint is to decide how long a robot wait should be. Based on this model, for the first time, we present the necessary and sufficient conditions under which a single-arm cluster tool with residency time constraints is schedulable, which can be checked analytically. Meanwhile, a closed form scheduling algorithm is developed to find an optimal periodic schedule if it is schedulable. Also, a simple method is presented for the implementation of the periodic schedule for steady state, which is not seen in any previous work.

Modeling and deadlock avoidance of automated manufacturing systems with multiple automated guided vehicles

An automated manufacturing system (AMS) contains a number of versatile machines (or workstations), buffers, an automated material handling system (MHS), and is computer-controlled. An effective and flexible alternative for implementing MHS is to use automated guided vehicle (AGV) system. The deadlock issue in AMS is very important in its operation and has extensively been studied. The deadlock problems were separately treated for parts in production and transportation and many techniques were developed for each problem. However, such treatment does not take the advantage of the flexibility offered by multiple AGVs. In general, it is intractable to obtain maximally permissive control policy for either problem. Instead, this paper investigates these two problems in an integrated way. First we model an AGV system and part processing processes by resource-oriented Petri nets, respectively. Then the two models are integrated by using macro transitions. Based on the combined model, a novel control policy for deadlock avoidance is proposed. It is shown to be maximally permissive with computational complexity of O(n/sup 2/) where n is the number of machines in AMS if the complexity for controlling the part transportation by AGVs is not considered. Thus, the complexity of deadlock avoidance for the whole system is bounded by the complexity in controlling the AGV system. An illustrative example shows its application and power.

A Closed-Form Solution for Schedulability and Optimal Scheduling of Dual-Arm Cluster Tools With Wafer Residency Time Constraint Based on Steady Schedule Analysis

Because of wafer residency time constraints for cluster tools, it is very difficult to schedule them. This paper addresses their scheduling issues and conducts their schedulability and scheduling analysis. A Petri net (PN) model is developed to model them. With this model, to schedule a dual-arm cluster tool with wafer residency time constraints is to determine when and how long the robot should wait for. Based on the model, necessary and sufficient conditions under which the system is schedulable are presented. The conditions can be checked analytically. Meanwhile, an algorithm is developed for the optimal scheduling of dual-arm cluster tools. The algorithm finds an optimal periodic schedule with closed form expressions if it is schedulable. A method is also presented for the implementation of the obtained cyclic schedule by appropriately controlling the initial transient process. Examples are presented to show the application and power of the theory and algorithm.

Petri Net-Based Scheduling of Single-Arm Cluster Tools With Reentrant Atomic Layer Deposition Processes

For some wafer fabrication processes in cluster tools, e.g., atomic layer deposition (ALD), wafer revisiting is required. Typically, in such processes, wafers need to visit two consecutive processing steps several times. Such a revisiting process can be denoted as (mi, mi + 1)h, where i means the ith-step and mi and mi + 1 mean the corresponding quantity of the processing modules in i and (i+1)th steps, and h the number of visiting times. This paper conducts a study for scheduling single-arm cluster tools with such a wafer revisiting process. The system is modeled by Petri nets (PNs) to guarantee the feasibility of robot activities. Based on the model, a deadlock avoidance policy is presented. With the control policy, cycle time analysis for the revisiting process is made. With the fact that wafer processing times are much longer than robot movement times in cluster tools, it is shown that, when mi = mi + 1 = 1, i.e., each step has only one processing module, the optimal one-wafer cyclic schedule is deterministic and unique, and the minimal cycle time can be calculated by an analytical expression. It is also shown that, when mi = 1 and mi + 1 = 2 or mi = 2 and mi + 1 = 1, the optimal one-wafer cyclic schedule can be obtained by finding h deterministic schedules and the one with the least cycle time. A novel analytical method is finally presented to schedule the overall system containing such reentrant wafer flow. This represents a significant advance in single-arm cluster equipment automation.

Schedulability Analysis and Optimal Scheduling of Dual-Arm Cluster Tools With Residency Time Constraint and Activity Time Variation

With wafer residency time constraint of cluster tools in semiconductor manufacturing, activity time variation can make an originally feasible schedule infeasible. Thus, it is difficult to schedule them and schedulability is a vitally important issue. With bounded activity time variation considered, this paper addresses their real-time scheduling issues and conducts their schedulability analysis. A Petri net (PN) model and a control policy are presented. Based on them, this paper derives closed-form schedulability conditions. If schedulable, an algorithm is developed to obtain an offline periodic schedule. This schedule together with the control policy forms a real-time schedule. It is optimal in terms of cycle time and can be analytically computed, which represents significant advance in this area.

Petri Net Modeling and Cycle-Time Analysis of Dual-Arm Cluster Tools With Wafer Revisiting

There are wafer fabrication processes in cluster tools that require wafer revisiting. If a swap strategy is applied to dual-arm cluster tools handling wafer revisiting, a three-wafer periodical process is formed with three wafers completed in each period. Such a period contains three cycles in a revisiting process and three cycles in a nonrevisiting one. Hence, analysis and scheduling of such tools become very complicated. In this paper, a Petri net (PN) model is developed to describe their operations. Based on it, it is found that, if a swap strategy is applied, such tools are always in a transient state. A systematic method is then presented to analyze their performance. With the help of the proposed PN model, this work, for the first time, derives the optimality conditions of three-wafer period scheduling. Industrial application examples are given to show the results.