Journal of Manufacturing Systems Volume 13/No. 4 Augmented Timed Petri Nets for Modeling, Simulation, and Analysis of Robotic Systems with Breakdowns Kurapati Venkatesh, New Jersey Institute of Technology, Newark Mehdi Kaighobadi, Florida Atlantic University, Boca Raton MengChu Zhou, New Jersey Institute of Technology, Newark Reggie J. Caudill, New Jersey Institute of Technology, Newark Abstract Flexible manufacturing and assembly systems con- sist of machines, robots, and automated guided vehi- cles aimed at meeting dynamically changing market needs. Numerous asynchronous concurrent actions involved in these systems make analysis of them difficult. Breakdowns of system components further complicate investigation of issues related to design, performance optimization, and control. This paper pro- poses a new class of modeling tools called augmented timed Petri nets (ATPNs) for modeling and analyzing robotic assembly systems with breakdowns. These models aid designers in better understanding the con- currency, synchronization, and sequential relations involved in breakdown handling and in system simula- tion for performance analysis. A flexible assembly system consisting of three robots with various break- down rates is used to illustrate modeling, simulation, and analysis with ATPNS. ATPN models for breakdown handling are presented and analyzed for estimating system performance and for designing the optimum number of assembly fixtures. ATPN models can also be used for real-time system control. Finally, possible extensions to this study are discussed. Keywords: Breakdown Handling, Flexible Assem- bly Systems, Petri Nets, System Modeling, System Design Introduction Over the last few years, industry has been con- stantly increasing its efforts to combine flexibility and automation to meet dynamically changing mar- ket needs. To this end, flexible factories capable of flexible and agile manufacturing and assembly have to be designed to achieve higher productivity with labor savings. This paper focuses on modeling and analysis issues related to flexible assembly systems (FASs) in such factories. An FAS often consists of a number of robots, automated guided vehicles (AGVs), part feeders, and magazines. An FAS is capable of agile produc- tion of small and medium-sized components and avoids many disadvantages encountered with fixed assembly systems. 1 An FAS increases resource utilization and maximizes production rate by avoid- ing unnecessary job transfers within the factory; however, to realize the full benefits of FASs, one has to consider their modeling and simulation to investigate problems relating to design, perfor- mance optimization, and control. An important issues in modeling an FAS is breakdown handling, which takes into account many design and control issue. Considering the importance of breakdowns in production control, Gershwin and Berman 2 pre- sented the analysis of transfer lines consisting of two unreliable machines with random failures. Groenevelt et al. 3 investigated issues related to estimation of economic lot size and safety stock levels for an unreliable manufacturing system with a constant failure rate. Glassey and Hong 4 presented a model for the analysis of behavior of an unreliable n-stage transfer line with finite size buffers. Most of the above researchers studied conven- tional manufacturing systems by estimating eco- nomic safety stocks to handle breakdown situations; however, to implement just-in-time manufacturing and to increase automation, there is a need to handle breakdowns by reducing safety stock levels and implementing efficient breakdown handling proce- dures. Breakdowns and other production interrup- tions stop a system's operation and pose difficulties for shop floor control in ensuring uninterrupted production. Furthermore, breakdowns increase sys- tem downtime and degrade performance. Due to breakdowns, the optimal system operational param- 289