Preliminary Evaluation of a Contract Net-Based Heterarchical Scheduling Approach for Flexible Manufacturing Systems A. Saad and K. Kawamura Department of Electrical and Computer Engineering email: fashraf,
[email protected] G. Biswas Department of Computer Science email:
[email protected] M.E. Johnson Owen Graduate School of Management email:
[email protected] Vanderbilt University, Nashville, TN 37235 A. Salama Austin Peay State University Clarksville, TN 37044 email:
[email protected] Abstract
The next generation of intelligent and exible manufacturing systems must be e ective and ecient in manufacturing products in small batches, even down to lot sizes of one unit. In order to achieve this objective, it is necessary to move factory control from the more rigid centralized and hierarchical manufacturing computer control architectures (MCCA) to fully decentralized, heterarchical architectures which can recon gure dynamically depending on production requirements. In addition to taking advantage of the decentralized nature of the manufacturing process, a truly decentralized control system is more extensible and fault tolerant than a centralized or a hierarchical one. However, this comes Abstract Published in the Proceedings of the UC/IAMS Workshop on Manufacturing Research, Cincinnati, OH, April 8, 1995.
with the price of designing the rules of interaction and cooperation among the agents comprising such a control system that ensure a satisfactory performance of the underlying manufacturing system. We present the preliminary results of our investigation into heterarchical scheduling for exible manufacturing systems. In our work, the workcells comprising the manufacturing process and the products to be generated are modeled as autonomous agents which interact dynamically to generate the production schedule for each product unit. The interaction scheme combines production reservation with a bidding mechanism using the contract net protocol. The e ectiveness of this approach is demonstrated using simulation experiments that compare its performance to heuristic dispatching rules commonly used in industry.
1 Introduction The next generation of intelligent and exible manufacturing systems must be e ective and ecient in manufacturing products in small batches, even down to lot sizes of one unit. This is necessary to keep up with ever increasing customer demands in a diverse global marketplace. While traditional mass production processes were geared to producing large quantities of the same product in continuous manufacturing environments, exible manufacturing moved the focus to more of a batch production. In order to achieve the objective of manufacturing eciently in small batch sizes, it is necessary to move factory control from the more rigid centralized and hierarchical manufacturing computer control architectures (MCCA) to fully decentralized, heterarchical architectures which can recon gure dynamically depending on production requirements. Heterarchical architectures present a number of inherent advantages such as improved fault tolerance, recon gurability, adaptability, and Extensibility. With the advent of intelligent manufacturing cells, robots and material handling systems, interconnected via computer networks, the implementation of heterarchical control architectures is now becoming more feasible. Our work focuses on constructing a testbed for modeling and analyzing the performance of heterarchical control schemes in a small to medium job shop environment.
2 Heterarchical Job Shop Control We studied the e ectiveness of heterarchical control scheme on a generic job shop process. To realize heterarchical control in this environment, the machines comprising the manufacturing system and the parts to be manufactured are modeled as autonomous agents. These agents use a contract net protocol to implement a bidding scheme which then allows the parts to dynamically and cooperatively determine, with the given set of machines, each part's production schedule. In the past, the contract net protocol has been used for task distribution 2
among a hierarchically organized set of manufacturing entities in the YAMS architecture, and heterarchical control architectures. Industrial case studies have proved the viability of heterarchical control architectures in real job shop environments. Other approaches have only tested the e ectiveness of rescheduling in a decentralized job shop environment after the original schedule was generated by a centralized scheduler.
2.1 A Generic Job Shop Model
A generic job shop model has been used for this study. Each workstation comprises a machine, an input bu er and an output bu er, interconnected via a material handling system. A conveyor or an AGV system picks up partially or completely processed parts from the output bu er of a workstation and transfers them to the input bu er of the next workstation. A speci c testbed has also been constructed for the performance studies of interest to our investigations.
2.2 Bidding as means for Scheduling
Within a heterarchical control framework, a cooperative negotiation mechanism has been established for generating and maintaining production schedules. Cooperative negotiation is implemented using two bidding perspectives, namely: Part-Centered Negotiation and Machine-Centered Negotiation. In order to take into account part delivery date as well as machines performance, both bidding perspectives are merged into a part-centered bidding scheme that relies on Production Reservation (PR) for scheduling and the Contract Net protocol for the interaction among the machines and parts agents.
3 Simulation Results SIMAN was used to construct a cellular model of the generic job shop. An object-oriented methodology was adopted for the design of the heterarchical framework, which was then implemented using C++. The link between the heterarchical scheduling mechanism and the SIMAN was established using a C-language structure. A set of simulation runs were performed to compare the performance of PR scheduling versus a number of heuristic dispatching rules that are typically used in job shop environments. The set of product related parameters that are of interest in evaluating the e ectiveness and eciency of most discrete manufacturing systems are the average lead time, ow time, production time (sum of lead and ow times), tardiness, and earliness. Three more measures of performance pertinent to the manufacturing system are also monitored. They are namely the percentage of tardy parts, machines utilizations, and bu er levels which are in turn measured under three categories: raw material Bu er, work in progress or WIP, and nished product bu er. The performance results are listed in Tables 1, 2, 3, and 4 below. They have been collected under the following conditions: a part necessitates a total of ve operations for its production, the average processing time for an operation is assumed to be 3
normally distributed with mean equal to 10 time units and standard deviation equal to 3, parts arrival times are exponentially distributed with mean of 6.5 time units, and the due dates for parts are tight and equal to just four times the expected total processing time of a part.
3.1 Heuristic Dispatching Rules
The heterarchical PR scheduling is compared to a number of traditional local dispatching rule schemes. Dispatching rules specify the criteria according to which a machine selects the next part for processing from its input bu er. Experiments were conducted with three disptaching rules, namely: First-InFirst-Out (FIFO), Earliest Due Date (EDD), and Shortest Processing Time (SPT).
3.2 Augmented Production Reservation
A second set of experiments were conducted to study the scheduler performance when Production Reservation is combined with the three FIFO, EDD, and SPT local dispatching rules. PR has also been combined with an additional heuristic, denoted by ERF which stands for Earliest Reservation First. In the case of ERF, the parts waiting in a machine's input bu er are ordered based on the time of their reservation. Hence, when a machine is available for processing, it will pick up from its input bu er the part with the closest reservation time to the current time.
4 Analysis The results have demonstrated the superior performance of the job shop when PR scheduling is in e ect. The lead times are higher but savings on the total production time are achieved and the maximum WIP bu er levels are signi cantly lower than in the case of any of the dispatching rules. Further improvements of the system performance were obtained by augmenting the PR algorithm with dispatching rules. However, improving the system performance along one dimension led to the degradation of the performance along one of the other dimensions. The performance of the system actually degraded in all dimensions when PR was combined with FIFO. Combining EDD and PR achieved lower average tardiness and percent of tardy jobs than the PR alone but at the expense of a longer production time and higher WIP levels. In both cases of SPT and (PR+SPT), the percent of tardy jobs is lower than the EDD and PR cases, respectively, but at the expense of a much higher average tardiness. When (PR+ERF) is applied, the performance of the system is improved over the PR case at the expense of higher WIP levels and longer average ow time. However, (PR+ERF) does achieve the lowest percentage of tardy parts and average tardiness. 4
The results presented so far highlight the tradeo s of using PR vs the augmented PR cases. Based on these results, the objective of our future work is to try to achieve the improvements obtained by the combination of ERF with PR as of the initial scheduling phase, while maintaining lower WIP levels. The results therefore con rm the need for a more dynamic scheduling scheme which enables a machine to take into consideration those parts that are, or will be, waiting in its input bu er when computing the earliest delivery time for a new part. This will be done by augmenting the production reservation for an incoming part by a mechanism that enables taking into account the other parts already within the manufacturing system. The resulting schedule should then re ect the best possible \operational compromise" for scheduling the incoming part under the current system status at that time. This will enable determining the best possible nish time of a product in order to achieve a JIT production scheme and to quote reliable delivery dates to customers.
5 Conclusions and Future Work We have highlighted the results of our initial investigation into heterarchical production scheduling. The results obtained for the augmented production reservation cases con rm the merits of the proposed bidding technique and point out the directions for extending it to improve the performance of the manufacturing system under consideration. In future simulations we will incorporate additional production parameters such as additional part types and priority levels for the parts being manufactured, as well as incorporating the setup and transportation times. We also envisage to allow parts to negotiate with the AGVs for transportation services as an integral part of the production scheduling. According to the contract net protocol, a part should pay the machines for their services. Since all parts have equal priorities in the current model, the price all parts are paying is constant. In future work, parts will be allocated budgets proportional to their priority levels to enable them of bidding for the services they need from the machines. Moreover, we are currently extending this work to explicitly address the issue of rescheduling in order to accomodate the production of new parts and to cope with machine breakdowns, variations in processing, setup or transportation times, as well as cancellation of orders, all within the same framework of heterarchical scheduling proposed in this paper. The results of this investigation will also be available in the very near future. The methodology discussed can also be extended to assembly-type of manufacturing applications. In this case, the components and subassemblies that make up a product can negotiate their production schedules with the machines in the same manner that the parts did in our approach.
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References [1] A. Saad, K. Kawamura, M. E. Johnson, G. Biswas, and A. Salama, \Evaluating a Contract Net-Based Heterarchical Scheduling Approach for Flexible Manufacturing", submitted to the 1995 IEEE International Symposium on Assembly and Task Planning (ISATP'95), August 10-11, Pittsburgh, Pennsylvania. [2] G. Biswas and K. Kawamura and A. Saad, "Intelligent Manufacturing Systems: State of The Art", Proceedings of the 10th Annual Technical and Business Exhibition Symposium (TABES 94), Paper No. 94-503, Huntsville, AL, May 1994. [3] A. Saad, A. Salama, K. Kawamura, M. E. Johnson, and G. Biswas, \A Bidirectional Contract Net for Production Planning and Scheduling", Proceedings of the UC/IAMS Workshop for Manufacturing Research, Cincinnati, Ohio, April 9th, 1994. [4] G. Biswas, S. Bagchi, and A. Saad, \Recon gurability for Holonic Manufacturing", CIS Technical Report, Vanderbilt University, Nashville, Tennessee, January 1995. [5] A. Salama and A. Saad, \Simulation of Heterarchical Manufacturing Systems using Siman", Proceedings of the 1994 meeting of the Tennesse Academy of Science, in press. [6] A. Saad and K. Kawamura, \A Survey of Architectures for Intelligent Teamwork among Robots", CIS Technical Report (CIS-93-03), Vanderbilt University, Nashville, Tennessee, September 1993.
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