Robust supervisory control for production systems with multiple resource failures

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Robust Supervisory Control for Production Systems With Multiple Resource Failures
Song Foh Chew, Student Member, IEEE, and Mark A. Lawley, Member, IEEE

Abstract—Supervisory control for deadlock-free resource allocation has been an active area of manufacturing systems research. To date, most workassumes that allocated resources do not fail. Little research has addressed allocating resources that may fail. In our previous work, we assumed a single unreliable resource and developed supervisory controllers to ensure robust deadlock-free operation in the event of resource failure. In this paper, we assume that several unreliable resources may fail simultaneously. In this case, a controller mustguarantee that a set of resource failures does not propagate through blocking to stall other portions of the system. That is, the controller must ensure that every part type not requiring any of the failed resources should continue to produce smoothly without disruption. To do this, the controller must constrain the system to states that serve as feasible initial states for: 1) a reduced system whenresource failures occur and 2) an upgraded system when failed resources are repaired. We develop the properties that such a controller must possess and then develop supervisory controllers that satisfy these properties. Note to Practitioners—For the past decade or so, researchers have begun to actively address the issue of ensuring smooth and continuous operation for flexibly automatedmanufacturing systems. This research effort has been motivated by the many failed attempts to implement flexible automation throughout the 1980s. During this time, much has been learned about modeling the control functions of a flexible, automated system. In fact, ladder logic control code can now be generated automatically from mathematical models, such as Petri nets, which compactly capture the requiredoperating system logic. Because the code is based on a formal model with well-established properties, it is guaranteed to ensure proper operation without significant startup troubleshooting. One area that has not been investigated is controlling these systems when machines or tools “fail.” The question is not how to fix what has failed, but rather how to control the system so that if something does fail,the system can continue producing items that do not require the failed elements. This is essential work since automated manufacturing systems consist of thousands of components, any of which are subject to failure. If failures in the system are not handled gracefully, it becomes difficult to keep the automated system running, in which case, system production does not meet expectations. In ourprevious work, we investigated ensuring smooth operation for systems with a single unreliable resource. We developed supervisory controllers to guarantee this requirement for these systems. In this paper, we extend the previous results to a more general class of systems where there are multiple unreliable resources. We establish a set of desired properties that the supervisory controller must possessManuscript received October 25, 2004; revised February 22, 2005 and August 30, 2005. This paper was recommended for publication by Associate Editor M. Zhou and Editor N. Viswanadham upon evaluation of the reviewers’ comments. This work was supported by the National Science Foundation under Grant GOALI 0085047. The authors are with the School of Industrial Engineering, Purdue University, WestLafayette, IN 47907-1287 USA (e-mail:; Digital Object Identifier 10.1109/TASE.2005.861397

in order to guarantee robust operation for these systems, and then develop a number of controllers that satisfy these properties. Index Terms—Deadlock avoidance, fault tolerance, flexible manufacturing systems, robust control.

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