| A ?exible manufacturing system(FMS), consisting of the uniform information system,material storage and transportation system, and a set of numerical control processing equipments, is a kind of automated manufacturing systems. The introduction of FMS is intended to tackle the balance problems between the e?ciency and ?exibility of highly automated manufacturing systems, especially, in the small batch manufacturing.However, the existences of insu?cient system resources, inappropriate processing sequences, and improper resource allocations can lead to the competition for resources and result in deadlocks. Petri nets have been recognized as a powerful, graphical,and mathematical tool and widely used to model FMS. By the Petri net model of an FMS, system characteristics and properties, including deadlock behavior, can be well analyzed. With an o?-line computational mechanism, deadlocks can be dealt with during the design stage of a system. Therefore, deadlock prevention has raised increasingly researchers’ concerns in recent years.In general, deadlock prevention can be realized by adding monitors to a Petri net model.In order to obtain a structurally simple supervisor, researchers try their best to reduce the number of added monitors, such as the theory of elementary siphons. Meanwhile,to realize the maximally permissive behavior, the theory of regions is applied. Consequently, the theory of siphons based on structural analysis and the theory of regions based on reachability graph analysis become the two dominant methods for deadlock control. Due to the inherent complexity of siphon-based approaches, a complete siphon enumeration can not be avoided and the controlled net looks a bit complex.This thesis concentrates on the liveness-enforcing supervisor synthesis based on the function con?gurations. The research contributions of this thesis are proposed as follows.1. To solve the problem of deadlock prevention for timed Petri nets, an e?ective deadlock prevention policy based on elementary siphons is proposed. Without enumerating reachable markings, deadlock prevention is achieved by adding monitors for elementary siphons, increasing control depth variables when necessary, and removing implicit,liveness-restricted and redundant control places. The ?nal supervisor is live. First, a timed Petri net is stretched into an SPN. Unchanging the system performance, each transition in the SPN has a unit delay time. For each elementary siphon, a monitor is added such that it is invariant-controlled. After testing the controllability of dependent siphons and enlarging the depth variables when necessary, each siphon is successfully controlled and no emptiable control-induced siphons can be produced. Then implicit and liveness-restricted places are removed. Finally, the SPN can be reverted into a Td PN. After the removal of redundant control places, the ?nal Td PN with simple structure is a liveness-enforcing supervisor.2. Based on the reachability graph and priorities of transitions, a new approach is presented to implement the liveness of the controlled net system. First, given a Petri net, its reachability graph is computed where markings are categorized into four classes.Then, to prevent the system from reaching the deadlock and bad markings, a new class of Petri nets with transition priorities is de?ned. According to the priorities of transitions, if several transitions are enabled simultaneously, the one with the highest priority level may ?re. An algorithm is developed to separate the deadlock and bad markings from the reachability graph. A set of priorities is obtained. Considering the existence of local isolated loops(LIL), another algorithm is proposed to eliminate them.Under the ?ring sequences decided by priorities, the resulting net has the simplest structure and is live.3. Based on the capacity restrictions for activity places in the Petri net model of an FMS, a new approach is presented to implement the liveness of the controlled net system. First, a new class of Petri nets, called Petri nets with capacity(PNWC) is de?ned.Second, for two special subclasses of Petri nets, namely, WS3 PR and S4 R nets prone to deadlocks, their initial capacity vectors are decided. Then, an algorithm is proposed to compute their ?nal capacity vectors iteratively. At each iteration, the capacity of one activity place is decreased by one. The objective is to prevent the excessive occupancy of one resource by one production process, which is an important reason causing a deadlock. This process is carried out until the net model becomes live. With the constraints by the ?nal capacity vector, the controlled net is live. It is proved the iterative algorithm is convergent. Considering the redundance of capacity restrictions, another algorithm is given to eliminate it. The presented method for FMS control guarantees its live operation and high performance in terms of resource utilization. Furthermore,the new proposed method does not need to compute the reachability graph and strict minimal siphons. A case study shows that a PNWC has a simpler structure and lower computational cost than the controlled net obtained by traditional approaches.Generally, the proposed method is applicable, easy to use, e?ective, and straightforward.4. To solve the problem of deadlock for ?exible manufacturing systems(FMS), an e?ective deadlock prevention policy based on time constraints is presented. The proposed approach is for a special class of Petri nets, named LS3 PR. Reachability analysis is a fundamental method applied widely into analyzing Petri nets model. The deadlockfreedom is achieved by preventing the system from reaching the bad, deadlock markings and local isolated loops(LIL). With the time constraints, the ?ring priorities of enabled transitions can be controlled to ensure the deadlocks never occur. The introducing of time Petri nets makes the control strategy here di?erent from traditional reachability graph-based methods. Compared with the traditional deadlock control policies, the controlled net by the presented method has simpler structure.
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