Iterative deadlock prevention strategies based on siphons have drawn increasing attention. For iterative strategies, selecting which siphon to control at each iteration has an influence on the final supervisor in structural complexity, computational complexity, and behavioral permissiveness. In this paper, we define two kinds of emptiable siphons and provide two modified mixed-integer programming (MIP) formulations to compute such siphons. On the basis of them, a three-stage iterative deadlock prevention policy that specifies the siphon control order is proposed. The experimental results show that a supervisor with a simpler structure, higher behavioral permissiveness, and lower computational complexity can be obtained by the proposed strategy since neither the exhaustive siphon enumeration nor the reachability analysis is required.

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A Siphon-Based Deadlock Prevention Strategy for S 3 PR

This paper focuses on the liveness analysis and deadlock control for automated manufacturing systems (AMSs) with multiple resource requirements. Such an AMS is modeled by a class of generalized Petri nets called systems of simple sequential processes with multiple resources (S3PMR). It is shown that a deadlock of the considered AMSs can be characterized by the saturation of a structural object in S3PMR, called perfect resource transition-circuit (PRT-circuit). As a consequence, an S3PMR is live if and only if no PRT-circuit is saturated at any reachable marking. To ensure the system liveness, one has to prevent all PRT-circuits from being saturated at all reachable markings. To develop a structurally simple Petri net deadlock controller, we present the concept of an effective transition cover, which is a special subset of PRT-circuits that may be saturated. Then by designing a control place with a proper control variable for each PRT-circuit in an effective transition cover, we obtain a deadlock controller for the system. The needed control variables are determined by an integer linear program. Since the number of PRT-circuits in an effective transition cover is much less than that of all PRT-circuits that need to control, our controller is of small structural size. For an AMS with saturable PRT-circuits, there exists at least a transition cover. An algorithm is presented for checking the effectiveness of transition covers, and transforming noneffective transition covers into effective ones. Finally, some examples are used to illustrate the proposed method.

Liveness Analysis and Deadlock Control for Automated Manufacturing Systems With Multiple Resource Requirements

Owing to large production capacity and high efficiency, multibridge machining systems (MBMSs) have gained increasing attention in industry. Their multiple bridge machines work concurrently in their serially arranged and partially overlapping workspaces. To solve totally the job scheduling and collision resolution problems of MBMS, our prior work proposes a serial-colored traveling salesman problem (S-CTSP)-based method. Each salesman in S-CTSP visits his exclusive cities and some cities shared with his neighbor(s). To endow MBMS with fault-tolerance ability, this paper presents a two-stage method for both static path planning and dynamic collision avoidance of multiple machines. At the first stage, the path planning problem abstracted as an S-CTSP is solved by a population-based incremental learning (PBIL) algorithm. The PBIL introduces a local search operation, two types of possibility vectors, and a selection strategy of exclusive and shared cities. The second stage uses a Petri net supervisor to dynamically avoid any emerging collision when performing the scheduled work. This work also provides a novel priority net structure to prevent mutual waiting of two neighboring machines ready to enter their overlapping workspace. Then, we apply the presented method to a large tribridge waterjet cutting case to show its performance.

A Two-Stage Approach to Path Planning and Collision Avoidance of Multibridge Machining Systems

Deadlock is a rather undesirable case in automated manufacturing systems (AMSs). The appearance of deadlock can cause the partial or total stagnation of a system. So far, a large number of deadlock control policies have been developed; nevertheless, the majority are dependent on the assumption that allocated resources cannot break down. In the real world, an AMS consists of a set of concurrent production routes that share and compete for a limited number of resources, such as automated material/component handling devices, buffers, robots, and machines. Resource failures occur unexpectedly. If reasonable control does not exist, a simple resource failure can lead an entire system to stagnation, which can cause enormous economic loss. Therefore, researchers have gradually paid attention to AMSs allowing resource failures in recent years. In this paper, we focus on reviewing and comparing various robust supervisory control policies from the perspective of their structural complexity, behavioral permissiveness, and computational complexity. Some potential future directions are explored. This paper provides a reference source of robust supervisory control of AMSs for researchers and practitioners in this area. Note to Practitioners —In automated manufacturing systems (AMSs), resource failures are common. Their occurrences can lead a system to stagnation, which can cause unnecessary downtime and bring vast economic loss for enterprises. To resolve such stagnation issues, a great number of robust supervisory control policies have been proposed for AMSs with unreliable resources. These policies guarantee that a controlled system can continue to progress without deadlock and blocking states even if some unreliable resources fail to work. By a thorough review of existing robust supervisory control policies for AMSs with unreliable resources, this paper compares and analyzes these policies in terms of their structural complexity, behavioral permissiveness, and computational complexity. The goal of this paper is to provide a reference source in the area to help researchers and practitioners choose a suitable method for solving industrial application problems that are subject to resource failures.

A Survey on Robust Deadlock Control Policies for Automated Manufacturing Systems With Unreliable Resources

Deadlock resolution strategies based on siphon control are widely investigated. Their computational efficiency largely depends on siphon computation. Mixed-integer programming &#x0028 MIP &#x0029 can be utilized for the computation of an emptiable siphon in a Petri net &#x0028 PN &#x0029 . Based on it, deadlock resolution strategies can be designed without requiring complete siphon enumeration that has exponential complexity. Due to this reason, various MIP methods are proposed for various subclasses of PNs. This work proposes an innovative MIP method to compute an emptiable minimal siphon &#x0028 EMS &#x0029 for a subclass of PNs named S4PR. In particular, many particular structural characteristics of EMS in S4PR are formalized as constraints, which greatly reduces the solution space. Experimental results show that the proposed MIP method has higher computational efficiency. Furthermore, the proposed method allows one to determine the liveness of an ordinary S4PR.

Computation of an emptiable minimal siphon in a subclass of Petri nets using mixed-integer programming

Modern complex systems require intensive application of sophisticated supervisors Structural simplification techniques are one of the fundamental researches in the context of flexible manufacturing systems (FMSs) They can reduce implementation cost, mitigate fabrication complexity, and improve reliability Several typical methods have been developed along this direction In order to thoroughly explore their effectiveness and performance, we not only conduct a comparison investigation but also develop some new theoretical results Several analytical results and performance measures are proposed for their qualitative and quantitative comparison Our approach can assist researchers and practitioners to better comprehend the inherent mechanisms and relative merits of these simplification methodologies as well as their applicability in FMSs This paper is motivated by FMSs’ control; however, it is also applicable to other systems with discrete event controllers

Ling Yuan

Papers

Supervisor Simplification in FMSs: Comparative Studies and New Results Using Petri Nets