Models Of Flexible Manufacturing Systems Research Articles

Robust Total Energy Optimization of Flexible Manufacturing Systems Based on Renyi Mean-Entropy Criterion

To deal with uncertainties in energy optimization of flexible manufacturing systems (FMSs), this paper presents a robust total energy optimization problem with productive and idle powers of resources being random variables (RVs). Using the weighted p-timed Petri net model of FMS, the robust energy-optimal schedule is determined by searching the robust shortest path of reachability graph based on Renyi mean-entropy criterion. The Renyi quadratic entropy is proposed as a robustness measure which does not depend on the Gaussianity of RVs and can be computed easily from samples. Next, Bellman's dynamic programming is applied to find the shortest path in Renyi mean-entropy criterion. The effectiveness of Renyi mean-entropy criterion is verified with mathematical rigour as well as extensive numerical results from simulations and an industrial stamping system, in terms of computational complexity and deviation from optimality.

A Petri net-based particle swarm optimization approach for scheduling deadlock-prone flexible manufacturing systems

This paper proposes an effective hybrid particle swarm optimization (HPSO) algorithm to solve the deadlock-free scheduling problem of flexible manufacturing systems (FMSs) that are characterized with lot sizes, resource capacities, and routing flexibility. Based on the timed Petri net model of FMS, a random-key based solution representation is designed to encode the routing and sequencing information of a schedule into one particle. For the existence of deadlocks, most of the particles cannot be directly decoded to a feasible schedule. Therefore, a deadlock controller is applied in the decoding scheme to amend deadlock-prone schedules into feasible ones. Moreover, two improvement strategies, the particle normalization and the simulated annealing based local search, are designed and incorporated into particle swarm optimization algorithm to enhance the searching ability. The proposed HPSO is tested on a set of FMS examples, showing its superiority over existing algorithms in terms of both solution quality and robustness.

Optimization of Total Energy Consumption in Flexible Manufacturing Systems Using Weighted P-Timed Petri Nets and Dynamic Programming

Schedule optimization is crucial to reduce energy consumption of flexible manufacturing systems (FMSs) with shared resources and route flexibility. Based on the weighted p-timed Petri Net (WTPN) models of FMS, this paper considers a scheduling problem which minimizes both productive and idle energy consumption subjected to general production constraints. The considered problem is proven to be a nonconvex mixed integer nonlinear program (MINLP). A new reachability graph (RG)-based discrete dynamic programming (DP) approach is proposed for generating near energy-optimal schedules within adequate computational time. The nonconvex MINLP is sampled, and the reduced RG is constructed such that only reachable paths are retained for computation of the energy-optimal path. Each scheduling subproblem is linearized, and each optimal substructure is computed to store in a routing table. It is proven that the sampling-induced error is bounded, and this upper bound can be reduced by increasing the sampling frequency. Experiment results on an industrial stamping system show the effectiveness of our proposed scheduling method in terms of computational complexity and deviation from optimality.

New Petri Net Structure and Its Application to Optimal Supervisory Control: Interval Inhibitor Arcs

This paper presents a new Petri net structure, namely, an interval inhibitor arc, and its application to the optimal supervisory control of Petri nets. An interval inhibitor arc is an arc from a place to a transition labeled with an integer interval. The transition is disabled by the place if the number of tokens in the place is between the labeled interval. The formal definition and the firing rules of Petri nets with interval inhibitor arcs are developed. Then, an optimal Petri net supervisor based on the interval inhibitor arcs is designed to prevent a system from reaching illegal markings. Two techniques are developed to simplify the supervisory structure by compressing the number of control places. The proposed approaches are general since they can be applied to any bounded Petri net models. A marking reduction approach is also introduced if they are applied to Petri net models of flexible manufacturing systems. Finally, a number of examples are provided to demonstrate the proposed approaches and the experimental results show that they can obtain optimal Petri net supervisors for some net models that cannot be optimally controlled by pure net supervisors. Furthermore, the obtained supervisor is structurally simple.

A livelock control policy for a flexible manufacturing system modeling with a subclass of generalized Petri nets

AbstractLivelocks, like deadlocks, can result in the serious problems in running process of flexible manufacturing systems (FMSs) as well. Current deadlock control policies based on the approaches of siphon detection and control, cannot cope with livelocks in a system of sequential systems with shared resources (S4R), a typical subclass of Petri nets that can model FMSs. On the basis of the mixed integer programming method, this study proposes a livelock control policy (LCP) that can not only solve the new smart siphons (NSSs) associated with livelocks or deadlocks in an S4R system directly, but also make the solved NSSs max′-controlled by adding the corresponding control places (CPs). As a result, an original S4R system with livelocks or deadlocks can be turned into the live controlled one in which no NSSs can be found. The related theoretical analysis and several examples are given to demonstrate the proposed LCP. Compared with the existing methods in the literature, the proposed one is more general and...

Parameterized ACD Modeling of Flexible Manufacturing Systems

Activity cycle diagram (ACD), which is essentially a timed Petri net, is one of the oldest formal modeling tools for discrete-event systems, and a flexible manufacturing system (FMS) is a highly automated job shop that is used widely in mechanical and electronics industries. Previous FMS modeling studies have indicated that formal modeling of real-life (or industrial) FMSs with classical ACDs (or Petri nets) is almost impossible. This paper presents an incremental modeling procedure for building a formal simulation model of a real-life FMS with parameterized ACD (P-ACD) that was proposed recently. The incremental modeling procedure consists of job flow modeling, job routing modeling, dispatching rule modeling, and refixture operation modeling. In this paper, a P-ACD model of a real-life FMS was constructed and an FMS simulator was implemented from the proposed P-ACD model. A simulation experiment was conducted in order to demonstrate the usefulness of the FMS simulator.

Optimal Supervisory Control of Flexible Manufacturing Systems by Petri Nets: A Set Classification Approach

Supervisory control is usually considered as an external control mechanism to a system by controlling the occurrences of its controllable events. There exist Petri net models whose legal reachability spaces are nonconvex. In this case, they cannot be optimally controlled by the conjunctions of linear constraints. For Petri net models of flexible manufacturing systems, this work proposes a method to classify the legal markings into several subsets. Each subset is associated with a linear constraint that can forbid all first-met bad markings. Then, the disjunctions of the obtained constraints can make all legal markings reachable and forbid all first-met bad markings, i.e., the controlled net is live and maximally permissive. An integer linear programming model is formulated to minimize the number of the constraints. A supervisory structure is also proposed to implement the disjunctions of the constraints. Finally, examples are provided to illustrate the proposed method.

A Deadlock Prevention Policy Based on Complementary Places

For Petri net models of flexible manufacturing systems,this work proposes a method to design liveness-enforcing supervisors based on complementary places.Firstly,a finite-capacity net珦Nis obtained from a given Petri net N by adding a capacity function K.Then,by increasing K to Kmax,the maximal reachability set of live finite-capacity net珦Nis computed and the complementary-place transformation is used to obtain the live system N′.Finally,an example of S3PR net is provided to compare with some other approaches proposed in prior work.Comparison results show that the liveness-enforcing supervisor obtained by the proposed method has simpler structure and more permissive behavior in some cases.Furthermore,another example of S3PGR2net shows that the proposed method can be applied to the general Petri nets.

Architecture Model of FMS Based on Multi-Agent Technology

The concept of flexible manufacturing system (FMS) has been purposed to improve flexibility and efficiency of manufacture system. Characteristics and operating mechanism of FMS is analyzed in this paper. Fuzziness and distributive of the Multi-agent system can meet the requirements of building FMS platform. On the basis of the theory mentioned above, this paper proposed an architecture model of FMS based on multi-agent technology. At last, the communication between the agents in the system was researched and designed. The introduction of multi-agent technology not only provides a feasible method of building FMS platform system, but also helps to lucubrate the architecture of FMS.

Deadlock-free genetic scheduling for flexible manufacturing systems using Petri nets and deadlock controllers

In this paper, a new deadlock-free scheduling method based on genetic algorithm and Petri net models of flexible manufacturing systems is proposed. The optimisation criterion is to minimise the makespan. In the proposed genetic scheduling algorithm, a candidate schedule is represented by a chromosome that consists of two sections: route selection and operation sequence. With the support of a deadlock controller, a repairing algorithm is proposed to check the feasibility of each chromosome and fix infeasible chromosomes to feasible ones. A feasible chromosome can be easily decoded to a deadlock-free schedule, which is a sequence of transitions without deadlocks. Different kinds of crossover and mutation operations are performed on two sections of the chromosome, respectively, to improve the performance of the presented algorithm. Computational results show that the proposed algorithm can get better schedules. Furthermore, the proposed scheduling method provides a new approach to evaluate the performance of different deadlock controllers.

Flexibility development – a personal retrospective

This paper describes the development of flexibility through my personal views and involvement in researching this complex concept. It differentiates between two modelling perspectives: the bottom-up perspective (from the flexibility of elements to integrated flexibility) of the early researchers for modelling flexible manufacturing systems (FMSs) and the more recent top-down perspective, from flexibility as a strategy to resources needed for implementing it. The bottom-up perspective is represented here by three papers. One paper focuses on Petri nets as a modelling tool, another offers a multi-factor design of experiments (DOE) view on factors affecting the performances of an FMS, and the third one presents a framework of logistic flexibility. The top-down perspective focuses on supply chain flexibility and uses quality function deployment (QFD) as a hierarchical modelling technique. An original framework for analysing the flexibility of generic objects, utilising cloud diagrams, is also presented. In all these papers a variety of changes that flexibility has to cope with are modelled, enabling the viewing of flexibility as a powerful ingredient for responding to changes and reducing risks in uncertain and changeable environments.

Sequence Control of Essential Siphons for Deadlock Prevention in Petri Nets

Deadlock prevention is crucial to the modeling of flexible manufacturing systems. In the Petri net framework, deadlock prevention is often addressed by siphon-based control (SC) policies. Recent research results show that SC methods can avoid full siphon enumeration by using mixed integer programming (MIP) to greatly increase the computational efficiency so that it can be applied in large systems in computable time. Besides, maximally permissive control solutions can be obtained by means of iterative siphon control (ISC) approaches and MIP. Then the remaining problems are redundancy and MIP iterations. Redundant controllers make the closed-loop system more complicated and each MIP iteration increases the total computational time. This article proposes a revised ISC deadlock prevention policy which can achieve better results than the other reported methods in terms of redundancy and MIP iterations while maintaining the maximal permissiveness. Several benchmark examples are provided to illustrate the proposed approach and to be compared with the other reported methods.

Parsimonious Monitor Control of Petri Net Models of Flexible Manufacturing Systems

Most approaches for deadlock prevention and liveness enforcement in Petri nets rely on siphon control methods or the theory of regions to derive monitor-based supervisors. These techniques raise methodological and computational issues, from the existence of feasible solutions to the hardness of guaranteeing maximal permissivity and optimality in the size and cost of the control subnet. Recently, the supervisor design problem has also been reformulated as a direct monitor optimization task based on integer linear programming, which can more effectively deal with the mentioned issues and objectives. This paper introduces an efficient branch-and-bound scheme for the exploration of the solution space of the direct monitor optimization problem. An extensive computational analysis on a set of benchmark instances demonstrates the efficiency of the approach.

Hybrid Liveness-Enforcing Policy for Generalized Petri Net Models of Flexible Manufacturing Systems

This paper proposes a hybrid liveness-enforcing method for a class of Petri nets, which can well model many flexible manufacturing systems. The proposed method combines elementary siphons with a characteristic structure-based method to prevent deadlocks and enforce liveness to the net class under consideration. The characteristic structure-based method is further advanced in this work. It unveils and takes a full advantage of an intrinsically live structure of generalized Petri nets, which hides behind the arc weights, to achieve the liveness enforcement without any external control agent such as monitors. This hybrid method can identify and remove redundant monitors from a liveness-enforcing supervisor designed according to existing policies, improve the permissiveness, reduce the structural complexity of a controlled system, and consequently save the control implementation cost. Several examples are used to illustrate this method.

A deadlock prevention policy for a class of Petri net models of flexible manufacturing systems

This paper focuses on the problem of deadlocks in automated flexible manufacturing systems (FMS). Based on Petri nets, a deadlock prevention policy is proposed for a special class of Petri nets, S3PR. We embed the deadlock avoidance policy (DAP) of conjunctive/disjunctive resources upstream neighbourhood (C/D RUN) into the deadlock prevention policy (DPP), and allocate the underlying (sequential) resources reasonably to guarantee the absence of deadlock states and processes. First, siphons in a net model are distinguished by elementary and dependent ones. From the set of elementary siphons, a set of linear inequality constraints expressed by the state vector can be formalised. After being modified by the proposed policy, a set of generalised mutual exclusion constraints (GMEC) expressed by the marking vector can be found. Then monitors based on the GMEC are added to the plant model such that the elementary siphons in the S3PR net are all invariant-controlled and no emptiable siphon is generated due to the addition of the monitors. This novel deadlock prevention policy can usually lead to a more permissive supervisor by adding a smaller number of monitors and arcs than the existing methods for the design of liveness-enforcing Petri net supervisors. Two manufacturing examples are utilised to illustrate the proposed method and compared with the existing ones.

Simultaneous scheduling of machine and tool in automated manufacturing system using genetic algorithm

Part and tool flows are the two major dynamic entities in the operation of a flexible manufacturing system (FMS). They are the key factors and their management plays an important role. The aim of this paper is to consider three off-line scheduling algorithms that generate optimal or near optimal simultaneous machine-tool finite production schedules for the FMS model with makespan criterion. In this context, three heuristics, priority dispatching rule algorithm (PDRA), simulated annealing algorithm (SAA) and genetic algorithm (GA) are proposed to find optimal or near optimal solutions with minimum makespan criterion. The analysis reveals that GA-based heuristic provides an optimal or near optimal solution with reasonable computational time.

General mixed integer programming-based liveness test for system of sequential systems with shared resources nets

For a system of sequential systems with shared resources (S4R), a typical class of Petri net models of flexible manufacturing systems (FMS), current deadly marked siphon (DMS) detection approaches based on mixed integer programming (MIP) cannot detect siphons that cause livelocks in systems. Thus a deadlock-free system cannot be further controlled. Moreover, most of these methods cannot obtain a minimal DMS directly. This study proposes the concept of extended DMS (EDMS) on the basis of max″-controllability condition of siphons. Accordingly, a more general MIP testing approach that can detect the minimal EDMS that cause deadlocks or livelocks in S4R is presented to lay foundations for further analysis and control. The authors conclude that an S4R is live if there is no feasible solution to this new MIP test. Compared with the existing methods, the proposed one is more general and powerful. Experimental studies are used to illustrate it.

Modelado con redes de Petri e implementación con GRAFCET de un sistema de manufactura flexible con procesos concurrentes y recursos compartidos

<span>En este trabajo, se presenta el modelado de un Sistema de Manufactura Flexible (SMF), con procesos concurrentes y recursos compartidos mediante Sistemas a Eventos Discretos (SED), específicamente Redes de Petri (RdP), y GRAFCET. El SMF se plantea como un modelo hipotético que se modela con una RdP con el objeto de identificar su dinámica y hallar la secuencia óptima de funcionamiento del sistema. Se desarrolló un modelo matemático que permite estimar el vector de tiempo acumulado de un proceso modelado mediante una RdP, el cual constituye la base para hallar la mejor secuencia posible del sistema modelado. Por último, se realizó una implementación en el Laboratorio de Automatización de la Universidad Militar Nueva Granada, que simula el funcionamiento del SMF modelado por un montaje electrohidroneumático controlado con GRAFCET mediante PLC.</span>

Applying multi-agent technique in multi-section flexible manufacturing system

In the highly competitive market, cooperative multi-agent transaction and negotiation mechanism have become an important research topic. This paper uses multi-agent technology to construct a multi-section flexible manufacturing system (FMS) model, and utilizes simulation to build a manufacturing environment based on JADE framework for multi-agent to combine with dispatching rules, such as shortest imminent processing time (SIPT), first come first serve (FCFS) earliest due date (EDD), and Buffer Sequence. This paper finds that using multi-agent technique for multi-section FMS model can enhance the production efficiency in practice. Meanwhile, in this study, multi-agent systems combined with dynamic dispatching can be used to identify the best dispatching rules combination for achieving largest throughput, and thus it can provide the reference for production scheduling in advance.

Self-liveness of a class of Petri net models for flexible manufacturing systems

As a highly undesirable situation, deadlocks usually occur in flexible manufacturing systems because of the limited resources. To develop a deadlock control policy, Petri nets (PN) are widely used to model these systems. For a class of PN, WS3PR-weighted systems of simple sequential processes with resources, this study reveals the relationship between its siphons and deadlocks and explores the conditions under which it is live. The concept of self-max'-controlled siphons is proposed and a sufficient condition for self-max'-controllability of siphons is developed. The authors conclude that a WS3PR is live if each strict minimal siphon is self-max'-controlled. Examples are used to demonstrate the proposed concepts and methods.