Exclusion Constraints Research Articles

Reduction approaches for a generalized line balancing problem

An NP-hard optimization problem dealing with the assignment of operations to the workstations and spindle heads of a machining transfer line is studied. Precedence, inclusion and exclusion constraints among the operations, capacity and cycle time constraints exist. The objective is to minimize the cost of the line being designed. This problem has been referred to as the Transfer Line Balancing Problem (TLBP) and is a generalized case of well-known simple assembly line balancing problem. In literature, few exact methods have been proposed to solve it. Since the problem is NP-hard, those that do are too time-consuming for solving real life industrial cases. This paper presents effective pre-processing methods which can reduce the size of the initial problem in order to shorten the solution time required. These methods are evaluated on three datasets of new generated problem instances and known benchmarks. The results obtained show that for certain cases the solution time can be significantly reduced when these pre-processing methods are applied.

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.

Enhanced mixed integer programming model for a transfer line design problem

Modular machining lines with multi-spindle workstations are considered. A multi-spindle head executes a set of operations. The problem of optimal design or reconfiguration of such lines is considered here. The set of all available spindle heads, operations executed by each spindle head, spindle head times and costs are assumed to be known. There are operations which can be executed by one of several candidate spindle heads, i.e., in different configuration with other operations. The problem consists in the choice of spindle heads from the given set and their assignment to workstations. The goal is to minimize the line cost while satisfying the precedence, inclusion and exclusion constraints. This problem is an extension of well known assembly line balancing and equipment selection problem. In our previous work, we proposed a MIP model which was significantly limited as to the size of the problems treated. In this paper, quite a few original approaches are suggested to improve the previous MIP model. The numerical tests reported show that the calculation time is drastically decreased, thereby expanding the model to larger and more realistic industrial problems.

On the Complexity of Authorization in RBAC under Qualification and Security Constraints

In practice, assigning access permissions to users must satisfy a variety of constraints motivated by business and security requirements. Here, we focus on Role-Based Access Control (RBAC) systems, in which access permissions are assigned to roles and roles are then assigned to users. User-role assignment is subject to role-based constraints, such as mutual exclusion constraints, prerequisite constraints, and role-cardinality constraints. Also, whether a user is qualified for a role depends on whether his/her qualification satisfies the role's requirements. In other words, a role can only be assigned to a certain set of qualified users. In this paper, we study fundamental problems related to access control constraints and user-role assignment, such as determining whether there are conflicts in a set of constraints, verifying whether a user-role assignment satisfies all constraints, and how to generate a valid user-role assignment for a system configuration. Computational complexity results and/or algorithms are given for the problems we consider.

On Expressing Value Externalities in Position Auctions

We introduce a bidding language for expressing negative value externalities in position auctions for online advertising. The unit-bidder constraints (UBC) language allows a bidder to condition a bid on its allocated slot and on the slots allocated to other bidders. We introduce a natural extension of the Generalized Second Price (GSP) auction, the expressive GSP (eGSP) auction, that induces truthful revelation of constraints for a rich subclass of unit-bidder types, namely downward-monotonic UBC. We establish the existence of envy-free Nash equilibrium in eGSP under a further restriction to a subclass of exclusion constraints, for which the standard GSP has no pure strategy Nash equilibrium. The equilibrium results are obtained by reduction to equilibrium analysis for reserve price GSP (Even-Dar et al. 2008). In considering the winner determination problem, which is NP-hard, we bound the approximation ratio for social welfare in eGSP and provide parameterized complexity results.

Leveraging Early Aspects in End-to-End Model Driven Development for Non-Functional Properties in Service Oriented Architecture

In Service Oriented Architecture (SOA), each application is designed with a set of reusable services and a business process. To retain the reusability of services, non-functional properties of applications must be separated from their functional properties. This paper investigates a model-driven development framework that separates non-functional properties from functional properties and manages them. This framework proposes two components: (1) a programming language, called BALLAD, for a new per-process strategy to specify non-functional properties for business processes, and (2) a graphical modeling method, called FM-SNFPs, to define a series of constraints among non-functional properties. BALLAD leverages aspects in aspect oriented programming/modeling. Each aspect is used to specify a set of non-functional properties that crosscut multiple services in a business process. FM-SNFPs leverage the notion of feature modeling to define constraints among non-functional properties like dependency and mutual exclusion constraints. BALLAD and FM-SNFPs free application developers from manually specifying, maintaining and validating non-functional properties and constraints for services one by one, reducing the burdens/costs in development and maintenance of service-oriented applications. This paper describes the design details of BALLAD and FM-SNFPs, and demonstrates how they are used in developing service-oriented applications. BALLAD significantly reduces the costs to implement and maintain non-functional properties in service-oriented applications.

Low-Cost and High-Performance Supervision in Ratio-Enforced Automated Manufacturing Systems Using Timed Petri Nets

In the context of automated manufacturing, this work proposes a new special class of timed Petri nets, namely, Timed ratio-enforced Augmented Marked Graph (TAMG) and its low-cost and high-performance supervisor synthesis methodology. A supervisor is composed of a set of control places (monitors), each of which is easy to be algebraically specified by a generalized mutual exclusion constraint (GMEC) to prevent certain siphons from being undermarked. In order to make a good tradeoff between the supervisor implementation cost and system performance, a mixed integer programming (MIP) approach is formulated to synthesize the monitors. An example is used to validate the effectiveness and efficiency of the proposed method. The results show that the proposed method remarkably outperforms any existing ones.

Fault tolerant control design via hybrid petri nets

AbstractThis paper proposes a novel fault tolerant control (FTC) scheme for hybrid systems modeled by hybrid Petri nets (HPNs). The HPNs model consists of discrete and continuous PNs. The faults are represented by unobservable discrete transitions or the normal observable discrete transitions with abnormal firing time in discrete PNs. First, an observer‐based fault diagnosis method is proposed to estimate the marking in discrete places with unknown initial marking and diagnose the faulty behavior simultaneously. Then, an adaptive fault tolerant controller is designed to maintain the general mutual exclusion constraints (GMEC) of discrete PNs, and a scheme that adjusts firing speeds of continuous transitions is provided to maintain the optimality of continuous PNs. Finally, an example of an intelligent transportation system consisting of automated vehicles on a bridge is included to demonstrate the effectiveness of our developed techniques. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society

Algebraic Synthesis of Timed Supervisor for Automated Manufacturing Systems Using Petri Nets

For practical automated manufacturing systems (AMSs), the time dimension is of great significance and should be integrated in their plant models. Reasonably, many of the realistic general mutual exclusion constraints (GMECs) imposed on these discrete models should be timed rather than merely algebraic or logic. In the past, such a problem was studied on the basis of the Ramadge-Wonham supervisory control technique (SCT) and the theory of regions. It proves to be NP-hard since it necessitates the generation of reachability graphs. This paper shows that it can be solvable in polynomial time by using generalized linear constraints, which are originally proposed to increase the expressive power of the linear marking constraints. By dividing each constraint into marking, firing vector, and Parikh terms, its respective control place can be synthesized algebraically without considering the separation of dangerous states and events. Several examples are used to validate the effectiveness and efficiency of the proposed approach.

Constraint Propagation in Propositional Planning

Planning as Satisfiability is a most successful approach to optimal propositional planning. It draws its strength from the efficiency of state-of-the-art propositional satisfiability solvers, combined with the utilization of constraints that are inferred from the problem planning graph. One of the recent improvements of the framework is the addition of long-distance mutual exclusion (londex) constraints that relate facts and actions which refer to different time steps. In this paper we compare different encodings of planning as satisfiability wrt the constraint propagation they achieve in a modern SAT solver. This analysis explains some of the differences observed in the performance of different encodings, and leads to some interesting conclusions. For instance, the Blackbox encoding achieves more propagation than the one of Satplan06, and therefore is a stronger formulation of planning as satisfiability. Moreover, our investigation suggests a new more compact and stronger model for the problem. We prove that in this new formulation many of the londex constraints are redundant in the sense that they do not add anything to the constraint propagation achieved by the model. Experimental results suggest that the theoretical results obtained are practically relevant.

Optimizing glass coating lines: MIP model and valid inequalities

Glass coating is a specific transformation aiming at improving glass performance. The work presented in this paper deals with the determination of the optimal configuration of the production lines used to perform this operation. We propose a first MIP formulation of the problem and then discuss several types of valid inequalities for improving it. The main idea is to exploit explicit or implicit binary exclusion constraints to derive stronger valid inequalities: the maximal clique constraints. Efficient (polynomial time) separation algorithms exploiting special structure of the problem are described, giving rise to a cutting-plane generation procedure for strengthening the initial formulation. The computational study carried out shows that, with the enhanced formulation, good solutions can be obtained within reasonable computation times using currently available integer programming software.

Synthesis of Structurally Simple Supervisors Enforcing Generalized Mutual Exclusion Constraints in Petri Nets

Generalized mutual exclusion constraints (GMECs) are a typical class of specifications for the supervisory control of discrete event systems in a Petri net formalism. This paper classifies the given constraints into elementary and dependent ones according to the linear dependency of their characteristic transition vectors that indicate the token count change of the concerned places. The dependent constraints are further divided into strongly and weakly dependent ones. A constraint is usually enforced by explicitly adding a monitor to a plant model. This research develops the conditions under which a dependent constraint is enforced due to the enforcement of the elementary constraints. The results developed in this paper are applied to the existing manufacturing-oriented Petri net classes. An algorithm is also proposed to identify a set of elementary constraints. Examples are used to demonstrate the proposed methods. Some potential extensions and applications are also discussed. This research improves the existing methods in computational efficiency and structure simplification of the supervisor, given a set of GMECs.

Control Synthesis for Partially Controlled Marked Graph Subject to Marking Exclusion Constraints

The goal of control synthesis is to build a control law which forces the system to respect given specifications. In this paper we propose a new type of specification called Marking Exclusion Constraint (MEC). The main advantage of MEC specification is an increased modeling power regarding General Mutual Exclusion Constraints (GMEC). We define two types of MEC: MEC-OR and MEC-AND and we propose a technique to build the controller which enforces MEC specifications for discrete events systems modeled by marking graphs.

Supervisory Control of partially observable Marked Graph based on Marking Exclusion Constraint

This paper presents a control synthesis approach for discrete event systems modelled by partially observable marked graphs. It solves the forbidden state problem for discrete event systems modelled by partially observable event graphs subject to Marking Exclusion Constraints (MEC). Two kind of MEC specifications are considered: MEC-AND or MEC-OR. The main advantage of MEC specifications is an increased modelling power regarding to General Mutual Exclusion Constraints GMEC. We provide a simple optimal controller to enforce MEC specifications with independent critical places. We also propose a suboptimal controller for MEC-OR with dependent critical places belonging to the same direct path.

Balancing lines with CNC machines: A multi-start ant based heuristic

We are working on a machining line balancing problem involving specific constraints. The studied lines are paced and serial, i.e. a part to be machined passes through a sequence of stations. The stations are equipped with CNC (Computer Numerical Control) machines. A CNC machine is a mono-spindle head machine which can use sequentially different tools and can rotate the part in order to perform different tasks. Such a machine is guided by a computer numerical controller system. As usual with machining lines, this problem is subject to precedence constraints as well as exclusion and inclusion constraints. Moreover, the station workload depends on the sequence in which the tasks are assigned because of set-up times related to the change and displacement of tools, rotation of the part, etc. In addition, accessibility constraints have to be considered. Two types of CNC machines with different characteristics can be used. Several tasks require a particular type of machine. The objective is to assign a given set of tasks, required for part machining, as well as a given set of machines to a sequence of stations while minimizing the total cost of the line. In this paper, a multi-start heuristic is proposed and tested on real life industrial problems.

Deadlock free Control Synthesis for Partially Controllable Marked Graphs

This paper deals with the problem of deadlock avoidance in control synthesis for a discrete event systems modeled by marked graphs. The specifications are modeled by General Mutual Exclusion Constraints. We show that, even that the system to be controlled is live, the restriction introduced by the controller may generate deadlocks. Therefore, the research work we present in this paper focuses on enhancing an existing control synthesis technique in order to provide a deadlock free maximal permissive control law.

Liveness Enforcing Supervision of Video Streaming Systems Using Nonsequential Petri Nets

Internet-motivated video streaming systems face such complicated issues as a high degree of network-resource sharing amongst many flows, which potentially leads to deadlocks. Using siphons and their corresponding dangerous markings, this work investigates a method to enforce control iteratively. At each iteration, a generalized mutual exclusion constraint is produced to keep only those markings under which liveness is enforced. Furthermore, a generalized elementary siphon control method is proposed such that the final supervisor is structurally simple. Examples are used to illustrate the proposed approach.

Supervisor Synthesis for Enforcing a Class of Generalized Mutual Exclusion Constraints on Petri Nets

The considered class of generalized mutual exclusion constraints (GMECs) on a controlled Petri net are such that the influence-uncontrollable subnets are forward-concurrent-free nets. Some structural properties of forward-concurrent-free nets are firstly proposed. Utilizing these properties, a method is then proposed to transform a given conjunction of GMECs into a conjunction of admissible GMECs. Furthermore, the necessary and sufficient condition of the existence of the permissive supervisor is obtained, and the optimal supervisor with a complexity of polynomial time is designed. The theoretic results are illustrated by an example that synthesizes a maximally permissive supervisor for a manufacturing system.

Branch and bound algorithm for a transfer line design problem: Stations with sequentially activated multi-spindle heads

In this paper, a new transfer line balancing problem is considered. The main difference from the simple assembly line balancing problem is that the operations are grouped into blocks (batches). All the operations of the same block are carried out simultaneously by one piece of equipment (multi-spindle head). Nevertheless, the blocks assigned to the same workstation are executed in series. The line cost consists of the sum of block and workstation costs. At the considered line design stage, the set of all available blocks is given. The aim is to find a subset from the given set of available blocks and to find a partition of this subset to workstations such that each operation is executed once and the line cost is minimal while all the precedence, cycle time, and compatibility (operation inclusion and block exclusion) constraints are respected. A new lower bound based on solving a special set partitioning problem is presented and a branch and bound algorithm is developed. The experimental results prove the quality of the lower bound and applicability of the suggested branch and bound algorithm for medium sized industrial cases.

On suboptimal supervisory control of Petri nets in the presence of uncontrollable transitions via monitor places

Recently, a study has been presented for suboptimal supervisory control of Petri nets via monitor (control) places in the presence of uncontrollable transitions. To enforce a generalized mutual exclusion constraint (GMEC) suboptimally on a plant Petri net with uncontrollable transitions, a set of monitor places has been provided to choose from. There are two main results of this study, the first of which is that “there is not an optimal solution to the problem of computing a monitor-based controller to enforce a GMEC on a plant net in the presence of uncontrollable transitions”. The second result deals with the computation of monitor places for the suboptimal supervisory control in such Petri nets. In this paper, it is shown that there may be an optimal solution to the problem of computing a monitor-based controller to enforce a GMEC on a plant net in the presence of uncontrollable transitions. To do this, an example Petri net is considered and then two different optimal (maximally permissive) solutions are provided for this Petri net.