Reliability Assessment of Distribution SystemBased on Discrete-event System

Discrete-event system simulation technology is used to analyze distribution system reliability in this paper. A simulation model, including entity state models, system state models, state transition models, reliability criterion model, is established. ‘Next happen event’ is taken as impulse principle of simulator clock to determine the sequence of random event occurrence dynamically. The results show this method is feasible.

Queue reduction in discrete-event systems by relabeling

The customer population of entities potentially requesting to join a queue for service often have identical structure, i.e., the same state set and isomorphic transitions. The state size of the automaton modeling a queue will grow rapidly with increase of the size of this population. However, by relabeling the queue arrival events and service events to the same symbols respectively, the automaton model of a queue will be converted to the structure of a buffer, which is proved to be independent of the total size of the customer population, as long as the queue size is held fixed. We propose the model of a dynamic buffer to embody order and shift of a queue. The result is applied to a manufacturing facility with a dynamic buffer to manage the repair of broken down machines.

Infinite-and K-Step Opacity Verification of Discrete-Event Systems Under Nondeterministic Observations

In some practical applications modeled by discrete-event systems(DES),the observations of events may be no longer deterministic due to sensor faults/failures,packet loss,and/or measurement uncertainties.In this context,it is interesting to reconsider the infinite-step opacity(∞-SO)and K-step opacity(K-SO)of a DES under abnormal conditions as mentioned.In this paper,the authors extend the notions of∞-SO and K-SO defined in the standard setting to the framework of nondeterministic observations(i.e.,the event-observation mechanism is state-dependent and nondeterministic).Obviously,the extended notions of∞-SO and K-SO are more general than the previous standard ones.To effectively verify them,a matrix-based current state estimator in the context of this advanced framework is constructed using the Boolean semi-tensor product(BSTP)technique.Accordingly,the necessary and sufficient conditions for verifying these two extended versions of opacity are provided as well as their complexity analysis.Finally,several examples are given to illustrate the obtained theoretical results.

Supervisory control of concurrent discrete-event systems

Concurrency is a common feature in most industrial systems,where several components can execute different actions simultaneously.In this paper,we first introduce a new feasible nonblocking concurrent supervisory control map for a concurrent system,and subsequently,new concepts of concurrent controllability and concurrent observability.Then we present a sufficient and necessary condition for the existence of a feasible nonblocking concurrent supervisory controlmap to achieve a given concurrent language.After introducing a new concept of prefix-closed concurrent normality,we show that the supremal concurrently controllable and prefix-closed concurrently normal sublanguages exist and are computable.Our emphasis here is not to address the computational efficiency of solving concurrent supervisory control problems,which is still a technical challenge,but to show that the commonly used Ramadge–Wonham asynchronous control is simply a special case of our proposed concurrent control framework by providing an insightful treatment on compound events.

THE EIGEN-PROBLEM AND PERIOD ANALYSIS OF THE DISCRETE-EVENT SYSTEM

In this paper we provide a sufficient and necessary condition for the eigenvalueand eigenvector of a general reducible matrix in a discrete-event system described by the“max”algebra,analyse the steady periodical performance of the system,and obtain ananalytic solution of the dynamic equation.We propose the conception of“order-d-(?)-block-periodical matrix”and obtain its sufficient and necessary condition and provide an algorithmof (?) matrix.

A discrete-event model to simulate the effect of truck bunching due to payload variance on cycle time, hauled mine materials and fuel consumption

Data collected from truck payload management systems at various surface mines shows that the payload variance is significant and must be considered in analysing the mine productivity,energy consumption,greenhouse gas emissions and associated cost.Payload variance causes significant differences in gross vehicle weights.Heavily loaded trucks travel slower up ramps than lightly loaded trucks.Faster trucks are slowed by the presence of slower trucks,resulting in‘bunching’,production losses and increasing fuel consumptions.This paper simulates the truck bunching phenomena in large surface mines to improve truck and shovel systems’efficiency and minimise fuel consumption.The study concentrated on completing a practical simulation model based on a discrete event method which is most commonly used in this field of research in other industries.The simulation model has been validated by a dataset collected from a large surface mine in Arizona state,USA.The results have shown that there is a good agreement between the actual and estimated values of investigated parameters.

A Survey of the Use of the Discrete-event Simulation in Manufacturing Industry

In this work, a survey of companies has been produc ed from a questionnaire and in-depth inter-views. The aimof the paper is to give a current view of the use of DES (Discrete- Event Simulation) in the indust ry. The companies have been selected in a wide range in e.g. size, capital turno ver and employees. The survey was restricted to Sweden, although the results may be applicable to other countries with similar industrial structure. Some of the companies investigated are also multinational with branches around the world. T o illustrate the differences in production systems different sectors have be en investigated, e.g. the manufacturing industry and energy sector. The manufact uring systems generally differ in that the energy sector is more known for high reliability in their systems and the manufacturing sector tends to work more wit h lean production. Previous surveys of manufacturing efficiency illustrate different aspects. Accor ding to one survey, more than one-third (34%) of the disturbances in two countr ies (the U.S. and Finland) were mainly caused by design-based errors and flaws. The survey also showed the scope of safety problems experienced in the implemen tation and use of flexible manufacturing systems (Jrvinen et al., 1996). Another survey in Germany reports a gap between users and non-users of DES. Man y companies that are not familiar with simulation are not aware of its benefits. The study suggested integrating simulation into the planning processes as a reg ular tool. So far, DES is mostly used in system planning, resource planning and scheduling and production planning (Hirschberg & Heitmann, 1997). The disposition of the accomplished questionnaire had been multiple-choice ques tions in a strict order. The respondents gave their views on what the interviewe r considered relevant for the survey. The questionnaire was mailed to 220 person s in May 2001; a second round for unanswered surveys was dispatched in August 20 01. Replies came from 114 respondents; thus, the reply rate to all questionnaire s sent out was 52%. The replies came in from 80 different companies. Questionswere raised regarding what kinds of decisions are taken from a simulation projec t and questions were also asked regarding the tool itself. Some common problems encountered based on the authors’ knowledge were asked in the questionnaire. The questions were sent to those who had reasonable knowledge of how to work with D ES. Supplementary in-depth interviews were also accomplished. On the question if DES facilitates the decision-making process 79% of totally 3 8 answered to a very large or a large extent. Regarding the question if DES is a useful tool for measuring the availability of equipment for example system verification 73% of 33 responded to a very large or a large extent. A majority o f the respondents genuinely see an advantage with the use of simulation, which i s an important result from the study. The cost reduction aspect of the productio n is also an advantage of DES. On the question eight answered to a very large ex tent and 23 to a large extent of total 39 replies. The question demonstrates tha t the results of a simulation model are indeed used in the real world. Finally, in the question of DES is used for staffing it is indicated that it is not so co mmonly used as for other functions asked for. The staffing issue requires good k nowledge about time measurement of manual labor. The option, however, to include manual work in a simulation model is in many programs extensive and must be a v aluable option to evaluate work intensive parts of production lines and their im provement. To conclude, it could be mentioned that DES is a tool with potential. The use is increasing and 12 of 80 companies investigated in Sweden 2001 are using it, an increase compared to previous stud-ies. Of those 12 companies, four are using t he tool to a large extent. The main advantage according to the survey beside the visualization part is that the knowledge about a system is investigated and doc umented.

Discrete-event modeling for internet multi-robotics

Internet multi-robotics is a typical discrete-event system. In order to describe joint activities between multiple operators and multiple robots, a 4-level discrete-event model is proposed in this paper based on the controlled condition/event Petri nets (CCEP). On the first or mission level, the task splitting of the system is defined; on the second or multi-operator level, a precedence graph is introduced for every operator to plan his or her robotic actions; on the third or coordination level, the above precedence graphs are translated and integrated into the corresponding CCEPs in terms of specific rules; and on the last or multi-robot level, operators can select their control range by setting the corresponding control marks of the obtained CCEPs. As a consequence, a clear mechanism of operator-robot collaboration is obtained to conduct the development of the system.

A Parallel Discrete Event Simulation Engine for the Low-Earth-Orbit Satellite Constellation Networks

Low-Earth-Orbit satellite constellation networks(LEO-SCN)can provide low-cost,largescale,flexible coverage wireless communication services.High dynamics and large topological sizes characterize LEO-SCN.Protocol development and application testing of LEO-SCN are challenging to carry out in a natural environment.Simulation platforms are a more effective means of technology demonstration.Currently available simulators have a single function and limited simulation scale.There needs to be a simulator for full-featured simulation.In this paper,we apply the parallel discrete-event simulation technique to the simulation of LEO-SCN to support large-scale complex system simulation at the packet level.To solve the problem that single-process programs cannot cope with complex simulations containing numerous entities,we propose a parallel mechanism and algorithms LP-NM and LP-YAWNS for synchronization.In the experiment,we use ns-3 to verify the acceleration ratio and efficiency of the above algorithms.The results show that our proposed mechanism can provide parallel simulation engine support for the LEO-SCN.

Timed Petri Net Models of Shared-Memory Bus-Based Multiprocessors

In shared-memory bus-based multiprocessors, when the number of processors grows, the processors spend an increasing amount of time waiting for access to the bus (and shared memory). This contention reduces the performance of processors and imposes a limitation of the number of processors that can be used efficiently in bus-based systems. Since the multi-processor’s performance depends upon many parameters which affect the performance in different ways, timed Petri nets are used to model shared-memory bus-based multiprocessors at the instruction execution level, and the developed models are used to study how the performance of processors changes with the number of processors in the system. The results illustrate very well the restriction on the number of processors imposed by the shared bus. All performance characteristics presented in this paper are obtained by discrete-event simulation of Petri net models.

Modeling and Performance Analysis of Weighted Priority Queueing for Packet-Switched Networks

Weighted priority queueing is a modification of priority queueing that eliminates the possibility of blocking lower priority traffic. The weights assigned to priority classes determine the fractions of the bandwith that are guaranteed for individual traffic classes, similarly as in weighted fair queueing. The paper describes a timed Petri net model of weighted priority queueing and uses discrete-event simulation of this model to obtain performance characteristics of simple queueing systems. The model is also used to analyze the effects of finite queue capacity on the performance of queueing systems.

Discrete-event stochastic systems with correlated inputs: Modeling and performance evaluation

In the majority of the previous works on discrete-event stochastic systems,they have been assumed to have independent input processes.However,in many applications,these input processes can be highly correlated.Furthermore,the performance measures of the systems with correlated inputs can be significantly different from those with independent inputs.In this paper,we provide an overview on some commonly used methods for modeling correlated input processes,and we discuss the difficulties and possible future research topics in the study of discrete-event stochastic systems with correlated inputs.

Object-oriented World-view for Discrete-Event Simulation Modelling

Object-oriented world-view provides a natural and powerful paradignl for expressing system elements and their dynamic behaviour for discrete-event simulation. Simulation gives the critical ability to study the dynamic behaviour of the models that are defined with object-voriented means. This paper first examines the ways of applying the object-oriented world-view to three major discrete simulation modelling approaches, then considers the process and merits of object-oriented simulation modelling.

On the design and selection of vehicle coordination policies for underground mine production ramps

Traffic management in underground mines,especially on production ramps,is a difficult problem to optimize and control.Most operations use one of a few common policies;e.g.,the so-called ‘‘lock-out" and‘‘loaded-vehicle-priority" policies.The work presented in this paper uses discrete-event simulation to study the efficiency of multiple policies.Based on simulation results,an improvement to the common lock-out policy is proposed.This new policy utilizes the rules of the lock-out policy but integrates an option that allows a group of vehicles to be given temporary priority in a certain direction of travel.Quantitative results are provided and,based on these,a systematic technique for ramp design that aims to optimize the efficiency of underground mine ramp traffic flow is described.

A STUDY OF THE CONCEPT OF DYNAMIC EQUIPMENT IN THE EARTHMOVING PROCESS BY A CONSIDERATION OF TIME,COST AND EMISSIONS

The earthmoving process is usually handled by a combination of multi-functional excavators and delivery trucks.According to the principles of excavation,a specific machine is selected to dig the earth.Sometimes,due to the difficulties in the process,the digging process is split between different machines with different capabilities.This paper aims to introduce the concept of dynamic equipment for allocating different tasks in an operation to a machine.This concept is discussed and modelled via a discrete-event simulation method and is tested in a real earthmoving operation from three different perspectives:time,cost and emission.The results from the case study show that the use of dynamic equipment will lead to a decrease in cost and emissions and an increase in productivity.

Warehouse automation by logistic robotic networks:a cyber-physical control approach

In this paper we provide a tutorial on the background of warehouse automation using robotic networks and survey relevant work in the literature.We present a new cyber-physical control method that achieves safe,deadlock-free,efficient,and adaptive behavior of multiple robots serving the goods-to-man logistic operations.A central piece of this method is the incremental supervisory control design algorithm,which is computationally scalable with respect to the number of robots.Finally,we provide a case study on 30 robots with changing conditions to demonstrate the effectiveness of the proposed method.

PERFORMANCE EVALUATION OF DISCRETE EVENT SYSTEMS INVOLVING HENSTOCK-KURZWEIL INTEGRAL

This paper presents a study on the performance of flexible manufacturing systems （FMSs）, by using discrete event system （DES） models, considering resource losses modelled by a parameter entitled coverage factor. We conclude that the resources cell loss distribution between the tasks of a FSM is a real function that cannot be integrated, in order to calculate its primitive, in the classical sense of Riemann or Lebesgue, but only in the sense of Henstock-Kurzweil integral. Our result allows one to study more general processes where highly oscillatory functions occur. The method used to deduce the functi6n describing the resources cell loss distribution is compared with a classical method related in the literature, respectively rational interpolants. An example has been constructed to emphasize what we believe to be, new approaches.

ESTIMATING CROSS-TRAINING CALL CENTER CAPACITY THROUGH SIMULATION

Call centers have grown world-wide during the past decade. One of the most important aspects considered by call center managers is the optimization of its operators, which implies covering the highly variable demand and f＇mding an efficient way to assign people to certain shifts in order to achieve a desirable service level and abandonment rate. Another challenge is determining which system setup is appropriate for the specific call center. Should we have a single-skill call center or multi-skill call center？ If we do have the latter, how many multi-skill agents should we have on staff？. In this case study, we generate and analyze discrete-event systems simulation-optimization models to test the behavior of a real-world call center under the actual configuration and under different levels of cross-training. The model results help call center managers by： 1） determining the optimal number of operators needed for different staff configtmations in order to achieve the targets for service level and abandonment; 2） providing information about the trade-off between the key measurements in the call center; and 3） providing useful information about the number of operators needed and used for each hour of operation to estimate the number of four-hour shifts required to achieve the performance targets. Our experimental f＇mdings from this case study suggest that a bi-skill call center is economically better in the long-rtm compared to a full-skill or single-skill call center. This case study augments the call center body of knowledge by providing additional managerial insights for the operations management community.

A learning-based synthesis approach to decentralized supervisory control of discrete event systems with unknown plants

In this paper, we consider the problem of automatic synthesis of decentralized supervisor for uncertain discrete event systems. In particular, we study the case when the uncontrolled plant is unknown a priori. To deal with the unknown plants, we first characterize the conormality of prefix-closed regular languages and propose formulas for computing the supremal conormal sublanguages; then sufficient conditions for the existence of decentralized supervisors are given in terms of language controllability and conormality and a learning-based algorithm to synthesize the supervisor automatically is proposed. Moreover, the paper also studies the on-line decentralized supervisory control of concurrent discrete event systems that are composed of multiple interacting unknown modules. We use the concept of modular controllability to characterize the necessary and sufficient conditions for the existence of the local supervisors, which consist of a set of local supervisor modules, one for each plant module and which determines its control actions based on the locally observed behaviors, and an on-line learning-based local synthesis algorithm is also presented. The correctness and convergence of the proposed algorithms are proved, and their implementation are illustrated through examples.

DEVS and MBSE:A review

We review Discrete-Event system Specification(DEVS)in the context of Model-based Systems Engineering(MBSE)and discuss an application of DEVS methodology to MBSE.We outline support for an envisioned MBSE development cycle of DEVS top-to-bottom MBSE capability and offer an example of mapping UML activity diagrams into executable activity-based DEVS models.We close with conclusions and future research directions.