Revision on ‘Maximally permissive deadlock prevention via an Invariant Controlled Method’

All current design approaches of maximally permissive supervisors assume that forbidden and legal markings are linearly separable. Based on that implicit assumption, Liu et al. propose a novel method of optimal control of a problematic siphon to synthesise maximally permissive controllers. However, there are counter examples for which we could not produce optimal solutions. This occurs when forbidden and legal markings are not linearly separable even though it rarely happens. Verifying linear separability involves solving a large set of linear inequalities and is quite time consuming. This paper aims to relieve this problem via enhancing our critical-siphon approach. Specifically, we propose to: (1) find an efficient method to detect linear inseparability; (2) construct best suboptimal control models; (3) find all lost legal markings; and (4) identify the linear equations that violate linear separability. Each above contribution is unique and pioneering.     

New modelling formalism for control programs of flexible manufacturing systems

Proposed in this paper is a modelling formalism, called the FMS control model, which is able to provide a detailed guideline for the implementation of the FMS control program (PLC program). Although, there are numerous simulation models for FMSs describing the system dynamics at the level of discrete event simulation, they cannot be fully utilised for the generation of a real control program involving sensors and actuators. Due to this, PLC programs for FMSs are usually done by the repetitive method of code writing, testing, and debugging until the control objectives are achieved. This is an error-prone and time-consuming task. The proposed FMS control model is described at the level of sensors and actuators. It can support the PLC program generation from the model. Since the FMS control model has been developed by expanding the DEVS formalism, it has a well-defined formalism. The proposed methodology has been implemented and test runs were made.     

Production planning for flexible manufacturing systems with multiple machine types: A practical method

We study three important production planning problems for flexible manufacturing systems (FMSs) that consist of multiple types of machines. Namely we address machine grouping, workload allocation among machine groups, and batch sizing that maximize system throughput, when part types are selected for the upcoming production period. This differs from earlier related works in that earlier works give either only qualitative characteristics of the optimum solution or provide a solution method for FMSs consisting of only one type of flexible machine. In this paper, we provide both the optimum and heuristic methods to simultaneously solve the three problems for realistic FMSs. Computational results show that the heuristic method always finds the optimum solution at a fraction of computation time and that batch sizing can significantly affect throughput. Development of the heuristic method is necessary since the optimum method can be time-consuming for a moderate size of problems.     

Deadlock prevention technique using additional transitions for Petri nets

ABSTRACT

Siphon control has been utilized as a general methodology for systems of simple sequential process with resource (S³PR) deadlock systems. Although siphon control imposes additional control places on S³PR systems, it cannot guarantee that the maximum permissiveness is achieved. To solve the problem of deadlock prevention for S³PR, a new policy to design a deadlock-free supervisor with the maximally permissive system is proposed using additional transitions. Additionally, this approach can solve the problem of deadlock prevention for systems of sequential systems with shared resources (S4PR) system models.     

基于大涡模拟的大气边界层湍流强度对低矮房屋风荷载特性影响研究

对德州理工大学(Texas tech university,TTU)低矮房屋标准模型,以已有现场实测以及缩尺模型风洞实验数据为验证对比,基于大涡模拟(Large-eddy simulation,LES)方法研究了大气边界层湍流强度对低矮房屋风荷载特征的影响机理。采用CDRFG (Consistent discretizing random flow generation) 人工合成湍流方法生成大气边界层湍流,研究了来流湍流度对低矮建筑表面的平均、脉动以及极小值风压分布以及风压非高斯特性的影响,并利用LES能提供非常场流动全流域信息的优势,结合瞬态湍流场结构对大气边界层湍流对低矮房屋风荷载特征的影响机理进行了阐释。结果表明:LES数值模拟得到的平均、脉动及极小值风压系数与实验以及实测结果一致,平均风压结果包络在实测误差范围以内,极小值风压系数最大误差小于10%,脉动风压系数最大误差小于20%且误差区域较小。在来流湍流度增大的过程中,低矮房屋屋面平均风压系数变化较小,脉动风压系数呈显著的线性增加;极小值风压系数变化规律相对复杂,呈现出非线性减小的趋势,风压系数极小值可达?5.0;屋面涡脱强度逐渐被抑制,锥形涡迹线与屋面迎风前缘的夹角由14.4°下降至8.7°。屋面风压非高斯特性主要与屋面形成的涡旋结构相关,表现出典型的右偏软化非高斯过程,且随着来流湍流度的增加风压非高斯特性逐渐减弱。从流场的角度来看,湍流度的增加抑制屋面迎风前缘柱状涡以及锥形涡的形成,加快流动分离的再附,减少分离泡尺度,同时提高了屋盖周围的湍流高频能量成分,从而使脉动风压增加,极小值风压减小以及风压非高斯特性减弱。该研究阐明了大气边界层湍流对低矮房屋风荷载特性的影响机理,有助于进一步理解低矮房屋风致破坏机理,并且为低矮房屋的抗风设计及抗风性能优化提供重要参考。     

湍流边界层中三维山丘地形风场大涡模拟

利用谐波合成法生成粗网格的脉动风速时程,通过双线性插值得到入口节点时程数据,考虑时程互相关性对时程进行修正得到大涡模拟湍流入口。采用谱元法对两种不同坡度的三维山丘地形进行大涡数值模拟,将结果与风洞试验及有限体积法数值模拟进行对比。结果表明:大涡模拟能较准确地预测山丘地形的风场及湍流特性;与有限体积法相比,谱元法的计算效率更高,在复杂山地地形的风场预测上有较好的应用前景。     

Optimal design for flexible manufacturing systems: generalized analytical methods

Advanced manufacturing systems such as Flexible Manufacturing Systems (FMSs) are capital-intensive. Designing functional yet cost-effective FMSs is a challenging task because it involves the solution of a complex series of interrelated problems. In this paper, we study a design problem for FMSs that consist of multiple types of machines. Using closed queueing network models for FMSs, this problem seeks the minimum cost design subject to meeting throughput requirements. The design decisions include the number of machine groups, the number of machines for each group, the workload allocation among machine groups, the number of pallets, the number of transporters, and the batch size. Since these design decisions are highly interdependent, we present the optimum and heuristic methods that simultaneously determine them. To our knowledge, these methods are the most general analytical methods for FMS design. Development of the heuristic methods is crucial because of the time-consuming nature of the optimum method as the number of machine types increases. Computational results show that the heuristic methods are both effective and efficient.     

Avoidance of deadlocks in flexible manufacturing systems using a capacity-designated directed graph

Deadlocks in flexible manufacturing systems (FMSs) refer to the stop state of job flow that may often occur while operating the systems. This state should be avoided to provide the efficient way of job flow and to achieve full utilization of high-capital resources. In this paper, we developed a capacity-designated directed graph (CDG) representing machine resources and job routeings in FMSs. Using the CDG, a deadlock-avoidance method is proposed to prevent the system from satisfying the necessary condition for deadlocks. This deadlock-avoidance method is implemented into static and dynamic strategies. A simulation study demonstrates the applicability of the two strategies to an FMS.     

基于POD的大涡模拟入流脉动合成方法研究

针对并行计算特点,发展适用于流体并行计算的大涡模拟(Large Eddy Simulation,LES)入流脉动直接合成方法。基于特征正交分解(Proper Orthogonal Decomposition, POD)型谱表示法合成入流面主要网格点脉动风速时程,采用有限元形函数空间插值获得入流面所有网格点风速时程,采用UDF(User Defined Functions)编程实现Fluent软件平台流体并行计算时合成的脉动风速时程读入及赋值。进行B类1:500缩尺比风场内宽高比1：6的单体方形截面高层建筑非定常绕流LES计算,将数值模拟所得风剖面、风速谱及结构风压系数统计值、自谱、相干性等,与刚性模型测压风洞试验及文献数值模拟结果比较。研究表明,该合成方法可较好模拟紊流风场,预测结构风荷载具有一定精度。     

基于谱元法的复杂地形风场大涡模拟

以Askervein山为研究对象,基于开源平台Nek5000,自编程序完成复杂地形下谱元法的网格建模,添加计算湍流粘性项子程序,对复杂地形风场进行大涡模拟,并与场地实测数据及其它数值结果进行对比。结果表明,谱元法的大涡模拟结果与Askervein山的场地实测结果符合较好,表明该方法在复杂地形风场的预测上有较高的精度,可用于复杂地形的风能资源评估。     

Multiperiod tool and production assignment in flexible manufacturing systems

Flexible manufacturing systems (FMSs) are usually composed of general purpose machines with automatic tool changing and integrated material handling. FMSs offer the advantages of high utilization levels and simultaneous production of a variety of part types with minimal changeover time. Although the trend in FMS operation appears to be towards unmanned production, there is much diversity in the policies under which FMSs can be operated. These policies are governed to some extent by the available technological resources. In this paper several policies are described. Tool loading and production assignment models for setting production plans over a short term planning horizon when using these policies are then formulated. Heuristics for the solution of these models are detailed and computational results are presented.     

Flexible manufacturing systems: a structural approach

In the last five years, developed countries have seen more rapid advancement in the application of high technology to manufacturing than in the two preceding decades. Many of these applications have occurred under the banner of a new type of manufacturing called flexible manufacturing systems (FMSs). With the implementation of FMSs, a new awareness of the opportunities for manufacturing-cost reduction is emerging. In this paper, the FMS taxonomy and structure is presented. To exploit this structure, FMS process and problem views are presented, and various aspects of flexibility are discussed. To show the complexity of an FMS, its design and operational problems are defined. Basic functions of robots and vision systems are also presented. Finally, FMS modularity issues and future trends in FMS development are outlined.     

基于社会交换理论的组织公民行为影响因素研究

为探讨社会交换理论对组织公民行为的影响因素,对来自12个高科技组织的294名员工和52名主管进行了问卷调查,研究结果表明：员工的组织支持感知与组织公民行为之间具有正向关系;领导-下属交换与组织公民行为之间具有正向关系;员工的社会交换意识是组织支持感知与组织公民行为之间以及领导-下属交换与组织公民行为之间的调节变量.     

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.     

Reaching most states via refining controller regions for supervisors of two well-known S3PRs

Deadlock control of flexible manufacturing systems (FMS) using Petri nets has been a very active research topic for the past two decades. Siphon-based deadlock control of FMS suffers from reaching fewer states than those of optimal control. This paper proposes three sets of siphon-based liveness-enforcing supervisors (LES) for a well-known FMS to obtain near optimal solutions. The first set includes 11 control places (monitors) providing 21,363 good states. The second set consists of 14 monitors providing 21,562 good states, the same as that using the method of the first-met bad marking (FBM) (based on an iterative method using reachability graph analysis) but with fewer monitors and control arcs. This result is achieved by refining some monitors into several monitors with smaller controller regions, causing fewer disturbances to the original uncontrolled model. The third set contains 15 monitors providing 21,573 good states – the best suboptimal solution so far in the literature. This result is obtained by further shrinking the controller region of the second set. Formal proof of the correctness is provided. Although reaching eight states fewer than that by the set-covering approach, it does not employ weighted control arcs and runs more efficiently. It also computes all lost states based on invariant without reachability analysis. Application to another well-known example is also illustrated.     

大气边界层大涡模拟入口湍流生成方法综述

随着计算资源的飞速发展以及数值模拟技术的不断进步,大涡模拟被越来越多地应用于结构风工程领域的研究。运用大涡模拟准确模拟结构风效应的关键问题之一是生成满足大气边界层风场特性的入口湍流条件。预前模拟法和人工合成法是目前主流的两类大涡模拟入口湍流生成方法。该文阐述了不同入口湍流生成方法的基本原理,并梳理其在结构风工程领域的发展。从结构风工程研究的角度出发,对比分析不同方法的特点及适用性。最后,针对当前大气边界层大涡模拟入口湍流生成方法存在的问题,提出了未来研究的展望。     

Recognition of Polynomial Plane Curves Under Affine Transformations

 We present a new method to decide if two algebraic plane curves are (or are not) affine equivalent. The method is based on describing the curves by means of local parametrizations around related points. To that end we introduce a new type of parametrizations, called Ancochea parametrizations, which are canonical under affine transformations. Received: February 1, 2001; revised version: February 8, 2002     

Effect of convection heat transfer on the design of vapor-cooled current leads

A general optimization method for vapor-cooled current leads is presented with taking into account the effect of convection heat transfer and extended surfaces. This analytical work is considered as a unified design method, since one formulation calculates the minimum heat load and the corresponding optimal design condition for arbitrary heat transfer condition, spanning two limiting cases—the zero convection (or conduction-cooled leads) and the perfect heat transfer. It is clearly shown that the augmentation of the convective cooling can reduce the heat load to a certain extent, but the optimum lead parameter required to minimize the heat load for the finite heat transfer may not exist between the two limiting values. A new dimensionless parameter called the Ch number is introduced to conveniently incorporate the convection effect into the optimization. The present method is demonstrated for two specific lead designs that have been recently developed for 10 kA level of applications.     

Maximally permissive deadlock prevention via an invariant controlled method

Recently, researchers have proposed a novel and computationally efficient method to design optimal control places and an iteration approach that computes the reachability graph once to obtain a maximally permissive, if any, liveness-enforcing supervisor of flexible manufacturing systems (FMS). The approach solves the set of integer linear inequalities to compute control places. If, given a Petri net model, no solution exists, the optimal control place does not exist for the Petri net model. We discover that a solution always exists for systems of simple sequential processes with resources (S³PR), but not for the case of FMS modelled by generalised Petri nets (GPN). We propose a theory to prove that there are no good states that will be forbidden by the control policy for S³PR, in which live and dead states cannot have the same weighted sum of tokens in the complimentary set of a siphon. For a system of simple sequential processes with general resource requirements (S³PGR²) modelled by GPN, we find the reason why the integer linear programming (ILP) may not have solutions, which is consistent with the fact that optimal supervisor synthesis for GPN remains unknown. We show that live and dead states may have the same weighted sum of tokens in the complimentary set of a siphon in a GPN. These theoretical results are verified by case studies.