Modelling,simulation and scheduling of flexible assembly systems with coloured Petri nets

Scheduling in flexible assembly systems is a complicated phenomenon owing to the large variability in the operating parameters. In this paper, coloured Petri nets are shown to be useful in modelling, simulating and scheduling flexible assembly systems. Case studies from published literature are chosen to show its superiority over some of the heuristic and analytical tools. In addition, the tool is also shown to model and analyse systems with some realistic constraints.     

面向装配序列规划的装配建模方法研究

为实现自动化的装配序列规划,提出了一种应用面向对象和Petri网技术定义的产品装配模型.该模型由静态模型和动态模型构成.基于模型定义,提出了局部装配可行性的判定依据,并据此建立了一种基于静态模型的动态模型构造算法.该算法通过逐层分析可能状态的变迁,最终获得装配体所有几何可行的装配序列.通过实例验证了该建模方法的有效性.     

Deadlock prevention using Petri nets and their unfoldings

Unfoldings of Petri nets (PN) provide a method for the analysis of concurrent systems without restoring the state space of a system. This allows one to overcome the state explosion problem. Many properties of the initial PN (boundedness, safety, persistency and hazards) can be checked by constructing the unfolding. A deadlock prevention procedure first detects deadlocks using an unfolding. Then, the first method reduces the unfolding to a set of deadlock-free subunfoldings that cover all live behaviours. The second method uses a direct transformation at the level of the original PN. The methods are implemented as subroutines in the Berkeley program SIS. Although the deadlock detection problem is known to be NP-complete, experimental results show that for highly parallel specifications deadlock prevention by unfoldings is typically more efficient than deadlock prevention based on symbolic BDD (binary decision diagrams) traversal of the corresponding reachability graph.     

基于广义随机Petri网的装配可靠性建模

为了在装配环节保障产品的可靠性,提出可靠性驱动的装配技术的概念,并用广义随机Petri网对该技术进行了建模,通过同构的Markov链对模型进行定量分析。在可靠性驱动的装配技术建模分析中,采用结构分析和设计技术对产品进行功能分析并建立起完整的功能分析模型;将功能分析的结构分析和设计技术模型转化为相应的具有动态分析和定量研究能力的广义随机Petri网模型;为获得可信准确的分析模型,利用三角模糊数中的r截集对同构的Markov链进行优化;针对装配产品的可靠性特征,用装配可靠度指标对产品进行评价。以某加工中心的分度工作台为例,验证了该建模与分析方法的有效性。     

Load balanced scheduling and reliability modeling of grid transaction processing system using colored Petri nets

On-demand computing is a popular enterprise model in which the computing resources are made available to the users as needed. On-demand computing based transaction processing system which has grown rapidly in recent years is an information processing system with the stringent requirements of resources to meet the fluctuating demands. Concepts such as grid computing, utility computing, autonomic computing, and adaptive management seem very similar to the concept of on-demand computing. When demands of resources fluctuate, the system needs load balancing for the efficient utilization of the computational resources. Furthermore, scheduling is needed to assign the transactions to the appropriate resources. Thus, modeling of load balanced scheduling along with reliability analysis for this system is a challenging task.This paper presents the load balanced scheduling and reliability modeling in such an environment by using colored Petri nets (CPNs). CPNs which combine Petri nets with programming languages is a powerful modeling technique. The proposed CPN-based modeling pattern formally describes the process of transaction distribution and execution within the on-demand computing environment. Moreover, the CPN-based model uses the hierarchical modeling capability of CPNs, including different levels of abstraction (sub-modules). This helps easily handling and extending the model. Since, on-demand computing based transaction processing system executes a number of concurrent transactions. The CPN-based model is extended to express the concurrency, thus improving the reliability results. This paper takes the example of grid transaction processing (GTP) system with the problem of load balanced scheduling modeling and reliability evaluation.     

Discrete event control system design using automation Petri nets and their ladder diagram implementation

As automated manufacturing systems become more complex, the need for an effective design tool to produce both high-level discrete event control systems (DECS) and low-level implementations becomes more important. Petri nets represent the most effective method for both the design and implementation of DECSs. In this paper, automation Petri nets (APN) are introduced to provide a new method for the design and implementation of DECSs. The APN is particularly well suited to multiproduct systems and provides a more effective solution than Grafcet in this context. Since ordinary Petri nets do not deal with sensors and actuators of DECSs, the Petri net concepts are extended, by including actions and sensor readings as formal structures within the APN. Moreover, enabling and inhibitor arcs, which can enable or disable transitions through the use of leading-edge, falling-edge and level of markings, are also introduced. In this paper, the methodology is explained by considering a fundamental APN structure. The conversion of APNs into the IEC1131-3 ladder diagrams (LD) for implementation on a PLC is also explained by using the token passing logic (TPL) concept. Finally, an illustrative example of how APNs can be applied to a discrete manufacturing problem is described in detail.     

基于面向对象Petri网的多Agent系统交互协议建模

为了描述、分析和验证多Agent系统交互协议,提出了一种面向对象Petri网。与现有的形式化方法相比,面向对象Petri网能形象、直观地刻画多Agent系统复杂、并行的交互协议,可以描述其静态和动态语义,并可利用Petri网的数学分析方法对交互协议进行动态分析。面向对象Petri网还具有较好的模块性和柔性,能单独地分析参与交互的每一个Agent,而不会影响其他Agent和环境的状态。最后,利用面向对象Petri网描述了智能物理代理基金会请求协议和合同网协议,并对一个买卖Agent交互实例进行了建模和动态分析,验证了面向对象Petri网具有较好的建模能力。     

基于确定时间连续Petri网的航空发动机总装资源调度

为解决航空发动机总装过程中不同工艺对设备资源的竞争问题,进行了面向资源冲突的Petri网建模,将离散模型转化为确定时间连续Petri网模型.提出采用路由函数化解设备资源冲突,并对给定路由函数下系统的周期性进行了分析.为求解全局资源利用率最大的调度策略,采用粒子群优化算法优化资源路由参数,并将最优路由参数以启发函数的形式还原到离散Petri网模型调度中.案例研究表明,调度算法实现了资源的全局最优化,大大缩短了航空发动机维修周期.     

Material flow and control sequence specification of flexible production systems using coloured Petri nets

Starting with the specification of each resource and the whole structure of a flexible production system, in this approach a special kind of coloured Petri nets is used for performing the modelling and the validation of the coordination control structure of the systems. In a second phase, it is proposed to modify the first models to synchronised Petri net schemas to facilitate the supervision and the interaction of the coordination model with the physical components of the system as well as the development and maintainability of the discrete-event control structures. The final result is a formal specification of coloured Petri net based coordination control of resources of the system, and logic control structures for control sequencing based on the use of synchronised subPetri net structures derived from the first one by refining transitions, i.e., their occurrence. Based on the proposed approach, the coordination control model of resources and a first skeleton of the logic control structures of a flexible assembly cell located at the Institute of Manufacturing Automation and Production Systems at the University of Erlangen-Nuremberg, Germany is elaborated and then the correctness of the obtained models with regard to material flow and control sequence specifications is validated by means of the structural analysis of the coloured Petri net-based models.     

服务机器人手臂轨迹规划的研究

利用D-H法建立坐标系、Matlab软件求解正逆方程和插值法处理关节变量来研究服务机器人手臂5个关节空间的轨迹规划问题,实验表明,这种轨迹规划方法是准确和可行的。通过对该方法分析,指出其实现轨迹规划的一般步骤和优缺点,最后展望该轨迹规划法的研究发展方向。     

基于Petri网的Web服务描述及其可替换性分析

为解决服务组合中的Web服务可替换性问题,在工作流网、开放网等Petri网模型的基础上,提出一种适合面向服务系统的Petri网模型——WS_Net.该模型包含了Web服务的两部分内容:内部流程和外部接口.WS Net可以准确描述顺序、循环、并发、调用、选择五种常见的服务组合方式.在Petri网互模拟等价的基础上定义了WS_Net内网的关于接口变迁的互模拟等价关系,关于接口变迁互模拟等价的WS Net可以实现上下文无关的相互替换并保证替换前后组合服务的行为是一致的.在Petri网化简技术的基础上提出了WS_Net的化简规则,证明了这些化简规则能够保持WS_Net化简前后的接口互模拟等价特性.在实际应用中能够通过化简的方法降低Web服务可替换性分析的复杂度.     

基于广义随机Petri网的可重组制造单元建模与分析方法

为揭示可重组制造单元的重组过程并计算重组方式对系统性能的影响,根据可重组制造单元的特征,提出了基于广义随机Petri网的模块化建模及分析方法。该方法将制造单元的不同资源及重组生产过程对应于相应的广义随机Petri网基本模块,并通过过渡变迁合成广义随机Petri网模型。在此基础上,采用马尔可夫理论及相关数学方法,分析可重组制造系统的性能指标,并通过实际应用,验证了该建模与分析方法的有效性。     

Synthesis of feedback control elements for discrete event systems using Petri net models and theory of regions

This paper describes a method for constructing a Petri-net-based controller for a discrete event system (DES) modelled by a Petri net. Assuming that an uncontrolled Petri net model of the DES and a set of forbidden state specifications are given, feedback control elements, i.e. a set of places and related transitions, with initial marking, are computed using the theory of regions, which is a formal synthesis technique for deriving Petri nets from automaton-based models. When feedback control elements are added to the uncontrolled Petri net model, the controlled (closed-loop) Petri net model of the system is obtained. The controlled Petri net model obtained is maximally permissive while guaranteeing that forbidden states do not occur. The proposed method is computationally efficient and does not suffer from the state explosion problem. Two examples are provided to show the applicability of the proposed method.     

基于Petri网的位置敏感工作流建模与正确性验证方法。

针对移动业务过程中对位置信息建模和分析的需求,结合目前已有的工作流建模方法,提出一种针对位置敏感工作流进行建模和正确性验证的方法。该模型以工作流网为基础,结合6种描述位置之间关系的原语,以位置约束的形式定义业务流程中的位置信息及其与活动之间的关系,然后基于Petri网的状态可达图得出对所建立模型进行正确性检查的方法。基于ProM平台开发了一个位置敏感工作流的建模工具,验证了所提建模方法的可行性和算法的正确性。     

Dynamic path planning and trajectory tracking using MPC for satellite with collision avoidance

This paper proposes a dynamic path planning and trajectory tracking algorithm for an autonomous satellite, released from the space station, to get to the desired position for performing space tasks. The complex construction of the space station results in the presence of a geometric channel constraint for the obstacles avoidance. In addition, a three dimension B-spline template with minimizing the curvature of the path is designed, which could guarantee the continuity of the curvature to make the trajectory smooth and avoid the satellite from stopping at discontinuities waypoints. Then, the reference states and inputs are solved by a new projection method, which provides a foundation for the subsequent trajectory tracking. Subsequently, a finite horizon model predictive control method is constructed for the path tracking. The benefits of this approach are to take constraints into consideration, and to get optimal performance by minimizing the fuel consumption compared with other tracking controllers. The closed-loop stability is guaranteed by the feedback controller, terminal penalty, and a newly terminal constraint set. In simulation experiments, results illustrate the effectiveness and practicality of the algorithm.     

PCB assembly planning using genetic algorithms

Printed circuit boards (PCB) are used extensively in industry for the manufacture of electronic and electromechanical products. One of the primary concerns in the manufacture of PCBs is the determination of the optimal assembly plan. This paper presents work that leads to the development of an approach to PCB assembly planning using genetic algorithms (GAs). The approach takes into consideration component insertion priority and sequencing decision rules. A polygamy reproduction mechanism with dual mutation has been proposed and implemented. Details of the approach are described. A PCB model extracted from the literature was used for performance evaluation. Details of the evaluation are presented.     

柔性装配制造系统中生产优化的Petri网方法

本文在基本Ｐｅｔｒｉ网的基础上,给出了一种进行周期性生产的柔性装配制造系统的模型;同时,针对该系统,给出了一个求稳定需求下最佳生产指量的启发式调度方法。     

装配顺序分层、分步规划方法

提出产品装配顺序生产采用分层、分步的规划方法。分层规划是指分别在部件层、组件层和零件层上进行装配顺序推理。在每层的规划中分步进行,第1步采用成熟的割集法,第2步、第3步则在特定领域经验知识的支撑下,运用规划进行推理。     

A hybrid approach to supervisory control of discrete event systems coupling RW supervisors to Petri nets

In this paper a hybrid approach is proposed for supervisory control of discrete event systems (DES) subject to forbidden states. Assuming that an uncontrolled bounded Petri net (PN) model of a (plant) DES and a set of forbidden state specifications are given, the proposed approach computes a maximally permissive and nonblocking closed-loop hybrid model. The first step is to simplify the given PN model by means of PN reduction rules. The simplified model and the specifications are then represented as buffers, and supervisory control theory (SCT) is applied to obtain a Ramadge–Wonham (RW) supervisor in the form of an automaton. After reduction of the latter’s state size by a ‘control congruence’, the simplified RW supervisor is represented by a so-called auto-net and coupled to the given uncontrolled PN plant model by means of inhibitor arcs to represent the disabling actions. The plant model and supervisor auto-net run concurrently, synchronizing on shared events. This procedure provides a maximally permissive and nonblocking ‘hybrid’ (mixed PN/automaton) closed-loop controlled system. The method is straightforward logically, graphically, and technologically. Its applicability is shown by two examples, one of them a workcell from the PN control literature.     

Simultaneous optimisation of control factors in automated storage and retrieval systems and FMS using stochastic coloured Petri nets and the Taguchi method

Automated storage and retrieval systems (AS/RS) are playing an important role in modern manufacturing systems such as flexible manufacturing systems (FMS). The integration of AS/RS systems and FMS is very complex owing to the complexity of the individual elements. Stochastic coloured petri nets (SCPN) can be used to model, simulate and analyse such a system efficiently and realistically. In this paper, SCPN based simulation, aided by the Taguchi method of design of experiments, has been used for analysing the influence of important factors on makespan, unproductive travel time, and the mean flow time, and is supported by an example.