Verification of distributed control systems in intelligent manufacturing

This paper presents an application of formal methods for validation of flexible manufacturing systems controlled by distributed controllers. A software tool verification environment for distributed applications (VEDA) is developed for modeling and verification of distributed control systems. The tool provides an integrated environment for formal, model-based verification of the execution control of function blocks following the new international standard IEC61499. The modeling is performed in a closed-loop way using manually developed models of plants and automatically generated models of controllers.     

The design of distributed control computer systems

The top-down approach in the design of Distributed Control Computer Systems, DCCS, is presented. The starting point of the study is three statements: (i) the design approach should be production process control oriented; (ii) for real time process control applications, computer networks require an efficient communication subsystem, the most important features of which are high reliability and short message transmission times and (iii) recent research has shown that the tools and methodologies used in both computing and control should be complementary to each other in order to solve some of the distributed control computer system/distributed computer control system design problems. The paper contains the design problems of distributed control computer systems, the presentation of the methods and tools for the study and design of the DCCS, the top-down approach in the design of the DCCS, and examples of two design stages.     

A software architecture for distributed computer control systems

Distributed computer control systems have a number of potential advantages over centralized systems, especially where the application is itself physically distributed. A computer station can be placed close to the plant being controlled, and a communications network used to enable the stations to communicate to coordinate their actions. However, the software must be carefully designed to exploit the potential advantages of distribution. This paper describes the software architecture of CONIC, a system to support distributed computer control applications. This architecture emphasizes the distinction between the writing of individual software components and the construction and configuration of a system from a set of components. A modular structure is used to separate programming from configuration. Typed entry and exit ports are used to clearly define module interfaces. Ports, analagous to the plugs and sockets of hardware components, permit modules to be interconnected in different ways. On-line modification and extension of the system is supported by permitting the dynamic creation and interconnection of modules. Message passing primitives are provided to permit modules to coordinate and synchronize control actions.     

基于数据流和控制流的分布式星载计算机

空间太阳望远镜是中国科学院国家天文台正在研制的我国第1颗太阳同步轨道天文卫星。以空间太阳望远镜高速科学数据处理系统为例,介绍了星载计算机系统分布式构型和基于数据流、控制流分开的设计思想和技术实现,分析了星载计算机系统的网络拓扑、通信并行度、响应时间和容错技术的应用。最后给出了该系统原理样机的调试结果和进一步工作的探讨。     

Braess-like paradoxes in distributed computer systems

We consider optimal distributed decisions in distributed computer systems. We identify a Braess like paradox in which adding capacity to the system may degrade the performance of all users. Unlike the original Braess paradox, we show that this behavior occurs only in the case of finitely many users and not in the case of infinite number of users     

A computer network based control architecture for autonomous distributed multirobot systems

This paper presents the specification and implementation procedure using a microcomputer network based autonomous distributed control architecture for industrial multirobot systems. The procedure is based on the concept of data flow network controlled by communicating sequential processes to perform coordinated tasks. Robots and other computerized industrial devices such as conveyors and manufacturing machines are defined as object-oriented Petri nets. A modular and hierarchical approach is adopted to define a set of Petri net type diagrams which represent concurrent activities of control processes for such devices. Asynchronous and synchronous interactions are modelled by places and transitions, respectively, in global process interaction nets. The control software is implemented on a computer network using Inmos transputers with true parallel processing and message passing primitives efficiently handled in hardware. Petri net based models are directly and efficiently transformed to corresponding codes in occam, the high level parallel programming language defined for the transputer.     

On security measures in distributed computer systems

Problems of data security are becoming increasingly acute. This study of these basic problems has been carried out in cooperation between scientists at the Department of Electrical Engineering at Linköping Institute of Technology and FOA 3 (Försvarets Forskningsanstalt Planeringsbyrån — Research Institute of National Defense, Operations Research Center).The situation considered is an office with individual work stations having desktop computers. These communicate among themselves and share a host computer of higher capacity including special purpose input/output equipment.Among questions discussed are:

• * problems related to the protection of data bases

• * ease of selectively designating addressees and authorisation to access data and messages; key distribution and protection

• * general requirements for equipment needed in such a local net environment.

No attempt has been made to predict when such distributed office computer systems will become commonly available in full scale. It does, however, seem clear that they are at least technically feasible already.     

Intelligent distributed and supervised flow control methodology for production systems

This paper deals with the development of an intelligent distributed and supervised control approach for high-volume production systems, in which the flow of parts can be approximated by a continuous (fluid) model. The proposed approach is based on the decomposition of the production system into elementary modules in order to reduce the control design computational complexity. In this context, a two levels control structure is proposed. At the local level, a surplus-based principle is adopted to regulate the production flow for each module according to the distributed structure. The proposed control methodology decides how to adjust the production rate in order to avoid system overloading and eliminate machine starvation or blocking. In this context, the local control law is synthesized by using the Takagi–Sugeno fuzzy systems. At the high level, a supervisory controller is designed to improve the overall system performances. A supervisor provides an additive component for each local controller when the overall system performances deviate from their acceptable domains (degraded mode). This is done by combining both local and global information into a unified formalism by using aggregation operators and according to fuzzy interval representation of the desired objectives. Finally, the feasibility of the proposed methodology is validated with simulation examples.     

Real-time task reallocation in fault-tolerant distributed computer systems

We address the issue of real-time task reallocation due to the failure of one or more processors in a software implemented fault-tolerant system. The incremental and total task reallocation problems are discussed, and a heuristic procedure for both problems is proposed. Computational experiments with the heuristic on the total task reallocation problem produce near perfect system reconfigurations. Further experiments with a slightly modified version of the heuristic show that excellent quality system reconfigurations are obtainable with minimal computational effort.     

Intelligent distributed control systems

ContextThe paper² deals with distributed reconfigurable embedded control systems following the component-based International Industrial Standard IEC61499 in which a Function Block (abbreviated by FB) is an event-triggered software component owning data and a control application is a distributed network of Function Blocks. Nowadays, limited related works have been proposed to address particular cases of reconfigurations without considering distributed architectures. Our first problem is to be able to handle all possible forms of reconfigurations that can be applied at run-time to distributed Function Blocks. In this case, a coordination between devices of the execution environment should be applied to guarantee safe and coherent distributed reconfigurations. A second problem is to find the sufficient solutions for the correct implementation of this reconfigurable distributed architecture.ObjectiveThe paper defines an implementable multi-agent architecture for automatic and coherent reconfigurations of distributed Function Blocks.MethodTo address all possible industrial forms, we classify the reconfiguration scenarios into three levels. The first level deals with additions–removals of Function Blocks to-from the system’s implementation. The second deals with updates of compositions of blocks, and the third deals with updates of data. We define a Reconfiguration Agent for each device of the execution environment, and a unique Coordination Agent for coordinations between devices. Each Reconfiguration Agent to be modelled by nested state machines applies local reconfiguration scenarios in the corresponding device after coordinations with the Coordination Agent. We propose an Inter-Agents Communication Protocol to support correct and coherent reconfigurations of distributed devices. This protocol is based on Coordination Matrices to be handled by the Coordination Agent in order to define all reconfiguration scenarios that should be simultaneously applied in distributed devices. We propose XML-based implementations for both kinds of agents where XML code blocks are exchanged between devices to guarantee safety distributed reconfigurations. The contributions of the paper are applied to two Benchmark Production Systems available in our research laboratory.ResultsThe communication protocol is successfully applied to our platforms where simulations are executed to check distributed and coherent reconfiguration scenarios. The Reconfiguration and Coordination Agents are implemented in this platform by following the International Standard IEC61499. We show in addition XML-based successful interactions between devices when distributed reconfigurations are applied.ConclusionThe paper successfully defines a multi-agent architecture for IEC61499 distributed reconfigurable embedded systems where Coordination and Reconfiguration agents are proposed to allow feasible and coherent distributed reconfigurations by using a defined communication protocol. This architecture is implemented in XML and applied to real industrial platforms.     

Supervisory control of concurrent discrete-event systems

When a discrele-event system P consists of several subsystems P₁,...,P_n which operate concurrently, a natural approach to the supervisory control problem is to synthesize a ‘local’ controller S_i , for each subsystem P_i , and operate the individually controlled subsystems S_i/P_i concurrently. Such an approach is called concurrent supervisory control and is closely related to decentralized supervisory control as studied by Cieslak et al. (1988) and Lin and Wonham (1988). In the present paper simple and easily computable conditions are developed which guarantee that concurrent supervisory control can achieve the optimal behaviour achievable by a global supervisor. To achieve this, two specific concurrent control strategies are introduced.     

Verification of distributed systems with local�Cglobal predicates

This paper describes a methodology for developing and verifying a class of distributed systems in which the state space may be discrete or continuous. Our focus is on systems where changes are local in that a small number of components change state while the remainder of the system is unchanged. A proof methodology is developed that ensures global properties, such as invariants and convergence, by guaranteeing local properties within subsystems. This methodology is used to prove the correctness of concrete examples. We present a PVS library of theorems and proofs that can be used to reduce the work required to develop and verify programs in this class. A transformation of these libraries to Java is also outlined.     

Software design methods for distributed computer control

The design of software for distributed computer control systems (DCCS) is a field which has benefited greatly from certain software engineering concepts and methodologies that are used in computer science. This paper aims at reviewing some of the more commonly used software tools that help the control engineer to simplify software design for distributed control. The paper describes the current situation, and the tools which are now available.     

Pursuit problem in distributed control systems

A pursuit problem in distributed parabolic control systems without mixed derivatives with variable coefficients is considered. The finite-difference method is used to solve this problem. Necessary conditions for the termination of a pursuit are obtained. Translated from Kibernetika i Sistemnyi Analiz, No. 2, pp. 153–158, March–April 2009.     

Configuring distributed computer systems with online database backups

Designers of distributed computer systems face many complex decisions in determining the system configuration. These decisions involve location of computing resources, determination of databases and allocation of one or more copies of these databases among computing facilities. This paper deals with the configuration problem for a class of organizations that are critically dependent on the availability of their computer systems, and have widely dispersed operations with localized information needs. These organizations require online back-up copies of their databases in order to continue their operations in case of failure of a computer installation or a communication channel. An integer programming formulation of the problem is presented and an effective solution procedure is developed. Computational experience with this procedure is reported.     

Flatness-based feedforward control for parabolic distributed parameter systems with distributed control

This paper deals with distributed parameter systems being described by inhomogeneous parabolic partial differential equations in one space dimension with a distributed control. The distributed control is presented by the right part of an equation, i.e. the source function. The source function can depend on the time as well as spatial variable. The approach for design of a feedforward control for the purpose of exact output tracking is presented. The design of the feedforward control is based on the examination of inverse system dynamics. The proposed technique utilizes the method of the variables separation and the representation of a solution by the power series in the time domain. Some examples and numerical simulations are included and demonstrate the efficiency of the proposed approach for developing the feedforward control.