Holonic Multiagent-Based System for Distributed

Holonic Multiagent-Based System for Distributed Control of Semi-industrial Pilot Plants Mieczyslaw Metzger, Grzegorz Polaków Faculty of Automatic Control, Electronics and Computer Science Silesian University of Technology, Akademicka 16, 44-100 Gliwice, Poland {mieczyslaw.metzger, grzegorz.polakow}@polsl.pl

Abstract. Semi-industrial pilot plants are the best experimental domain for advanced control systems testing with the real-world instrumentation. At the same time, inexpensive flexible process real-time simulators and virtual soft controllers are indispensable in the research and education field. Integrating control instrumentation of varying manufacturers, plant simulators, and virtual controllers into uniform system capable of flexible research and educational experiments is a complex problem to solve. A tool is needed to describe and organise knowledge on such integrated structure involving many communication channels and using distributed processing power. In the presented case-study, holarchy paradigm is applied, resulting in an untypical holonic multiagent system. The concept, architecture and development of application framework for the system are presented thoroughly in the paper. Key words: system integration, distributed control, multi-agent, holonic system, producer-distributor-consumer networking

1

Introduction

Over the past years automation systems designed for industrial plants became complex and distributed and usually consist of many components such as instrumentation, software and networking. A growing need for appropriate research and education techniques for such complex control systems results in creation of more advanced methods and tools. For example, real-time simulation plays an important role in the development of industrial process simulators [1]. Such kind of tools (virtual plants) can be very useful for operator’s training, testing specialised control software and especially for testing industrial controllers as well as virtual soft controllers. Software simulation-based experiments are not expensive, but they consider only limited part of the real world, depending on the model’s level of detail. Hence, such a method of investigation is suitable for initial tests only. Testing control software and hardware with the real-world processes gives more realistic results that take into account all possible problems occurring in industrial reality. Unfortunately, experiments with the real world industrial processes are not only very expensive, but are also limited by possibility of production disturbance and potential financial losses. A concept

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of specialised semi-industrial pilot plants – designed specifically for control systems investigation, can be considered a compromising solution. Such method of experiments conducting is less expensive than the one based on industrial processes during normal production. Moreover, it ensures nearly the same potential that is provided by an experimentation with real full-scale industrial processes (industry standard control instrumentation). Such laboratory with semi-industrial pilot plants has been developed for a few years [2]. The plants, which are of different types and distributed in several laboratory rooms, are attractive subject of investigation in the field of industrial network-based distributed control systems. Although the experiments with the pilot plants are less expensive than with the industrial ones, their costs are still considerably notable for educational entities. Hence, it can be very interesting to expand the number of plants’ users by the Internet. It has to be considered that a lot of experimenters (local and remote) will carry out theirs investigations simultaneously using different plants, different instrumentation (for example not connected to particular plant), and different industrial networking. Realisation of such demands is a very difficult control issue. An untypical holonic multi-agent system is proposed as a solution to the problem. Basic concepts of multiagent systems are discussed for example in [3],[4],[5], while industrial applications of multi-agent systems are presented in [6],[7],[8]. The word “holon” was proposed by Koestler [9] to describe an elementary unit of organisation in biological and social systems, whereas standardisation of holonic systems for manufacturing is proposed by HMS consortium [10]. Over the last decade several survey papers (see e.g. [11],[12]) as well as industry application papers (see e.g. [13],[14]) have been presented. It is not standard yet, but in several publications (see e.g [15], [16]) applications of FIPA communication connected with functional blocks (FB) programming are proposed. A holonic multi-agent system, which is proposed in this paper, is developed in the LabVIEW (LV) environment [17]. Although LV is not designed with agentural applications on mind, it is well-equipped for programming an artificial intelligence (neural networks, fuzzy logic, DB) and co-operation tasks (XML). Also, programming in the LV environment is done with the G graphical language, which is as powerful and versatile as any typical text-based C and C++ languages. It should be noticed that the LV’s supplier offers wide choice of instrumentation for measurement and distributed control. It appears to be the reason of the broad popularity of the LV environment in research and educational fields. Although function blocks for the basic TCP/IP clientserver communication exist in the LV environment by default, any more advanced communication schemes (like producer-distributor-consumer) adequate and necessary for multi-agent communication have to be developed additionally [18].

2

Hardware architecture

System under consideration consists of six physical semi-industrial plants, designed to enable research in the following industrial process domains: biological, heating, pH neutralisation, sedimentation, combustion, and hydraulics. Detailed information on functionalities of these plants is available in [2].

Holonic Multiagent-Based System for Distributed Control of Semi-industrial Pilot Plants 3 Biological process (SBR and SOCP)

Industrial 4-20mA

FlexLogix

Heating network and chemical reactor

pH neutralisation

Industrial 4-20mA

Industrial 4-20mA

ControlLogix

FlexLogix

Industrial networking (ind.net.): Ethernet/IP, ControlNet, DeviceNet

Real-time process simulators Users Virtual controllers

Industrial networking (ind.net.): Profinet, Profibus Simatic S7

Simatic S7

Industrial 4-20mA

Sedimentation

Simatic S7

Industrial 4-20mA

Combustion and heating process

Industrial 4-20mA

Variable structure hydraulic process

Fig. 1. Physical structure of system under consideration.

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Each of the six plants is hardwired to the separate commercial programmable logic controller (PLC) using classical industrial standard (4-20mA current loop or 0-10V signal). Three of the PLCs are Siemens Simatic S7 controllers and other three are various models of Rockwell Logix series. S7 controllers are connected together with the Profinet and Profibus network, while Logix controllers communicate with the DeviceNet, ControlNet and EtherNet/IP protocols. Physical structure of the system is shown in Fig. 1. Additionally, not shown in the figure, there are multiple desktop personal computers (PC) present in the vicinity of the plants, connected to each of the PLCs with available means (specialised network interfaces and networking/integration software: RSLinx, MPI, OPC). All these PCs are linked to the existing Ethernet-based local area network. Requirement of a system integration is an effect of the specific character of the pilot plants. Being part of the technical university resources, the system is expected to be useful both in research and education. It is a usual situation, when researchers or instructors need a process from the specific domain, connected to the exact model (or even specific unit) of the commercial programmable logic controller. These interconnection requirements are very dynamic, as they change between any classes and researches conducted by students, instructors, and researchers using the system. As a result, the most required functionality of the integrated system is possibility of connecting any available controller to any of the existing plants. Additionally, it is desirable to seamlessly integrate software instrumentation with the existing hardware, where software instruments are virtual controllers and processes simulators. There exist solutions for varying vendors’ industrial networks interconnection like specialised PLC modules, PC cards (see for example Anybus cards and modules for ProfibusDeviceNet transmission [19]), and network bridges (such as proposed in [20] or [21]). Unfortunately such solutions are usually limited to two network standards only and require extensive configuration, so they can not provide required degree of integration. Such uncommon structure of the system and additional specific requirements of its quick and flexible reconfiguration, are describable with the holarchy theory ([10]). Each of the semi-industrial plants together with its linked programmable logic controller can be treated as a holon. Requests of students, instructors and researchers are external rules. Intelligent agents employed by holons dynamically reconfigure the structure of inter-holon connections to form clusters of holons, which fit to the current constraints given by the rules. The hardware of the system is logically embedded in the holons, only unified interface of the agents is presented to other members of the holarchy. This makes it possible to integrate software instruments with the system, as they can be embedded in the uniformed holons, too. Achieved logical structure of the system is presented in the Fig. 2. While idea of the holonic integration of the system is very attractive, it requires extensive development of the unified inter-holon interface. A protocol of real-time process data exchange between boundaries of existing industrial networks is needed, and. a method of holons synchronisation and appropriate agent language are required. Proposed structure of the interface and progress of the development of all its layers is presented in the following section.

Holonic Multiagent-Based System for Distributed Control of Semi-industrial Pilot Plants 5 Bioprocess holon

Heating holon

ind.net.

pH neutralisation holon

ind.net.

Virtual control holons

Simulation holons

...

...

Multi-agent system

ind.net.

Sedimentation holon

ind.net.

Combustion holon

Hydraulic holon

Fig. 2. Holarchy as a method of system integration.

3

Communication framework

A need for exchange of sampled process data between holons not physically interconnected through the specialised industrial networks, implies a necessity of developing a method for such data transmission by the network of inter-holon connections. Therefore, interface agents have to be equipped with double communicational skills. On the one hand, advanced high level protocol is needed, so the holons are able to synchronise knowledge on the logical structure of the holarchy and on the other hand a low level real-time process data transmission protocol is required. When compared to the usually investigated holonic manufacturing systems, presence of the real-time

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protocol at the level of holon communication is a notable distinction. That is because sampled continuous processes are controlled by algorithms in the form of recursive difference equations, which in each iteration require knowledge of values of all input variables to be valid. On the contrary, manufacturing systems are not time critical – required periods of data exchange are large enough to be fulfilled by nearly any networking protocol. Required structure of the holon’s interface is shown in Fig. 3. As it is seen, internal structure of a holon is layered, each layer of the structure has corresponding level of communication. Structure of the interface agent is also layered to support the two types of communication. Upper level of agent’s thread is connected with cooperation control protocol, while lower one deals with the real-time data transmission. Interface agent

XML-based co-operation (TCP/IP) RT data (UDP/IP)

Agent’s intelligence (neural nets, fuzzy logic, DB support)

XML-based co-operation (TCP/IP)

Real-time engine

RT data (UDP/IP)

OPC or MPI+OPC

Controller algorithm Industrial networking DeviceNet, PROFIBUS

Industrial networking DeviceNet, PROFIBUS Industry standard 4-20 mA or 0-10V

Process

Fig. 3. Anatomy of holon.

3.1

Real time engine

An Ethernet-based protocol for real-time transmission of numerical values between LabVIEW developed software agents was developed earlier and is described thoroughly in [18]. The protocol was designed specifically for cyclical exchange of data between equally privileged network peers. Whole transmission framework is built according to the producer-distributor-consumer paradigm. Each of the interface agents is connected to scheduling agent, which manages timing of the communication performing the role of the distributor. Main task of the distributor is storing the list of variables

Holonic Multiagent-Based System for Distributed Control of Semi-industrial Pilot Plants 7

(each variable consist of a textual identifier and a value) and making it available to the interface agents performing the roles of producers and consumers. The list of identifiers is served constantly and may be downloaded by agents at any moment (usually once, when agent is started). List of values of variables is broadcasted cyclically. Period of broadcast cycle defines sampling time of all continuous signals in the system. Each specific broadcast synchronises cycles of work of distributed plants. Diagram of the real-time framework is presented in the Fig. 4. Continuous black arrows show the flow of control in agents’ threads; grey dashed lines indicate data flow inside and between agents. Global identifiers

Serve list

Scheduler agent (distributor)

Global values

Broadcast values

Collect values

Switched Fast Ethernet

Download list of identifiers

Wait for values broadcast

Extract required variables

Local variables

Send changed values

Synchronise values with the PLC Local copy of global identifiers

Local copy of global values

OPC

Interface agent (producer/ consumer)

Fig. 4. Control and data flow in real-time agent communication.

A presumption taken into account during framework design was compatibility with the existing Ethernet-based local area network. To ensure compatibility with existing network traffic, protocol was designed to run in the application layer i.e. on top of TCP and UDP protocols. List of the variables’ textual identifiers is transmitted with the TCP, because it is served on demand (TCP supports two-way request-answer transmission). Raw values are transmitted with the UDP protocol, because of its low control data overhead and capability of broadcasting, which is required in producerdistributor-consumer communication scheme.

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It is sometimes pointed that Ethernet’s transmission times are non determined because of used CSMA/CD method. Therefore, it should be noted that Ethernet networks built using switching technology does not use that algorithm and only source of possible data loss is overloading switches’ internal packet queues. Keeping queues safe from overfilling (by limiting amount of traffic in the network) allows to ensure that each time of packet transmission is determined. Summarising: presented framework is real-time capable when running on top of not overloaded Ethernet network built with the switching technology only. Distributor application is developed using LabVIEW and it is required to be running on one of the computers in the local network. Cooperation between distributor and the interface agents is made easy with the predeveloped set of functional blocks, which make transforming any LabVIEW program into framework capable agent trivial. Details on the framework operation, available G language functional blocks, network transmission formats and distributors manual are included in [18]. 3.2

Co-operation engine

Cooperation engine is superordinated to all the layers in the holon’s structure, including interface agent’s real-time engine layer. The role of cooperation engine is synchronising internal state of the holon with the state of the holarchy which is defined by current constraints. Changes of the structure of the holarchy are unpredictable and unscheduled, yet they are very important for the proper functionality of the system. Therefore it is proposed to implement transmission of this kind of information with the reliable TCP protocol. All advanced agent and holonic knowledge languages can be successfully implemented with the LabVIEW thanks to Internet Toolbox supplied by National Instruments. The toolbox contains (among others) complete XML support with the full functionality of the Document Object Model including namespaces support. In combination with the artificial intelligence methods supported by LabVIEW (neural networks, fuzzy logic, database integration, etc.) it is easy to develop a custom program able to synchronise holon with the rest of the holarchy. Additional possibilities are enabled by the web server embedded in the distributor application (not marked in the Fig. 4). The framework is supplied with the set of server side scripts enabling Web Services providing full access to all the variables of the system. This channel of system communication is preferred as the method of inputting holarchy constraints using standard web browser. 3.3

Interface agent – PLC connection

Each of the six plants of the system is connected to one of designated desktop PCs. Linking method depends on the specific PLC and include dedicated interface cards and retail integration software (e.g. RSLinx). Despite of the specific linking method, each of the links ends with the OPC standard compliant data server providing access to the internal variables of the PLC programs (which include plants process data).

Holonic Multiagent-Based System for Distributed Control of Semi-industrial Pilot Plants 9

Interface agents are resident and execute their threads in the computers directly connected to the process plants. Versatility of the typical PC (in contrary to the limited PLCs) allows for execution of LabVIEW developed software. There exist programmable logic controllers able to execute directly LabVIEW applications (National Instruments FieldPoint series), but in the case of presented system it does not apply. Applications executed by the PLCs present in the system have to react to changes of the logical role of the holarchy. Proposed method of implementing such reaction capability is to develop separate subprograms fulfilling every possible role of the specific plant in the holarchy. Then, all the subroutines have to be merged into single application for the PLC. Subprogram to being executed at the moment has to be chosen by the application basing on the value of variable. In such configuration, interface agent is able to choose subprogram of the PLC by simple modification of this specific variable. It is visible at the diagram in the Fig. 3 – agent’s cooperation engine is able to communicate with the PLC through the real-time variables layer. 3.3

Industrial networking

Industrial networks present physically in the system do not connect all of the holons, dividing them into two clusters. It is possible to employ the networks for communication in the range of the two clusters, however it would require careful developing of the PLC programs, so they supported two channels of real-time transmission (both industrial networks and holon layer) when needed. Such situation would complicate development process, so it was decided that industrial networks are not used for plant integration anymore Since industrial networking is not used to transfer process data it is possible to utilise them in other ways. There are lessons and researches, where industrial networking is not the tool for but subject of the investigation. Proposed structure of the holarchy supports such researches, because user has access to parallel real-time communication on higher level than industrial networks. Ethernet-based real-time transmission may be used to supervise industrial communication. Such approach turns existing industrial networks into additional holons being another research and education plants.

4

Concluding remarks and future work

Holonic approach to the presented system turned out to fulfil all required duties, so presented system is successful implementation of the holarchy paradigm in the process automation. Currently, only real-time protocol of the holons layer is codified and regulated to the extent comparable to retail software products. Other layers (i.e. PLC programs and cooperation layer of the holons) were developed as custom prototype applications, so only loose hints and advices are presented in the paper. A work is still ongoing to develop a general methodology and tools for those layers. It is also planned to implement holarchy paradigm at even lower level of the system. Each of the currently developed plant holons is planned to be modelled as a holarchy consisting of holons embedding elemental components of the process control system (i.e. actuators, sensors, and algorithms).

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Acknowledgments. This work was supported by the Polish Ministry of Scientific Research and Information Technology.

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