Collaboration support in a Web-based SCADA system Grzegorz Polaków Department of Automatic Control, Electronics and Computer Science Silesian University of Technology, ul. Akademicka 16, 44-100 Gliwice, Poland
[email protected]
Abstract. This paper presents the approach to develop the SCADA system supporting the expertise distributed amongst multiple persons. SCADA systems are briefly presented and the analysis of sources of conflicts in the multi-user SCADA system is provided. To assess the scale of conflicts two usage scenarios are assumed, i.e. the collaborative environment for skilled engineers and experts, and competitive environment for students and researchers. The implemented solution allows the users to take exclusive control of specified parameter of the plant by dividing the users into specific groups and using the token-based mechanism. The proposed conflict resolution mechanism has been implemented in the industrial pilot plant setup. Keywords: remote learning, cooperative research, multiuser system, conflicts.
1
Introduction
SCADA applications (Supervisory Control And Data Acquisition) are an essential component of the control systems for industrial plants. They enable a remote user to perform maintenance and supervision tasks over the communication link, which is nowadays a common requirement due to the scarceness of the expert knowledge. The formal definition of the Supervisory Control and Data Acquisition system is found in [1]. The typical structure of SCADA system (Fig. 1) is usually sufficient for the industrial applications in which there is only one remote control panel used by a single expert. The operator is able to read the values of controlled variables (CV) and process variables (PV) and to change the parameters of the system, i.e. the setpoints (SP) and the tuning parameters of the control algorithms (CP). However, nowadays the expertise is often distributed amongst multiple persons. It would be desired that the multiple experts involved could cooperate concurrently during the supervisory tasks. A multiuser SCADA system could also be useful in education and research, since there are educational industrial systems which attract the attention of multiple students and researchers at the same time (see [2]). To allow the multiple users to work concurrently with the system, a multiple access framework was developed, allowing the multiple users to connect to the plant using a Web browser (as in [3]). Web-based approach was chosen, because it allows the industrial data to be accessible in the broad range of higher level processes, e.g. in
education [4][5], or business [6]. However, during development, several issues of inter-user conflicts have arisen, since each of the parameters of the control system can be physically controlled by only one user at the time. Typically, this problem of a multi-user access to a SCADA system is solved by designating one privileged user which has the right to perform supervisory control task. In case of cooperative and collaborative experimentation such the approach is too simple and requires distribution of privileges amongst specific variables. Therefore, the mechanism of privileges was additionally augmented with the token-based procedure for each of the variables. This combined approach is novel in the field of continuous control systems and along with the test implementation proving the applicability of the method for cooperative control is the main contribution of this work.
object CV
Data Acquisition PV
network
SP controller
CP
Supervisory Control
Graphical User Interface
Operator
Fig. 1. The structure of a typical single-terminal SCADA system.
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Conflict resolution
As [7] states, the following sources of conflicts in collaborative work may arise: misunderstanding, lack of information, differing interests, and personal values. To assess the factors in the described applications, two usage scenarios are assumed. The first scenario is the collaborative environment. In this variant, operators of the plant are the skilled engineers and experts, who share the common goal i.e. possibly efficient operation of the plant. The second scenario is the competitive environment. Operators of the plants are the students and researchers, who want to perform various experiments according to their own agenda. In the first scenario the only sources of conflicts are misunderstanding and lack of information, while in the second case the most influential factor is differing interest. The proposed token mechanism allows the users to take exclusive control of the selected parameters of the plant. As long as the user holds the token, he is able to modify the parameter corresponding to the token. To eliminate the sources of conflicts, the GUI (graphical user interface) is employed and the parameters of token procedure are properly tuned: ─ Misunderstanding is eliminated by setting the token time-out in relation to the dynamic behavior of the controlled process. This ensures that the token owner can be held responsible for the effect of the atomic control task he performed while holding the token, and on the other hand it is ensured that the performance of the process at the moment is the result of actions of this user only. ─ Proper amount of information is provided by the GUI, which displays current state of the system, ownerships of tokens, and expected times in which each of
the token will be returned to the pool. ─ Differing interests are crucial in the competitive scenario. These conflicts are solved by extending the token lease time to be long enough to perform full operation. Additionally, queuing of the requests for tokens is required. This prevents the process from being taken over by a single user, and ensures that the other users waiting for the availability of the process have their opportunity to take the control. The exact parameters of the token mechanism (grouping of variables into clusters, time-out durations, number of user classes and their hierarchy) have to be determined depending on the character of the users concurrency. It can be concluded that for the cooperative system just one class of user is enough with the maximum token lease time of the order of seconds (depending on the dynamics of the system). For the competitive system a few user classes are needed (e.g. students, teachers, and researchers) and the token time-out of order of minutes.
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Implementation and conclusion Bioprocess plant
Heating plant
pH neutralisation plant
Supervisory Control
Sedimentation plant
Hydraulic plant
Data Acquisition
Conflict solving with Token Mechanism
network
Combustion plant
Web Server XML over HTTP
network
network network
network
Operators
Operators
Fig. 2. The structure of the system.
The mechanism proposed was implemented in the set of semi-industrial pilot plants available in the Institute of Automatic Control (Gliwice, Poland), which has already been the subject of research on the collaboration and cooperation during concurrent educational and research experimentation. The system consists of six semi-industrial pilot plants focused on various domains of continuous control. The core of the interfacing hardware is the desktop computer designated to fulfil the role of proxy.
From the pilot plants’ point of view it acts as a client, while from the office-grade local area network it is seen as a typical Web server providing the data read from the control instrumentation as adequately formatted XML files. The HTTP requests sent to the server main contain the supervising control commands, which are then relayed to the plants as the changes of values. It is the stage of the relaying, where the tokenbased mechanism is implemented. The flow of the data in the framework is graphically illustrated in the Fig. 2. The openness provided by the XML standard was primarily intended to support external automated modelling and simulation of the plants (actively investigated for a long time – see [8]–[10]), but it naturally allows for any modern external software to be integrated with the framework. The token mechanism, although uncommon in continuous process automation, proved useful; the system is conflict-free, and as long as the operators share the common goal, there are no disturbances in the plant. The differences of opinions on reaching the solution of a given control problem are solved by allowing the processes to reach their steady states before another control action may be applied. The approach adopted passed the preliminary test and will be developed further. Acknowledgments. This work was supported by the Polish Ministry of Science and Higher Education, grant no. N N514 296335.
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