- Email: [email protected]
Support Systems, Neural Networks & New Visualization Forms for S&C in Power Plants
Copyright @ IFAC Power Plants and Power Systems Control, Brussels, Belgium, 2000
SUPPORT SYSTEMS, NEURAL NETWORKS & NEW VISUALIZATlON FORMS FOR S&C IN POWERPLA TS
Peter F. Elzer
Badi Boussoffara
Technical University o/Clallsthal (TUC) Institute/or Process- and Production Control Technology (IP?) JlIlius-Albert-Strasse 6, D-38678 Clallsthal-Zeller/eld, Germany Tel.: ++49-5323-727501 Fax.: ++49-5323-727599 e-mail: [email protected]
Abstract: Several operator support techniques in power plants are presented that have been prototypically implemented at the IPP. Special emphasis is put on methods for early recognition and identification of unknown situations. A distinction is made between 'Advisory Systems' and 'Human-Machine-Interfaces (HCI)' proper. The first comprise software components that use 'knowledge' about the process to derive advice for the operators from it. The second mainly encompass new forms of visualization of process situations that enable humans to bener utilize their natural cognitive skills. The effectiveness of most of the presented approaches was demonstrated by user experiments. Copyright © 2000 IFAC Keywords: Alarm Systems, Computers, Displays, Expert systems, Human-machine interface, Learning systems, Multimedia, Neural networks, Power station control, Supervisory control
I. OPERATOR SUPPORT SYSTEMS
the beginning. This means that the optimal support of operation and maintenance of technical systems has already to be considered during their design and development. One aspect thereof is the design of optimal concepts for operation and maintenance on the organisational level, another one the instruction and training of the personnel, and a third one the provision of technical support for operation and maintenance. The laner comprises three classes of tools:
1.1 General Large technical systems - like e.g. power plants - put high demands on the skills of the personnel in charge of their supervision and control (S&C). Operator actions may also have significant financial consequences or even lead to dangereous situations. Therefore much effort has traditionally been devoted to research and development with the aim to optimally support the operating personnel and thus to make sure that the systems in question can be properly controlled by humans. One of the most important results of this work is the insight that the operation of technical systems is especially efficient and secure - and therefore cost effective as well - if they have been developed as 'human-machine-systems' right from
I The 'classical' S&C-System. It relieves the operators from most of the tedious and/or error prone tasks and realizes the control concept which in turn determines the behaviour of the plant as perceived by the operator. Therefore it has been formulated by Sheridan (1987) that the main task of humans in such systems is that of 'supervisory control'. This particular mode of operation has serious consequences for
323
der to shorten the time to return to a safe operating condition and to prevent errors during operation; warning of situations which have not yet occurred during operation, etc.
the way humans perceive the behaviour of a plant and has to be taken into account in the design of the following two classes of technical support systems. However, as it has been discussed for years in many places, it shall not be dealt with explicitely in this paper.
In order to achieve these goals practically all methods and tools have been applied that emerged during the development of information technology. The related research takes place on an international basis, covers a very wide range and includes practically all relevant main contractors, subsystem suppliers and technical organisations of the power industry. In the narrow space limits of this paper it is therefore only possible to describe a few of the results.
2 The HeI proper. In general understanding this consists of various - central or detached - control panel(s), the displays associated with them, and, in some cases, a mimic board. If the situation requires, however, the operating personnel has to to be able to complement or cross-check the indirect information provided by these devices by directly looking at the process ('external views'). 3 The advisory systems. This class has become more and more important since some years. One reason for this may be the sharp decrease of prices of data processing components and the immensely increased quality of graphical tenninals. Therefore it e.g. became cost effective to provide knowledge about the process, including documentation, training material, operation or maintenance manuals no longer in the form of unwieldy volumes of printed paper, but on computers. These in turn can support the operators in their search for important information.
1. J The Role ofExperiments for the Design of Human-Alachine-Systems
The design of human-machine-interfaces or advisory systems - despite of the large amount of available and verifiable knowledge in the domain of ergonomics and cognitive science - is very often still considered as an 'engineering art', which is supposed to be mainly based on personal experience and intuition. Partially this is still true, especially as experiments, in which human behaviour constitutes the main parameter, are difficult to plan and to evaluate. Besides, they are rather time-consuming and expensive.
Figure 1 illustrates the interrelations between these components. "External View" (Real Images)
Despite the problems mentioned it is absolutely necessary that engineering work is based on experimentally verified scientific knowledge. Therefore at the IPP it is tried to verify the results of own developments by means of user experiments whenever feasible. In several cases this has resulted in substantial corrections of design concepts.
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The authors also expect significant inputs from the results of an interdisciplinary joint-effort project, sponsored by the 'Volkswagen-Foundation' (Ref.nr. 1/69886), in which the IPP has served as project leader. The goal of this project was to examine the possible influence of the human-machine-interface on the development of incorrent assumptions of operators concerning the actual state of a technical system (Elzer, et al., 2000).
Operator
Fig. I. Overall structure of a 'Human-MachineSystem' 1.2 Research in the Area ofSupport Systems
The main research topics in this area over the past decades have been the support of operating personnel during routine operation and - in particular - the handling of critical situations. Some of the main development goals have been:
2. ADVISORY SYSTEMS 2.1 Alarm Filtering
reduction of the mental load of the operators, in particular the prevention of 'cognitive overload' in order to maintain the quality of the operators' decisions and actions; provision of an overview over the state of the plant that is as comprehensive and easy to interprete as possible; early warning of upcoming criticial situations, in order to gain time for planned counteractions and to prevent panic; offering aids to cope with such situations, in or-
As already mentioned above, the reduction of the operators' mental load and the prevention of 'cognitive overload' in critical situations has traditionally been an important aspect of the design of human-machine-interfaces. The 'reduction of infonnation', presented to the operator, has for many years been regarded as the best means to achieve this goal. An established technique for this purpose is 'alann filtering' (as e.g. described by Sill, et al. (1989)), which makes sure that only such information is presented to the operating personnel that is absolutely necessary
324
cess states known to the NN. Thus, at every point in time it is possible to provide the operator with an overview over the possible simultaneous occurrence of several abnormal process states as far as these have been taught to the NN before.
for the comprehension and handling of the operational condition at hand. However, alarm filtering has traditionally been criticized for a possible danger: there may be cases where independent alarms are lost which normally appear as consequences of others.
Implementation. On this basis a laboratory prototype was developed at the IPP (Elzer, et al., 1996) in order to demonstrate the applicability of an NN as an early warning system for the operating personnel in case of abnormal or unknown situations in technical systems. To this prototype those abnormal process states of a coal fired power plant were taught that could be presented by means of an existing simulator (Weisang and Elzer, 1988). In case of a deviation of the process state of the plant from the normal state, the NN very early gives an estimation as to which of the abnormal process states known to the NN might be similar to the actual situation.
2.2 Knowledge-based Systems Another approach are advisory systems on the basis of knowledge-processing techniques (e.g.: Elzer, 1989; Elzer et al,. 1989), which have been developed at various places. At the first glance such systems seem to lead to an increase of the information presented to the operators. However, they have the potential to decrease the mental load by offering helpful situation assessments as well as proposals for appropriate actions. It is even possible to warn the operating personnel of upcoming - probably critical - process situations so early that these situations can be prevented in time (Elzer, et al., 1992).
This situation assessment is continuously visualized in graphical form. The pre-warning time can be varied from several minutes to half an hour. Newly identified critical process situations can be additionally taught to the NN if deemed necessary.
However, the two approaches mentioned still have the disadvantage that they can only deal with situations that have already been known during the design of the support system. In unknown situations they cannot offer any help. As well, unplanned changes in the behaviour of the plant, e.g. caused by ageing, can only be taken into account with difficulty. Furthermore, as warnings occur as 'singular events', i.e. suddenly like in conventional S&C systems, they still may cause the operating personnel to feel insecure.
E;r:perience. However, the rather straightforward construction of that prototype did not allow the identification of unknown situations. For this purpose a classification methods with two levels is necessary. Further investigations are also necessary in order to solve some problems of 'upscaling' that are expected in the transition process to a system more similiar to reality, e.g. considering the number of process values to be supervised or that of abnormal process states known to the system. This is especially true as far as the expected learning times are concerned.
In cases of several abnormal operating conditions occurring simultaneously, it can also happen that indicators or knowledge-based advisory system give warnings which ccontradict each other or at least overlap to such an extent that the operating personnel has difficulties in distinguishing them or correctly assessing their plausibility. A possible solution for this problem has been demonstrated: a knowledge-based advisory system that shows the possible consequences of occurring disturbances to the operators on request (Elzer, et al., 1992). This enables them to assess these disturbances with respect to the seriousness of their consequences and thus to set priorities for counteractions.
2.4 The Selflearning System s4 Technical Principle. In order to demonstrate the possibility to build an advisory system without the above mentioned high effort for knowledge aquisition (as in the case of knowledge based systems) or initial teaching (as in the case of neural nets) the system S4 (Support System based on Situation Similarity) has been developed at the IPP in the course of a PhDthesis (Boussoffara, 1996; Boussoffara and Elzer, 1997a). S4 is able to learn abnormal process situations and the necessary countermeasures during operation under the guidance of the operator. The basis for this ability is the formation of 'feature profiles'.
However, a still unsolved problem (also with knowledge-based systems) is, how to inform operators that the advisory system does not know the situation at hand. It is therefore one of the main research topics of the IPP to find solutions for this problem.
A situation can be described by means of several of such feature profiles that have been recorded during a certain time interval since the occurrence of the first deviation. In order to recognize the situation again at a later point in time, these recorded feature profiles are stored and compared with the momentary feature profiles over a certain range of time.
2.3 Neural Networks Technical principle. The main reason for the use of neural nets ('NN's') for solving the two last-mentioned problems is that they are able to assign probability values to each process state they have been taught. These probability values are computed simultaneously during each classification step for all pro-
Support during Identification of Situations. In the case of unknown situations S4 informs the operator
325
that there is a new situation and prepares the fonnation of a new situation class. After having analyzed the situation and stabilized the process the operator can decide whether the new situation class is to be included in the infonnation base. If yes, he can assign a name to it and S4 learns it under this name. However, if the situation at hand is similar to at least one situation already known to the system, S4 tries to verify this and infonns the operator about this action and its results. This decision process is appropriately visualized.
has been developed at the IPP, which is particularly adapted to the human ability to quickly recognize and assess changes in graphical patterns: the 'mass data display' (MOD) (Zinser, 1993, Elzer, et al., 1995). The approach is main Iv based on the work of Grinstein (Grinstein and S~rgeron, 1989). Its basic principle is that the same graphical symbol (of relatively simple design) is assigned to each of the process values. It changes its parameters (form, size, colour etc.) according to the changes of the process value assigned to it. The operator does not observe one single symbol, but the entire pattern, which builds up while a number of symbols, belonging to technically connected process values, change their appearance simultaneously. This technique makes it possible to visualize an overview over the entire process with hundreds (to thousands) of values on one display screen of average size.
Support during Operator Actions. In case of process states that require actions, the operator first has to perfonn the necessary actions in order to bring the process into a stable state again. The system identifies these and compares them in chronological order to actions that have proven successful in comparable, earlier cases. In case any deviations to the currently observed procedure are detected, the operator is informed. This message does not only include a warning, concerning inconsistencies, but also a suggestion of alternative actions.
In order to support the operator during this procedure, another means of support has been introduced, the 'process notice' (Fig. 2). It mainly relieves the operator's short-time memory and therefore greatly contributes to the reduction of the probability of operator errors. The principle is that during the analysis of a process state the operator can chose the trend graphs of those process values he considers to be important and can group them around a polar diagram. Thus the operator is able to select those process values from the complex infonnation space he thinks to be important for the current analysis of the process behaviour (Boussoffara and Elzer, 1997b).
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Experimental evaluation. The effectiveness of the designed MODs was tested in a number of experiments. In particular it was evaluated how a) the operators' capability of diagnosis was enhanced by using MODs instead of 'classical' fonns of representation and b) different fonns of MODs perfonned in relation to each other with respect to the recognition of disturbances.
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In the course of these experiments in total 11.000 individual measurements were taken. Some of the results considerably exceeded the authors' expectations. So e.g. in some cases with an MOD disturbances could be recognized twenty times faster than by means of conventional fonns of representation. On the whole this fonn of representation proved extremely effective for the early recognition of disturbances, because at any point in time it helps the operator to get a very quick impression of the overall state of the process.
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Fig. 2. The Process Notice 3. THE HUMAN-MACHINE-INTERFACE PROPER
However, MODs should not be regarded as a replacement for the classical fonns of representation (like e.g. P&l-diagrams). They are an additional - though very effective - means for early warning. In order to quantitatively analyse a displayed deviation from the normal state and to decide which actions to take,
3.1 The 'Mass Data Display' Technical principle. In cooperation with the research laboratory of ASS in Heidelberg a new fonn of visualization of the behaviour of technical processes
326
operators should rather use the representation of single values in P&l-diagrams.
terms 'multi-media' and 'virtual reality'. It is not always quite clear whether and to what extent progress for process control can be expected from these techniques. Therefore they are also investigated at the IPP. The results partially confirm the impression that the state-of-the-art that has been customary in process control for many years is still better than that of the techniques nowadays termed 'multi-media' in the domain of office technology. But the cheap machines, available for this huge application area, can serve as a basis for extremely comfortable and user friendly support tools for training and maintenance in our area.
3.2 Three-Dimensional Representations Basic Principle. In order to solve the problem of giving an as complete as possible survey of the state of the plant on a (limited) display screen while simultaneously maintaining the possibility to assess the quantitative state of individual process values, another approach was tested within the scope of PhDthesis: three-dimensional representations (c. Beuthel, 1997; Beuthel and Elzer, 1997).
With respect to 'virtual reality' the situation is similarily ambivalent. In the course of a study, undertaken at the IPP (R. Beuthel, 1997), a number of ergonomic problems with the terminal devices, currently used in this field, were identified. This is especially true for their constant use on the shop floor. On the other hand, some techniques that are summarized under the term 'computer augmented reality' (CAR) appear to be extremely promising. In particular for diagnosis and maintenance they can be expected to have the potential for a breakthrough (Elzer, et al. 1999). Therefore at the IPP much emphasis is put on the establishment of test installations of such methods and devices, in order to be able to evaluate their usefulness.
For a general overview over the state of a process it is necessary to display a large number of process values. With current forms of representations this usually leads very quickly to a 'crowded' screen, where in most cases single values can no longer be interpreted in a meaningful manner. Therefore it has become customary to spread the whole representation over several screens. This again leads to new problems, like e.g. to loss of time while searching for a particular picture that is necessary for a certain action, to 'window effects' which complicate the comprehension of a situation by the operator, etc.. During the work at the IPP with three-dimensional representations for completely different applications (Krohn, 1996) the impression arose that these had the potential to display more information related to a certain situation in an easily comprehensible form than two-dimensional representations. The hitherto most plausible explanation for this effect can be derived from the principles formulated by Gibson (1986), referring to the influence of the natural environment on the development of the human perceptional capabilities. It appears is that these are based on the immediate perception of stimuli created by the virtually imitated three-dimensional spaces in which the users can therefore orientate themselves intuitively.
4. SUMMARY AND PROSPECTS To the authors' opinion it could be shown in this paper that there is still a huge potential for further improvement of the human-machine-interface in power plants. In particular it appears to be possible to reduce the up to now very high engineering costs for advisory systems by introducing self-learning systems and thus to considerably improve the safety of technical processes. 3D-representations can - by means of their direct connection to the natural perceptional capabili· ties of humans - help to facilitate situation assessment and therefore reduce the reaction times in critical situations. Techniques from the domains of 'multi-media' and 'computer-augmented-reality' can very effectively support training, diagnosis and maintenance.
implementation, Therefore, at the IPP a 3D display for S&C of technical systems, especially power plants, was prototypically implemented, using the above mentioned principles. During its design, a number of design principles for artificial environments were formulated, intensely discussed and finally applied.
Effectiveness and cost efficiency of the developed systems can be verified by means of quantitative evaluations on the basis of systematic user experiments.
Afterwards, the resulting process control interface was systematically evaluated with the intention to check the assumed positive effects of this technique compared to the usual P&l-diagrams by means of user experiments. The investigation included the reaction times of the test persons as well as their error rates. With respect to both parameters the 3D-representation yielded considerably better results.
REFERENCES Beuthel, C. (1997). Dreidimensionale Prozessvisualisierung zum Fuehren technischer Anlagen am Beispiel eines Kohlekraftwerks. PhD-Thesis, Papierflieger, Clausthal-Zellerfeld, Germany
3.3 Further Techniques The current discussion on the further development of the human-machine-interface is dominated by the
327
Beuthel, C. and Elzer, P. (1997). Advantages of3D Displays for S& C ofLarge Technical Systems. XVI. European Annual Confere nce on Human Decisio n Making and Manual Control, Kassel, German y. Beuthel, R. (1997). Eignung und Nutzung van VRTechnik enfuer Aufgaben der Prozessleiltechnik - eine Abschaetzung. Master' s Thesis, ClausthalZellerfe ld, Germany. Bousso ffara, B. (1996) Ein Verfahren zur Bereitstellung von Betriebserfahrungen zur Unterstuetzung der Bediener technischer Anlagen. PhDThesis, Papierflieger, Clausth al-Zelle rfeld, Germany. Bousso ffara, B. and Elzer, P. (1997a). A New Approach Supporting Operators to Evaluate Situations in Supervisory Control, in: 4th IFAC Internationa l Workshop on Algorit hms and Architectures for Real-Time Control, Vilamoura, Portugal, pp. 223-228. Bousso ffara, B. and Elzer, P. (l997b) . Reducing Operator Mental Load through Dynamic Icon Interfaces and Process Notice, in: 7th International Confere nce on Human -Compu ter Interaction, San Francisco, CA., pp. 391-394. Elzer, P. (1989). GRADIENT - Ein Schritt in Richtung intelligenter Prozessueberwachung, in: Brauer, W., Freksa ,e. (Eds.): Proceed ings of the '3. Internationaler GI-Kongress '89', Muenchen, German y, Informatik Fachberichte 227, Springer, Berlin-H eidelbe rg-New York, pp. 507515. Elzer, P., Weisang, C., Zinser K. (1989). KnowledgeBased System Support for Operator Tasks in S&C Environments. 1989 IEEE International Confere nce on Systems, Man and Cybernetics', Boston, MA. Elzer, P., Weisang, e., Zinser, K. (1992) Alarm Filtering vs. Failure Prediction - how to best reduce Operator Overload. 5th IF ACI IFIPIIFORS/IEA Sympos ion on Analysis, Design, and Evaluation of Man-M achine Systems, The Hague, Netherlands, Paper 3.1.5. Elzer, P., Beuthel, e., Boussoffara, B., Zinser, K., Tissen, A. (1995). Advantages ofmass-datadisplays in process S&C. 6th IF ACI IFIPI IFORSI IEA Symposium on Analysis, Design and Evaluat ion of Man-M aschine Systems, Boston, MA, pp. 439-444.
Elzer, P., Reuter, M., Berger, A. (I~~O). t:./nsatz neuronaler Netze zur Fruehwarnung des Bedienpersonals von technischen Anlagen. VDIND EGMA-C ongress , Baden-B aden, Germany, VDIReport 1282, pp. 499-508. Elzer, P., Beuthel, e. (1999).New Techniques in Maintenance and Training in Power Plants. Paper presented at: International Joint Power Generation Confere nce (IJPGC'99). San Francisco, CA. Elzer, P.F., Kluwe, R.H., Boussoffara, B. (Eds.) (2000). Human Error and System Design and Management. Lecture Notes in Control and Information Sciences, Nr. 253. Springer-Verlag, London, Berlin, Heidelberg. Gibson, J.J. (1986, Ist. Edition 1979). The Ecological Approach to Visual Perception. Lawren ce Erlbaum Associates, Hillsdale, NJ. Grinstein, G. and Bergeron, D. (1989). The Visualisation ofScientific Data, in: Proceed ings of the '19. GI-Jahrestagung', Muenchen, Germany, Informatik Fachberichte 222, Springer, BerlinHeidelb erg-New York, pp. 1-9. Krohn, U. (1996). Visualizationfor Retrieval of Scientific and Technical Information. PhDThesis, Techn. Univ. Clausthal, Papierflieger, Clausthal-Zellerfeld, Germany. Sheridan, T. (1987). Supervisory Control. In: Handbook ofhuman factors (G. Salvend y (Ed.». Wiley, New York. pp. 1243 - 1268. Sill, U., Elzer, P., Roghmanns, H. (1989). Informationsaufbereitung und -darstellung zur Stoerungserkennung und -behebung. VDE-E TG Confere nce 'Rechnergestuetzte Prozess- und Betriebsfuehrung im Kraftwerk', Saarbru ecken, Germany, VDE-Verlag, ETG-Te chnical Report 28, pp. 147-172. Weisang, e. and Elzer, P. (1988). Die Bedeutung eines Prozesssimulators in der Prototypphase eines wissensbasierten S & C Systems, in: Proceedings of the Technical Confere nce 'Prozessrechensysterve '88', Stuttgart, German y, Informatik-Fachberichte, Vo1.l67, Springer, BerlinHeidelberg-New York, pp. 524-533. Zinser, K. (1993). Neue Formen und Medien der Prozessvisualisierung, in: atp- Automatisierungstechnische Praxis, Vol. 35, No. 9, pp. 499504.
328
SUPPORT SYSTEMS, NEURAL NETWORKS & NEW VISUALIZATlON FORMS FOR S&C IN POWERPLA TS
Peter F. Elzer
Badi Boussoffara
Technical University o/Clallsthal (TUC) Institute/or Process- and Production Control Technology (IP?) JlIlius-Albert-Strasse 6, D-38678 Clallsthal-Zeller/eld, Germany Tel.: ++49-5323-727501 Fax.: ++49-5323-727599 e-mail: [email protected]
Abstract: Several operator support techniques in power plants are presented that have been prototypically implemented at the IPP. Special emphasis is put on methods for early recognition and identification of unknown situations. A distinction is made between 'Advisory Systems' and 'Human-Machine-Interfaces (HCI)' proper. The first comprise software components that use 'knowledge' about the process to derive advice for the operators from it. The second mainly encompass new forms of visualization of process situations that enable humans to bener utilize their natural cognitive skills. The effectiveness of most of the presented approaches was demonstrated by user experiments. Copyright © 2000 IFAC Keywords: Alarm Systems, Computers, Displays, Expert systems, Human-machine interface, Learning systems, Multimedia, Neural networks, Power station control, Supervisory control
I. OPERATOR SUPPORT SYSTEMS
the beginning. This means that the optimal support of operation and maintenance of technical systems has already to be considered during their design and development. One aspect thereof is the design of optimal concepts for operation and maintenance on the organisational level, another one the instruction and training of the personnel, and a third one the provision of technical support for operation and maintenance. The laner comprises three classes of tools:
1.1 General Large technical systems - like e.g. power plants - put high demands on the skills of the personnel in charge of their supervision and control (S&C). Operator actions may also have significant financial consequences or even lead to dangereous situations. Therefore much effort has traditionally been devoted to research and development with the aim to optimally support the operating personnel and thus to make sure that the systems in question can be properly controlled by humans. One of the most important results of this work is the insight that the operation of technical systems is especially efficient and secure - and therefore cost effective as well - if they have been developed as 'human-machine-systems' right from
I The 'classical' S&C-System. It relieves the operators from most of the tedious and/or error prone tasks and realizes the control concept which in turn determines the behaviour of the plant as perceived by the operator. Therefore it has been formulated by Sheridan (1987) that the main task of humans in such systems is that of 'supervisory control'. This particular mode of operation has serious consequences for
323
der to shorten the time to return to a safe operating condition and to prevent errors during operation; warning of situations which have not yet occurred during operation, etc.
the way humans perceive the behaviour of a plant and has to be taken into account in the design of the following two classes of technical support systems. However, as it has been discussed for years in many places, it shall not be dealt with explicitely in this paper.
In order to achieve these goals practically all methods and tools have been applied that emerged during the development of information technology. The related research takes place on an international basis, covers a very wide range and includes practically all relevant main contractors, subsystem suppliers and technical organisations of the power industry. In the narrow space limits of this paper it is therefore only possible to describe a few of the results.
2 The HeI proper. In general understanding this consists of various - central or detached - control panel(s), the displays associated with them, and, in some cases, a mimic board. If the situation requires, however, the operating personnel has to to be able to complement or cross-check the indirect information provided by these devices by directly looking at the process ('external views'). 3 The advisory systems. This class has become more and more important since some years. One reason for this may be the sharp decrease of prices of data processing components and the immensely increased quality of graphical tenninals. Therefore it e.g. became cost effective to provide knowledge about the process, including documentation, training material, operation or maintenance manuals no longer in the form of unwieldy volumes of printed paper, but on computers. These in turn can support the operators in their search for important information.
1. J The Role ofExperiments for the Design of Human-Alachine-Systems
The design of human-machine-interfaces or advisory systems - despite of the large amount of available and verifiable knowledge in the domain of ergonomics and cognitive science - is very often still considered as an 'engineering art', which is supposed to be mainly based on personal experience and intuition. Partially this is still true, especially as experiments, in which human behaviour constitutes the main parameter, are difficult to plan and to evaluate. Besides, they are rather time-consuming and expensive.
Figure 1 illustrates the interrelations between these components. "External View" (Real Images)
Despite the problems mentioned it is absolutely necessary that engineering work is based on experimentally verified scientific knowledge. Therefore at the IPP it is tried to verify the results of own developments by means of user experiments whenever feasible. In several cases this has resulted in substantial corrections of design concepts.
i------------------------------------~,
,.----'---,
"',
~
a>ca>
\
.- e. .co u_ "'e. ~a>
'u e",
"''to
Ea>
I-=
::0-
The authors also expect significant inputs from the results of an interdisciplinary joint-effort project, sponsored by the 'Volkswagen-Foundation' (Ref.nr. 1/69886), in which the IPP has served as project leader. The goal of this project was to examine the possible influence of the human-machine-interface on the development of incorrent assumptions of operators concerning the actual state of a technical system (Elzer, et al., 2000).
Operator
Fig. I. Overall structure of a 'Human-MachineSystem' 1.2 Research in the Area ofSupport Systems
The main research topics in this area over the past decades have been the support of operating personnel during routine operation and - in particular - the handling of critical situations. Some of the main development goals have been:
2. ADVISORY SYSTEMS 2.1 Alarm Filtering
reduction of the mental load of the operators, in particular the prevention of 'cognitive overload' in order to maintain the quality of the operators' decisions and actions; provision of an overview over the state of the plant that is as comprehensive and easy to interprete as possible; early warning of upcoming criticial situations, in order to gain time for planned counteractions and to prevent panic; offering aids to cope with such situations, in or-
As already mentioned above, the reduction of the operators' mental load and the prevention of 'cognitive overload' in critical situations has traditionally been an important aspect of the design of human-machine-interfaces. The 'reduction of infonnation', presented to the operator, has for many years been regarded as the best means to achieve this goal. An established technique for this purpose is 'alann filtering' (as e.g. described by Sill, et al. (1989)), which makes sure that only such information is presented to the operating personnel that is absolutely necessary
324
cess states known to the NN. Thus, at every point in time it is possible to provide the operator with an overview over the possible simultaneous occurrence of several abnormal process states as far as these have been taught to the NN before.
for the comprehension and handling of the operational condition at hand. However, alarm filtering has traditionally been criticized for a possible danger: there may be cases where independent alarms are lost which normally appear as consequences of others.
Implementation. On this basis a laboratory prototype was developed at the IPP (Elzer, et al., 1996) in order to demonstrate the applicability of an NN as an early warning system for the operating personnel in case of abnormal or unknown situations in technical systems. To this prototype those abnormal process states of a coal fired power plant were taught that could be presented by means of an existing simulator (Weisang and Elzer, 1988). In case of a deviation of the process state of the plant from the normal state, the NN very early gives an estimation as to which of the abnormal process states known to the NN might be similar to the actual situation.
2.2 Knowledge-based Systems Another approach are advisory systems on the basis of knowledge-processing techniques (e.g.: Elzer, 1989; Elzer et al,. 1989), which have been developed at various places. At the first glance such systems seem to lead to an increase of the information presented to the operators. However, they have the potential to decrease the mental load by offering helpful situation assessments as well as proposals for appropriate actions. It is even possible to warn the operating personnel of upcoming - probably critical - process situations so early that these situations can be prevented in time (Elzer, et al., 1992).
This situation assessment is continuously visualized in graphical form. The pre-warning time can be varied from several minutes to half an hour. Newly identified critical process situations can be additionally taught to the NN if deemed necessary.
However, the two approaches mentioned still have the disadvantage that they can only deal with situations that have already been known during the design of the support system. In unknown situations they cannot offer any help. As well, unplanned changes in the behaviour of the plant, e.g. caused by ageing, can only be taken into account with difficulty. Furthermore, as warnings occur as 'singular events', i.e. suddenly like in conventional S&C systems, they still may cause the operating personnel to feel insecure.
E;r:perience. However, the rather straightforward construction of that prototype did not allow the identification of unknown situations. For this purpose a classification methods with two levels is necessary. Further investigations are also necessary in order to solve some problems of 'upscaling' that are expected in the transition process to a system more similiar to reality, e.g. considering the number of process values to be supervised or that of abnormal process states known to the system. This is especially true as far as the expected learning times are concerned.
In cases of several abnormal operating conditions occurring simultaneously, it can also happen that indicators or knowledge-based advisory system give warnings which ccontradict each other or at least overlap to such an extent that the operating personnel has difficulties in distinguishing them or correctly assessing their plausibility. A possible solution for this problem has been demonstrated: a knowledge-based advisory system that shows the possible consequences of occurring disturbances to the operators on request (Elzer, et al., 1992). This enables them to assess these disturbances with respect to the seriousness of their consequences and thus to set priorities for counteractions.
2.4 The Selflearning System s4 Technical Principle. In order to demonstrate the possibility to build an advisory system without the above mentioned high effort for knowledge aquisition (as in the case of knowledge based systems) or initial teaching (as in the case of neural nets) the system S4 (Support System based on Situation Similarity) has been developed at the IPP in the course of a PhDthesis (Boussoffara, 1996; Boussoffara and Elzer, 1997a). S4 is able to learn abnormal process situations and the necessary countermeasures during operation under the guidance of the operator. The basis for this ability is the formation of 'feature profiles'.
However, a still unsolved problem (also with knowledge-based systems) is, how to inform operators that the advisory system does not know the situation at hand. It is therefore one of the main research topics of the IPP to find solutions for this problem.
A situation can be described by means of several of such feature profiles that have been recorded during a certain time interval since the occurrence of the first deviation. In order to recognize the situation again at a later point in time, these recorded feature profiles are stored and compared with the momentary feature profiles over a certain range of time.
2.3 Neural Networks Technical principle. The main reason for the use of neural nets ('NN's') for solving the two last-mentioned problems is that they are able to assign probability values to each process state they have been taught. These probability values are computed simultaneously during each classification step for all pro-
Support during Identification of Situations. In the case of unknown situations S4 informs the operator
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that there is a new situation and prepares the fonnation of a new situation class. After having analyzed the situation and stabilized the process the operator can decide whether the new situation class is to be included in the infonnation base. If yes, he can assign a name to it and S4 learns it under this name. However, if the situation at hand is similar to at least one situation already known to the system, S4 tries to verify this and infonns the operator about this action and its results. This decision process is appropriately visualized.
has been developed at the IPP, which is particularly adapted to the human ability to quickly recognize and assess changes in graphical patterns: the 'mass data display' (MOD) (Zinser, 1993, Elzer, et al., 1995). The approach is main Iv based on the work of Grinstein (Grinstein and S~rgeron, 1989). Its basic principle is that the same graphical symbol (of relatively simple design) is assigned to each of the process values. It changes its parameters (form, size, colour etc.) according to the changes of the process value assigned to it. The operator does not observe one single symbol, but the entire pattern, which builds up while a number of symbols, belonging to technically connected process values, change their appearance simultaneously. This technique makes it possible to visualize an overview over the entire process with hundreds (to thousands) of values on one display screen of average size.
Support during Operator Actions. In case of process states that require actions, the operator first has to perfonn the necessary actions in order to bring the process into a stable state again. The system identifies these and compares them in chronological order to actions that have proven successful in comparable, earlier cases. In case any deviations to the currently observed procedure are detected, the operator is informed. This message does not only include a warning, concerning inconsistencies, but also a suggestion of alternative actions.
In order to support the operator during this procedure, another means of support has been introduced, the 'process notice' (Fig. 2). It mainly relieves the operator's short-time memory and therefore greatly contributes to the reduction of the probability of operator errors. The principle is that during the analysis of a process state the operator can chose the trend graphs of those process values he considers to be important and can group them around a polar diagram. Thus the operator is able to select those process values from the complex infonnation space he thinks to be important for the current analysis of the process behaviour (Boussoffara and Elzer, 1997b).
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Experimental evaluation. The effectiveness of the designed MODs was tested in a number of experiments. In particular it was evaluated how a) the operators' capability of diagnosis was enhanced by using MODs instead of 'classical' fonns of representation and b) different fonns of MODs perfonned in relation to each other with respect to the recognition of disturbances.
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In the course of these experiments in total 11.000 individual measurements were taken. Some of the results considerably exceeded the authors' expectations. So e.g. in some cases with an MOD disturbances could be recognized twenty times faster than by means of conventional fonns of representation. On the whole this fonn of representation proved extremely effective for the early recognition of disturbances, because at any point in time it helps the operator to get a very quick impression of the overall state of the process.
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Fig. 2. The Process Notice 3. THE HUMAN-MACHINE-INTERFACE PROPER
However, MODs should not be regarded as a replacement for the classical fonns of representation (like e.g. P&l-diagrams). They are an additional - though very effective - means for early warning. In order to quantitatively analyse a displayed deviation from the normal state and to decide which actions to take,
3.1 The 'Mass Data Display' Technical principle. In cooperation with the research laboratory of ASS in Heidelberg a new fonn of visualization of the behaviour of technical processes
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operators should rather use the representation of single values in P&l-diagrams.
terms 'multi-media' and 'virtual reality'. It is not always quite clear whether and to what extent progress for process control can be expected from these techniques. Therefore they are also investigated at the IPP. The results partially confirm the impression that the state-of-the-art that has been customary in process control for many years is still better than that of the techniques nowadays termed 'multi-media' in the domain of office technology. But the cheap machines, available for this huge application area, can serve as a basis for extremely comfortable and user friendly support tools for training and maintenance in our area.
3.2 Three-Dimensional Representations Basic Principle. In order to solve the problem of giving an as complete as possible survey of the state of the plant on a (limited) display screen while simultaneously maintaining the possibility to assess the quantitative state of individual process values, another approach was tested within the scope of PhDthesis: three-dimensional representations (c. Beuthel, 1997; Beuthel and Elzer, 1997).
With respect to 'virtual reality' the situation is similarily ambivalent. In the course of a study, undertaken at the IPP (R. Beuthel, 1997), a number of ergonomic problems with the terminal devices, currently used in this field, were identified. This is especially true for their constant use on the shop floor. On the other hand, some techniques that are summarized under the term 'computer augmented reality' (CAR) appear to be extremely promising. In particular for diagnosis and maintenance they can be expected to have the potential for a breakthrough (Elzer, et al. 1999). Therefore at the IPP much emphasis is put on the establishment of test installations of such methods and devices, in order to be able to evaluate their usefulness.
For a general overview over the state of a process it is necessary to display a large number of process values. With current forms of representations this usually leads very quickly to a 'crowded' screen, where in most cases single values can no longer be interpreted in a meaningful manner. Therefore it has become customary to spread the whole representation over several screens. This again leads to new problems, like e.g. to loss of time while searching for a particular picture that is necessary for a certain action, to 'window effects' which complicate the comprehension of a situation by the operator, etc.. During the work at the IPP with three-dimensional representations for completely different applications (Krohn, 1996) the impression arose that these had the potential to display more information related to a certain situation in an easily comprehensible form than two-dimensional representations. The hitherto most plausible explanation for this effect can be derived from the principles formulated by Gibson (1986), referring to the influence of the natural environment on the development of the human perceptional capabilities. It appears is that these are based on the immediate perception of stimuli created by the virtually imitated three-dimensional spaces in which the users can therefore orientate themselves intuitively.
4. SUMMARY AND PROSPECTS To the authors' opinion it could be shown in this paper that there is still a huge potential for further improvement of the human-machine-interface in power plants. In particular it appears to be possible to reduce the up to now very high engineering costs for advisory systems by introducing self-learning systems and thus to considerably improve the safety of technical processes. 3D-representations can - by means of their direct connection to the natural perceptional capabili· ties of humans - help to facilitate situation assessment and therefore reduce the reaction times in critical situations. Techniques from the domains of 'multi-media' and 'computer-augmented-reality' can very effectively support training, diagnosis and maintenance.
implementation, Therefore, at the IPP a 3D display for S&C of technical systems, especially power plants, was prototypically implemented, using the above mentioned principles. During its design, a number of design principles for artificial environments were formulated, intensely discussed and finally applied.
Effectiveness and cost efficiency of the developed systems can be verified by means of quantitative evaluations on the basis of systematic user experiments.
Afterwards, the resulting process control interface was systematically evaluated with the intention to check the assumed positive effects of this technique compared to the usual P&l-diagrams by means of user experiments. The investigation included the reaction times of the test persons as well as their error rates. With respect to both parameters the 3D-representation yielded considerably better results.
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3.3 Further Techniques The current discussion on the further development of the human-machine-interface is dominated by the
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