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A Graphical Language for Batch Control Johnsson, Charlotta

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A Graphical Language for Batch Control

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To Hans and Josefine

Published by Department of Automatic Control

Lund Institute of Technology

Box 118

SE-221 00 LUND

Sweden

ISSN 0280-5316

ISRN LUTFD2

/TFRT-1051-SE c

1999 by Charlotta Johnsson

All rights reserved

Printed in Sweden by Lunds Offset AB

Lund 1999

Contents

Acknowledgments .......................... 9

1. Introduction............................ 11

1.1 ContributionoftheThesis ................. 15

1.2 Published Papers....................... 16

1.3 Outline of the Thesis.................... 18

2. Petri Nets.............................. 21

2.1 BasicConcepts ........................ 22

2.2 Formal Definition...................... 24

2.3 Petri Net Properties..................... 26

2.4 Petri Net Analysis Methods................. 26

2.5 GeneralizedPetriNets ................... 29

2.6 OtherPetriNetClasses................... 30

2.7 Summary ........................... 32

3. Grafcet................................ 33

3.1 Syntax............................. 33

3.2 DynamicBehavior...................... 37

3.3 Formal Definition...................... 40

3.4 GrafcetvsPetriNets .................... 46

3.5 GrafcetvsFiniteStateMachines ............. 47

3.6 Summary ........................... 49

5

Contents

4. High-Level Nets.......................... 50

4.1 ColouredPetriNets ..................... 50

4.2 ColouredGrafcet....................... 57

4.3 ObjectPetriNetsandLOOPN............... 60

4.4 OBJSANets.......................... 62

4.5 Other High-Level Languages Influenced by Petri Nets . 63

4.6 Summary ........................... 65

5. Grafchart.............................. 66

5.1 GraphicalLanguageElements............... 68

5.2 Actions and Receptivities.................. 73

5.3 ParameterizationandMethods............... 74

5.4 Grafchart -the basic version................ 76

5.5 Grafchart -the high-level version............. 86

5.6 ErrorHandling........................ 100

5.7 FoundationsofGrafchart.................. 104

5.8 Grafchart vs Grafcet

/PetriNets.............. 106

5.9 Implementation........................ 118

5.10 Applications.......................... 128

5.11 Summary ........................... 130

6. Batch Control Systems...................... 131

6.1 Batch Processing....................... 132

6.2 BatchControl......................... 134

6.3 TheBatchStandardISA-S88.01.............. 134

6.4 Scheduling........................... 141

6.5 IndustryPractice....................... 142

6.6 Summary ........................... 147

7. Batch Control Systems and Grafchart............ 149

7.1 RelatedWork......................... 149

7.2 GrafchartforBatchControl ................ 151

6

Contents

7.3 ABatchScenarioImplementedinG2........... 155

7.4 Summary ........................... 163

8. Recipe Structuring using Grafchart............. 164

8.1 ControlRecipesasGrafchartFunctionCharts...... 165

8.2 ControlRecipesasObjectTokens ............. 170

8.3 Multi-dimensional Recipes................. 175

8.4 ProcessCellStructuredRecipes .............. 180

8.5 ResourceAllocation ..................... 184

8.6 Distributed Execution.................... 186

8.7 Grafchart Structured Recipes vs Industry Practice . . . 188

8.8 OtherBatchRelatedApplicationsofGrafchart ..... 191

8.9 Summary ........................... 192

9. Recipe Analysis.......................... 193

9.1 TheDeadlockProblem.................... 194

9.2 BatchRecipes......................... 196

9.3 BatchRecipeTransformation................ 199

9.4 Analysis............................ 204

9.5 Using the Analysis Results................. 206

9.6 Grafchart Supervisor.................... 213

9.7 Other AnalysisApproaches................. 221

9.8 ResourceAllocationandScheduling............ 224

9.9 Summary ........................... 224

10. Conclusions............................. 225

10.1 FutureResearchDirections................. 227

11. Bibliography............................ 230

A. Abbreviations............................ 239

B. Petri Net Reduction Methods................. 240 7

Contents

C. Formal Definition of Grafchart................ 243

C.1 Notation............................ 245

C.2 Grafchart -the Grafcet version............... 247 C.3 Grafchart -the basic version................ 255 C.4 Grafchart -the high-level version............. 272

C.5 Remarks............................ 280

D. Dynamic Simulator........................ 281

D.1 TotalMassBalance ..................... 282

D.2 ComponentMassBalance.................. 282

D.3 EnergyBalance........................ 282

D.4 LevelandVolume ...................... 284

D.5 ReactionRates ........................ 284

D.6 Implementation........................ 285

D.7 NotationandConstants................... 287

E. An Introduction to G2...................... 289 8

Acknowledgments

Acknowledgments

The work presented in this thesis started in August 1993 when I started as a PhD-student at the department. Now, in February1999, when the thesis is completed, there are several people that I would like to extend my special thanks to. I have enjoyed my employment at the department very much. A great part of this is thanks to the persons working here; four cheerful and caring secretaries, a handful very helpful technicians, many skillful professors, associate professors and lecturers, and a lot of friendly, joyful, crazy and funny PhD-students. Thanks to all of you! Throughout the project, Karl-Erik Årzén, my advisor, has helped and en- couraged me. He has been an excellent advisor with many great ideas and a deep knowledge in my research area. Thank you Karl-Erik! of Automatic Control and for this I am very grateful. He has also been a never ending source of inspiration and optimism. During the work with the thesis, I have had theopportunity tovisit and work with colleagues from universities abroad. I have enormously appre- ciated these sessions and I would like to thank Professor René David and Professor Hassane Alla at Laboratoire d'Automatic de Grenoble, Institut National Polytechnique de Grenoble, France, Professor Venkat Venkata- subramanian at Laboratory for Intelligent Process Systems, Purdue Uni- versity, USA, and Jo Simensen at Department of Engineering Cybernetics, Norwegian University of Science and Technology, Norway. An industrial reference committee has been linked to the project and the feedback from the members has been very valuable. Participating in this committee were: Tage Jonsson

HABB Industrial SystemsI,Lars

Pernebo

HAlfa Laval AutomationI, Stefan JohanssonHAstra Production

Chemicals

mark

HKabi PharmaciaI.

The work has been supported by NUTEK, the Swedish National Board for Industrial and Technical Development, Project Regina, 97-00061 and by TFR, the Swedish Research Council for Engineering Sciences, 95-759. I would also like to thank some people, not directly involved in the project. Thanks to my friends for giving me many great moments far away from batch control systems. Thanks to my parent and my sisters for always 9

Contents

supporting and encouraging me. The last and most important thanks go to Hans, my beloved husband, for his love, enthusiasm and help through times of hard work. As a sign of my gratitude, the thesis isdedicated to him and to our newborn daughter Josefine. 10 1

Introduction

Industrial manufacturing processes and production processes can gener- ally be classified as continuous, discrete or batch. The classification de- pends mainly upon the appearance of the process output,

JFisher, 1990K.

The output of a continuous process is a continuous flow. The output from a discrete process is individual units or groups of individual units. The output of batch processes appears as lots or quantities of materials. The product produced by a batch process is called a batch. Batch processes are common and important in, e.g., the chemical, food and pharmaceutical industries. Batch processes are often of the multi- purpose, multiproduct type, which means that different batches of differ- ent products can be produced in the same plant at the same time. The specification of how to produce a batch is called a recipe. The batch plants can have a network structure which means that the batch can take sev- eral different paths when passing through the plant. The combination of multiproduct and network-structured batch plants is the most effective and the most economical profitable plant structure. However, the multi- product and network-structured batch plants are also the most difficult plants to control. Economical interestshave causedan increasedinterest for smallscalepro- ductions, quick product changes, and order-based production. This means that there is a need for a flexible way of modifying the production in a plant. Recipe-based production is suitable for these purposes. Batch pro- cesses and the control of such processes have therefore lately received great interest. Even though batch production traditionally has belonged to the food, pharmaceutical and chemical industries, the idea of having a recipe-based system is applicable also in many other industries. Recipe-based control can be seen as a special type of sequential control. Sequential control is important, not only in batch production but also in the continuous and discrete industries. All industries have devices that 11

Chapter 1. Introduction

haveto be controlled. Thisis usuallydone sequentially,e.g. a device should first be opened and then closed, or first turned-on and later turned-off. Industrial production runs in different modes, e.g. start-up, operation and shut-down. In addition to this it is sometimes possible to decompose the production into sequential steps. Sequential control is therefore important both on a local level, compare the device control, and on a supervisory level, compare different operation modes. Grafcet is a well known and well accepted representation format used for sequential control at the local control level. Grafcet has a well accepted and intuitive graphical syntax, and a well specified semantics. Two in- ternational standards, IEC 848 and IEC 1131-3, define Grafcet and as a resultof this Grafcet hasbecome well known in industry. In the standards, however, Grafcet is refered to as Sequential Function Charts

HSFCI.In

industry, Grafcet is mainly used for PLC implementations, i.e., for local control. No commonly accepted representation format exists for sequen- tial control at the supervisory control level. Grafcet is based on Petri nets. Formal analysis methods that can be used to verify properties of asystem exist for Petri nets. In parallel with the development of Grafcet, Petri nets have been developed into high-level Petri nets, a richer and more elabo- rate version of Petri nets. Even though a strong relation exists between Grafcet and Petri nets, Petri nets are not well-known in industry. From a system theoretic point of view sequential control logic belongs to the area of Discrete Event Dynamic Systems

HDEDSI. This is a relatively

new area in the control community where, currently, a large amount of research is performed, e.g.,

JGunnarsson, 1997K,JCharbonnier, 1996K,JTit-

tus, 1995 K,JCassandras, 1993K,JHo, 1991K, etc. A large number of alter- native approaches have been introduced for modeling, analysis, and, in certain restricted cases also for synthesis of DEDS. However, their practi- cal use is severely restricted. This is due to the combinatorial complexity which makes it very difficult to scale up DEDS approaches so that they can handle industrial size applications. The research on discrete event dynamic systems can be divided into two main areas:

1. Formal methods for verification and synthesis.

2. Improved tools and languages for controller implementation.

In the first research area, formal specification languages and formal mod- eling techniques are used to describe the behavior of a system. Formal analysis techniques are then used to verify certain critical properties. A number of approaches have been developed. They are based on, e.g., state 12 machines, Petri nets, logics, and process algebras. The approaches must provide appropriate means for modeling the process and the controller. The model must be able to represent the dynamics and reactive nature of the process, and allow for proper expression of timing properties. Hence, a strong focus of formal methods is modeling and modeling languages. Most of the proposed approaches are only concerned with verification. The verifier is presented with a formal model of the process and the control system and a specification of the desired behavior of the system. The verification problem consists of demonstrating that the model satisfies the specification. In the synthesis problem a specification is given that the process should satisfy. The synthesis problem consists of the construction of a controller that ensures that the combined model of the process and the controller fulfills the specification. One formal method approach that has gaineda lot of interest in the control community is the Supervisory Control Theory

HSCTIJRamadge and Won-

ham, 1989 K. This approach has also been combined with GrafcetJChar- bonnier, 1996 K,JCharbonnieret al., 1995K. SCT has also been developed in to Procedural Control Theory

HPCTIJSanchezet al., 1995K.Petrinets

have been used as the basis for controller analysis and synthesis, in the form of Controlled Petri Nets

JHolloway and Krogh, 1994K,JHolloway and

Krogh, 1990

K. The second research area focuses on development of tools and languages for implementation of discrete event controllers. With better abstraction and structuring possibilities it is easier to get a good overview of the con- trol problem and the implementation is simplified drastically.The focus in this research area is, in most cases, on graphical programming languages rather than on modeling and specification languages. The two research areas complement each other. Most formal approaches only support verification. The designer must develop and implement the controller manually. If the approach also supports synthesis, a controller is constructed and presented in terms of a specification language. Also in this case, the user must implement the controller. It is therefore important to have good structuring mechanisms and good programming languages for controller implementations. Of course, it is also important to be able to use formal methods for verification of systems that have already been designed and implemented with, e.g., a high-level implementation lan- guage. Two lines of research have been pursued in this thesis ·Developing a programming language and tool for sequential control applications. 13

Chapter 1. Introduction

·Finding suitable ways to represent recipe-based control. The appli- cation area has been batch processes. The two lines can be regarded as two separate activities. However, there has been a strong interaction. Both research lines belong to the latter of the two research areas of DEDS. However, elements better categorized to the first area can be found. The work of developing a programming language and tool for sequen- tial control applications has consisted in providing better abstraction and structuring mechanisms for Grafcet. A secondary aim of the work has been to make use of the available formal methods that exists for Petri nets and Grafcet. The first goal is met by Grafchart, a new high-level pro- gramming language for sequential control applications presented in this thesis. Grafchart can, unlike Grafcet, be used to implement all levels in a control system, supervisory as well as local. The situation, of having bet- ter abstractionand structuringpossibilities,can be compared with moving from assembly languages to object-oriented high-level languages in ordi- nary programming. Grafcet can be compared to an assembly language whereas Grafchart is a high-level object-oriented language. The approach taken for the secondary goal is to show that Grafchart can be translated into Grafcet and /or into Petri nets. The formal methods for analysis of

Petri nets or Grafcet can then be applied.

The development of Grafchart has, as much as possible, followed the Grafcet standard. Grafchart has a syntax similar to Grafcet, i.e., it is based on the concepts of steps, representing the states, and transitions, representing the change of state. Grafchart exists in two versions: one basic version and one high-level version. The basic version is a Grafcet- based model with an extended syntax with improved abstraction facilities. A toolbox implementation exists. When the work presented in this the- sis started, the basic version of Grafchart was already available,

JÅrzén,

1994a
K. The high-level version of Grafchart contains additional high level programming language constructs and features inspired by high-level Petri nets. Also for the high-level version of Grafchart, a toolbox has been implemented. Recipe-based control can with advantage be represented sequentially by a graphical programming language. The language should have an intuitive and clear syntax and must have good abstraction facilities. This is pro- vided by Grafchart. In the thesis it is shown how Grafchart can be used for recipe structuring both at the lowest level of control and for the over- all structuring. A recent international standard, IEC 61512

JIEC, 1997K,

provides a formal definition of terminology, models and functionality of 14

1.1 Contribution of the Thesis

batch systems. The standard was originally an ISA standard, ISA S88.01, JISA, 1995K, and is therefore often refered to as the ISA S88.01 standard. The standard actually mentions the possibility to use Grafcet to control a batch process, i.e., to structure a recipe and to perform the actions associ- ated with it. However, it is only suggested for the lowest level of control. No suggestions are made for how to do the overall structuring of a recipe. The main application area of Grafchart and of this thesis is recipe-based control of batch systems. In the thesis it is shown how Grafchart can be used for recipe structuring both at the lowest level of control and for the overall structuring. By using the features of Grafchart in different ways, recipes can be represented in a number of alternative ways. All comply with the ISA S88.01 standard. The different structures are presented and their advantages and drawbacks are discussed. A simulation of a multi- purpose, network structured batch plant has served as a test platform. The thesis also shows how the recipes, structured with Grafchart, can be transformed into a Petri net and analyzed with respect to deadlock situ- ations. The results can be used for static deadlock prevention or dynamic deadlock avoidance. Grafchart, the recipe-execution system, and the batch plant are implemented in G2, an object oriented programming environ- ment. G2 is also an industrial environment which makes it possible to directly use the results in industry.

1.1 Contribution of the Thesis

The contributions of this thesis are the following:

·The syntax of Grafchart is formally defined.

·The semantics of Grafchart is defined and the translation betweenGrafchart and Grafcet and between Grafchart and Petri nets is pre-sented.

·It is shown how Grafchart can be used in recipe-based batch control both at the recipe level and at the equipment control level. A num- ber of alternative ways of representing recipes are presented and discussed.

·It is shown how resource allocation can be integrated with recipeexecution using ideas from concurrent programming and Petri nets.

·It is shown how the batch recipes can be analyzed using existingPetri net methods. 15

Chapter 1. Introduction

The work presented in this thesis is a continuation of the work presented in the licentiate thesis: * Johnsson C. H1997I: "Recipe-Based Batch Control Using High-Level Grafchart",Licentiate thesis, TFRT - 3217, Dept. of Automatic Con- trol, Lund Institute of Technology, Sweden,

JJohnsson, 1997K

The work has received a lot of interest by the members of the ISA S88.01 standardization committee.

1.2 Published Papers

The work presented in this thesis is primarily based on the following conference presentations and journal articles: ·Johnsson C. and Årzén K.-E.H1994I: "High-Level Grafcet and Batch Control",Presented at ADPM'94 (Automation of Mixed Processes: Dy- namical Hybrid Systems), Brussels, Belgium,

JJohnsson and Årzén,

1994
K Johnsson C. and Årzén K.-E.H1996I: "Object-Tokens in High-Level Grafchart",Presented at CIMAT'96 (Computer Integrated Manufac- turing and Automation Technology), Grenoble, France,

JJohnsson

and Årzén, 1996b K Årzén K.-E. and Johnsson C., andH1996I: "Object-oriented SFC and ISA-S88.01 recipes",Presented at World Batch Forum, Toronto,

Canada,

JÅrzén and Johnsson, 1996aK

Johnsson C. and Årzén K.-E.H1996I: "Batch Recipe Structuring us- ing High-Level Grafchart",Presented at IFAC'96 (International Fed- eration of Automatic Control), San Francisco, USA,

JJohnsson and

Årzén, 1996a

K Årzén K.-E. and Johnsson C.H1996I: "Object-oriented SFC and ISA- S88.01 recipes",ISA Transactions, Vol. 35, p. 237-244,

JÅrzén and

Johnsson, 1996b

K Årzén K.-E. and Johnsson C.H1997I: "Grafchart: A Petri Net/Gr- afcet Based Graphical Language for Real-time Sequential Control Applications",Presented at SNART'97 (Swedish Real-Time Systems

Conference), Lund, Sweden,

JÅrzén and Johnsson, 1997K

16

1.2 Published Papers

·Johnsson C. and Årzén K.-E.H1998I: "On recipe-based structures using High-Level Grafchart",Presented at ADPM'98 (Automation of Mixed Processes: Dynamical Hybrid Systems), Reims, France,

JJohns-

son and Årzén, 1998e K Johnsson C. and Årzén K.-E.H1998I: "On recipe-based structuring and analysis using Grafchart",Accepted for publication in European

Journal of Automation,

JJohnsson and Årzén, 1998fK

Johnsson C. and Årzén K.-E.H1998I: "Grafchart and batch recipe structures",Presented at WBF'98 (World Batch Forum), Baltimore, USA,

JJohnsson and Årzén, 1998aK

Johnsson C. and Årzén K.-E.H1998I: "Grafchart applications",Pre- sented at GUS'98 (Gensym User Society), Newport, USA,

JJohnsson

and Årzén, 1998c K Johnsson C. and Årzén K.-E.H1998I: "Grafchart and its relations to Grafcet and Petri nets",Presented at INCOM'98 (Information Control Problems in Manufacturing), Nancy, France,

JJohnsson and

Årzén, 1998b

K JohnssonC. and Årzén K.-E.H1998I:"Grafchart for recipe-based con- trol",Computers and Chemical Engineering 22:12,1998,

JJohnsson

and Årzén, 1998d K Johnsson C. and Årzén K.-E.H1998I: "Petri net analysis of batch recipes structured with Grafchart",Presented at FOCAPO'98 (Foun- dations of Computer Aided Process Operation), Snowbird, USA,

JJohnsson and Årzén, 1998gK

Johnsson C and Årzén K.-E.H1999I: "Grafchart and batch-recipe structures" Hextended version of the WBF'98 paperI,Accepted for presentation at Interphex'99, New York, USA,

JJohnsson and Årzén,

1999a
K Johnsson C and Årzén K.-E.H1999I: "Grafchart and Grafcet: A com- parison between two graphical languages aimed for sequential con- trol applications",Accepted for presentation at IFAC'99, Beijing,

China,

JJohnsson and Årzén, 1999bK

17

Chapter 1. Introduction

Grafchart has also been used in other areas of batch control. This work is only presented marginally in this thesis. More information can be found in the following conference presentations and journal articles:quotesdbs_dbs8.pdfusesText_14