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1、_归纳总结汇总_ - - - - - - - - - Visualization of PLC Programs using XML M. Bani Younis and G. Frey Juniorprofessorship Agentenbased Automation University of Kaiserslautem P. 0. Box 3049, D-67653 Kaiserslautem, Germany Abstract- Due to the growing complexity of PLC programsthere is an increasing interest
2、in the application of formalmethods in this area. Formal methods allow rigid proving ofsystem properties in verification and validation. One way to apply formal methods is to utilize a formal design approach inPLC programming. However, for existing software that has tobe optimized, changed, or porte
3、d to new systems .There is theneed for an approach that can start from a given PLC program.Therefore, formalization ofPLC programs is a topic of current research. The paper outlines a re-engineering approach based on the formalization of PLC programs. Thetransformation into a vendor independent form
4、at and the visualization of the structure of PLC programs is identified as an important intermediate step in this process. It is shownhow XML and corresponding technologies can be used for the formalization and visualization of an existing PLC program.I. INTRODUCTION Programmable Logic Controllers P
5、LCs are a specialtype of computers that are used in industrial and safetycritical applications. The purpose of a PLC is to control a particular process, or a collection of processes, by producing electrical control signals in response to electrical process- related inputs signals. The systems contro
6、lled by PLCs vary tremendously, with applications in manufacturing,chemical process control, machining, transportation, power distribution, and many other fields. Automation applications can range in complexity froma simple panel to operate the lights and motorized window shades in a conference room
7、 to completely automated manufacturing lines. With the widening of their application horizon , PLC programs are being subject to increased complexity and highquality demands especially for safety-critical applications.The growing complexity of the applications within the compliance of limited develo
8、pment time as well as the reusability of existing software orPLC modules requires a formal approach to be developed I. Ensuring the highquality demands requires verification and validation procedures as well as analysis and simulation of existing systems to be carried out 2. One of the important fie
9、lds for the formalization of PLC programs that have been growing up in recent time is Reverse-engineering 3. Reverse Engineering is a process of evaluating something to understand how it works in order to duplicate or enhance it. While the reuse of PLC codes is being established as a tool for combat
10、ing the complexity of PLC programs, Reverse Engineering is supposed to receive increased importance in the coming years especially if exiting hardware has to be replaced by new hardware with different programming environments Visualization of existing PLC programs is an important intermediate step o
11、f Reverse Engineering. The paper provides an approach towards the visualization of 1 / 20 _精品资料_ - - - - - - -第 1 页,共 20 页_归纳总结汇总_ - - - - - - - - - PLC programs using XML which is an important approach for the orientation and better understanding for engineers working with PLC programs. The paper i
12、s structured as follows. First, a short introduction to PLCs and the corresponding programming techniques according to the IEC 61131-3 standard is given. In Section an approach for Re-engineering based on formalization of PLC programs is introduced. The transformation of the PLC code into a vendor i
13、ndependent format is identified as an important first step in this process. XMLand corresponding technologies such as XSL and XSLT that can be used in this transformation are presented in Section IV . Section V presents the application of XML for the visualization of PLC programs and illustrates the
14、 approach with an example. The final Section summarizes the resultsand gives an outlook on future work in this ongoing project. PLC AND IEC61131Since its inception in the early 70s the PLC received increasing attention due to its success in fulfilling the objective of replacing hard-wired control eq
15、uipments at machines. Eventually it grew up as a distinct field of application, research and development, mainly for Control Engineering. IEC 61 131 is the first real endeavour to standardize PLC programming languages for industrial automation. In I993 the International Electrotechnical Commission 4
16、 published the IEC 61131 Intemational Standard for Programmable Controllers. Before the standardization PLC programminglanguages were being developed as proprietary programming languages usable to PLCs of a special vendor.But in order to enhance compatibility, openness and interoperability among dif
17、ferent products as well asto promote the development of tools and methodologies with respect to a fixed set of notations the IEC 61131standard evolved. The third part of this standard defines a suit offive programming languages: Instruction List IL is a low-level textual language with a structure si
18、milar to assembler. Originated in Europe IL is considered to be the PLC language in which all other IEC61 131-3 languages can be translated. Ladder Diagram LO is a graphical language that has its roots in the USA. LDs conform to a programming style borrowed from electronic and electrical circuits fo
19、r implementing control logics. Structured Text STJ is a very powerful high-level language. ST borrows its syntax from Pascal, augmenting it with some features from Ada. ST contains all the essential elements of a modem programming language. Function Block Diagram FBD is a graphical languageand it is
20、 very common to the process industry. In this language controllers are modelled assignal and data flows through function blocks. FBD transforms textual programming into connecting function blocks and thus improvesmodularity and software reuse. Sequential Function Chart SFC is a graphical language. S
21、FC elements are defined for structuring the organization of programmable controller programs. One problem with IEC 61 131-3 is that there is no standardized format for the project information in a PLC programming tool. At the moment there are only vendor specificformats. This is also one reason for
22、the restriction of formalization approaches 2 / 20 _精品资料_ - - - - - - -第 2 页,共 20 页_归纳总结汇总_ - - - - - - - - - to single programs or algorithms.However, recently the PLC users organization PLCopensee http:/www.plcopen.org started a Technical Committeeto define an XML based format for projects accordi
23、ngto IEC 61131-3. This new format will ease the access offormalization tools to all relevant information of a PLC project. . RE-ENGINEERING APPROACH The presented approach towards re-engineering cf. Fig.1 is based upon the conception that XML can be used as amedium in which PLC codes will be transfo
24、rmed. This transformation offers the advantage of obtaining avendor independent specification code. Even if the PLCopen succeeds in defining a standardized format for PLC applications, there will remain a lot of existing programsthat do not conform to this standard. Based on this code a step-wise tr
25、ansformation to a formalmodel automata is planned. This model can then beused for analysis, simulation, formal verification and validation,and finally for the re-implementation of the optimizedalgorithm on the same or another PLC. Since re-engineering of complete programs will, in mostcases, be only
26、 a semi-automatic process, intermediate visualizationof the code is an important point. At differentstages of the process different aspects of the code and/orformal model have to be visualized in a way that a designercan guide the further work. XML with its powerful visualizationand transformation t
27、ools is an ideal tool for solvingthis task. IV. XML AS A TOOL FOR VISUALIZATION XML extensible Markup Language is a simple and flexible meta-language, i.e, a language for describing other languages. Tailored by the World Wide Web Consortium W3C asa dialect of SGML S , XML removes two constraints whi
28、ch were holding back Web developments 6.The dependence on a single, inflexible document typeHTML which was being much abused for tasks it wasnever designed for on one side； and the complexity of fullSGML, whose syntax allows many powerful but hard-to-programoptions on the other side. While HTML desc
29、ribes how data should be presented,XML describes the data itself. A number of industries andscientific disciplines-medical records and newspaper publishingamong them-are already using XML to exchangeinformation across platforms and applications. XML can betailored to describe virtually any kind of i
30、nformation in aform that the recipient of the information can use in a varietyof ways. It is specifically designed to support informationexchange between systems that use fundamentally differentforms of data representation, as for example betweenCAD and scheduling applications. 3 / 20 _精品资料_ - - - -
31、 - - -第 3 页,共 20 页_归纳总结汇总_ - - - - - - - - - Using XML with its powerful parsers and inherent robustnessin terms of syntactic and semantic grammar ismore advantageous than the conventional method of usinga lexical analyzer and a validating parser cf. Fig. 2, 7.The conventional method of analysis of
32、program coderequires a scanner lexical analyser which generates a setof terminal symbols tokens followed by a parser that checks the grammatical structure of the code and generatesan object net. In the object net the internal structure of theprogram is represented by identified objects and the relat
33、ionsbetween them. Both the scanner and the parser to beused in this method are document oriented which impliesthat analysis of different types of documents requires rewritingthe generated code for the scanner and the parser. Anexample of an application of this method can be foundin 8.The most promis
34、ing aspect of using XML instead is thatXML and its complementary applications for transformationsare standardized so as to provide maximum flexibilityto its user. The XML based method is advantageous, since the lexicalspecification is an invariant component of XML ； thereforethe well-formedness is i
35、ndependent from the respectiveindividual application. Hence, an XML-Parser also can transfer well-shapedXML documents in an abstract representation called DocumentObject Model DOM without using a grammar.DOM is an application programming interface APII forvalid HTML and well-formed XML documents. It
36、 definesthe logical structure of documents and the 4 / 20 _精品资料_ - - - - - - -第 4 页,共 20 页_归纳总结汇总_ - - - - - - - - - way a documentis accessed and manipulated. In the DOM specification, theterm document is used in a broad sense increasingly. XML is used as a way of representing many different kind o
37、f information that may be stored in diverse systems, andmuch of this would traditionally be seen as data rather thanasdocuments. Nevertheless, XML presents this data asdocuments, and the DOM can be used to manage this data5. XSLT, the transformation language for XML is capableof transforming XML not
38、 only to another XML or HTMLbut to many other user-friendly formats. Before the adventof XSLT, the transformation of XML to any other formatwas only possible through custom applications developedin a procedural language such asC+, Visual Basic or,Java. This procedure lacked the generality with respe
39、ct tothe structural variation of XML documents. Capitalizing onthe concept that the custom applications for the transformationsare all very similar, XSLT evolved as a high-leveldeclarative language 9. XSLT functions in two steps. In the first step, it performsa structural transformation so as to con
40、vert the XMLinto a structure that reflects the desired output. The secondstage is formatting the new structure into the required format,such as HTML or PDF cf. Fig. 3 . The most importantadvantage of this transformation is that it allows a simpleand easily-conceivable representation of the documento
41、r data structure embedded inside the well-structured buthard-to-understand XML to be produced. When HTML ischosen as the format of the transformed produce it is possibleto use the extensive ability of HTML to produce aneasily-conceivable and attractive visualization of a program. Every XML document
42、has its own syntax and vocabulary.Therefore, in addition to being well-formed, the XMLdocument needs to conform to a set of rules. According toW3C recommendations this set of rules has to be definedeither through a Document Type Definition DTD or anXML Schema. The rules defined in a DTD or an XMLSch
43、ema state the hierarchical and structural constraints ofthe XML document. The DTD is for defining the document grammars； morerecently a number of 5 / 20 _精品资料_ - - - - - - -第 5 页,共 20 页_归纳总结汇总_ - - - - - - - - - alternative languages have been proposed.The W3C XML Schema language replicates theessen
44、tial functionality of DTDs, and adds a number of features:the use of XML instance syntax rather than an ad hocnotation, clear relationships between schemas and namespaces,a systematic distinction between element types anddata types, and a single-inheritance form of type derivation.In other words sch
45、emas offer a richer and more powerfulway of describing information than what is possible withDTDs. Fig. 4shows the XML technologies discussedabove and the connection between them. V. AN APPROACH FOR THE VISUALIZATION OFPLC PROGRAMS A. Overview Since Instruction List IL is the most commonly usedPLC l
46、anguage in Europe, the presented approach is basedon this language. The proprietary IL dialect Siemens STEP5and the standardized version according to IEC 61131-3are considered.The generation of XML documents showing differentaspects of a PLC program is realized in the following threesteps cf. Fig. 5
47、: 1.Transformation of the PLC program to an XMLdocument 2.Validation of the XML against the XML Schemawhich sets the syntax of the XML 3.Identification of the Instruction elements of thetransformed XML according to the instruction setof the source PLC These three steps are discussed in sub-sections
48、B to Drespectively. Sub-section E explains the visualization of thedifferent XMLs obtained during the preceding steps. 6 / 20 _精品资料_ - - - - - - -第 6 页,共 20 页_归纳总结汇总_ - - - - - - - - - Throughout this Section an example is used to illustrate the presented concepts. Fig. 6shows a PLC code written in Instruction List Siemens S5. The PLC code is written in atabular form where each row element is either a delimited list consisting of address, label,