TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 12/38 pp71-76 Volume 14, Number S1, June 2009

Role Assignment and Cooperation of Ontology and Object-Oriented Principle in Construction of Digital Product Model*

SHANGGUAN Jingchang ( ), YAN Yan ( )**, LIU Haitao ( )† , WANG Guoxin ( ), ZHAO Bo ( )

School of Mechanical and Vehicle Engineering, Beijing Institute of Technology, Beijing 100081, China;

† R & D Center, Shandong Wuzheng Group Ltd. Co., Wulian 262300, China

Abstract: Powerful expressive ability of semantic information, to be easily computed and flexibility are basic

features of digital product model (DPM). Using ontology and object-oriented principle (OOP) together to

cope with problems in modeling is brought forward in this paper. The two are widely used and do well in

modeling, but they each alone cannot cope with all issues and new challenges. Three basic requests are

pointed out in DPM modeling. Status, problems, and root of current non-semantic and semantic models are

introduced. Ontology, OOP, and their difference are introduced. It is found that the two are entirely comple-

mentary with each other. How to assign the roles and to cooperate for the two in coping with the three basic

issues in DPM modeling are explained in detail.

Key words: ontology; object-oriented principle; digital product model; data representation; role assignment

and cooperation

Introduction

There is no doubt that modeling is crucial for the study and application of the IT systems, especially the information management systems, like product data management (PDM) system. Many enterprises, schools, and other kinds of organizations have paid a great deal of attention to it, and lots of achievements have been made. Essentially the model is a kind of description about the question domain, which is in fact a kind of existence. Currently, the ontology and object-oriented family are the two main kinds of mode to describe the model. They have their own features and so have their own application fields: the ontology in knowledge

management, and the object-oriented in informantion management or data management. However, it is quite easy to find that each alone cannot cope with all the issues and new challenges in modeling. The reason is that the boundary between the knowledge management and the data management is fading away. People need to find the knowledge hiding in the traditional data base, and to richen the data base with much semantic infomantion. An effective method to solve such related issues is needed. Considering the advantages and disadvantages of the ontology and the object-oriented family, the research was done as pointed out in the topic, i.e., to study the mode of the role assignment and cooperation of the two in the construction of the digital product model.

The reason that the ontology and object-oriented are called ‘family’ is that they have formed a relatively complete system, including theory, method, and language. Currently, words maybe used by people to describe the object-oriented including thought,

Received: 2008-11-09; revised: 2009-03-05

* Supported by the Ministries’ Basic Research Foundation, ChinaKnowledge Engineering Platform for Enterprise Innovative Design(No. B0920060901).

** To whom correspondence should be addressed. E-mail: yanyan331@bit.edu.cn; Tel: 86-10-68912718

Tsinghua Science and Technology, June 2009, 14(S1): 71-76 72

principle, method, mode, and mechanism. Here, the object-oriented principle (OOP) is uniformly used to represent them and has no specific distinction.

1 Three Basic Issues in Modeling

Basic issues in construction of general digital product model (DPM) are to be studied here. DPM is the pro-jection picture of the outside world to the IT world. Maybe, this is the most basic definition about DPM people can give, and is the place of its nature. The DPM’s validity lies in its consistency with the outside world. The outside world includes objective world and part of the subjective world, which mainly refers to the specific requirements of a people or an enterprise. Any information, generated during the formation of a prod-uct, i.e., from its non-existence to existence and throughout its life-cycle, may become part of its DPM. Considering the essential provision of the DPM, the following three basic issues must be coped with effec-tively in modeling.

1.1 Same logical structure with outside world

With the ceaseless complication and high-technicalization of the products, the DPM inevitably becomes more and more complex. The complexity contains two aspects: semantic relationships among objects related to a product, and property information of an object itself. Objects mean enterprise/ business elements, which are related to the formation of a prod-uct, including resource, personnel, and capital. The two aspects must be faithfully and accurately reflected in the DPM, in order to make it useful.

1.2 Easily processed in computers and web

This is a pretty realistic problem. The current computer architecture, with a very simple processing logic, is fixed and limited when Alan Mathison Turing and John Von Neumann invented it at the very beginning. When all the things have been ultimately converted into sets of byte, a computer can deal with it. This forms one couple of contradiction, which is between the complex logical structure of the model and simple syntax proc-essing mode of the computer. This leads to a multi-level abstract structure of the current computer system, and makes people have to balance between a suffi-ciently represented DPM and a quite easily computed

DPM. An effective mechanism, which can assure to parse easily and precisely semantic information to syn-tax information, is crucial for the enhancement of the processing efficiency of the computers and web.

1.3 Complete adaptability to changes

If what the paragraph 1.1 and 1.2 discuss means a static view on modeling, here is the dynamic one. That is to say, DPM must be able to successfully respond to the changes of the outside world. Sometimes changes are fearsome things to people, but they are obstacles to the software engineering for sure, and things do not get solved till now. Dynamic adaptability to changes from the outside world is very important for modeling.

These three issues generally reflect the relationship between the IT world and the outside world. In order to have a sustainable vitality, a DPM must satisfy the three points simultaneously.

In fact, people can consider these modeling issues at the very start from fundamental and philosophical per-spectives, but whether those issues can be solved ac-cordingly or not relies on the development level of the IT itself and the society needs of production and life. Now, the putting forward of these issues is the code-velopment result of the IT and the society. Tradition-ally, people only concern the results such as the PDM systems, but now, people want to accomplish a more complete description on the product data.

2 Status, Problems, and Root 2.1 Status and problems

2.1.1 Non-semantic model Currently, study on non-semantic model mainly re-flects on those commercial PDM systems such as Windchill PDMLink of Parametric Technology Corpo-ration (PTC), ProductManager of international busi-ness machine (IBM), Beijing Daheng DHPDM, and Wuhan Tianyu PDM. Almost all of these models are created by taking product function and structure com-ponents as the core, and based on the OOP[1].

A main problem existing in such a model is it cannot describe complex semantic information; the reason is that OOP only reserves logical relationships like “is-a (inheritance), part-of (composition)”. Of course, it is not to say that OOP cannot represent other kinds of

SHANGGUAN Jingchang ( ) et al Role Assignment and Cooperation of Ontology … 73

logic relationship, but it needs a lot of code to be de-veloped. This is why the current application systems are so huge and hard to change. Some other causes about the flexibility of the IT systems are not included in this topic. 2.1.2 Semantic model People paid a wide attention to the semantic model and created a lot of data representation methods. Among these studies, semantic models constructed by using ontology counts for a big scale, and got significant re-sults. Main problems existing in the ontology-based models include unable to describe some problems of subjective aspects and difficult to store large volume of data. It is still impossible for them to support tradi-tional industries such as manufacturing industry.

2.2 Root of problems

Through the analysis above, problems are found that ontology or OOP each alone cannot cope with all the new challenges in modeling, especially in the changing situation. However, ontology and OOP are entirely complementary with each other. Therefore, a method researching the mode of role assignment and the coop-eration of ontology and OOP is adopted.

3 Introductions and Comparison 3.1 Ontology

Ontology is a transcendent principle system[2]. The word “ontology” did not emerge until the 17th century, but it is usually regarded as the trunk of the Western philosophy tradition or as the first philosophy from Plato to Hegel. It is a unique shape of Western philosophy.

The leading reason of ontology being introduced into engineering field is that people encounter many practical problems like integration of different applica-tion systems, business processing, and enterprises net-work. Maybe unified norms and standards can be cre-ated only from the view of philosophical height and by using of ontology, which can establish consensus among different opinions, systems, and businesses. The first problem solved by using ontology is to elimi-nate differences among different languages and cul-tures. An ontology is an explicit specification of a con-ceptualization[3]. An ontology is a common, shared, and formal description of important concepts in a

specific domain[4]. The greatest contribution of the on-tology is to find some facts in research field, i.e., on-tology, scope, and border of the domain[5]. Information systems need to reason the model of the world, so the researchers use the ontology to describe the informa-tion of the target world[6]. Many other researchers and users have found their chances through ontology.

When applied in the engineering field, the philoso-phical nature of ontology should be fully maintained. That is to say, people should not arbitrarily tamper with the essential connotation of it. The ontology should fulfill two kinds of tasks in the engineering field: to provide shared concepts and to describe inherent laws of the field. The latter means the unchangeable things in a specific field. For example, design of different products may go through the same process, which in-cludes four phrases, i.e., requirement domain, function domain, physical domain, and process domain[7].

3.2 OOP

When traditional structured programming model could not cope with large-scale commercial application de-velopment, OOP was introduced to solve such prob-lems in programming[8]. OOP is a new way of observa-tion on programming. In OOP, a program is separated into a lot of small pieces or objects, which simultane-ously contain information and function (or attributes and methods). Application is developed through com-bining objects the same way as building a house with bricks[9]. OOP can cause accomplished reusability and scalability. Lots of nice words may have been used by people to express their evaluation on OOP. Almost all current applications are constructed based on OOP.

OOP emphasizes in constructing systems from what the objective world inherently has, in cognizing, un-derstanding, and describing the objective things from the thinking habit of the human. The software system is able to reflect the question domain, the objects and their relationships can reflect the things and their rela-tionships faithfully and accurately about the question domain[8]. The foundation of the object-oriented meth-odology in thinking science, epistemology, and meth-odology is discussed in Ref. [10]. The literature points out that this is the fundamental reason why the object-oriented methodology can keep the same structure as the question domain and its model and why it is ap-plied widely.

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3.3 Differences

The difference between ontology and OOP is that OOP is a simplified version of ontology. It has two aspects: OOP reserves only two kinds of logical relationship, “is-a” and “part-of”; OOP gives up some of its provi-sions of philosophy; it allows itself to describe some subjective things. This simplification formed not pre-meditatedly but naturally. The simplification provides a possibility for the two to assign their roles and to cooperate with each other.

Of course, it is not accurate to say that OOP is a simplified version of ontology. The reason is that they both have their own merit to be brought into the engi-neering field and have their own application fields. Saying like that is due to their common nature, they both can be taken as an existing state with a solid ob-jective foundation.

4 Role Assignment and Cooperation

Currently almost all the commercial systems are

created based on OOP. The systems become bigger and bigger, and unfortunately their functions are not ap-pended. Once a system is finished, it is impossible or at least quite difficult to make any changes on it.

Taking Windchill PDMLink of PTC as example, its model can only describe two kinds of logical relation-ship, use/use-by and replacement; it is impossible to acquire some more semantic information from data people put into; this obviously can not meet the need of people’s expectation and adapt to the development of the situation. By using ontology, disadvantages of OOP can be overcome, for instance, “properties and methods” is all packaged in objects; rich semantic de-scription can be realized and the code development volume can be reduced. In addition, it is possible to provide some more effective data service based on such a model. Through in-depth and detailed compara-tive analysis on the two, the model is given, that how the two assign their roles and cooperate with each other. Table 1 shows the detail.

Table 1 Role assignment and cooperation

Essence Role assignment Ontology

Philosophy (Objective) Semantic structuresCooperation

OOP half-Philosophy

(Objective + Subjective) Syntax elements

iii Complete representation on product information iii Effective parsing from semantic information to syntax one iii Whole digestion of changes from outside word

Note: 1. “Philosophy (Objective)” means that there is a scientific, solid, and sustainable origin and foundation. 2. “Role assignment” and “Cooperation” are just indicative. In fact, there is no absolute distinction.

4.1 Expression of complex logical structures

Construction of the DPM concerns two aspects: logical relationships among objects related to the formation of a product and attribute information of the objects themselves. The “is-a” and “part-of” are the two logi-cal types that are used quite often and easy to under-stand, and they are the only ones OOP support. It is quite clear that OOP cannot meet the increasing com-plexity of the product model. Luckily, current research on ontology can solve such a problem: to compensate the lack of OOP in semantic information representation.

OWL is introduced by the World Wide Web Consor-tium (W3C) [11]. The theoretical basis of the OWL in-cludes two aspects: the ontology, which qualify the description object and the description logic (DL), which is a system of knowledge representation

language. To the ontology, OWL can only be used to represent those unchangeable things in a specific field. Those things include two parts: concepts that will be shared among different people and systems; inherent logical relationship among those concepts. This is de-termined by the philosophical nature of the ontology. To the description logic, OWL has a strong expressive ability, so it can represent richer semantic information of products. According to the description logic, people have to balance between the expressive ability and computing efficiency for so many times. Using the ontology to represent the inherent part of an engineer-ing issue is quite valuable, the reason is that those things are independent with a specific field and un-changeable; therefore, it is very easy to develop proc-essing program for the DPM.

SHANGGUAN Jingchang ( ) et al Role Assignment and Cooperation of Ontology … 75

The DPM will inevitably contain that kind of information related to a specific organization or indi-vidual. The characteristics of that kind of information include time-related, region-related, simple structure, and large amount of data. Representation of such kinds of information is just what the OOP good at. OOP’s biggest advantage is that it can describe structural in-formation and processing program according to users’ requirements.

4.2 Processing in computer and network

For an effective DPM, the key is that its complex se-mantic information can be easily parsed into the syntax information which can be computed. OWL has solved the problem successfully, the reason is that its nature is XML and it is an expansion of XML. OWL has the potential to be used widely like XML, and can be eas-ily exchanged among different people, systems, and networks.

In OOP, people can define concepts at random so long as they need. This is different with the ontology. This also produces a benefit that people can map all the concepts defined in the ontology. This provides a possibility for people to deal with the semantic infor-mation and the syntax information in DPM respec-tively and to develop program respectively. Such a distinction makes people realize rich semantic infor-mation representation and efficient processing. It also makes people to take full advantages of existing appli-cations. Almost all of the current applications are con-structed based on OOP.

4.3 Digesting changes from outside world

Traditionally, software engineering is not prepared for the changes of the outside for their rigid models. The reason is that the survival mechanism of their models is inconsistent with the change mechanism of the out-side world. Ontology and OOP together can solve this problem. What leads the changes and how to digest those changes for a DPM constructed by using both the ontology and OOP are as follow:

Changes from technology, customers’ requirements, and so on may lead the changes of a DPM. If a DPM wants to maintain its validity, it must response to these changes. These changes will lead to two kinds of changes for a DPM: logical relationship among objects and attributes of object.

The logical relationship of a DPM represented according to ontology can be expanded or compressed from any point or edge of it. It is quite easy. These changes bring changes to its structure, but will not af-fect its processing mode. The attributes of an object can be appended and reduced quite easily. The volume of an object’s attribute does not affect its processing mode.

5 Conclusions

In order to improve the level of data management and service, some related issues in DPM construction are studied. Works and conclusions show as following:

Three basic issues have been pointed out in model-ing including same logic structure with outside world, easy to be processed in computer and web and com-plete adaptability to changes from outside world.

From non-semantic and semantic aspects, status, problems, and root in modeling are introduced. Neces-sity of role assignment and cooperation of the two is proved.

Detailed introduction and in-depth comparative analysis about the ontology and OOP are made. Differ-ences between the two make it feasible to work to-gether in modeling.

In the end, the mode of role assignment and coop-eration for the two is given in detail.

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