12.1 PDM standards

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Although PDM uses many standards, there are no standards that are exclusively intended for PDM systems. Many standards are however closely related to PDM and originate from PDM-related requirements. Standards are enablers of interoperability, and, as interoperability is one of the most important goals of PDM, it is natural that PDM acquires many standards that are used in other domains. Examples of these domains are configuration management, documentation and content management, product life cycle management, etc.

12.1.1 ISO 10303 STEP

STEP is an ISO standard, the aim of which is to provide a standard means of modeling, specifying, and exchanging product information. STEP consists of a number of parts, each identified by a unique part number under ISO 10303. These parts are classified in the main groups [2–4] shown in Figure 12.1:

• Description methods. This group provides specifications and methods for products models, product structures, and application protocols. Important parts of this group are the members of the EXPRESS language family.

• Implementation methods. This group contains bindings, from models built with description methods to implementations. Parts from this group are interchange file format (ISO-10303-21), repositories (ISO-10303-43), and programming language support, such as for C++ (ISO-10303-36).

• Conformance testing. This group provides a framework for testing that is based on other parts of the STEP standard. Examples of units in this group are “abstract test methods for application protocol implementations“ or “abstract test methods for standard data access interface implementations.”

• Integrated resources. This group contains concepts that are commonly used by different application protocols, such as “fundamentals of product description and support,” “geometric and topological representation,” or “product structure configuration.”

  
  Figure 12.1: High-level structure of STEP.

• Application protocols. This group contains domain-specific data models for the exchange of data between applications. Some examples of these parts are “core data for automotive mechanical design processes,” “plant spatial configuration,” “technical data packaging core information and exchange,” or “systems engineering data representation.”

The core part of STEP is EXPRESS, a modeling and product specification family of languages. EXPRESS has is roots in entity-relationship modeling and database modeling. The basic elements are entities, attributes, and the relationship between the entities. The language semantic in reminiscent to some extent of object-oriented languages with data encapsulation and relationship principles. The language also includes the object-oriented concept of inheritance. Figure 12.2 shows graphical elements of the language EXPRESS-G.

Figure 12.2: Basic elements of EXPRESS-G.

The fundamental elements of EXPRESS-G language are entities. An entity describes an object that may represent a product, a component, or an abstract object. The language includes basic types (such as STRING or INTEGER), to which further types can be added. A relationship is defined as an attribute of the entity from which it originates. A data type can be a basic type, a user-defined type, or another entity. Entities can be grouped in sets designated selections and enumerations. The only difference between lists and enumerates is that an element is always selected in a selection, but not in an enumeration.

EXPRESS-G uses graphical notation and is suitable for product or information modeling. Figure 12.3 shows an example of a schema expressed in the EXPRESS-G language.

Figure 12.3: Example of EXPRESS-G language.

Another language designated EXPRESS-X is a textual representation of the EXPRESS language, suitable for automatic interpretation. The corresponding expression in EXPRESS-X appears as follows:

SCHEMA PHONE_BOOK_V1;  ENTITY Phone_book;     Name : STRING;    Revision : STRING;    Date : STRING; Content : LIST [1;?] OF Phone_book_index;    END_ENTITY;      ENTITY Phone_book_index;    Name : STRING;      Id : INTEGER;   END_ENTITY;    END_SCHEMA;  

By using EXPRESS, STEP is able to provide information models in which product definitions and product definition structures are specified in a standard format. The standardized format makes it possible to exchange information between different systems, either by on-line conversion and communication or by saving information in a so-called STEP file and then loading the file using the application protocol. When the information is exchanged between two databases or tools, the information models need not necessarily be the same. If they differ, particular data or the entire model can be transferred to the destination database through the mappings defined by the mapping schema and by a data converter. The information models and the mappings are specified in EXPRESS (see Figure 12.4).

Figure 12.4: System interoperability using EXPRESS specifications.

STEP is a set of standard specifications, and not an implementation. To implement STEP means to implement an application that uses these standards to exchange product information or that makes it possible for other applications to do so. STEP is acquired by many PDM tools, but also by other tools, (e.g., CAD/CAM tools or change management and CM tools). Many of these tools comply with a particular STEP standard (e.g., Standard Data Access Interface [SDAI]). SDAI defines the functionality of an API of a product model. API is standardized and implemented in different programming languages, such as C, C++, and Java, and also for CORBA/IDL. The development of applications that manage information within a system or between systems is greatly facilitated by using SDAI.

For many PDM systems, STEP is the main means of achieving interoperability between different tools. A disadvantage of using STEP is its relative isolation; it is used in PDM and related domains, but is unknown in other domains. Further, with the emergence of SGML and XML, the use of STEP has decreased—even in PDM-related domains—even though neither SGML nor XML can replace STEP (see Section 12.5).

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