A PDES/STEP-based model and system for concurrent integrated design and assembly planning

Abstract

Product data exchange and interfacing between different CAD/CAM systems are of great importance to the development of concurrent integrated design environments and computer integrated manufacturing systems. This paper presents a STEP-based method and system for concurrent integrated design and assembly planning. An integrated object model for mechanical systems and assemblies is first defined by a hierarchy of structure, geometry and feature. The structure is represented as a component-connector or joint multi-level graph with both hierarchical functional and assembly relations. These hierarchical relation models are then used for uniformly describing their causal relations both for assembly level and feature based single part level. The generic product assembly model is organized according to STEP, using mostly the entities of integrated resources and partly self-defined entities, which are necessary for design and assembly planning. Based on the generic product assembly model, STEP-based strategies and agent concepts are used for agent-based concurrent integration of design and assembly planning. A prototype system, consisting of a CAD system, a product modeling system, an assembly planning system, and an assembly evaluation system is developed, in which product data can be exchanged between these subsystems. Details about the implementation of the system are addressed. The integrated design and assembly planning system can support the introduction of a new product. The results of assembly planning are feedback to the stage of assembly design to improve on the design. A case study is carried out for assembly-oriented design of a gearbox, to illustrate the proposed approach and to validate the developed system.

Introduction

The product development process is characterized by one-of-the-kind complex products; numerous participants which usually are changing from project; various forms of organization of the projects and varying responsibilities; the use of numerous different types of computer applications. These characteristics set special requirements for the product information management process, and the need for intensive data exchange and information sharing based on an open generic integration framework for the applications. Data exchange and information sharing on the level of product data instead of document data eliminate the need for human interpretation, errors and rework in data exchange. Product information management and sharing can be achieved by using the product data exchange standards PDES/STEP (Standard for the Exchange of Product Model Data, officially ISO 10303 [23], [24], [25], [26], [27], [28], [29], [30], [31]), which provide a basis for flexible exchange of product model data. The ISO/STEP-work aiming at the development of international standards (IS) for representation and exchange of product data between CAX (CAD/CAE/CAM) systems has resulted in a new product data technology. Thus, STEP can provide on the one hand methods for the development of product data descriptions and methods for data exchange and information sharing, and on the other hand it provides data exchange standards for various application domains [53], [42], [16].

The computer-aided assembly-oriented design has served as a forum for the common industry goal, the development of an open information system, which would enable better utilization of information technology for the management of design and production information process [2], [8], [69], [47], [48], [71]. There is no doubt that modeling assembled products is of increasing importance in computer integrated systems. However, current research on product data exchange is mainly limited within single-piece parts [18]. While current feature concepts involved in an assembly are all connector or joint-oriented with no consideration of various subdivisions of a machine, there are no universally acceptable representation schemes for assemblies, in particular the assembly features involved. Assembly features should be represented as different degree of ‘transparency’ as a computer system for representing a designer's intention on various levels [14], [6], [60], [59]. The semantics of assembly features as usually defined, therefore, should not be the same at different modeling levels, and the significance of communication at assembly levels should not be underestimated [57]. There will be an absolute need for communications of different types of data at assembly level between real intelligent design systems in the future. From the development of general framework model, prototype applications and partial application product data models, the work for assembly design is now shifting to the development of practical data exchange standards for assembly in computer-integrated systems.

On the other hand, assembly planning has been the focus of numerous research efforts that result in many algorithms and methods for determining feasible assemblies. Most of these algorithms, however, did not include the design into the process. Even though some computer-aided assembly planning systems were integrated with CAD systems [9], [10], [12], [70], [32], the product data standard, e.g. STEP is not used. To ensure the success of the integrated design and planning for assembly, the communication and coordination among various disciplines in computer-aided assembly systems are essential issues. STEP can provide an avenue for data exchange and information sharing between different assembly application systems. It is therefore potential to be interpretable or to be informational complete for all assembly application systems.

The overall objective of this research is to develop a product data modeling method for integrated design and assembly planning using STEP. The data exchange and information sharing are achieved between the applications using the applicable parts of the STEP, such as information modeling methods, modeling tools, data exchange formats, etc. The modeling approach is to use a generic (i.e. application domain independent) product data model for which the assembly application specific information can be mapped. This generic data model can provide flexible support for data exchange and can be implemented using various software tools. The objective can be decomposed into subtasks as follows:

• Development of product/production data models describing the information to be created, used, maintained, etc. in the design/assembly planning process.

• Development of a model for integrated management of assembly information using object-oriented methods and tools.

• Implementation of the models in the form of generic tools and an object-oriented product model environment for integrated assembly information management.

• Integration of existing design/assembly planning applications to the product modeling environment.

This paper will present an integrated method and system for concurrent design and assembly planning based on the STEP. A generic product model is built in standard language EXPRESS/EXPRESS-G, which consists of information required by integrated design and planning for assembly. The implementation and software development will be done using object-oriented methods and tools in PC Windows environment. The developed model and system will be tested by data exchange demonstration based on the product model database with pilot data. The remaining sections will discuss:

• an integrated object model for assembly design and planning;

• the entities defined in STEP reference models that are organized to represent the minimal product definition data for mechanical part and system from the assembly viewpoint;

• the approach to implement STEP compatible application for concurrent engineering, and the proposed architecture of a STEP compatible assembly schema;

• the assembly schema and the part data model are translated into the classes that both C++ language and object-oriented database can accept; and

• the examples that illustrate PDES/STEP-based part and assembly definition; the gearbox and its axle system are used as examples to demonstrate the capability of STEP compatible assembly application protocol class library and test the developed system.