Abstract There are many difficulties in realizing the CAD model of the product model. Developing a dedicated CAD system for enterprise products is becoming an ideal choice for many manufacturers. This article discusses how to use an object-oriented approach to design and implement such specialized CAD systems through examples.
Keywords object-oriented method, product model, assembly model
APPLICATION OF O-O METHOD
IN ENTERPRISE-ORIENTED CAD SYSTEM MODELING
Miao Yi Bu Fentlin Zhang Shensheng
Shanghai Jiao Tong University, Shanhai 200030
Abstract There are many diffculties in developing CAD system which can implement the help the model of real products. To develope a special CAD system suitable for a given enterprise becomes an idesl choice in many situations. This article discusses how to develop a special CAD system with Of object-oriented method and shows it through an example.
Keywords Object-oriented method,Product model,Assembly model
With the increasingly fierce market competition, how to improve product design efficiency and adapt to the rapidly changing market demands has become a matter of great concern to the company. Because of this, computer-aided design has become an indispensable tool in design. With the increasing perfection of CAD systems based on geometric models, the requirements for computer-aided design systems are getting higher and higher. Therefore, the concept of product models is proposed and it is hoped that it will give full support to the entire design process. In fact, due to the complexity and variability of the actual situation, the product model basically remains on the basis of theoretical discussion. The actual demand and theoretical lag lead people to find another way. Thus, designing a dedicated CAD system that can realize the product assembly model to a certain extent has begun to become a natural choice for many companies that have a specific design process. This article shows how to use an object-oriented approach as a tool to achieve this goal through a design example of a dedicated CAD system for product models.
1 Design Example - Air Conditioning Cabinet CAD System
Air conditioning cabinets are an integral part of large central air conditioners. It is mainly composed of a fan room, a coil room, and a gas mixing room, and provides a set of heat exchange facilities for air conditioners. External air is drawn through the fan room, mixed in the mixing chamber, and then heat exchanged with the air conditioner in the coil chamber. Large-scale air conditioning is different from home air-conditioning. It is not a mass-produced product. It is often a single product. Generally, the air-conditioning cabinet manufacturer designs and customizes it according to the user's different needs. Due to different installation forms and installation locations and different design parameters (such as power and other requirements), the product graphics are basically similar, but the local small structures are different, the dimensions are also different, and the specifications are complex and diverse. For each user, the company must design multiple sets of brand-new drawings, which are time-consuming and laborious, making the design capabilities often fail to meet actual production capabilities and fail to meet user needs. For this reason, the manufacturer hopes to have a dedicated CAD model based on the product model and achieve the following functions:
. Automatically generate the original assembly drawing according to the user's needs;
. Ability to interactively modify components on the assembly drawing as a basic element, and automatically maintain the consistency of the assembly relationship and the consistency of the three-dimensional view;
. According to the revised assembly drawing, it can automatically generate processing information, such as process card, NC code, etc.
2 The choice of object-oriented methods
The object-oriented approach is based on the idea that anything can be represented by objects. In system analysis and design, objects combine data with data manipulation behavior and are treated as an integrated whole. Changes to the system only need to change and add or delete some operations, and its basic object structure does not change. Object in the object-oriented analysis method is the direct mapping of the objective world object. The system that uses the object-oriented method for analysis and design is easier to understand than the system designed with the process-oriented analysis method, and enhances the maintainability of the system.
The advantages of object-oriented methods make it adapt to the needs of modern software engineering, but also make it very suitable for product assembly model design. The object is the abstraction of objective language by computer language; the basic relationship between objects, such as association, dependence, tolerance, inheritance, etc., is the abstraction of the complex and complex relationships between objective things. In the process design of the product model, mapping the mechanical components directly to objects and mapping the assembly relationships between them as an association between objects is a very intuitive representation. In the system design, more emphasis can be placed on how to use the computer language to accurately describe the real world, rather than the processing of data, making the entire design process more natural.
According to our actual situation, we decided to carry out secondary development on a parameter-based 2D CAD system and use an object-oriented approach to modeling.
Nowadays, the more popular object-oriented design methods in China are OMT, BOOCH, etc. This design uses UML-based ROSE modeling tools. Various modeling ideas have their own emphasis, but their basic ideas are the same. What is important here is not what kind of object-oriented modeling methods are used, but how to abstractly describe the objective world in computers.
3 Design of special CAD system
3.1 Principle Structure of the System Fig. 1 shows the basic module division of the simplified special CAD system and its functional diagrams. among them:
Rule base: Accumulated by the manufacturer's many years of production experience. It contains rules for the generation of dimensions and positions of various parts under different models and different requirements. Is the key to automatic drawing.
Fig. 1 Schematic diagram of the CAD system for air-conditioner frame
Reasoners: Extract rules from the rule base, analyze them, and generate 3D models of parts.
3D model: It is the core of the entire dedicated CAD system. It not only contains the geometric information of the parts, but also contains all the useful information such as parts assembly information, production information, etc. It is the mapping of components in the computer.
Two-dimensional view: It is the projection of a component on a plane. It is the basic unit for interactive modification on the drawing. The three-dimensional model forms a one-to-many relationship with it.
Blanking module: According to the three-dimensional model, general blanking and special blanking are performed for a flexible two-dimensional view.
Processing information generation module: Generates processing information according to the three-dimensional module, such as a CN code.
3.2 Specific Implementation of the Modeling Section The following sections discuss some of the key points in the design and implementation.
(a) The establishment of a rule base The rule base is the key for the automatic generation of drawings, and is also a database for the manufacturer's multi-year production data. These data are stored in the form of examples (queries for examples are not discussed here). The following is a simplified example of the main part of the air-conditioning cabinet - angle iron, to illustrate the basic ideas of the example of the establishment of the system:
Instance name: The name of the instance.
Room type name: The air conditioner cabinet room name, such as the aforementioned fan room, coil room and so on.
Generate rules: equivalent to the IF language name of the rule. When the conditions are met, the angle iron entity is established based on the information in the rule.
Angle iron ID: Record the ID number of the angle iron to be generated for reference by other angle irons (panels or other parts).
The head is connected to the angle iron ID, and the tail is connected to the angle iron ID: the assembly relationship of the recording angle iron.
Dimensional Formula: An expression for each dimension of the recorded angle iron.
Based on the retrieved examples, an assembly drawing is generated based on the information of each of the parts.
(b) Three-dimensional model The three-dimensional model is the core of the entire CAD system and is the hub for connecting each module. It is an abstract description of a part in a computer language. The information that three-dimensional models need to record is divided into the following categories:
Geometry information: record the spatial position of the part;
Assembly information: record the assembly relationship between the part and other parts;
Production information: record the processing information of the part, such as size;
Drawing Information: Describes how the part expresses itself on the drawing.
The three-dimensional model is not only the recorder of information, but also the maintainer of information. There is a complicated constraint relationship among various kinds of information, and the maintenance of the consistency between them is naturally responsible for the three-dimensional model. For example, when the user changes the position of a view in the component on the drawing, the other two views must also be modified accordingly. This must be done by maintaining the consistency of the drawing information and the geometric information. Another example is when a part's position or size changes, parts that have an assembly relationship with it must also be modified accordingly so that the assembly relationship continues to be maintained. This must maintain the consistency of assembly information and production information. The following uses the simplified angle iron object as an example to illustrate the design of the angle iron three-dimensional model:
Class JT
{
Geometry information:
Point1, Point2: Position of the two anchor points in the space Rotation: Angle of rotation of the angle iron in the space Assembly information:
HeadID, TailID: The ID number of the angle iron connected to the end of the angle iron. JointIDArray: The ID number sequence of the angle iron connected to the middle of the angle iron. Production model:
Width,Length,Thick: Three basic dimensions of angle iron, width, length, and thickness:
ViewID[3]: IDs of the three views of the angle iron (top, top, left) Maintaining geometry and drawing information:
TranslateViewToModal(): Converts a point in the model to a plane point in a view TranslateModalToView(): Converts a point in the view to a 3D point...
}(c) Assembly relationships between assembly parts of an assembly relationship can be expressed abstractly by the association, inheritance, and tolerance of the objects. The connection between the angle irons is a connection, and the relationship between the room and the angle iron is inclusive.
(d) Establishment of a system model
Figure 2 Object-Oriented System Modeling Diagram
Through the above analysis, system modeling is shown in Figure 2 (simplified).
4 Conclusion
Really complete product assembly of the CAD system, in theory and in reality there are great difficulties; for a certain design of a customized CAD system has become a natural choice for many manufacturers. This article uses a successful design example to illustrate how an object-oriented method can be used to model a dedicated CAD system for reference by professionals preparing to design a product-specific CAD system.