Research on variant design based on topological entity compression algorithm

In order to improve the efficiency of product design and shorten the design cycle of product, the research progress in recent years is introduced. The basics of variant design such as the entities of topology, the equation’s structure and the compression/decompression are analyzed. An algorithm, which can compress redundant parts, based on the conditions of algebraic equations completely is put forward. A variant design system of volumetric heat exchanger is designed by using VB and SolidWorks API (Application Programing Interface). The results show that the variant design system based on algebraic equations can be quickly and accurately modeled according to different process parameters, which can improve the efficiency of product design and shorten the design period.


Introduction
In 3D graphics, entity refers to all objects composed of visual graphics, such as point, line and surface elements, stretching, rotation and other elements, product components, with the topology relationship between various entities.The current mainstream design software of CAD (Computer Aided Design) to the present and the design of visual graphic, such as CATIA SolidWorks and Pro/e are using the way of topological entities.The variant design refers to the basic principle of the product does not change, only through specific parameters changing entities or different entities topological relation, and the rapid design of new products [1][2].
Qin chao ruan [3],Li fen liang [4]  In this paper, based on the document [6][7], under the SolidWorks software environment, a topology entity compression algorithm is proposed, and the variant design based on topology entity compression is studied at the assembly level.

Solidworks orks variant design basis 2.1 Entity topology structure
The SolidWorks' entities topology structure in general as shown in Figure 1, the S assembly contains a sub assembly S1 ,S2 and nesting child assembly S3, and the component P1, P2.The cooperation parts of components belongs to the two common parent file [8], such as with the relationship between P1 and P2 which belong to S3, if P1 move up one level, and tied with S3,it will move to S1; if the P1 moved to S2, with the relationship will move to the top assembly S. " Diameter"=40 "Part1-1.part"=suppressed"H@Sketch1@Part-1.Part"=unsuppressed

Compression and decompression
Compression of topological entities (suppressed) refers to the parts removed from the model in computer memory, and hidden from the assembly, but does not remove it, before relieving compression (unsuppressed), it makes temporarily unavailable.Compression can reduce the storage and memory, compression plays an important role in the graphic image file storage, transmission and processing.
There are 2 general ways of compression or decompression of an entity, unconditional execution command, such as the above formula gives the suppressed or unsuppressed command directly on the right end; second is the condition of compression, that meet compression or compression of entities will be existed if certain conditions satisfied.
"Part2-1.Part"=IF("Diameter" <50,suppressed,unsuppressed) In this paper, an algorithm for condition compression of topological entities is proposed.The components used in the previous stage are added to the assembly, and the constraints are set.The input condition is used to determine whether to compress or decompress of the entity.

Composition of heat exchanger
Volume type heat exchanger is a key product of the centralized water supply system, widely used [10] with the water storage capacity is large.Its shape as shown in

Ellipsoidal head
The head of heat exchanger is usually ellipsoidal head, whose plane projection is elliptical, and the semi long

Flange name
For example, the flange is named "FL-nominal diameter", and the identification code for the discharge flange is 2.
The algebraic equation for the flange with nominal diameter 32mm is shown as follows "FL-32-2.Part"=IF("Outlet diameter" =32,unsuppressed,suppressed) When the nominal diameter of the discharge pipe is equal to 32mm, the flange is decompress.Otherwise it has been in a compression state.Thus, the flange assembled with the discharge pipe will change accordingly with the change of the parameters of the sewage pipe.

Multiple conditional judgment
For parameters that vary in gradient according to global variables, the IIF function can be used to establish equations for handling multiple conditions, such as the height of the bearing and the nominal diameter, which correspond to the relation shown in Table 1.

Algorithm analysis
The variant design provided the mainstream CAD the design software, variant design based on the parameter configuration only give to the entity parameters discrete values ,parameter values can't be changed continuously, the latter use is restricted, the life cycle of variant design system is short, the preliminary design is mainly applied to small variant entities; variant design based on API develop of two times need to use advanced programming language called software's API, and the API number is large, complex structure, the long development cycle.
The proposed algorithm based on topological entity compression between the parameters of different parts established algebraic equation, the parameter changes continuously, no parameter group can significantly extend the discrete additions, prolong the system life cycle; only by programming in terms of parameter transfer and interactive interface, avoids the complex API programming, greatly shorten the system design cycle.In addition, the algorithm of top-down, variant design with global in the assembly level, is suitable for the sketch, parts and assembly, also avoids the shortcomings of the algorithm only applies to parts of entities in the literature [6][7].

Conclusion
A algorithm based on redundant topology entity conditions compression, the study of variant design in assembly hierarchical top-down, parameters in different parts were correlated by algebraic equations.Using VB language to implement the algorithm, and design the heat exchanger variant design system.The variant design system base on topological entities compressed can quickly and accurately process variant design according to different parameters, which can avoid the complex programming and a large number of parameters calculate, and can make the parameters change continuously, improve the efficiency of product design, shorten the design cycle of the system.
This work was partially supported by the Basic Research Fund of Central University (No.2682014ZT28).
E-mail addresses:1021734635@qq.com(Y.Shuai) have studied the variant design based on parameters configuration, by giving some parameters multiple discrete values, establishing different parameters, Form different configurations of an entity.When the technological parameters are changed, selecting the corresponding configuration of physical parameters to change the parameters.However, the method can only assign discrete values to the entity parameters and parameter values can't be changed continuously.Chen qiang luo [5] of the variant design based on the two development of the way, mainly through the VB, C++ and other high-level programming language called 3D software driver software of API drawing, can achieve larger.But the method involves variant design function of API, many built-in the complex structure and long development cycle.The variant design based on algebraic equation can establish algebraic equation in different parameters, which can avoid the complicated programming, and can make the parameters continuous change, can avoid the deficiency of the 2 kinds of variant design., Gao Jie[6] ,Song Long [7] of variant design based on algebraic equations, greatly improves the design efficiency of graphics.But only in the entity hierarchy which topological entities are not involved in assembly level compression.

Figure 3 ,
Figure 3, which is mainly composed of a cylinder body 1, cylinder head 2, the heat exchanger core component 3 (including tube box, flange head, etc.), hot import and export component 4, the refrigerant components import and export group5, tube box and outlet6, lug 7, pressure gauge8, instrument tube 9, emission components 10, base 11 and other parts.There are a large number of driven parameters, whether early design or late manufacturing engineering drawings, all need to spend a lot of time, variant design of its products is necessary.

Figure. 3
Figure. 3 axis a and half short axis b meet a=2b Elliptical semi long axis a and barrel diameter D meet a =D/2As shown in Figure4,the OA is X axis, the OB is Y axis, establishment of coordinate system, and safety valve pipe length is 0 L , coordinates of M with the highest point branch need to obtain, and then taking M as the starting point down the stretch entity to the exchanger head.

Figure. 4 4 . 3
Figure.4The distance between one point of the ellipse (x, y) and the long axis and the short axis is 0 y and