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An Implementation of a 3-tier Hierarchical Wireless Sensor Network

Urban Bilstrup and Per-Arne Wiberg

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©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Bilstrup U, Wiberg P. An Implementation of a 3-tier Hierarchical Wireless Sensor Network. In: 2006 IEEE International Conference on industrial Informatics. IEEE; 2006. p. 138-143. DOI: http://dx.doi.org/10.1109/INDIN.2006.275749 Copyright: IEEE Post-Print available at: Halmstad University DiVA http://urn.kb.se/resolve?urn=urn:nbn:se:hh:diva-2107

Research on Data Transformation in Integrated Virtual Assembly

Environment

Wen-hua ZHU l, Deng-zhe MA 2

1 CIMS and Robot Research Center, Shanghai University, 200072, China

2. CIM Research Institute, Shanghai Jiao Tong University, 200030, China Abstract- This paper presents a new method of data transformation in an integrated virtual assembly environment. Information extraction and model conversion method is put forward. In this method the process of assembly model transformation is divided into two processes; information extraction and model conversion. Model data information is transferred into an integrated virtual assembly environment by application development of CAD systems and model conversion. Treating the feed device of a cone crusher as a virtual assembly object, the result of data transformation has been verified an in integrated virtual assembly environment. This concludes that the method is accurate and the effect of data transformation has been validated. I. INTRODUCTION

W ITH the globalization and networking of the manufacturing industry, the virtual manufacturing technique of a product becomes increasingly important, and virtual assembly is one of the key techniques of virtual manufacturing. It is universally acknowledged that product assembly is to assemble the scattered parts and subassemblies to be an integrated product. It is the last step of a product manufacturing process. Generally the traditional assembly of a product is completed with the real models, any small change will bring the result that a real model has to be rebuilt, so it is a process to spend more time and more work, which leads to the waste of capital and material. With the development of the virtual assembly technique, a new low cost and rapid method is provided to solve the question. Virtual assembly is a kind of technique that combines CAD techniques, visualization techniques, simulation techniques, manufacturing techniques, assembly techniques and virtual reality and so on [l]. Nowadays many research institutes and enterprises have Foundation item: Project supported by the Grand Science & Technology Program, Shanghai, China. (No.025115007) Wen-hua ZHU is with CIMS and Robot Research Center, Shanghai University, 200072, Shanghai, China. (e-mail: Deng-zhe MA is with Computer Integrated Manufacturing Institute, Shanghai Jiao Tong University, 200030, Shanghai, China. (e-mail: mdz(cims situ.edu.cn).

studied the virtual assembly technique and have gained a lot of progress. VADE [2,3] (Virtual Assembly Development Environment) which was developed by Washington University and the America Research Institute of Standard Technology. CODY [4] (Concept Dynamics) virtual assembly system was developed by Bielefeld University in Germany. VDVAS [5] (Virtual Design and Virtual Assembly System) which is based on disassembly was developed by ZheJiang University in China. TsingHua University put forward and realized an assembly system ASMLS [6] (Assembly SiMuLation System) in a concurrent environment. Northwest Industry University put forward an assembly simulation based on operation models [7]. These above-mentioned researches on virtual assembly have received progress respectively, but they are not mature. Virtual assembly technique is still developing. Integrated Virtual Assembly Environment (IVAE) is developed by Shanghai Jiao Tong University, in the environment an operator can use virtual reality communication apparatus to interact the assembly operation. It supports scene navigating based on position tracker, and realizes real-time collision detection and optimum path plan in the environment, so that we can ensure the actual assembly process, increase the assembly efficiency and decrease the assembly cost. To build a flexible assembly model is the first assignment in an integrated virtual assembly environment (IVAE). Although current virtual reality software has some ability of building models, if we want to create a complex shape part or a large assembly model, it cannot meet the requirement of modeling. At present the virtual model is mainly generated by 3D CAD system (e.g. UniGraphics and Pro/Engineer etc.), but IVAE cannot directly make use of the solid model which come from 3D CAD software. The model must be converted and be sent to IVAE. How to convert the model data into IVAE is the first step of virtual assembly, and is also the key step that will affect the display effect, assembly validation and required precision. This paper will focus on data transformation in IVAE. II. INFORMATION EXTRACTION AND MODEL CONVERSION METHOD [8]

Owing to the virtual reality does not have a strong ability to build models, many researchers have studied on the data 288

0-7803-9701-0/06/$20.00 (C2006 IEEE

conversion techniques from CAD systems to a virtual assembly environment. Their researches can be mainly classified into two kinds. One is that data exchange is depended on a neutral file, e.g. IGES (Initial Graphics Exchange Specification); STEP (STandard for the Exchange of Product model data); STL (STereo Lithography) and VRML (Virtual Reality Modeling Language); etc.. Although this method is very simple and direct, it has some obvious disadvantages. First, sometimes the model is simplified and approximated in the neutral file, and information can be easily lost. Moreover, sometimes there is much redundant information in the neutral file, and the data conversion efficiency is lower. In particular, there is a lack of required information of virtual assembly in the neutral file. The other kind is that they directly develop a new CAD system by themselves, and combine CAD design system and virtual assembly system to share the same data structure basis. This method can realize modeling and assembling in a system, and does not need data conversion, but it is difficult to integrate with the main CAD software systems. Each above-mentioned method has its advantages and disadvantages, in order to translate model from CAD systems to a virtual assembly environment better and more effectively, this paper put forward information extraction and model conversion method, as Fig. 1 shown, the method is divided into two processes, which are information extraction and model conversion, and then take these two processes data into IVAE. This method not only keeps the advantages of neutral file, but also compensates the disadvantages and can integrate with the main CAD systems. This method is characterized with data reliability and information integrality.

I3D CAD Assembly model

F p End Fig.2 Flow chart of information extraction program

CAD system

t IVAE environment

hierarchy structure, traversing assembly tree again can obtain mated constrains information and fill reference information of geometry item to the related parts model. According to the model data rule in IVAE, information conversion is carried on, and is written to the initial file of assembly information, it fits the complicated assembly tree to adopt recursive transversal. The functionality of the information extraction is to transfer the assembly information of CAD model to virtual environment, and make the model data which is taken into the IVAE environment more accurate, more credible and with overall information.

c

Fig. 1 Diagram of information extraction and model conversion method

A. Information extraction process By the application development of CAD systems, we can directly access the inside data of a CAD model and transmit out the hierarchy structure, parts information and constraints information of the assembly model. As Fig.2 shows, traversing assembly tree can obtain all parts information and

B. Model conversion process In the IVAE environment the assembly model can be rendered rapidly and check any conflict, therefore the model should have precise topology and geometry information, and assembly constraints information is also defined on the precise model. In the IVAE environment we adopt two kinds of models to represent the same part; one is facet model to display, and the other is convex model to detect collision. The functionality of model conversion is to display the product model and render it rapidly. Virtual reality apparatus can interact with the model to detect collision in real-time, allocate the parts position by constraint and optimize the assembly path. The conversion of the facet model exports a STL model through the model interface of CAD software. The parameterized solid model which is faceted can be taken into a virtual environment directly. The conversion of a collision model is indirectly generated by facet model, with the help of graphical system a STL model can be transferred to a neutral geometry POLY (POLYgon face) model, then carry on the convex decomposition and generate a DCP (DeComPoser) model to detect collision, a DCP model can take into the IVAE environment directly.

2006 IEEE International Conference on Industrial Informatics

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III. DETAILED REALIZATION AND APPLICATION EXAMPLE

A. Software and Hardware Environment In order to realize the information extraction and model conversion method, software environment we use UniGraphics NX as a CAD system to build a assembly model. We use Visual C++6.0 and UG/OPEN API as a development platform, using OpenGL Performer 3.0 and VR Flier developed by our project group as a graphical platform, using IVAE developed by our group as a application platform. For the hardware environment we use the Power Wall system. It mainly includes an arc screen projection system, a SGI Onyx 3200 work station and VR peripheral apparatus e.g. data glove(pinch glove), 3D position tracker FOB (Flock of Birds), etc.. The data glove is shown in Fig.3(a), there are 10 touching sensors in the finger tips, and the FOB position tracker is bound on the backs of the hand, as shown in Fig.3(b).

(a) Pinch Glove (b) 3D position tracker FOB Fig.3 Data glove and Position tracker

B. Information extraction Through the UG/OPEN API application program interface, UG NX is application developed by Visual C++6.0 to access the model data basis, the model assembly tree is traversed twice, and obtain the hierarchy structure, parts information, constraints information and referenced geometry items. According to the model data rule of IVAE, information conversion is carried on, and is written to the initial file of the assembly information. In UG NX software the assembly model is expressed with a tree like data structure which is called an assembly tree. Its assembling process is to build the hierarchical connections among the components. This connection relationship includes the assembly sequence implicitly. The component models are referred to assembly and the actual geometry data is not stored in the corresponding component in stead of assembly [9]. The assembly must be traversed in order to extract the assembly hierarchical structure and parts information. There is only one assembly tree for an assembly model, that means there is only one root, obviously the assembly root is very import to traverse the parts, it is the starting position of traversal. UG/OPEN API is a User Function toolkit that allows programs have access to object models in the database. In addition it provides a compatible way to compile and link programs with the UG environment. It supports the C/C++ language, the customer accesses the object model with UF_initialize( and returns through UF terminate(. In UG/OPEN API we can get the identifier of an assembly root with the function UF ASSEM ask root_part occ, and get the identifiers of the children component of root with the

290

function UF ASSEM ask_part occ children. For every child component we make use of seek children identifier function again, such circulation is going on until the component is constituted by a part only, then the traversal is complete. Based on this theory sub-program void CycleAssemblytree(tag_t child tag, tag_t parent tag, ivae appdata* appd) is designed with Visual C++ to traverse the assembly by using a recursion method, we can obtain the parts information with the function void sub-program UF_ASSEM_ask_part_data, AssemblyMembers(tag_t tag, ivae assembly* pa, ivae appdata* appd) is used to record the component member and part to get the hierarchy structure. In UG NX software constraints relationship is expressed by mate condition. When mate conditions between two components are defined, the system can solve the constraint automatically and parts positions are allocated. There are several constraints in mate condition, each constraint describes the mate type between the component features. Generally there are eight mate types which are mate, align, angle, parallel, distance, tangent, perpendicular, and center. Mate condition has different degrees of freedom based on the number of constraints. The mate information which should be extracted include such information as: mate component name, mate type, referred geometry item, and geometry item information etc.. Since mate condition is always stored in the component, we must traverse the assembly tree again to get the mate constraints information by using function UF ASSEM ask mc data of compnt. However these mate constraints information is not available in IVAE, they should be restructured, Table I is the mapping relationship between UG NX and IVAE. TABLE I

MAPPING RELATIONSHIP FROM UG NX TO IVAE

Mate Constraint Type in UG NX Mate

Align

Type Value in UG

Type Value in IVAE

3

Mate Constraint Type in IVAE Mate

0

4

Align or insert

2 or 8

Angle

5

Angle

4

Distance

Mate Offset 6 Distance 6ororAlign Align

NX

I or 3

1 or 3

Remark

If cylinder then 8

If the direction of

mate face is

coincident then 3,

is opposite then I

If the direction of

Parallel

7

Angle

4

mate face is

coincident then 0,

is opposite then PI

Perpen

8

Angle

4

Angle is PI/2

Center

9 10

Align Tangent

2 7

Transform

dicular

Tangent

According to relationship as shown in TABLE I, we make of UGconstrainttolVAE sub-program (UF_ASSEM_constraint_s constraint, ivae_constraint&

use


pcdata) to transfer the constraints information, make use of sub-program void PartSelectAddltem(ivae appdata& appdata) to get referred geometry item information, and utilize sub-program PartAddltem(int tag, int tag2, int type, ivae_part* part) to add geometry item to corresponding parts information. C. Model conversion The assembly model built in UG NX is exported an STL model through the rapid prototyping interface. The STL model is mainly applied in rapid prototype, here we use it to facet the solid models. In a STL file all geometry are dispersed as a set of triangle facet, every triangle are described by the coordinate of three vertex and a normal vector. The precision of triangle facet can be controlled by distance deviation and angle deviation. A STL file has less model information, and it can be put into a virtual assembly environment directly, to display the model and rapidly render. On the basis of a STL model we extract the facet information of referenced geometry items through application development of UG/OPEN API, and write it to the initial assembly information file to supplement the facet information. The collision detect model of a virtual assembly is indirectly generated by facet model, with the help of the graphical system Open GL Performer, the STL model is converted to a neutral geometry POLY model by Gsettopoly program, then the POLY model is converted a collision detect model DCP by convex decompose program Decomposer, and the DCP Model can be taken into IVAE directly, Fig.4 is flow chart of model conversion from UG NX model to the IVAE environment.

NX system to IVAE. Click UGtoIVAE menu customized in UG NX to run the Export UGtoIVAE File, convert the assembly model information, export the initial assembly information file in the IVAE environment, click Display IVAE File menu and it can show the initial assembly information converted, then carry on the conversion of model to get the corresponding STL model and DCP model. The initial assembly information file is read in the IVAE environment, and the assembly scene is presented in Fig.5. According to hierarchical structure and assembly sequences we can carry on the assembly process, snap the part with pinch glove, then move it to some position that a constraint can be highlighted to some extent. At the same time a constraint is recognized and confirmed. In the assembly process collision is detected automatically and the movement of parts will form the assembly path. Finally the assembly of feed device is completed.

UG NX1 Assembly model

Fig.5 Assembly scene of the feed device of cone crusher IUG/OPEN API | eeence geometry item facet model

Control deviationI

Exporti

STL model

IV. CONCLUSION

Gsettopoly_, |Neutral geometry |

Decomposer

Convex decomposer DCP modell

Initial assembly info TXT file

| nironmen|

Fig.4 Flow chart of model conversion

D. Application Example Treating the feed device of a cone crusher as a virtual assembly object, we build the assembly model in UG NX environment, use the method of information extraction and model conversion, and realize the data conversion from UG

This paper study on the data transformation in IVAE, information extraction and model cornversion method is put forward, and detailed realized the process ofthe method. By the application development of a CAD system, we can directly access the inside data of a CAD model and transmit the hierarchical structure, parts information, constraints information and referenced geometry item of assembly model. Facet model of STL style is exported through model interface of CAD software. With the help of a graphical system the STL model can be transferred to a neutral geometry POLY model, then carry on the convex decomposition and generate a DCP model to detect collision. This method can be applied in multi main CAD systems, probably there are some distinction among the different application development. Treating the feed device of a cone crusher as a virtual assembly object, the result of data transformation has been verified in the integrated virtual assembly environment. This concludes that the method is accurate and the effect of data transformation has been validated.


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ACKNOWLEDGEMENT

The authors gratefully acknowledge the contributions of Prof. Xiu-min FAN, PhD. Yang-xing OU, Dian-liang WU, and Run-dang YANG, et al. REFERENCES [1] Jun-qi YAN, Xiu-min FAN, Deng-zhe MA, et al. Theory, technical foundation and practice of virtual manufacturing. Shanghai Jiao Tong University Press, Shanghai: 2003 (in

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