Research of data acquisition process and reconstruction for constructing prototype surface*

Dong－man YU，DiWANG，Xiao－peng CHEN，Xiao－jing LI Henan Polytechnic Institute，Nanyang 473009，China;

2 Henan North Star Mechanical and Electrical Co.LTD.，Dengzhou 473000，China

Research of data acquisition process and reconstruction for constructing prototype surface*

Dong－man YU?1，DiWANG1，Xiao－peng CHEN2，Xiao－jing LI11Henan Polytechnic Institute，Nanyang 473009，China;

2Henan North Star Mechanical and Electrical Co.LTD.，Dengzhou 473000，China

As a modern design method that contrary to traditional design，Reverse engineering(RE)plays an important role in accelerating conceptual design and physical design.The key problem of RE is how to obtain point data，p rocess data and reconstruction data for a prototype.Therefore，this study focuses on the key steps of processing point cloud and reconstruction mode.Based on RE system ofmechanical structural design，the dim ensional surface module is established.The scattered point cloud data are acquired by optical scanning the physical object using three-dimensional coordinate measuring machine(CMM)and an optical scanning device.After the establishmentof selected model，data process for prototype，includes noise elim ination，data interpo lation，data smoothing，data filtering，data splicing and surface reconstructing，are conducted subsequently.Through processing ofmeasurementdata，a CAD model is created and fabricated in stereo lithography apparatus(SLA)machine.

Reverse engineering，Constructing surface，Coordinate m easuring machine

*Project supported by Provincial Science Foundation for Distinguished Young Scholars of Henan(Grant No.2012GGJS－248)，and Provincial Key Research Plan of Science and Technology(Grant No.112102210207)

? Dong－man YU，PhD.E－mail:yudongman@126.com

1.Introduction

Fabrication of pioneer products with lower cost and shorter cycle is a priority choice that an enterprise should conduct in such a competitive market circumstance，and reverse engineering plays an important role in propelling product development and borrowing novel ideals from other structure to meet such a requirement.In contrary to traditional design method，RE technology adopts special measuring technology and apparatus to get dimensional data［1－3］.After building a stereo model with measuring data，themore detailed analysis，modification，verification and fabrication would be carried out subsequently.In additional，combined RE and RP(rapid prototyping)technology，a particular manufacture technology，additivematerial forming process，which is different from the formerly removalmaterial forming technology，we can replicate ormanufacture the improved production and quickly evaluate ormodify the production［4 －6］.The RP system could produce working route from standard data or meshed STL model from the RE model.However，due to special structure and complex topology relation，obtain full surface data of a prototype，in most case，is a complicated work and should carry out a series of data processing procedure［7］.

The starting point of RE technology for an engineer，is to obtain the full point cloud data.The measuring technique of point data，inmost case，can be divided into 3 kinds in accordance with its feature and application，which are of contactmeasurement，non－contactmeasurement and layer－by－layer scanning method，respectively.The accuracy of contactmeas－urement is quite high and reliable，and one of the common tools used for surface reverse is CMM machine.The non－contact measurement can scan surface with high－speed and avoid of probe radius compensation，which has been widely used in industrial detection.However，the two measurement methods mentioned above also have some deficiency.For example，it is quite difficult tomeasure the inner contour of model body. The layer－by－layer scanning method，developingwith photoelectric technology and graph manipulation，can solve this problem，which has higher precision and could quickly measure the inner structure.The typical devices include laser scanner system，structural light scanner system，industrial computed tomography or stereo－radiography.

In thiswork，amodule of fish sample is selected as research object and used to obtain surface data.Figure 1 shows the whole devices of the point cloud acquisition system，which belongs to non－contact measurement method.The optical imaging device，CS－0400，offered by Beijing 3D CaMega Co.，Ltd，is installed in a CMM machine，CLY－CT2H，which is offered by Chengdu APICo.，Ltd.The CMM machine scans the workpiece surface from one point to another，namely by picking points through scanning the line.The scanning line is an intersection of the scanning plane and the workpiece.The photo source mounted on the scanning head is a filament lamp，and the double－side of the scanning head is an optical pickup tube.The measurement system，a non－contact device herein，adopts the projection grating method with triangulation principle and coded scanningmode.The scanning accuracy reaches 0.03 per frame，the global accuracy reaches 0.1 mm permeter.

Figure 1.Point cloud acquisition system，CMMand auxiliary attachment

2.Digitization of the surface

Two main scanning modes are frequently used for RP technology，which are radial scanning and axial scanning，respectively.Measurement path planning is concerned with determining the position of surface points，the directions of probing vectors and a collision－free path.In order to achieve the best point cloud，it is essential to arrange the measuring path properly.In this project，themost important surfaces are some free－form surfaces，such as discontinuity surface and transition surface.Selection of probe orientation is a key step for this system，while improper probe orientation method maymake a reachable point become unreachable.The CMM should be adjusted with different poses，at the same time，lowering the step size can get more points in this free－form surface.The point cloud derived from scanning of sample surface by optical non－contact device is shown in Figure 2(a).In this figure，somemalformed areas of point cloud in fins and tailsmust be processed subsequently.

Several datum－points should be created at different position of the surface andmarked beforemeasuring.For example，eightmarked points，served as index，are distributed in different areaswhich is shown in Figure 2(b).With the scanning head moved from one position to another position formeasuring the feature，three datum pointsmust be measured again for each step.The surface reconstruction is an indispensable step for this course，themain reason is themachining procedure needing the triangular mesh created by smooth surfacemodel or good point cloud.The result of a mechanical tracing process is a structural point sequences with a large number of points and a line structure.The output data of the optical device are coordinate values of the center of the probe and normal vectors in the X，Y and Z direction at the position of the surface points.Additionally，the output format of themeasured data does not accord with the demanding of the creation of a CAD model.Thus we must process themeasurement data.

3.Processing ofmeasurement data

Adequatemeasuring pointswould lift the surface accuracy and reduce the deviation betweenmodel and prototype.However，plenty of noise mixed in point cloud will increase rough error.The data of prototype might contains some errors that caused by anthropogenic factor or enhancement factor，especially in sharp edge and boundary point.Constructing the surface with error or bad pointmight lead to surface deflection or structural interference，even will lead to surface fitting failure.Therefore，the data obtained from physical model should be processed before building model.The data processing includes noise elimination，radius compensation，data splicing，data resampling and data boundary treatment.Generally，the data process for thin－wall big－area physicalmodel should take the following steps.

Figure 2.The point cloud of the samplemodule after splicing of several blocks

3.1.Data splicing and elim ination

In most case，some complicated workpiece cannot be described by a single surface，should be constituted by several surfaces.Therefore，the acquired point cloud should be divided into blocks according to covering power and construct each block into surface model.These surface models should be converted and aligned in one coordinate system.Figure 2(a)shows that four data blocks are spliced together to form whole data，the shade regions are overlapped by two or three regions.Three kinds of data division methods are in common use，which are based on edge，based on surface and based on edge＆ surface，respectively.

Noise is a fundamental component of surface image，especially prevalent in dense prototype.In the course of product numeralization，quantity noiseswill be introduced bymeasurementerror and operation error.For the reason of reducing or removing noise that influencing on model，the noise points would be picked out and removed away from cloud points.In this course，the visual inspection and spatial domain are common methods used for eliminating noise.Noise interrupts the boundary，resulting in incomplete surfaces in 3D reconstruction that typically appears rough. However， eliminating noise points would also delete some regular points，sometimes even result in porous surface，which is illustrated in Figure 3(a).

Figure 3.The point cloud of the samplemodule

3.2.Data interpolation and smoothing

Data interpolation method is used for refilling the vacant data for some areas that cannot be measured directly.Two kinds of surfacemodels should be take into account，one is surface reverse modeling based on point＆ spline，another is surface fitting based on point.The point cloud of the sample with data interpolation is shown in Figure 3(b).Figure shows some porous surfaces have been refilled.

Data smoothing aims to erase hash points and cuspidal points randomly distributed along the boundary.The data smoothing technology contributes to obtain good feature extraction result and establish accuratemodel.

3.3.Data filtering and surface reconstructing

Further improving surface precision technology is data filtering.Themethods of neighborhood averaging and median filtering are common used for eliminating system noise，among which the former can smooth the signal and the later can eliminate the noise.The experiment has confirmed data filtering could refine the measuring quality.A 3D model of the sample after data smoothing and filtering is shown in Figure 4(a).

A complex 3D model can be assembled by mul－tiple simple surfaceswith different features.After the digitization of themodel and a series of steps for processing，we can get plenty of spatial divergent points randomly distributed.However，the data still need bemodified or repaired furthermore for better results.From Figure 4(b)，we can see a 3D model of the sample after surface reconstructing，some bad points and overlap points have been repaired ormoved.

Figure 4.The 3D model of the sample after data

4.Fabrication ofmodel

After building themodel，the 3D model is converted into special code，STL format，which has been widely accepted as industrial standard.In the translation process，STL model would be discreted along special direction and generated by tessellating outside skin of the CADmodel.With the help of RP technology，we can manufacture the production with standard 3D model and could quickly evaluate or modify the production with shorterwork cycle.In this study，we choose a stereo lithography apparatus(SLA)machine as rapid prototyping device to fabricate the model.The whole device of SLA system is shown in Figure 5.

Figure 5.Whole fabrication system for 3Dmodel

Selection or optimization of forming orientation is a necessary step for fabricating a model accurately with high－speed.Choosing proper parameters and slicing the model with slice software，then we can get plane information of each layer and grid vector data，which is used to control scanning trace of laser beam.The slicing parameters have a significant effect on finished quality.Therefore，in this course，the slicing thickness，scanning mode，scanning speed and radius compensation must be assured.Table 1 and Table 2 listed all the important technical parameters of fabrication process.

Table 1.Parameters of support and STL

Table 2.Parameters of SLA process

5.Conclusion and further research

Based on previous research of RE technology，the authors have explored the engineering application of RE technology in designing and manufacturing of complex prototype.This paper takes a toy prototype as an example，describes the whole processes from object digitization to CAD model reconstruction，and have gained a good result.With the help of RE technology，we can accelerate product design and borrowing ideals from other business.It assumed that this technology would havemore extensively application in mechanical fields.

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構建原型表面的數據獲取處理與重構研究*

余東滿?1，王 笛1，陳曉鵬2，李曉靜1

1.河南工業職業技術學院，河南 南陽 473009;
2.河南北方星光機電有限責任公司，河南鄧州 474150

逆向工程是一種與傳統設計相反的現代設計方法。它的重要作用就是加速概念設計和物理設計。逆向工程的關鍵問題是如何獲得點云數據、處理數據和重建數據原型，因此著重研究處理數據點云和重建曲面模型，并經過基于逆向工程系統的機械結構設計最終建成空間曲面模型。使用包含三坐標測量機(CMM)和光學儀的三維光學掃描設備，完成實物對象分散點云數據的獲取。獲取點云數據后再進行數據處理操作，包括噪聲消除、數據插值、數據平滑、數據篩選、數據拼接和曲面重建。處理測量數據創建出CAD模型，將模型在激光快速成型機(SLA)上制造出來。

逆向工程;曲面構建;三坐標測量機

TN919.5

10.3969/j.issn.1001－3881.2014.12.022

2014－01－15

book=115,ebook=259