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14. Reverse Engineering
(Thanks to Yvan Germain and to Creaform for their contribution )
Reverse engineering is a relevant topic for this book, since with thedevelopment of recent data acquisition technologies, it allows more and moredesign and analysis possibilities.
This chapter does not intend to cover all software capabilities or workingmethodologies, but rather to give a quick overview of what can be done inthis eld. To do this, four dierent workbenches will be used: DigitizedShape Editor , Quick Surface Reconstruction, Free Style , and GenerativeShape Design , and it would be a good practice to have them readily availableby customizing the user environment and having the Workbenches toolbardisplayed on the screen to save time when moving from one workbench toanother.
DSE:Digitized Shape Editor
QSR:Quick Surface Reconstruction
GSD:Generative Shape Design
In all workbenches, tools will be available from toolbars or from the Insertpull-down menu. To facilitate the association of the tool with its respectiveworkbench, each workbench acronym (DSE , QSR, FS, or GSD ) will beincluded within parentheses in the command name.
In order to illustrate some commands or behaviors, an STL le is used as ascanned support. This cloud of points le could come from a laser scanner orfrom another source.
This chapter will cover the following:
Importing/exporting a cloud of points
Aligning the scanned model
Generating geometry from the cloud of points
14.1. Importing/Exporting a Cloud of Points
The rst thing to do when working with a cloud of points is to import it into apart. Since the data le cannot be opened directly in CATIA , it is necessaryto launch a new part and to activate the Digital Shape Editor workbench inorder to be able to access related tools. Note that a cloud of points can alsobe imported into a product if needed.
CLOUD IMPORT TOOLBAR (DSE)
As shown in Figure 14-1, the Import tool brings up a dialog box that allowsthe user to select the le containing the point data and to format it in orderto get the desired working elements in the 3-D environment.
Info: Imports a cloud lePrompt to User: Select a format and a le to preview or import
To start, it is necessary to select the scan Format to get the right leextension from the list of available formats; here, STL will be used. The le toopen then has to be loaded. This can be done by clicking the ellipsis ()button and browsing the available folders. After this, the Scale Factor mustbe checked in order to reect scan le units and system units; for example, ifa scan was done using mm as reference units and is loaded on a system thatuses inches as working units, it will be necessary to use a 1/25.4 factor inorder to have the le read at the proper scale. Finally, clicking the CreateFacets check box will generate a mesh of triangular facets that will be usefulfor some operations. Clicking Apply will allow previewing the result, whileclicking OK will conrm and end the operation. The entire scanned lecontent will then appear on the screen, as shown in Figure 14-2.
Figure 14-1. Import dialog box.
Exporting a cloud of points is sometimes necessary in order to decrease theamount of data to work with or to create partial data les. In the Export tooldialog box, shown in Figure 14-3, the user is allowed to choose theElement(s) to export and browse in order to locate the folder in which the lewill be saved. Clicking the OK button will complete the operation. It isimportant to note that the data will be exported using the system's units andwill not necessarily keep the original units used.
Figure 14-2. Imported scan.
Info: Exports a cloud to a lePrompt to User: Select visible clouds or meshes to export
14.2. Aligning the Scanned Model
A model that is scanned is not always properly oriented or positioned withrespect to the coordinate system when it is inserted into a .CATPart le. Ifsome scan post-processing is necessary, it is good to know how to manipulatea cloud of points in the 3-D environment since it can be required for furtheroperations later on. This is done by creating reference elements from areasor features of the cloud of points and using them to locate the entire cloudwith respect to these references.
CLOUD EDITION TOOLBAR (DSE)
In many cases, a cloud of points will need to be broken in smaller point setsin order to be manipulated or reworked. This will usually not be done bydeleting elements from the set, but by activating or deactivating theelements that are necessary to perform a certain task.
Figure 14-3. Export dialog box.
Info: Activates or deactivates cloud areasPrompts to User:When the tool is activated : Select the cloud of points or the mesh toprocessWhen a cloud is selected : Message varies with selected mode
The Activate tool allows the user to select only scanned elements he or shewants to work with. The displayed dialog box, shown in Figure 14-4, allowsthe user to select among dierent options to make the element selection. TheBrush option, used in the illustration to prepare elements in order to createa planar reference, selects a number of elements with a single stroke and isvery ecient to use. Note that if too many elements are selected, it ispossible to deselect some of them by pressing the SHIFT button while movingthe cursor. If the Trap option is used, there is a possibility that elementslocated at a distance from the area of interest are selected at the same time.The Flood option then allows the user to select the ones that are connectedtogether. By clicking the OK button, only the selected elements will stayvisible on the screen, with the others being deactivated. To retrieve thecomplete cloud denition, it is necessary to reuse the tool, to select the cloudof points in the 3-D environment or in the Part Specication Tree , and to usethe Activate All button.
Figure 14-4. Activate dialog box.
As mentioned previously, while selecting elements to activate them, it ispossible to use the SHIFT button to deselect some elements. The Remove toolpermanently removes elements from the cloud of points kept in the le andthen creates a hole in the mesh, as shown in Figure 14-5. Elements removedcannot be recovered. If a removed element needs to be used again, theoriginal cloud of points must be imported again.
The tool oers a dialog box similar to the one used by the Activate command,as shown in Figure 14-6.
Info: Removes points/vertices/trianglesPrompts to User:When the tool is activated : Select the cloud of points or the mesh toprocessWhen a cloud is selected : Message varies with selected mode
Figure 14-5. Removed points from scan.
SURFACE CREATION TOOLBAR (QSR)
In order to be able to realign the cloud of points with respect to the partcoordinate system, it is necessary to create reference elements to performthis task.
Basic Surface Recognition (QSR)
The Basic Surface Recognition tool oers predened primitive-like options tocreate surfaces from an activated portion of the cloud of points. When usingthe Plane option, the green arrows appearing on the surface boundaries can
Figure 14-6. Remove dialog box.
Info: Recognizes Basic SurfacePrompt to User: Select a cloud to be analyzed or a line to set the directionof the axis or a point to set the center
be used to resize the planar element you want to create, while the greencircle surrounding the surface can be used to rotate it in the 3-D space, asshown in Figure 14-7.
When creating a surface using this tool, it is important to click the Morebutton in order to get information about the quality of the surface objectcreated, as shown in Figure 14-8, when using the Cylinder option. Mean andmax errors can then be known.
Figure 14-7. Basic Surface Recognition dialog box.
When creating reference elements, it is important to create a geometry thatwill not lead to a mismatch of the cloud of points with its former set ofreferences. For example, in the proposed model, the hole in the propeller wasobviously created centered to it and normal to its forward face. By creating acylindrical surface from points taken on the face of the hole, there is apossibility that the Cylinder 's axis is shifted from center and not normal tothe forward face of the propeller, as shown in Figure 14-9.
Instead of using this approach, which could lead to errors, it is a betterpractice to dene the center and the axis of the propeller by taking anapproach using the initial reference planar face created to extract a sectionand get the center location from it. To create the section in the proposedmodel, it will be necessary to reactivate the entire cloud. In someapplications, activating a portion of the cloud could be enough.
SCAN CREATION TOOLBAR (QSR)
Planar Sections (QSR)
Figure 14-8. Surface recognition of a cylinder.
Figure 14-9. Geometrical mismatch.
Info: Cuts a cloud of points into planar sectionsPrompts to User:
The Planar Sections tool allows the dynamic creation of one or many sectionsin the cloud of points with respect to a planar reference, as shown in Figure14-10. It creates objects in the model that can be post-treated but that cannotbe exported as geometry. When launching the command, it is necessary toselect the cloud of points to consider and a planar reference to dene thesection. By using the green arrows displayed on the plane icon appearing onthe cloud of points, it is possible to dynamically locate the level of thesection. By populating the corresponding edition boxes