This lab exercise has two parts: (a) scan a part using a laser scanner, (b) construct a surface model from the scanned data points.

Transcription

1 1 IIEM 215: Manufacturing Processes I Lab 4. Reverse Engineering: Laser Scanning and CAD Model construction This lab exercise has two parts: (a) scan a part using a laser scanner, (b) construct a surface model from the scanned data points. (a) Laser scanning We shall use a Vorum laser scanner. The scanner uses a red LED laser that is beamed to the surface of a part. It has a set of cameras (at least three, though our scanner uses a set of eight) that record the image of the working region of the scanner, and can identify the image of the laser spot where it is reflected from the surface of the part. Using some simple mathematical calculations, the coordinates of the point at which the laser beam is shining can be calculated. This calculation is repeated, at a very rapid rate, for a large set of points all over the part surface. The collection of points (called a point cloud) will be used to create a CAD model of the part using a CAD system (in our case, CATIA). The TA will help you to locate and scan the part. Some noisy points (due to system error) can be deleted, and eventually the software connected to the scanner generates a smoothed point cloud. You can control the level of precision in the smoothing if the precision is high, then the surface is more irregular since noisy points are treated as true data if the precision is low, then points with larger noise fall inside the tolerance zone, and are smoothed out to give smoother surface, but less precise geometry. (b) Construct a CAD model of the part in CATIA Here, we shall use a simple method of constructing the part geometry. The main steps are: (i) Join the points into small triangles (called a tessellated surface). (ii) Generate smooth curves along several cross-sections of the tessellated model. (iii) Create a smooth surface that interpolates the series of smooth curves (called a loft-surface). (iv) Fill the surface to form a solid (v) Save the solid model in a format that will be used to manufacture the model using Rapid Prototyping in a future lab. Here are the details of these steps. (1) Start CATIA CATIA will display (a) the part that you are working with; and (b) a Feature tree on the left side of the window, that displays the geometric entities that you create, step by step.

2 2 Mouse functions: - Left button CLICK: select the object/entity at the point of the CLICK (e.g. part, feature tree, point, curve, etc.) - Middle button DRAG: drag the entity that is selected (i.e. part, feature tree, ) - Middle and Right button DRAG: rotate the part - Middle Button PRESS + Right button CLICK + Middle button drag: zoom in/out (2) Import the point cloud of the scanned object Start Shape Digitized Shape Editor, and then Insert Cloud Import In the window that comes up, Select the file containing your point cloud, press Apply, and then OK. The imported point cloud may be outside the current view, but you can see it by View Fit All In. You will see the point cloud; also, the imported point cloud is shown in the Feature tree, in Geometry.Set.1

3 3 (3) Tessellate the point cloud Select the point cloud (click on it, and a box will outline it), then Insert Mesh Mesh Creation In the box, Increase the size of the Neighborhood to a large number, e.g. 10mm; Apply OK

4 4 - Don t worry if there are holes at the two ends of the surface we will discard that portion later; - You can see the Mesh.Creation.1 object in the Feature tree. We don t need the Point Cloud now, so hide it: In the feature tree, Right Click the object for the imported point cloud, and click Hide/Show. (4) Create a few smooth cross-section curves In this lab, we shall only create an approximate shape of the part. To do so, only a few cross-section curves, say five or six, are sufficient.

5 5 Select the tessellated part, then Insert Scan creation Planar sections In the Planar Sections window: Number = 1 Curve creation tab (at bottom) ON (this will open the Curves from Scans window) Plane Definition: Select the plane that is parallel to the cross sections we want; here: XY plane) CATIA displays a plane, and also its cross-section with the tessellated part. Drag (left mouse button) the Green-Dot in the middle of the plane to locate the plane where you want the cross-section. Then press Apply. If you want a smoother cross-section, you can increase the Tolerance 1mm, and Apply; If you want sharper corners, decrease Tolerance 0.2, and Apply; For this lab, 0.2mm is considered ok. - The three buttons on bottom of Curve from Scans widow toggle on/off the tolerance and error displays. Slide the plane to the next cross-section region, and click Apply in the Plane Sections window. Repeat this action to create SIX cross-section curves in the positions shown approximately as follows. After making all six curves, click OK. You will see the curves in the Feature tree on the right side.

6 6 (4) Create a smooth surface through the cross-section curves: LOFT operation We will create the surface in two steps: (i) loft a surface between the first three sections; (ii) loft a surface between the remaining sections, such that the two surfaces join smoothly. [Note: you can use a single step for a simple shape like this screw-driver, but for more complicated shapes, it is better to make several surfaces that are joined by a surface-fill operation]. Select Start Shape Generative Shape Design. Insert Surfaces Multi-section surface

7 7 In the Multi-sections Surface Definition window, click in the top text-area. Then select the curves you want by Left-mouse click on the curve. I have selected three consecutive curves. IMPORTANT: - The Closing Point on each section is approximately aligned. If not, the loft surface will be twisted. - The direction of all curves is same: The arrow on each curve must point in the same direction. In the above, the arrow on Section3 (Curve.9) is in opposite direction to the other two. You can reverse the direction of the curve by clicking on the head of the arrow. All curves point in same direction, OK!

8 8 Click OK: the surface is created. If you want to see the surface clearly, Hide the tessellated object in the Feature tree Right Click on the Mesh Creation object Hide/Show. Complete the surface creation: Insert Surfaces Multi-section surface Select the section curves; There are two IMPORTANT things to note 1. The direction of all four curves I selected is the same 2. We want this surface to be smoothly connected to the previous surface (Multi-sectionsSurface.1 in my example); for us, it is sufficient that the two surfaces are C 1 smooth (i.e. tangent is same at all points where the surfaces meet). The two surfaces will meet at the joining curve -- Curve.9 in my example. In the Multi-sections Surface Definition window, after you add this curve to the Sections, click anywhere on the previous surface. See figure below: notice that Multi-sectionsSurface.1 appears in the Tangent column of the definition.

9 9 Click OK, to get the outer surface of the screw-driver: (5) Close the Ends: Fill surface The ends of the screw-driver surface are open; we want to close them to make a solid body. We use the Fill Surface operation. Insert Surfaces Fill The Fill Surface Definition window pops up. On the part, select the boundary of the hole that you want to fill with a surface. In our case, the boundary is a single curve click on the curve (Curve.6 in my example) OK.

10 10 Use the Fill surface on the other end of the screw-driver also now you have a closed surface (a shell). (6) Fill the shell to make a solid The screwdriver is a solid part to construct a solid from the surfaces we have constructed, we need to fill up the surfaces with solid. To do so, we need to: (i) join all surfaces in our shell into a single surface (to make a single shell), and then (ii) fill the shell. (i) From the Feature tree, select all surfaces on the shell: multiple objects can be selected using CTRL-CLICK of the left mouse: Insert Operations Join The Join Definition window pops up, with the selected surfaces shown in the list of Elements to Join. Change the join tolerance to a large enough value (e.g. 0.1mm), and click OK.

11 11 (ii) Make a solid: Insert Volumes Close Surface The Close Surface Definition window that pops up. Select the join-surface you have created earlier (Join.1 in my example) by clicking on it (on the part, or on the Feature tree). Click OK. (7) Round the sharp corners to make a more realistic part: Fillet. We will use Fillet operations to round off the sharp edges at the two ends of our part. First, we put a 1mm fillet on the front end of the screw driver: Insert Operations Edge Fillet The Edge Fillet Definition window pops up. Select the edge on the front end of the screwdriver body (Leftclick the mouse on the edge); enter the Fillet Radius: 1mm. OK. Using the same method, put a 4mm fillet on the back end of the screwdriver. [Hint: in the figure below, I have used the Hide/Show to hide all the curves and surfaces] (8) Save the part (i)file Save As use CATpart format. (ii) File Save As and this time, save in STL format. You will use the stl file in a later lab.