David W. Bell
3D scanning consultant
Chapel Hill, N.C.

Digital shape sampling and processing (DSSP) converts 3D scan data into digital models for design, visualization, analysis, and manufacturing. Combining the advantages of DSSP with featurebased modeling provides a hybrid method of modeling.

An example of a damaged pump impeller shows how to combine DSSP and CAD to create a parametric model that can be manufactured into a replacement impeller.

Combining strengths
Traditional CAD models are made by defining a sequence of 2D and 3D geometric features or entities and specifying the dimensions and characteristics of these entities. Parametric models make it easy to generate part variations useful in experiments of form and function.

While traditional CAD works well when modeling from scratch, shortcomings arise when it’s used to reconstruct complex surfaces. Reconstructing complex objects in CAD takes a lot of time and there is no guarantee that the finished model will be accurate. In some cases, it is almost impossible to use a feature-based approach to reconstruct surfaces because it’s so difficult to identify and quantify parameters that control the object’s shape.

Hybrid modeling fixes these shortcomings. Basic reference geometry, such as datums, curves, and primitive features can be measured and extracted from 3D scan data. Hybrid modeling moves easily between CAD and DSSP software to take advantage of their respective strengths.

The first step in rebuilding the impeller is scanning the existing part. The example impeller is scanned using a GOM ATOS II, a whitelight scanner that uses two cameras to capture projected fringe patterns on the part’s surface. Since the shiny impeller surface could interfere with the projected patterns of the scanner, it’s coated with a powder to reduce reflectivity. The scanner collects millions of points to generate a point cloud.

The complex shape requires taking 20 scans from different positions to provide line-of-sight to all of the impeller’s surfaces. An array of registration targets are placed on the impeller to help align the scans. A technique called photogrammetry detects the target’s center points from the camera images. The collection of visible points as seen from any position provides enough information to align the scans with each other.

The scanner captures 15 million points. These are aligned and merged in Geomagic Studio software to create a polygon model. The software is also used to clean and repair the scan data. Typical cleaning includes removing extraneous data and noise, decimation of data into smaller file sizes, filling holes, and repairing intersections. In this case, the entire polygon model is repaired, but in many cases only portions of a model that will be used in a parametric reconstruction must be cleaned.

Extracting curves and datums
The polygon model is saved as an STL file and imported into CAD software to model the impeller hub. Users start by detecting the center axis of the impeller, defining the hub profile with an extracted curve, and then generating a surface of revolution with the curve revolved about the axis.

The axis can be found by selecting the impeller’s perimeter surface. This should be a perfect cylinder and will generate a stable datum axis. Next, create the profile curve that defines the hub. In this case, a simple planar cross-section curve cannot be extracted because blades interfere with the hub’s cross section. Fortunately, some CAD programs allow another approach: a variable section sweep. We generate the surface by extruding a half-circle along the hub axis, and control the diameter of the half-circle with a nonplanar curve extracted from the hub surface.

The bottom surface of the impeller is not obstructed by the blades, so a simple planar curve can be extracted and used to create a surface of revolution. To describe more parameters that control the shape of the surfaces, consider using orthogonal edge boundaries of the swept surfaces. These would also generate new, idealized curves.

Combining free-form surfacing
The blades are the most complex elements of the impeller’s design so their surfaces are difficult to measure or define with basic parameters. However, a blade can be quickly surfaced using Nurbs-surface functions in Geomagic. The completed surfaces are then imported into CAD as IGES or STEP features.

After importing a single blade, it can be copied around the imported datum axis to generate all the blades on the hub. Because the turbine has blades of two different designs, the importand- copy process is repeated with the second blade.

A direct angular measurement could define blade spacing, but a better way defines a parameter that relates angular spacing to the total number of blades. This allows changing the number of blades and letting the software calculate their spacing.

After incorporating the blade surfaces into the solid model, define a parametric radius where the blade base intersects the hub. This radius can be adjusted at any time. CAD functions allow other blends, such as variable radius and rolling ball.

Trimming and blending
After positioning the blades, define additional datums and curves to perfect the contour along the outer surfaces of the impeller. To do so, extract a nonplanar profile curve in Geomagic Studio, and use a variable section sweep to generate the surface and trim the outer surface of the blades.

Then generate a cylinder to trim the outer surface of the entire impeller. Center the cylinder on the datum axis. Cylinder diameter can be measured by constructing a 3D feature cylinder from the polygon surface. After trimming the outer cylinder, we can be assured the design has a perfectly centered and symmetric impeller.

With modeling complete, we can use a computer-aided inspection program to compare the final CAD model to the original scan data to verify accuracy.

Key benefits
The hybrid modeling method provides full parametric control over the shape of the hub surface, the blend radii, and the blade number and spacing. The complete process takes less than a day, including scanning, repair, and modeling. What’s more, the hybrid modeling process provides several key benefits. For instance, it:

Uses 3D investments. CAD vendors have invested thousands of man-years to create useful systems for digital design and creation. Hybrid modeling takes advantage of existing modeling systems and the skills of CAD users and experts, and augments it with 3D measurement and rapid surfacing.

Helps quickly create new designs. When compared to traditional CAD, the hybrid measurement and modeling approach greatly reduces the amount of time needed to copy an existing design. In some cases, hybrid modeling takes only hours as opposed to days or weeks using conventional techniques.

Produces native parametric CAD geometry. Using the CAD system, various aspects of the design can be parametrically driven by numerical values or constraints such as assembly-mating conditions. Many file I/O translation issues are avoided because the user’s CAD system produces the core of the geometry.

Generates accurate results. Free-form surfaces can be generated from point clouds containing millions of sample points. This technique reproduces subtleties in surface structures that would otherwise be lost. The process is repeatable because scanned data is less operator dependent than typical hand measurements.