Thursday, June 3, 2010

Everthing You Always Wanted to Know About 3D Scanning Part 3

Chapter 3: Digital Modeling
Converting Raw Point Clouds into CAD Formats

You’ve read your overview, figured out how to collect data, and now that you have this data, what do you do with it? Chapters Three through Five will cover how this data can be processed.

As you learned in Chapter Two, Direct Dimensions breaks data collection into two common methods: Laser Scanning and Digitizing. We also categorize the processing of the collected data into two main categories: Digital Modeling and Reverse Engineering. In this chapter, we’ll tell you what you need to know about Digital Modeling.

Digital Modeling: the process of creating a computer model of an object that exactly replicates the form of the object. Laser scanners are used to capture the 3D data of the object, and this data is transferred to the computer where it is aligned, edited and finalized as a complete 3D model.

Polygonal Models and Dumb Solids

So, when does something fall into the category of Digital Modeling as opposed to Reverse Engineering? At DDI it generally depends on a couple of factors: shape (organic vs. geometric) and desired file output. As a general rule of thumb, organic shapes tend to fall into the Digital Modeling category, as do polygonal models (STL Files) and Rapid NURBS Dumb Solids.

A Polygonal Model is a faceted (or tessellated) model consisting of many triangles. Facets are formed by connecting points within the point cloud. STL files can be used for visualization, rapid prototyping, design, milling, and analysis software.

A Rapid NURBS ‘Dumb’ Solid (usually in IGS format) starts with the polygonal model. NURBS surfaces are wrapped over the polygonal wire frame. This wrapped surface model is smoother than a polygonal model. The NURBS model can be brought into parametric modelers such as SolidWorks (albeit with no parametric history – which is why we call it dumb).

The bridge between Digital Modeling and Reverse Engineering is the Hybrid Model. A Hybrid model is a polygonal model that has been converted in a rapid NURBS surface model and then also uses traditional solid modeling techniques. It is commonly used when basic geometric features, such as holes & edges, blend with complex organic contours, such as a machined casting.
Do Reverse Engineering and Digital Modeling ever Overlap?

In addition to Hybrid Models, there are instances when it is appropriate to use both Digital Modeling and Reverse Engineering techniques. For example, when collecting data of a large object (such as a plane) for Reverse Engineering, it is necessary to use a laser scanner to capture the massive amounts of surface data. The data output from a laser scanner is a point cloud, but point clouds cannot be brought into any CAD packages. Before the data can be transferred into CAD it must be digitally modeled into either a polygonal model or a Rapid NURBS dumb solid.

The Digital Modeling Advantage

There can be a fine line between Digital Modeling and Reverse Engineering and sometimes both methods can be a valid solution to 3D problems. Some advantages of Digital Modeling are:

* Digital Modeling generally offers a faster and more cost effective solution.
* It presents a great solution for creating solid models when an object has organic contours.
* Offers excellent dimensional accuracy and can be utilized for comparative analysis.
* While it is true that Rapid NURBS Dumb Solid models do not have parametric history, they can be utilized as a base for design work and Boolean functions are possible.
* Unlike raw point clouds, Digital Models can be visualized in rendering software as a solid object, which is great for seeing the overall shape and contour of the model.

Getting Started

In Chapter Two we discussed data collection and various brands of scanners and digitizers that we use on a daily basis. Don’t worry! We won’t leave you hanging on what software packages we recommend for digital modeling. We use the following packages every day at DDI (in alphabetical order):

* Geomagic Shape Studio: Polygonal and NURBS modeling and point cloud to CAD comparison. Geomagic can automatically generate an accurate digital model from any physical part.
* PolyWorks Modeler: Polygonal modeling and point cloud to CAD comparison. PolyWorks can process large point clouds over 100 million points and easily integrates with all standard digitizers and articulating arms.
* Rapidform: Third generation point processing software for creating native parametric "design-intent" CAD models directly from scan data. At Direct Dimensions we often use Rapidform in Hybrid Modeling but it also has a great Rapid NURBS function.

Tackling Reverse Engineering

Now that you know a bit more about what we call Digital Modeling and why it can be a great option, you are ready to tackle Reverse Engineering.