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TUTORIAL • October 30, 2000
From Concept to Reality
by Anshuman Razdan
The CAD revolution has shortened product life cycles, increased product complexity, and spurred global competition with growing innovation. This revolution presents a challenge to create more usable products at a lower cost, and that challenge is creating new opportunities for 3D artists and animators. If you specialize in representing real-world objects, or objects that might possibly exist in the real world, these opportunities can increase your bottom line. The key is to take advantage of 3D at every stage of product design, right up to manufacturing.
A virtual prototype extends the usual requirements of creative modeling, surfacing, and animation to embody true-to-life characteristics of an object that will, if all goes well, become a bona fide manufactured product. The virtual object can be subjected to analysis and simulated use almost as though it were a physical prototype, but much more quickly and at a lower cost. Making a virtual prototype involves representing not only the object's form (precisely to scale, of course) and look, but also the way its parts interlock and move and possibly other functions as well.
In its fullest manifestation, virtual prototyping (VP) becomes a basis for another cost-saving process known as rapid prototyping (RP). A rapid prototype is a physical object generated directly from virtual prototype data. Generally, an RP machine uses the data to apply layers of plastic or another substance to build a real-world object of the desired form. This is an exciting new area, and various technologies exist for getting the job done. Currently, all of them are in their infancy, but they promise to change the dynamics of industrial design and manufacturing in a wide range of fields.
How do you assemble a viable virtual prototype? And how do you prepare that work to be used as data for an RP system? We'll survey the process, offer some pointers, and examine a few actual projects in which 3D models are serving to demonstrate, prove, and realize product designs.
Surfaces & Solids
In traditional industrial design, to create a prototype, you'd supply a product specification on paper and outsource fabrication to, for example, a machine shop. Unfortunately, a fully functional prototype can be very expensive to build. In addition, it can take enormous amounts of time. While you wait for the machine shop to call, you can't help but wonder, "Will it look different in reality than it does on paper? Will it fit with other parts? Will it function as designed? Will it have the qualities customers want? Can it be manufactured in large numbers?"
FIGURE 1. The typical design cycle. |
VP and RP answer these questions before you commit yourself to high expenses and drawn-out schedules. However, VP and RP are late steps in the product development cycle (Figure 1), which begins with a product concept and ends with a plan for recycling or disposal, given the constraints of environmental resources or hazards the product may pose.
The first step beyond the product concept is to turn it into a 3D model. Here, the 3D content creation tools most familiar to artists and animators are helpful to a point. Softimage, Alias|Wavefront Maya, Discreet Studio 3D MAX, and the like are designed for sculpting surface geometry and covering it with a realistic surface—essential for VP, to be sure, but insufficient for RP.
The key to creating a VP model that can be used for RP, and the feature that distinguishes CAD tools from artistic modeling tools, is solid modeling. While artistically oriented 3D programs specialize in surface modeling, solid modeling raises the level of abstraction, describing a form as a solid object. Solid modeling tools do surface modeling, of course, but the surfaces have thickness. They can define an object that's solid or hollow, with or without holes, but either way they maintain a clear distinction between the object's inside and outside.
An important advantage of solid modeling is the ability to do Boolean operations such as addition and subtraction of two solids. If you want to create a cube with a cylindrical hole in the center, for instance, you'd start by creating two solids—the cube and the cylinder. Then, using a Boolean operation, you'd subtract the cylinder from the cube.
 
FIGURE 2. The Mobius strip and Klein bottle are two examples of nonmanufacturable solids. To be defined as normal, a surface must have a distinct inside and outside. |
| Sidebar: Designing a Better Bottle
Back in the '60s, Alfred Heineken had a vision. He believed his beer bottles could be designed in such a way that empties could be nested for use as building materials for developing countries. His WOBO (world bottle) project made it to prototype stage and demonstrated potential, but the bottle never went into mass production. Esther Ratner believed it was time for Heineken's vision of 30 years ago to become a reality.
(Top) CAD model of the Eco/Ergo Bottle. (Middle) RP bottle, with ledge for users with limited grip. (Bottom) RP models stacked. |
Ratner's Eco/Ergo Bottle represents a new bottle shape that's both ecological and ergonomic. The ecological goal was to create a shape that will nest with itself, so when it's no longer useful as a container, it can be stacked on other bottles with mortar (like a glass brick) to create a translucent building wall. The ergonomic goal was to create a shape that's easier to hold for a person with limited grip strength. An integral ledge allows the bottle to rest on the fingers, thus facilitating beverage consumption without requiring actuation of the small muscles of the fingers. Using RP, Ratner created prototypes to test the design.
RP proved the feasibility of the concept without the need to form multiples by hand. This allowed for a full test, which included filling and pouring—obviously impossible with VP, although it could have been simulated. Technical drawings and concept images of the bottle and its uses were developed, but the RP models communicated the benefits of the concept most succinctly and convincingly. |
Solid modeling packages range from many multithousand-dollar products such as SDRC I-DEAS, PTC ProE, SolidWorks, and Autodesk Mechanical Desktop to less expensive ones such as Robert McNeel & Associates Rhino and auto•des•sys form•Z. Good CAD software will detect dangling edges, disconnected geometry, paper-thin surfaces, and self-intersections. Surfaces and curves must match exactly. Most CAD tools also provide advanced texturing, lighting, and rendering capabilities. It's easy to overlook wrinkles, gaps, and other minor blemishes. As you get farther along in the process, you'll find it more difficult to detect and fix these anomalies, so be sure to address them early.
A model destined for RP must be not only solid but valid. In short, a valid model is leak-proof; that is, if water were poured into it, it wouldn't leak. It must have distinct outside and inside surfaces, and their surface normals must be properly oriented. Contrast this with the Klein bottle and Mobius strip, two examples of invalid forms that can't be fabricated (Figure 2). The better CAD modeling packages include a validate function that helps ensure that your work is up to snuff for RP.
An Incredible Simulation
Having created a valid CAD model, you're ready to turn it into a prototype. Although it's possible at this stage to go straight to making a physical prototype via RP, in most situations, it's a good idea to make a virtual prototype first.
A virtual prototype differs from a CAD model in three critical ways. First, it has a true-to-life surface, including color, texture, and reflective properties. Second, moving parts in the design are represented with accurate animation. Third, it includes interactive features that simulate the product's operation in its intended environment. These three factors form a continuum, and not every virtual prototype is taken to the extreme of being wrapped into a fully interactive simulation complete with audio and force feedback. However, relatively inexpensive off-the-shelf tools exist that make a simulation less exotic than it may sound.
FIGURE 3. Various steps of prototyping. Clockwise from top: design > virtual prototype > simulation > rapid prototype. |
To create a surface that represents the finished product, it's important that the mesh be smooth and precise. Non-uniform rational B-spline (NURBS) tools can be very helpful in this regard. In addition, full-featured mapping capabilities and rendering via raytracing, radiosity, and other high-precision techniques contribute to a useful prototype. These capabilities let you conjure different material types, finishes, and lighting conditions quickly, making it possible to give the product a thorough visual inspection. It's a lot easier to run through what-if scenarios on the computer than in fabrication, but beware: Texture maps can hide a multitude of imperfections that become glaringly obvious in a rapid prototype. Again, the best remedy is to make sure the CAD model is flawless.
Animation adds a great deal to the value of VP for any product that has moving parts. It's very helpful in evaluating form, stress points, and usability so you can identify problems before they become costly mistakes. A good example is provided by a new rotor design for a helicopter that I once saw in VP form. Animating the model showed immediately that the design was flawed: The bolts holding the rotor hub were upside-down, and they stuck out far enough to interfere with the rotational motion of the blades. Maya, MAX, and many other 3D animation tools have a built-in capability that can turn the static CAD model in to an animated machine.
A simulation uses the full resources of the computer to generate as lifelike an experience as possible, with an emphasis on interacting with the product and representing influences of the surrounding environment (Figure 3). If the concept calls for a button that opens a rolltop cover, this is the time to prototype the interaction. To take it one step further, if the button is supposed to beep when it's pressed, you might add interactive audio as well. The recent development of inexpensive force, or haptic, feedback devices (such as Immersion FeelIt Mouse and the Phantom from Sensable Technologies) lets you prototype the feeling of holding or handling the part. The surrounding environment comes into play in products such as airplanes or cars, in which it's desirable to test the way the form interacts with air or water at high speeds.
| Sidebar: Rapid Prototyping Technologies
RP processes divide into two categories: additive and subtractive. In an additive process, each layer of material must be sprayed, laid down, cured, or sintered. Examples include stereolithography (SLA), the process developed by 3D Systems; Fused Deposition Modeling (FDM) by Stratasys; and Sintering Layer System (SLS) by DTM Corp. In a subtractive process, each layer starts at a standard size, and the excess is trimmed away. Examples include Laminated Object Modeling (LOM) by Helisys.
Although each process differs, the fundamental difference is the materials used. SLA uses liquid polymer resin and a laser to harden it. LOM cuts away sheets of paper. SLS, which uses lasers to sinter powdered material, offers a range of materials from plastics to metal called RapidSteel, a combination of copper and steel. FDM can make parts from casting wax, ABS plastics, and medically approved ABS plastics. Ceramic parts have been produced in research labs. Although the build envelope (the theoretical maximum size a machine can build) of the biggest RP machines is limited to roughly 25x25x20 inches, it's big enough to accommodate a wide variety of types of parts.
Recently, a new breed of RP machines reached the market: desktop modelers, also known as concept modelers and 3D printers. These tabletop machines are safe enough for an office environment and priced around $60,000. They're meant to be used like paper printers, for quick validation of designs, complete or incomplete. The typical build envelope is 10x10x10 inches. Other limiting factors include surface finish quality and tolerance.
Although these processes are called rapid, rapid is a relative word. A small model may take a few hours to produce. However, compared to the time required to produce a prototype via traditional means, rapid prototyping lives up to its name. |
This degree of virtual prototyping requires interactive authoring or simulation authoring tools. Macromedia Director is the most popular interactive authoring tool, but it's not designed for simulating precise real-world phenomena, so it's best suited to crude simulations. In any case, it doesn't handle 3D natively; a plug in from Shells Interactive is required. VRML authoring tools such as Cosmo Worlds may be ideal. For precise mechanical stress testing and the like, tools are available that map simulation data on the surface of the CAD model part to give visual cues about the validity of the model. Typically, these tools are sold as a plug-in to the CAD software. Proper simulation tools are bound to be overkill in many situations, but if you can handle the programming overhead, they're available from MultiGen, EAI, and others.
Obviously, virtual mechanical stress testing requires skills beyond the usual modeling, animation, and rendering. If you don't have the necessary expertise, you can call on the services of companies that specialize in this kind of work (see "RP & VP Resources").
Layers of Reality
If you've gone to the trouble of creating a surfaced, animated, interactive virtual prototype that looks, feels, and acts like the real thing, why take the final step and fabricate a physical prototype? The answer lies in the physical prototype's unique ability to deliver form, fit, and function.
In this context, form refers to the ability to get your hands an object. Tactile feedback can make all the difference. Consider a new cel phone design: Does it feel fragile? Difficult to manipulate? Awkward to carry? Moreover, it might be necessary to show a sample to the target consumers or investors.
Fit refers to the assembly of parts. Continuing with the cel phone example, it consists of three parts: the phone body, the plastic lens over the LCD screen, and the battery pack behind it. All three must fit together snugly. Tolerances may be very tight for the lens and body. A virtual prototype doesn't account for deviations due to fabrication, so physical prototypes of all three components are essential. For complicated assemblies such as engines, a physical prototype assembly can reveal things like whether commonly replaced parts are sufficiently accessible.
FIGURE 4. The process builds in layers from the design to the RP part. |
Function can be tested to some degree in a virtual prototype, but there's nothing like the real thing. If a physical prototype is made of the same materials and to the same scale as specified in the design, real-world testing will get you as close as possible to knowing how the finished product will perform. If the materials and scale are significantly different, a scientific methodology must be applied to extrapolate from test results using the prototype.
Most physical prototypes are built in conventional ways, but it's not an overstatement to say that RP has revolutionized physical prototyping. In the last 10 years, this technology has reduced the time required to make a physical prototype from 16 weeks to a matter of hours. 3D Systems introduced the first RP process, SLA (stereolithography), which uses a laser to harden photosensitive liquid polymer resin. Since then, many methods have been developed, and today RP can be accomplished using wax, ABS plastic, nylon, and even metal (see "Rapid Prototyping Technologies").
All RP technologies are based on the simple method of building one layer at a time (Figure 4). For instance, in the FDM (fused deposition modeling) process, the machine can be thought of as an automated glue gun that spreads layer upon layer of glue. The gun's nozzle traverses a platter and extrudes molten plastic. As a layer is finished, the platter steps downward a fraction of an inch and the nozzle applies the next layer. The new layer bonds with the previous one as it cools.
RP hardware is driven by software that reads CAD models in a file format known as .stl, stereolithography triangulation language (see "Preparing a Model for RP"). The software slices the model into horizontal cross sections, each of which corresponds to a layer the machine will build. The height of the cross sections determines resolution on the axis assigned to be the build direction. Typical resolution is 8 to 10 thousandths of an inch, but some machines can achieve 6 thousandths of an inch.
As you can imagine, resolution is important because the layering process can result in jaggies, analogous to aliasing in a low-resolution raster image. On the other hand, the higher the resolution, the longer the build time. Current market prices for RP services fluctuate between $60 and $80 per hour, so shop around to find the best price. More than 2000 RP service bureaus operate in the U.S. alone.
You can orient the build along any axis you want, depending on the shape you're creating, but be careful—build direction can have a big impact on final surface quality, depending on the shape being built. (Vertical walls have the best surface quality.) During building, a shape may require supports that aren't part of the product design. If a shape includes a cantilever-type feature, some RP systems automatically build scaffolding. This support is easy to remove after the part is built.
Assuming your CAD model is valid (and not too large), all it takes to build a rapid prototype is to load the model. Then get a cup of coffee, put your feet up, and watch the part take form.
| Sidebar: Preparing a Model for RP
Most RP hardware is driven by software that reads the .stl file format. The format's initials stand for stereolithography triangulation language, the first RP process on the market. Although this file format was developed for use with the stereolithography technique, today it is used with a variety of other technologies.
To generate an .stl file, it's necessary to start with a valid CAD model. The model must have distinct inside and outside surfaces and a boundary, and its surfaces must define an enclosed volume unambiguously. Solid modeling will ensure the validity of the model.
The next step is to convert the CAD model. In the export options of your modeling program, look for an option to save as .stl. In the conversion, free-form surfaces will become triangular facets.
Most export operations will prompt you for a tolerance. The smaller the tolerance, the closer the .stl model will be to the resolution of the original model. Of course, a smaller tolerance also means larger file size. The tolerance you choose depends on how fine the original model's features were and how much detail you want to retain in the RP output.
The export software may also ask if you want to save the file in binary or ASCII format. Binary format results in a smaller file size. ASCII files at the same tolerance are several times bigger than their binary counterparts, but they can be useful for debugging. Here's a little trick: You can check whether or not a binary file is written incorrectly based on its size; the last two digits always should be 84.
Once you have an .stl file, you're ready to load it into the computer running the RP machine, where it will be sliced and verified and then sent to the RP machine. In a matter of hours, you'll have an RP realization of your concept in your hands.
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Virtually Real
Prototyping is an integral aspect of modern product design, and the rise of VP and RP means that 3D professionals will play an increasing role in it. The cost savings achieved by automation are too great to ignore, as are the opportunities to explore design alternatives quickly and inexpensively. Moreover, the ability to share CAD and RP data over networks, including the Internet, eliminates the distance between members of the design and manufacturing teams and facilitates the contribution of professionals from other disciplines to the design process.
At present, the possibilities of VP appear to be virtually limitless. Ongoing increases in desktop computer power have paved the way for virtual reality technology to occupy an important place in mundane activities such as product design. The tools and techniques exist today, and they'll only become more widespread and easier to use tomorrow. 3D professionals will be at the forefront of any mainstream application of this technology, and VP is no exception.
On the other hand, a gulf stretches out between the interactive world offered by VP and the current output of RP machines. Much development remains before a full-blown VP model can be realized automatically. Nonetheless, the current state of RP is impressive and cost-effective, and it's highly useful for a variety of purposes. Certainly, a CG movie character can be transformed readily into a statue destined to be marketed as a souvenir or toy. More complex scenarios are common in industrial design and are bound to develop apace.
Many common product design goals can be accomplished through VP, while others are addressed by RP. Nonetheless, complex designs may demand a combination of VP, RP, and conventional prototyping. Even in these cases, 3D models and animations play a pivotal role. They're uniquely malleable and portable, and the cost savings are undeniable. Those savings will flow directly to 3D professionals who are prepared and inspired to take part. In the world of prototyping, 3D is here to stay.
Anshuman Razdan is technical director of the Partnership for Research in Stereo Modeling (PRISM) at Arizona State University in Tempe.
Thanks to Arati Rao (illustrations), Phoenix Analysis and Design Technologies, Esther Ratner, Z Corp., and 3D Systems for their assistance with this article.
(This article originally appeared in the July, 1999 issue of 3D magazine.)
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