A LINK BETWEEN VIRTUAL AND PHYSICAL PROTOTYPING

A LINK BETWEEN VIRTUAL AND PHYSICAL PROTOTYPING Georges Fadel, Darren Crane, Larry Dooley Mechanical and Bio–engineering Departments Robert Geist Computer Science Department Clemson University Clemson, South Carolina

ABSTRACT This paper describes how a link between Virtual Reality and Rapid Prototyping was established at Clemson to enhance the designer’s understanding of manufacturing issues. It surveys the literature in the area of virtual reality and discusses some of the issues of translating an STL model into the appropriate format for Virtual Reality. The paper then speculates on the usefulness of such a tool for designers and manufacturing engineers.

INTRODUCTION Computers have profoundly affected the way engineers perform their jobs. Presently, aspiring engineers are taught how to use the computer to solve equations, to simulate processes or actions, and to design or to interface with machines. The computer enabled the designer to move away from the pen–based graphical representation of design to the screen based three dimensional solid representation. This evolution provided the designer with a more powerful tool for virtual prototyping since components to be manufactured could be represented on the screen, rotated, assembled with other parts and more recently, with the advent of rapid prototyping, built on a rapid prototyping or free form fabrication machine. One limitation of this environment is still the two dimensional representation of a three dimensional object. Rotating an object on the screen cannot be compared to having a prototype in hand and coordinating the eye – hand relationship to detect design problems, manufacturing problems, tolerances, fits, etc. The rapid prototyping technology partially solves this problem, however, building the prototype still requires time and cost. Virtual Reality (VR) removes the two dimensional representation barrier and allows the designer to look at a virtual object represented by a CAD drawing, to move around it, in it, to illuminate it where needed, and to get a better visual feedback because of the true three dimensional representation. VR technology is still evolving, and using the state of the art in VR already provides benefits that profoundly affect design and will influence manufacturing.

PREVIOUS WORK The topic of Virtual Reality has been extensively covered in the popular literature such as Popular Science, Popular Mechanics and Science. In the technical literature, articles dealing with the subject of Virtual Reality and engineering report on the issues of training [1, 2, 3], flow visualization (wind tunnel and flow visualization)[4], maintenance [5], ergonomics and some applications of CAD[6]. The issues of virtual reality and manufacturing are mentioned in a number of articles [2, 3, 6], but technical papers are scarce. The reference on Caterpillar’s use of CAD based models for training [6] shows the potential of the technology. Users “test” virtual models before the prototypes are built. This “virtual testing” process allows the designer to study the ergonomics of novel designs and modify the models if needed to satisfy the customer or user. Boeing’s work on design and maintenance using virtual reality [5] on the Boeing 777 has been described and talked about in a number of magazines, newspapers and articles on the Internet. They used a virtual environment to ensure that parts could be accessed and easily removed during maintenance. The plane is reputed to be the first aircraft designed without the use of a full physical mock–up. In England, Rolls–Royce has worked with ARRL [7], a VR based company in Suffolk to investigate design and maintenance of aircraft engines. They allow the designer or user to be immersed in an environment to study problems of assembly and maintenance. Sandy Ressler from NIST published a report [8] on the application of virtual reality to Manufacturing. Many examples cited earlier are summarized in this report. On the topic of Virtual Reality and Rapid Prototyping, no written material has been found in the literature. Our intent is to show how a technology such as VR, albeit immature to many, can help in the manufacturing process and contribute to a reduction of the time to market of a product.

IMPLEMENTATION Rapid Prototyping technology evolved as a result of a desire to build parts by material addition rather than subtraction under the control of a computer. In order to achieve this goal, geometric modelers (surface based and solid based) were given the capability to generate files that are easy to manipulate mathematically. The tessellation representation is one that approximates a surface by a number of triangles of various sizes. Once this approximation is performed, it is relatively straightforward to generate slices that represent the intersection of the triangles with a horizontal plane, and surfaces inside a solid can be hatched to solidify a resin or sinter a powder. In our lab, an object designed on a CAD system and prepared for the rapid prototyping environment, i.e. formatted in the de–facto industry standard STL format, can be directly translated and displayed in the virtual environment. Figure 1. shows a user holding a prototype of a ship–in–a–bottle created in a stereolithography machine. Figure 2. shows the view of the designer in the virtual environment. The user is wearing a head mounted display and is “sitting” in the ship in the bottle. He is observing the bottle from inside the model that was used to create the plastic prototype.

Figure 1. User holding ship in the bottle prototype

Why would such a feature attract designers and manufacturing engineers? Because of the ability of the brain to interact with visual information without input from the hands. The user is immersed in the object. He or she can walk around it, inside the object, look up, down, sideways, and see features that are not directly accessible using two dimensional representations of a three dimensional object. Just this benefit is significant since a better front end for computers signifies better understanding of the model and a reduction in design time through a reduction of design iterations. Another very significant benefit of the VR technology as applied to the Rapid Prototyping manufacture is the ability to identify errors in the STL files. Typically, STL files list the coordinates of three corners of a triangle in the order corresponding to the right hand rule. This allows a direct computation of the surface normal which points outside the solid. Additionally, the surface normal is included in the STL format. Figure 3. illustrates an ASCII STL format. Errors in the STL file often prevent the Rapid Prototyping machine from operating. These errors are typically triangles that are oriented towards the inside of the part because of the order of the points. These cause significant problems with hatching algorithms and result in a failed part. Another error is the absence of triangles in the tesselation process. Should this happen, holes in the finished part represent a visible defect, and may lead to a failed part that cannot be built. Our intent is to first be able to visualize these problems. Once visually identified, they can be remedied algorithmically. We are working to be able to interact with the STL file and correct it in the virtual environment.

Figure 2. View from the inside of the model in the VR environment

THE VR / STL CONNECTION Translating an object from the STL format generated by CAD systems to the VR space is facilitated by the wealth of information held in the STL file format. A STL file consists of a list of triangles in 3D–space. Each triangle definition contains coordinates in 3D–space for each point of the triangle as well as a 3D–space vector defining the normal of the triangle (figure 3). This format is ideal for Virtual Reality since the representation of objects in VR is itself a tesselated representation. The VR application reads in the STL file consisting of a list of triangles which represent the object to be built in rapid prototyping. First, minimum and maximum points in (x, y and z) for the object are calculated for scaling and translation purposes. The triangles are then translated and scaled such that (0,0,2) becomes the center of the object which then fits into a 4x4x4 cube. Units are arbitrary, but one could imagine a four foot cube as the virtual space. Translating the object has the effect of centering it in the VR space, just below the tracking transmitter at the user’s eye level. Scaling the object to fit the VR space tracking range is necessary such that the visual space does not extend beyond the ability to track the user’s movements, and is large enough to be useful to the user.

... facet normal outer loop vertex vertex vertex endloop endfacet ...

0.000000e+00 0.000000e+00 1.000000e+00 2.029000e+00 1.628000e+00 9.109999e–01 2.229000e+00 1.628000e+00 9.109999e–01 2.229000e+00 1.672000e+00 9.109999e–01

Figure 3: example STL file.

OpenGL is used for all the VR graphics. Directional lights are positioned beyond the VR tracking range (a distance of 10 from (0,0,0)), and pointed toward the object center (0,0,2). The triangles are drawn using an OpenGL compiled call list to facilitate real time updates. This has the effect of imposing static lighting model on the object while still allowing real time changes of eyepoint and viewport. As the users movements are tracked in real space, coordinates representing the user ’s position and vectors representing the user’s pitch, roll, and yaw are passed via a serial port to the computer. This input is fed to OpenGL calls to change the eyepoint and viewport, and then the OpenGL compiled list is called to redraw the object in VR space. The image is then sent to the users head mounted display. Using a Silicon Graphics Onyx as the compute engine, a level well above 30 updates per second for objects consisting of 45,000 triangles or less can be maintained. Above 45,000 triangles the update level drops to below 30 a second, but despite the apparent lag, the system is still quite usable.

SIGNIFICANCE AND BENEFITS Holding an object in one’s hand, and rotating it in order to observe its operation, to understand the way it is assembled or designed can only be performed with a physical prototype. The CAD representation of a design on a computer screen, even three dimensional and solid based, is far from giving the designer the visual to hand coordination needed to truly assess a design. The ability to visualize the CAD prototype in a virtual environment, penetrating the prototype, examining it from different angles and eventually manipulating it, offers significant benefits to the designer , to the manufacturing engineer and to the mold maker or tool maker. This technology will reduce the time of market of products by drastically reducing if not eliminating the iterative process

of design and manufacturing. The designer can not only study the part he or she is designing, but also try fitting it, design tools to make the part, and ensure that the prototype is as close to the final product as possible. The ability to study a part that has been designed for manufacture in a rapid prototyping environment and to identify errors in the mathematical representation of the part is significantly enhanced by the use of tools such as Virtual Reality. In this area of design and manufacturing, rapid prototyping processes have established themselves as essential contributors to the quest to produce parts rapidly. Whether parts are directly formed in the RP machines, or molds are created from the RP parts, the time to market of a product has the potential to be significantly reduced. This technology gives the designer more time to mature the product and reduces if not eliminates the communication gap between design and manufacturing. Issues of accuracy, materials, size are still, and will continue to be researched to improve the parts manufactured, but the available technology is producing results today. The tie between computer aided design (CAD) and RP significantly enhances the designer’s productivity.

Bibliography [1] Kozak, J.J., Hancock, P.A., Arthur, E.J. and Chrysler, S.T. “Transfer of training from virtual reality” Ergonomics, 1993, Vol 36, No.7, 777–784 [2] Bricken, M. “Virtual Reality Learning Environments: Potentials and Challenges.” Human Interface Technology Laboratory, University of Washington, Seattle, WA, 1991. [3] Nash, I. “Visions for a Braver World. Virtual Reality Computer Simulations as an aid to Teaching Science”. Times Educational Supplement, n.3923, p12, Sept 6, 1991. [4] Robertson, G. Card, S., Mackinlay, J. “The Information Visualizer.” Proceedings of CHI 91. ACM, NY. pp 181–188, 1991. [5] Adam, J. A., “Virtual reality is for Real” IEEE Spectrum Magazine, Vol 30, No 10, October 1993, pp 22–29. [6] Tanner, S., Miller, K. “The use of High Fidelity CAD Models as the basis for training on Complex Systems”, Proceedings of the 1993 conference on Intelligent Computer–Aided Training and Virtual Environment Technology,. NASA Houston, [7] Stone, Robert J., “A Year in the Life of British Virtual Reality, ..” Virtual Reality World, January/ February 1994. [8] Ressler, S. “Applying Virtual Environments to Manufacturing” NISTIR 5343, 1994