August 31, 2012
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1. Introduction to Reverse Engineering
Reverse engineering is the process of examining an end product to determine how it was brought into existence in order to replicate the product, etc. In most cases, reverse engineering is referred to engineering practice of measuring mechanical structures and then creating the tangible object on CAD (Computer-Aided Design) software. In the past, reverse engineering involved physical measurements of a part, using calipers and micrometers, to measure the dimensions.
1.1 Introduction to 3D Scanning
Now with the help of 3D scanning technology, engineers are able to avoid the tedium of measuring the dimensions manually. Non-contact reverse engineering is a term that refers to modern 3D scanners that can measure the dimensions of a mechanical part, from a distance, which saves time and increases accuracy. 3D scanners have the ability of determining the coordinates of each point that constitutes the exterior shell of the scanned subject. The scanner records the astronomical number of points in the 3D system, and stores it as a point cloud. The point cloud then uses algorithms from 3D modeling software that â€œconnects the dotsâ€ in order to interpolate the entire system, since the computer processing power cannot yet compute the infinite number of points in a real-world system.
1.2 3D Scanning Example
3D scanners are helpful in the medical industry to quickly and accurately measure patients for orthotics and prosthetics, along with facial reconstruction surgery and more. 3D scanning helps archeologists and historians to bring the physical world into the digital by 3D scanning, or digitizing, archaic buildings and fossils (Grimm). One example that shows the convenience of 3D scanning is when determining the drag coefficient of real-world objects. With the help of virtual wind tunnels used by multi-physics software, designers are able to determine drag coefficients of systems such as bicyclers without even needing a physical prototype or a test dummy, allowing for rapid design iterations and faster product development overall. It would be simple to measure the wind drag effects on the bicycle alone, because it can be designed on CAD software and easily imported into the virtual wind tunnel. However, measuring the wind drag effects on the human riding the bicycle requires digitizing a human- bicycle system. Currently, the best practice of digitizing humans involves 3D scanning the human- bicycle system and then converting the 3D point cloud to a CAD model.
1.3 3D Scanning in Industry
3D scanning is a growing field with several new software and hardware advancements found in the last few years. Each technique has its own set of advantages and disadvantages, usually compromising speed for resolution and compromising cost for accuracy. The purpose of this report is to demonstrate the differences associated with todayâ€™s modern 3D scanning techniques to show which technique is appropriate for various applications.
2. Photogrammetry Mechanics
The most accessible form of 3D scanning is using and image-based approach, known as photogrammetry. The reason it is so accessible is because the only hardware required is a camera, which also allows it to be extremely portable. Although any optical camera can render a 3D model, cameras that take higher resolution and more focused photos are preferred, such as a DLSR. Photogrammetry uses a series of a few dozen photos of the subject to be modeled. The photos consist of overlapping photographs, recorded on a camera 360 degrees around the 3D subject. It is recommended to take photos in 360 degrees from a low elevation, a medium elevation and a high elevation relative to the subject. Photogrammetry is useful for a range in sizes of 3D subjects, in contrast to other techniques that have a small range of focal length. The most user-friendly photogrammetry software is Autodeskâ€™s 123D Catch. Since the program resizes photos to 3 MP, most camera phones on the market are sufficient to produce 3D models, as long as the 3D subject remains in focus throughout all the photos taken.
2.1 Photogrammetry Review
Considering the software is free, and most consumers in the US have access to a medium-quality camera phone or better, photogrammetry is the most accessible form of 3D scanning. However, photogrammetry is the most time-consuming and the least precise of todayâ€™s modern 3D scanning techniques. Photogrammetry software attempts to create a point cloud by using common reference points throughout the photo, which is why it is important for the photos to overlap. Since photogrammetry uses only visible light, it does not require a radiation source such as a laser of projector. However, if the subject is either transparent or shiny, then the program is not sufficient because the photos taken from different angle perceive different scenes, even if pointed at the same reference point. For this reason, it is recommended to use photogrammetry for subjects that are opaque using diffused and consistent lighting, therefore should not be used outside on a sunny day.
3. Time of Flight Mechanics and Review
Time of flight is arguably the most commonly used 3D scanning technique. With time of flight scanners, a point cloud is generated by sending pulses of laser light and determining the distance between the transmitter and the 3D subject by recording the time is takes for the laser pulse to reflect back to the scanner. Time of flight has a range can be used for a range of several hundred meters, which should give note to the achievable accuracies (FrÃ¶hlich & Mettenleiter). The phase comparison technique for 3D scanning is similar to time of flight on the outside, but it is much more complicated.
3.1 Time of Flight- Phase Comparison Mechanics and Review
Phase comparison sends a harmonic beam of light that is modulated by a harmonic wave. The distance between the transmitter and the 3D subject is measured by the phase difference between the transmitted and received wave once it bounced off the subject and returns. Since this technique uses more complicated algorithms used for detecting sinusoidal differences, the processing time may be much longer than that of a time of flight system (BOEHLER & MARBS).
4. Triangulation Mechanics
The most rapidly advancing form of 3D scanning today is known as triangulation. Triangulation uses a transmitter that projects a light pattern on the 3D subject, and then a camera that is separated by a distance that records the distorted light pattern and translates the distortion pattern into a point cloud.
4.1 Triangulation with Laser Light
The most common form of light that is projected onto the 3D subject is focus laser light. The focused laser light sweeps across the 3D subject using a point, a line, or an array of lines. The camera is situated at a fixed distance from the light source, and depending on how much the 3D subject bends the laser pattern, the camera is able to precisely detect how far away each point is from the light source, creating a point cloud. This technique is more precise but also much more costly, usually present only in research labs and engineering firms for reverse engineering and quality control. Also, the laser light is harmful to humans because the light is focused with intense beams that are capable of damaging a retina under exposure of the laser.
4.2 Triangulation with Infrared Light
Thanks to Microsoftâ€™s Xbox Kinect sensor, triangulation is being mass-produced. The Kinect is an Xbox device that allows the gamer to be the controller by tracking their motion. The Kinect uses triangulation in the form of infrared light in order to construct a dynamic point cloud of the gamer in motion. The transmitter projects a pattern of infrared light onto the 3D subject and the on-board camera detects the ever-changing pattern that is distorted by the subject. 3D scanning enthusiasts have been using the Kinect to scan themselves in 3D, most commonly for the application of consumer 3D printing. Considering the Kinect is optimized to detect motion, the 3D scanning accuracies are poor.
4.3 Triangulation with Visible Light
A similar inexpensive form of triangulation uses a visible light projector to display the light pattern, known as structured light. Structured light proves to be the most promising technique listed due to its high precision relative to its low cost (Gupta, Agrawal, Veeraraghavan & Narasimhan). The hardware necessary for a structured light 3D scanner is a high contrast LCD projector, and a webcam. It is quicker and more accurate as well because a projector allows for a more complicated array of stripe patterns than a series of single line lasers can produce. Structured light is also safer for humans than laser scanners since it uses unfocused visible light rather than focused laser light (Fofi, Sliwa & Viosin).
4.4 Triangulation with Visible Light in Industry
Few companies are using structured light 3D scanners. One reason is that the technology is underdeveloped. Not enough research has been performed on structured light to verify its feasibility. Another reason why it is not popular is because it is still very new technology (Robinson, Alboul & Rodrigues, 2003). With its low price, and the ability to use it for accurate point clouds, the software is the only missing factor.Â One company that is taking advantage of structured light scanning is David-LaserScanner.com. This company sells the software that you use with your own projector and webcam, and they also sell the complete package.
5. Comparison of 3D Scanning Techniques
Photogrammetry provides a simple, portable and low-cost method for 3D scanning opaque subjects, as long as the lighting conditions are ideal. However, the resolution is relatively poor and the post-processing of photogrammetric models is time consuming. Photogrammetry is ideal for beginners and for situations that require extreme portability. Time of flight scanning is ideal for long ranges due to its high precision. This technique can only scan one point at a time, however it is still capable of scanning relatively quickly. Triangulation using laser light can be very accurate due to the precision of focused laser light, however it can be time consuming to sweep a laser across the entire subject. Also, laser light can be harmful when scanning humans. Triangulation using structured visible or infrared light is capable of much quicker scans since the projector can display an array of very thin lines on the object. Although it is under development currently, structured light scanning has high potential due to its high precision and low cost.
As 3D scanning technology continues to arrive to consumers in the form of low cost consumer electronics, such as the Xbox Kinect and LCD projectors, we should expect to see advancements from the DIY (Do It Yourself) community. The same way that the DIY enthusiasts found ways to make 3D printers more and more affordable, 3D scanning should start to become commonplace. Now that 3D scanning is achievable by a new demographic, we should also begin to see a large shift in the way that we use 3D scanners.
BOEHLER, W., & MARBS, A. (n.d.). 3d scanning instruments. Unpublished manuscript, Institute for Spatial Information and Surveying Technology, FH Mainz, Mainz, Germany.
Fofi, D., Sliwa, T., & Viosin, Y. (n.d.). A comparative survey on invisible structured light. Informally published manuscript, Le Creusot, Retrieved from le2i.com
FrÃ¶hlich, C., & Mettenleiter, M. (n.d.). Terrestrial laser scanning â€“ new perspectives in 3d surveying. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, XXXVI(8), W2.
Grimm, T. Z Corporation, (n.d.). 3d scanners selection criteria for common applicationsT. A. Grimm & Associates, Inc.
Gupta, M., Agrawal, A., Veeraraghavan, A., & Narasimhan, S. (n.d.). Structured light 3d scanning in the presence of global illumination. Unpublished manuscript, Robotics Institute, Carnegie Mellon University, Pittsburgh, USA.
Robinson, A., Alboul, L., & Rodrigues, M. (2003). Methods for indexing stripes in uncoded structured light scanning systems. Informally published manuscript, Sheffield Hallam University, Plzen, Czech Republic. , Available from Journal of WSCG.
by: Jesse DePinto, CEO at Voxel Metric, Inc.
Voxel Metric, Inc. is a 3D scanning software company based in Milwaukee, WI