2.1 Surface and Solid Model Reconstruction
2.1.1 Scanning Instruments and Technology
One of the biggest challenges of reconstructing a mechanical part is to cap- ture its geometric details. Fortunately, advanced devices have been devel- oped to image the three-dimensional features of a physical object and translate them into a 3D model with high accuracy. Data can be obtained directly using a digitizer that is connected to a computer installed with reverse engineering software. The two most commonly used digitizing devices are probes and scanners. They both measure the part external fea- tures to obtain its geometrical and dimensional information. Probes obtain data by either a direct-contact or noncontact imaging process. The contact probe is an arm with a tiny ball attached to the end that comes into direct contact with the part being digitized. The noncontact probe is tipped with a small laser probe that never makes direct contact with the subject, and is usually used for more delicate or complex parts. The contact probe is the most economical 3D digitizer. It measures a limited number of points across a target part, and feeds the data back to a computer where the information is processed by software to build an electronic image of the part. It works best for small parts, up to the size of a book, in simple geometric shapes, and it usually provides high accuracy. The user can simply move the stylus, trac- ing over the contours of a physical object to capture data points and recreate complex models.
A scanner usually does not contact the object and obtains the data by a digital camera. To scan a physical part in a reverse engineering practice, sometimes the only required manual actions are just to point and shoot. All other actions, such as focusing and topographic features imaging, will be processed automatically by the scanning instrument itself. The liquid crystal display (LCD) viewfinder and autofocusing technology are used in modern scanning instruments to frame the object being digitized. Figure 2.3 illus- trates the schematic of the scanning process by a 3D non-contact scanner.
The laser beam is projected through an emitting lens and reflected by a mir- ror that is rotated by a galvanometer to sweep the laser light across the entire target object. The reflected laser light from the surface of the scanned object passes through a receiving lens and a filter, and then is collected by a video camera located at a given triangulation distance. The captured images are saved to flash memory. Most scanning instruments are also bundled with digitizing software to help engineers modify and scale the data. Its imaging process is based on the principle of laser triangulation. The Konica Minolta vivid 9i scanner measures 640 × 480 points with one scan, and can capture the entire object image in a few seconds, then convert the surface shape to a lattice of over 300,000 vertices. A file beyond just point cloud, such as a polygonal mesh, can be created with all connectivity information retained, thereby eliminating geometric ambiguities and improving detail capture. A photo image is also captured at the same time by the same camera.
Digitized scanning is a very dynamic and rapidly evolving field. More com- pact new instruments with still higher resolution and more functionality are introduced to the industry every year. Within just a few years, ATOS III was introduced over ATOS II and ATOS IIe by GOM mbH, and Konica Minolta Range7 was introduced over Range5, vivid 9i and 910 scanners. Figure 2.4a is a photo of ATOS II in operation, and Figure 2.4b shows a photo of Konica Minolta Range7. The Range7 scanner is a lightweight box digitizer; the laser beam is emitted from the right and a camera is installed on the left when facing the object. It provides an accuracy up to ±40 μm with a 1.31-million- pixel sensor. The installed autofocus functionality can automatically shift the focus position to provide sharp, high-accuracy 3D measurement data.
The implemented sensor and measurement algorithm provides an expanded dynamic range up to 800 mm, and can measure the objects with a wide range of surface reflections, from shining glass and metallic surfaces to dark surfaces with a reflectance as low as 2.5%. The 3D digitizer Range7 can be used with various software packages, such as Geometric, PolyWorks, and Rapidform. The data output of Range7 is in the format of ASCII or binary, including normal vectors, and can be imported to various CAD systems.
The improvement in scanning rate and processing speed, and the advance- ment of graphical user interface have made real-time scanning possible. The users can instantaneously check the scanned data on the preview screen to see if any data are missing. This allows them to make timely adjustments and take a sequential scanning for the missing data if necessary. Several sources of illumination are available for 3D scanning. The white-light digi- tizing system is an optical 3D digitizing system that measures the subject surface geometry using a white light. Because white light covers a spectrum of frequency, it usually provides the best-quality data, compared with other measurement technologies, such as infrared or X-ray.
Subject part
Mirror
Emitting lens Laser beam Receiving lens
Filter
Camera
FIgurE 2.3
Schematic of scanning process.
The probes and scanners are available in a variety of forms and brands, provided by many manufacturers in various models. Some examples include the Faro laser probe, Konica Minolta laser scanner, and Leica T-scan/tracker.
Scanning is often conducted using a coordinate measuring machine (CMM) that is structured like an extended arm with various degrees of freedom to provide the necessary flexibility for digitizing. For example, the Faro Laser ScanArm V3 is a seven-axis, fully integrated contact/noncontact digitizer
(a)
(b)
FIgurE 2.4 (See color insert following p. 142.)
(a) ATOS II in operation. (b) Konica Minolta Range7 scanner.
with an accuracy of about 35 μm. The Leica T-Scanner can digitize all types of surfaces by projecting a laser beam onto them.
Compared to the contact probe, the noncontact laser scanner is more sophisticated and expensive. It captures millions of data points to better define larger parts with free-form shapes and contours. It is usually used for large parts with complex or curved lines. A large number of data points are needed to accurately capture the intricacies of the design to create digi- tal representations of these objects. The automatic process and feature-rich software requires less training and supervision, and provides faster feed- back. The metrology technology and devices are becoming more environ- mentally friendly: less and less hardware has to be prepared with coating or paint for proper measurements, and thus eliminating the adverse envi- ronmental effects of coating and paint due to their chemical contents. The quality of these tools has also improved with every new generation, allow- ing for higher precision and the measurement of smaller dimensions. The latest scanning technology is more intelligent and can integrate data collec- tion and feature identification together. For instance, the reverse engineer- ing software Rapidform XOR does not just capture the geometric shape of the scanned object; it also captures the original design intent. It automati- cally detects features, such as revolves, extrusions, sweeps, and fillets, on the scanned object.
Both the contact probe and laser scanner work by digitizing an object into a discrete set of points. In contrast to analog signals that are continuous, digi- tal signals are discrete. Therefore, a digital image can only be an approxima- tion of the object it represents. The image resolution depends on the area and rate of scanning. To obtain the required information within a reasonable amount of time, the engineer usually tracks the sharp edges of the object with a direct-contact probe for better-defined details and combines this information with the results of a laser scan.
Small and economically affordable contact probes, as well as sophisticated laser scanners, are available for reverse engineering applications. This afford- ability has eased many engineers’ reliance on outside services, allowing them to purchase their own devices and keep their digitizing design work in-house. Work in-house means shorter turnaround times, better control of the design process, and better security of proprietary information. New digi- tizers are also packaged with feature-rich software that makes it easier to turn static raw physical data into dynamic computer images.