December 03, 2007
Laser Scanning Makes Manufacturing Inroads
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Focus Scan is the driver, data acquisition, and point cloud pre-processing software for Metris laser scanning. The software controls the Metris laser scanner on the CMM and is integrated with Focus Inspection and Focus Inspection Automation for automated CMM inspection stations. Focus Scan 5.0 now also supports indexing rotary tables increasing the application range for CMM based laser scanning to even more complex part geometries. There is also a Focus Scan Offline that module enables the user to create, modify, and prove out part programs using only 3D CAD models instead of using the actual CMM hardware and waiting for real parts.
Although I have never actually used the Metris laser scanner and its software, I do have experience with other similar devices and software. In many peoples’ minds, reverse engineering is the illegal act of copying, in effect stealing, an original design, in this case a physical object. That old perception and definition is something I wanted to help dispel with a more positive image based on my experiences.
Reverse engineering (whether used for inspecting or ultimately physically reconstructing an object) is a process for digitally reconstructing a physical part. This is significant because recent estimates are that up to 80% of new designs come from existing ones, usually from existing parts and assemblies. Reverse engineering is part of a larger scheme becoming known in some circles as digital shape sampling and processing (DSSP).
DSSP data for manufacturing purposes can be viewed as having two somewhat similar yet definitely distinct ways of describing and representing 3D forms. Reverse engineering does it by handling geometry as sets of discrete points, whereas traditional CAD does it with shapes defined by continuous curves and surfaces. Combined, reverse engineering and CAD are complementary to each other and are what transform physical objects to digital objects, and ultimately back to physical objects in a workflow. In other words, reverse engineering extracts geometric information from physical objects and CAD reconstructs it into a digital form that can be used for creating physical objects based on the
Reverse engineering is a process of examination only, the part under consideration is not modified during the scanning/digitizing stage (which would make it re-engineering), although it can be modified in downstream applications, such as CAD.
The past fifteen years have seen explosive development of many different types of 3D digitizing devices, as well as the reverse engineering software that allows the data produced by the digitizing devices to be manipulated into a useful form.
There are two parts to data-related portion of the reverse engineering process – scanning and data manipulation. Scanning, also called digitizing, is the process of gathering geometric point data from an object. Several different contact and non-contact technologies are used to collect three dimensional data. Each technology has its advantages and disadvantages, and their applications and specifications overlap. What eventually comes out of each of these data collection devices is a description of the physical object in 3D space called a point cloud.
There is usually too much data in the point cloud collected from a scanner/digitizer, and some of it may be unwanted “noise.” Without further processing, the data cannot be used by downstream applications such as CAD/CAM software or in rapid prototyping. Reverse engineering software is used to edit point cloud data, establish the connections of the cloud points, and translate it into useful formats such as surface and solid models or STL files. It also allows several different scans of an object to be combined so that the data describing the object can be defined completely from all sides.
Usually, collecting an objects data is the shortest and easiest part of any RE procedure. Normally, most scanning only requires a few seconds or a few minutes. On the other extreme, manipulating that scanned data can be quite time-consuming and labor-intensive, but like RE in general, this scanned data manipulation techniques are rapidly improving and becoming much more time and cost effective.
Although there are several digitizing technologies available today each has its own distinct advantages and disadvantages.
Feature-based scanning is a method where features of an object are scanned by physical contact. A mechanical touch probe, also known as a contacting digitizer, is a physical part contact device and method well-suited for prismatic parts, such as an automotive transmission housing. The touch probe is actually a pretty basic device that is connected to a computer and simply lets you know when and where in space contact is made with an object.
Touch probes, are usually very accurate over a wide measurement volume. There are contact digitizers that are positioned manually to yield a single measurement at a time, or may be scanned across a surface to produce a series of measurements. There are also touch probe instruments available which can automatically scan an object using a variety of mechanical drives. Contact instruments usually employ an articulated arm that allows for multiple degrees of freedom of movement.
In feature-based scanning, basically, you can randomly scan sections of a part that are automatically turned into lines and splines for creating a digital surfaces and solids.
A couple of disadvantages of contacting devices include the fact that they can distort soft objects. They also can be too slow for digitizing organically-shaped parts, because they usually require too much time and labor for scanning complex curved surfaces, although there are work-arounds. On the other hand, they are not affected by the color, transparency, or reflectivity of a surface the way laser systems can be. And while they can be relatively slow, contacting devices are often the fastest way to digitize simple surfaces where just a relatively few data points are required.
Point Cloud Scanning
Point cloud scanning is performed using a laser scanner that is a physical part non-contact device and method well-suited for organic, freeform, artistic parts, such as statues. It is also well-suited for scanning soft objects with surfaces that could be distorted by a touch probe. Point cloud scanning is not as well-suited, however, for prismatic or sharply faceted parts as is feature-based scanning because too many points are captured that need to be dealt with.
Laser scanners most often use relatively simple geometric triangulation to determine the surface coordinates of objects. A laser line is scanned on a target object and integrated sensors, such as CCD arrays, image the line, usually simultaneously from each side of the line. Where the laser line's image falls on each sensor is determined as trigonometry is applied to calculate the position of the target surface at each point on the laser line.
The relative simplicity of the laser technique and its ability to quickly digitize a large object accurately with good resolution have made laser scanners increasingly popular for reverse engineering purposes. Laser scanner products are available as complete systems, and as self-contained measuring heads for mounting to standard touch-probe arms or in other ways, including customized mechanical fixtures.
A significant challenge for point cloud scanning is that there is often more clean up required of the scanned data because of the number of points captured, but this is being addressed on different levels by several vendors.
Reverse engineering is a fascinating field that is really starting to take off from several different perspectives – hardware and software – largely because there are an increasing number of legitimate applications for it. Because more companies in more industries are starting to realize its benefits, I expect reverse engineering to continue to grow at an impressive rate for both potential users and hardware and software vendors servicing those users.
The Week’s Top 5
At MCADCafé we track many things, including the stories that have attracted the most interest from our subscribers. Below are the five news items that were the most viewed during last week.
SpaceClaim Announces Immediate Availability of SpaceClaim Professional 2007+ and Trial Program
SpaceClaim Corp. announced the availability of SpaceClaim Professional 2007+ and the Explore SpaceClaim 30-Day Trial program. SpaceClaim Professional 2007+ is a complementary 3D design solution that enables users to originate concepts, make better use of existing models and freely modify mechanical designs. The release includes the ability to create sheet metal parts, as well as open and modify sheet metal parts originating in other CAD systems. Additionally designers can work with lightweight assemblies, drive 3D modifications via any 2D section, add and modify driving dimensions and integrate with CAD-neutral partner
applications. SpaceClaim is also introducing a trial program. The Explore SpaceClaim Trial program enables participants to download and use SpaceClaim Professional 2007+ for 30-days.
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-- Jeff Rowe, MCADCafe.com Contributing Editor.
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