By Orlando Perez, Orlando Perez is Senior Technical Product Manager for FARO Technologies
CAD introduced the capacity for design engineers to visualize in 3D, but it did nothing to help them interface with real-world products. For this, they had to await shape digitizers. Digitizers complete the feedback loop between a work-in-progress and CAD. Using these instruments, designers can make new iterations almost as quickly as revised shapes can be scanned or touched.
The earliest digitizers were contact instruments. A user would touch the object to be measured with the stylus on an articulating arm, press a button on the stylus’ handle, and record a point in space. Rotational transducers in the arm “joints” marked the position and orientation of the point, to an accuracy of 0.0002 in. By dragging the stylus along the object and depressing the button, a user can record a straight or curved line.
Images saved in the digitizer’s software are a collection of points or a wire frame, and are compatible with current surfacing software where they can be converted into 3D images suitable for CAD.Because they are portable and don’t require programming, contact digitizers created an instant niche for themselves, offering an alternative to large portage-type coordinate measuring machines (CMMs) that had been the standard of 3D measuring for over a decade. They continue to hold this niche and are favored for applications where 1000 or fewer points are necessary to describe an object.
The second basic type of digitizer employs lasers to capture surfaces. These can be further subdivided into those that capture relatively few points and those that create “point clouds.”
A hybrid type, known as a Laser Tracker, uses a spherical reflector to bounce a projected laser back to the emitter. Software in the instrument collects precision vertical and horizontal angles of the path of the beam, along with the distance to record the 3D position of the reflector, to an accuracy of 0.001 in.
Like conventional contact digitizers, the Tracker is used more for point and line capture than point clouds. But Trackers are designed for capturing the image of large objects, at distances of up to 230 ft-far too great for contact digitizers. Design specs loaded into the instrument’s software become a checking tool to measure incremental changes as the hardware is built. For instance, the 50 ft Canadarm used by NASA to inspect the Space Shuttle in flight is built under the dimensional guidance of a Tracker.
Trackers have largely replaced theodolites that were previously used for large-object dimensional measurement.They can be found checking specs on auto body assemblies, machine foundations, whole process-line setup (paper mills), and the curvature of yacht hulls.
The latest digitizer generation creates 3D images with photographic realism. The images are typically two orders of magnitude more dense than those generated by contact digitizers or Trackers. Two types are available: Those for near imaging-ScanArms-and those for imaging larger or distant objects-Laser Scanners-such as whole airplanes, statues, tunnels, and architectural objects.
ScanArms are for close-up and small-object imaging, and are built with the same type of articulating arm that is found on contact digitizers. Hybrids of sorts, ScanArms have the ability to be used as either a scanner or contact digitizer, simply by rotating the head of the instrument. In the scan mode, the head is “flown” close to the surface or part. A laser is bounced off the surface and is recorded in the scan head. Extreme detail is gathered. Even surface textures such as knurls and grip patterns are captured. Typically, they are used to capture images that range in size from a button to a water bucket, to a tolerance of ±0.0030 in.
If ScanArms are equivalent to the close-up 35-mm cameras of the metrology world, then Laser Scanners (LSs) are analogous to Ansel Adams’ view camera. These LSs are big- scene imagers, for covering everything from monuments to airplanes to crime scenes.
Putting the images to work
Dimensional verification: Perhaps the first application of digitizers was as parts and tooling checkers. Embedded CAD files of a part or tool became the go-gage in the instrument’s software. Technicians measure critical points on a part and the software compares them to the CAD standard, all the while building a running quality report. This data can be communicated between sister installations or between OEM and customers, almost instantly.
|Instrument||Physical range||Typical accuracy||Imaging rate||Software compatibility||Applications|
|Articulating arm||4-12 ft (1.2-3.65 m)||0.0005 in. (0.013 mm)||na||PolyWorks
* Windswept surfaces of
|Laser tracker||230 ft
|350 pts/sec||FARO CAM2
|* Process machines
* Marine hulls
* Part inspection
* Tool building
* Aerospace tooling and parts and assemblies
* Automotive tooling, parts and assemblies
* Automated assembly systems
|Scan Arm||4-12 ft
|19,000 pts/sec||* Consumer products
* Hand tools
* Potted electronics
|Laser Scanner||249 ft
|±0.118 in. @ 82 ft
(±3mm @ 25m)
|120,000 pts/sec||* Monuments
* Whole aircraft
Reverse engineering: Once upon a time, “reverse engineering” was an uncomplimentary phrase that implied the copying of competitors’ products. Now, copying a physical object or model by taking a dimensional snapshot of it is an indispensable step in rapid prototyping. Digitizers capture changes in the shape of a model, in minutes instead of hours or days. Where two sides of an object are mirror images of each other, the designer has to copy only one side, then flip the image in software to create the other side.
Dimensional interaction: The old story about the fellow who built a boat in his basement — then discovered that it was too big to go out the door — happens at every level of engineering. How two objects move past each other is often critical to their function. Digitizers can recreate an existing 3D space, to almost any degree of detail necessary and in perfect proportion, with all surfaces measurable. Using this file, designers can model material flow, pipe layout, machine positioning, and clearances between interacting elements. Objects can be downloaded into these programs to check for clearance and interaction between them. One consulting group recreated the cockpit of an F-16 fighter so that the virtual cockpit could be used to position radios and other instrumentation ergonomically. In another case, a consultant recreated the entire containment room of a nuclear reactor so that a virtual “walk through” could be performed without endangering personnel.
Flow analysis: Almost every flow process, from the air stream over NASCAR racers to the sprues in metal molds, can be evaluated by computational fluid dynamics (CFD) instead of physical modeling because it is faster, cheaper, and safer. Often, the number of design iterations required to produce the final product drops an order of magnitude. The process is similar to reverse engineering.
A digitizer captures the shape to be evaluated-from an extrusion die to the external surface of a supersonic airplane. Then, after applying a surface to the virtual model, the user applies an FEA grid to the surface and downloads it to a CFD program where flow patterns are evaluated. In the case of extrusion dies, the number of design iterations drop from about twenty to two or three, greatly reducing the cost of machining “trial” dies; in the case of aircraft design, the cost of flying the plane is eliminated along with the risk.
Filed Under: 3D CAD, Software, Test + measurement • test equipment