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Laser Scanning Helps Shrink Digital Projector

Photonics Spectra
Mar 2005
Judy Dancer

Projectors are no longer large, clumsy boxes. They have slimmed down so much that some digital models fit neatly into a briefcase. They work side by side with portable and handheld computers in conference rooms, classrooms and meeting places throughout the world.

The challenge in producing an ultracompact projector, such as the 2.05 × 3.69 × 9.75-in. LP120 from InFocus Corp. of Wilsonville, Ore., is in engineering the internal components to fit into the designed case. The difficulty is increased by the fact that the geometry of the case is so complex, with numerous contoured 3-D surfaces, making it difficult to get accurate measurements. And yet that accuracy is important because the numerous features on the case must mate with the internal components.

Laser Scanning Helps Shrink Digital Projector
Figure 1. Laser Design Inc. scanned the initial design of InFocus Corp.'s LP120 portable digital projector.

Laser scanning offers the necessary measurement accuracies. Using the technique to model critical components of a prototype enables manufacturers to quickly assess whether a design will meet production tolerances, shortening the time to market and increasing revenues.

For InFocus, building an assembly model of the LP120 projector during the design process presented problems. Notably, the tolerances involved in stereolithographic rapid prototyping are rather large -- typically around 0.02 in. -- whereas those of the production process for the LP120 are around 0.005 in. As a result, when the engineers made and assembled the prototypes, they could not determine whether any misfits they encountered were because the prototypes did not conform to the production tolerances or because of problems with the design.

Laser Scanning Helps Shrink Digital Projector
Figure 2. The final design incorporated the changes made following the scanning.

Moreover, they could not determine whether the parts that fit properly would also fit once they went into production.

Of the five components of the projector that must fit precisely, two are the die-cast magnesium parts that form the case. The tooling for each of these parts could cost $50,000 and take weeks or even months to produce. If the tooling turned out to be wrong, the engineers would have to modify the tools, which could cost an additional $10,000 or more and add a few weeks to the production schedule. Even worse, the tools could be so far off that they would have to be rebuilt from scratch, at an expense of another $50,000 and with further delay in the introduction of the product.

Delay is of particular concern in this market because the life cycle of a product typically is only 18 to 24 months. The potential cost of a delay thus is much greater than the tooling cost because it would sacrifice a significant amount of the revenues that a product otherwise would generate.

InFocus scans a relatively small number of parts, so it could not justify the cost of purchasing a laser scanner. Instead, it worked with Laser Design Inc. of Minneapolis to have preproduction prototypes of the critical components scanned and returned as CAD files. Each point was accurate to within 8 µm, and the surfaces generated from the point cloud were accurate to at least 0.001 in.

Laser scanning systems work by projecting a line of laser light onto the surfaces being measured while cameras continuously triangulate the changing distance and profile of the laser line. A computer translates the video image of the line into 3-D coordinates, providing real-time data renderings that give the operator immediate feedback on areas that might have been missed. Rather than collecting points one by one, the scanner picks up tens of thousands per second, so reverse engineering the most complicated parts often may be accomplished in an hour or two.

Software simplifies the process of moving from point cloud to a CAD model, making it possible to rapidly generate a model that faithfully represents the original part. Additional software can be used to compare original design geometry with the scanned physical part, creating a graduated-color error plot that shows in a glance where and by how much the surfaces deviate from the original design.

Laser scanners have an advantage over touch probes in that they can quickly measure large parts while generating a greater number of data points. And because no probe must physically touch the object, problems of depressing soft objects and measuring cavities that are smaller than the probe are eliminated.

With feedback from the prototype, the InFocus engineers made design changes and adjusted tolerances in several areas. Laser scanning validated the prototype parts to the master solid model, so that the engineers could use them to accurately evaluate the design. Once a critical area was identified, the laser scanning service bureau could zoom in, making it possible to determine which fit problems were caused by the prototypes being out of tolerance and which were the result of the design.

The projector designers saved a considerable amount of time because they could zero in on problems and not waste any effort on prototype inaccuracies. The product went to market weeks faster with a perfect fit.


GLOSSARY
digital
Denoting the use of binary notation; i.e., the representation of data by bits (1 or 0).
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