BizJet Emerges from Laser-Made Data Cloud

Today, the initial stage of aircraft development is almost completely digital, and thousands of details are settled before any metal is bent or riveted. In this paradigm, virtual 3D models are evaluated for performance, strength and flying characteristics in programs that simulate the forces and stresses of flight. Now, if engineers want to see how much lift is gained by adding winglets to an existing plane, they add the winglets to a digital model. Then, the model is inserted into a simulated wind tunnel where a computerized fluid dynamic program gives a close approximation of actual change in lift and drag. Good or bad design usually makes itself evident right away.

Virtual development is a faster, less expensive means of evaluating aerodynamic and structural concepts than building and testing physical models. The problem is, many current aircraft have no digital facsimiles because they were built long before virtual development existed. This was the case with a business jet used for radar testing. The basic design of the aircraft is more than 40 years old, conceived in an era when digital imagery as we know it was nonexistent. When engineers needed to evaluate design modifications to this family of business and military jets, no digital models were available, but engineers knew where to get this expertise. They turned to a company in Baltimore, known as Direct Dimensions, Inc. (DDI).

MODELS IN DIGITAL CLOUDS

Direct Dimensions has been on the "curl of the wave" of the decadeold revolution in 3D laser scanning and digital image manipulation. Their reputation for being out front with new technology has attracted unusual projects, a number of which have national or historical significance. From recreating an exact duplicate of the propeller of the Wright Brothers 1903 Flier - from a remaining segment of it - to precisely capturing the original Liberty Bell to make a casting pattern; they are exploring the broad expanse of digital ether between ideas and hardware. This has led to new ways of visualizing things. In their eyes, objects become wire-frame CAD images, arrays of 3D points, or even a point cloud with millions of very dense points, to be converted by surfacing programs and then adapted to full prototype models.

What the business jet project offered, however, challenged even DDI. They were asking for a digital model of the outer airflow surface, with sufficient detail, to perform virtual wind tunnel analysis known as CFD (computational fluid dynamics). Over the years, numerous modifications had already been made to the plane, from access doors to speed brakes, so this current virtual model would make the process of experimenting with new concepts easier.

Direct Dimensions has been creating 3D models of other aircraft and aircraft parts for more than 10 years. By originally using the new generation of contact- type portable CMMs from FARO Technologies, Inc. (Lake Mary, FL), they had digitized critical dimensions of cockpits, fuselages, wings and other aerostructure components. By carefully measuring the part surfaces, they captured enough data to generate a 3D image that could be converted by surfacing programs to a workable CAD model.

These models were subsequently used in flight simulation software and in virtual "mockups" to test seating location, pilot visibility, critical switch positions and other human factor issues.

However, because contacttype CMMs are best for capturing geometric points and lines, recreating a complete aircraft via this technique would have taken over a week just to gather the data. And several more days would have been needed at the computer to flow in the contours of the fuselage, wing and tail. Thus, Direct Dimensions decided that the data density necessary for the business jet was beyond the scope of contact CMMs. They needed something that could capture entire surfaces and everything on them, not just lines - and that could do it very quickly.

A BETTER MOUSE TRAP

The solution lay in a new device known as the FARO Laser Scanner LS. Using an optical transmitter with a beam divergence of only 0.25mrad, the device has a range of 76 meters and gathers enormous quantities of 3D data – 120,000 points per second with 3mm linear accuracy. The result is image resolution that, in its extremes, a hawk would envy – 0.009° vertical and 0.00076° horizontal.

Metrology specialists accustomed to thinking in terms of measurements of a few hundred to a few thousand points must reorient their thinking when using the FARO LS. A typical scan image has millions of 3D pixels. Also, the image is "photographic" compared to those generated with contact CMMs. This is relatively unimportant on geometric (flat or rectangular) surfaces where hole position or flatness is being measured, but it is a huge advantage when it comes to capturing complex surfaces with gradually changing slopes.

"What we are talking about is a step-change in data density," explains Michael Raphael, owner of Direct Dimensions. "Because the FARO LS brings in so much data at once, it really compresses the time that we spend on-site collecting data, tying up the client's asset."

Project size-both physical and in terms of detail-determines when to use one type of digitizer versus another. Some things are too large to capture with a contact-type CMM. Direct Dimensions' guideline suggests that if an object is as big as a car, they generally scan it with a laser. If it is of a size that can be shipped, they will use either a contact digitizer or an arm-based laser scanner, depending on the complexity of the surface.

EVOLUTION OF AN IMAGE FROM POINT CLOUD TO DESIGN TOOL

Like portable CMMs, the Laser Scanner can be carried to remote locations to gather data, such as to an aircraft hangar where the target bizjet was housed. This instrument, however, can take the concept of portability one step farther by incorporating its own computer hard drive on board. Although Direct Dimensions normally uses a laptop computer with the device, collected data can go straight into the body of the FARO LS, so no second box is required on the scanner site.

Laser imaging is a line-of-sight process. Since the exterior of the target jet could not be captured from a single location, this planning is required. It was necessary to figuratively divide the plane into multiple sections and establish common reference points that could be used to reconnect the sections later. Because the scans are 3D, the sections can be rejoined to provide an accurate perspective of the entire plane from any point of view.

Direct Dimensions set up the laser and went to work, overlapping the scans of the sections of the plane. From eight positions, they took more than 50 scans of the plane, each 1/8-resolution scan taking only five minutes. In spite of having to reposition the scanner, only six hours were needed to cover the entire surface, nose to tail. When they were finished, they had a collective point-cloud image estimated to be 10-million points.

"We could even see the gaps in the flaps and the lug nuts on the wheels," saysRaphael.

Back in their lab, over the next few days, DDI software specialists set to work, uniting the image sections and eliminating background data. This is similar to outlining a photo, but in three dimensions. The next step was to convert the image to a clean single point cloud and then into a polygonal model in which the points become a faceted mesh of triangles.

At this stage, a model can be used in several ways, such as rapid prototyping, machining or analysis. The aircraft builder wanted a model that they could manipulate in CAD, so the polygonal model was converted to a wire frame. Then, using reverse-engineering surfacing software, they converted it to a NURBS model so that it could be imported into CAD. Altogether, the bizjet project took about a week, from the initial planning, to actual scanning, to image manipulation, to finished model that could be pulled into the company's CAD program.

FREEZING HISTORY

Business jets are not the only huge objects that Direct Dimensions has digitally captured with the Laser Scanner.

"Because the details of the images are so precise, we use the Laser Scanner to capture the features of many of our national monuments," explains Raphael. Direct Dimensions now works with the National Park Service to create images of the statuary, monuments, and architectural facades that are American icons. "For many of these objects, the park service has no documentation at all – no design drawings, nothing," notes Raphael. "If they were damaged, the only things that could be used to guide restoration would be photographs. Now, with laser scans, we are able to cover these huge objects and give the park service 3D documentation that could be used to recreate them in case of a disaster."

FARO Technologies, Inc. designs, develops, and markets portable, computerized measurement devices and software. For more information, please visit www.faro.com.

Direct Dimensions, Inc. (DDI) specializes in the application of geometric measurement technolog y. Visit them at www.directdimensions.com.