Chopper
checker
Photogrammetry, CAD-based inspection bring speed,
accuracy to helicopter panel assembly
The larger things are, the more challenging they are to inspect. Traditionally there
were only two choices--bringing the measuring system to the parts or taking the parts to
the measuring system. Now, this problem is being solved by combining two proven
dimensional-measurement technologies--photogrammetry and CAD-based inspection. The result
is a new type of coordinate measuring machine (CMM).
Here is the Navy variant of GKN Westland's EH101 helicopter.
Neither of the original options for measuring large objects was very appealing from a
productivity standpoint. Moving the measuring system to the part requires recalibration.
At a minimum, on-the-fly recalibration doubles the time needed to do the measurements.
Worst case, measurements can take 10 times longer. Plus, if vibration or movement are
present, the production line must be halted. The alternative, moving the pieces to be
measured, increases the risk of bending or damage and raises the problem of fixtures.
Fixtures add another source of variability into the process--another potential point of
failure.
Either way, moving the pieces or moving the CMM, measurement can be a painful prospect
when demand is high and the work force hard-pressed to meet schedules.
These problems can be especially acute in the aircraft industry. Such was the case at
GKN Westland Helicopters, Yeovil, Somerset, England. Westland is a European supplier of
rotary wing aircraft. At the end of 1999, the company's order book totaled $6.3 billion
and demand for the world-class range of aircraft is at an all-time high.
Westland's 5000 workers build the EH101, and the British Army's version of the Apache
attack helicopter plus the Lynx, Super Lynx and Sea King. Westland's corporate parent is
GKN plc, one of the UK's biggest manufacturers. Originally known as Guest Keen
Nettlefolds, GKN is an automotive, aerospace, and industrial conglomerate.
The need for Q/A data
Like all the other aerospace giants, Westland, needs a great deal of quality assurance
(Q/A) data from its assembly lines. Among other things, Q/A data on assembly line fit-up
is continually fed back to suppliers of complex and highly engineered components such as
skin panels, assembly fixtures, and the helicopter's airframe elements.
For years Westland, like all other aerospace manufacturers, struggled with the
challenge of inspecting large, intricate parts. Productivity and output were regularly
impacted by the need for data about these parts. In the background was another pressing
problem: the need to coordinate data from a wide range of measurement systems.
As engineers and Q/A managers at Westland sought a better solution, photogrammetry was
high on their lists. Triangulation is fundamental to photogrammetry's ability to measure
dimensions in 3D space. Precise point locations are determined mathematically using lines
that either intersect or converge. Triangulation also underpins theodolites and other
surveyor's instruments. Unlike them, however, photogrammetry can measure multiple points
at a time.
Early in 1999, Westland saw its opportunity in a fortuitous convergence of inspection
technologies:
- Low-cost digital cameras made inspection results instantly available, even online, and
in handy digital formats. Photogrammetry's front-end was revolutionized.
- CAD-based inspection technology generated foolproof point-by-point comparisons between
the inspection data and the part's original design file.
Recognizing the huge potential in these similar approaches, and noting that
photogrammetry had finally followed CAD in "going digital," Westland insisted
that photogrammetry and CAD-based inspection work together.
Photogrammetry uses bits of reflective tape attached to selected datum points on the
object or tool to be measured. Highlighted by a flash unit, the targets' positions are
recorded as digital images. Locations in 3D space are then determined to a high degree of
accuracy using triangulation. CAD-based inspection software is used with 3D CMMs and
inspection arms. It allows users to inspect complex machined parts, then automatically
compare the results with the original CAD models. Tabular, graphical and pictorial reports
are produced.
Geodetic Service's, Melbourne, FL, V-STARS gathered the data and Delcam's, Windsor,
Ontario, Canada, PowerINSPECT formatted it. The result was an entirely new approach to
coordinate measurement using scanned in, "clouds of points" data.
During assembly
One major measurement task on the EH101 is to verify the correct placement of skin
panels on the helicopter's airframe structure. This required that a variant of a CMM be
assembled around part of the helicopter. Since measurements from this system were made to
a fixed datum, the aircraft had to be held completely still. This immobilized operations
for hours.
It was here that Westland introduced photogrammetry, specifically Geodetic Services'
V-STARS system. "Photogrammetric equipment is much more portable than our traditional
CMM approach," said Dave Wakely, manager of central tooling at GKN Westland
Helicopters.
"Work can continue on another section of the aircraft while a measurement is
taking place."
Prior to using digital cameras, however, there was one big problem with
photogrammetry--the time to the process film. Once the film was developed, the negatives
had to be measured. This was done on a machine called a monocomparator--a non-portable,
extremely heavy instrument. It could take many hours or even several days before results
were obtained. Production managers faced a dilemma. They could either run the line and
risk a big rework problem or stop the line and risk the loss of half a day's production
for no good reason.
The forward sliding cowl being verified in situ on the aircraft.
"Photogrammetry provides capabilities to perform measurement tasks that cannot be
done with the other methods," says Gary Johanning, Geodetic Services general manager.
"In cases where data acquisition must be done quickly, usually the only option is
photogrammetry. Targeting and other preparation can be done concurrently with production
and the photography can be done during a shift break. If the environment of the
measurement is 'hostile'--footing unstable, scaffolding in the way, or any
vibration--photogrammetry is usually the best choice."
Before assembly
Naturally the skin panels were inspected prior to assembly, too. So were the patterns,
tooling and fixtures on which the panels were produced. "The information we get from
PowerINSPECT is laid out extremely clearly," Mr Wakely says. "The graphs and
pictures mean that the results are meaningful to everyone, not just to the specialist. The
system is also very fast. It can analyze thousands of points in under a minute. Being able
to see and understand the results so quickly allows us to start work on fixing the problem
that much sooner."
PowerINSPECT also integrated well with Westland's corporate CAD/CAM system which
traditional photogrammetry can not. Coupled to a portable CMM, PowerINSPECT compared point
data with the original CAD file, specifically CATIA from Dassault Systemes. "As a
sophisticated surface-fit software program, PowerINSPECT has proved invaluable in
determining minimum product deformation," Mr Wakely says.
Westland's CATIA files download quickly and easily into PowerINSPECT. "We
regularly take files from CATIA into PowerINSPECT and have not experienced any data
translation problems," he adds. What could not be done, however, was directly
comparing photogrammetry's results with data from CAD inspections.
Sliding cowl master tooling being verified using V-Stars Photogrammetry and
PowerINSPECT.
These initially incompatible measurements led Westland to bring Geodetic and Delcam
together to add vital capability. Photogrammetry, specifically Geodetic Services' V-STARS
system, gained a sophisticated digital interface to the world of CAD. PowerINSPECT
provided the crucial link, allowing the regeneration of inspection data in CAD formats
readily usable in CATIA.
"The training and use of software is an important feature," Mr Wakely notes.
"The tooling metrology section was instrumental in the selection process and they
have been pleased with the short integration and learning time scales. A final
consideration has been the back-up we have received from Delcam," he continues.
"The training package was excellent and quickly gave our metrologists confidence in
using the software. The support has also been very strong. Furthermore," he points
out, "we have regular contact with the [Delcam] development staff, who are keen to
use our ideas to make the software even better."
Westland has also loaded PowerINSPECT onto its CMM. This, Wakely noted, "is to
exploit the benefit of a common software package supporting two systems. Because of the
complex geometries in helicopters and the wide range of measurements to be taken, Westland
will use both photogrammetry and CMMs for the foreseeable future.
Geodetic Services Inc, Melbourne, FL. http://www. geodetic.com
Circle 364
GKN Westland Helicopters, Yeovil, Somerset England. http://www.gkn-whl.co.ukCircle 365
Delcam, Windsor, Ontario, Canada. http://www.delcam.comCircle
366
Digital cameras,
CAD interface expand
photogrammetry's scope
Photogrammetric principles are widely
recognized, but its digitization and subsequent interfacing to CAD-based inspection
systems offers highly effective solutions to the need for quick and accurate inspections
of large pieces.
"We are hard at work building on our mutual Westland success," says Gary
Johanning, Geodetic Services general manager. "Virtually all of our customers need
the capability provided by PowerINSPECT. Laser trackers have gained a stronghold on the
mainstream inspection applications. Nevertheless, photogrammetry solves many exotic
measurement problems. Our V-STARS system can be taken virtually anywhere. It is even more
portable than some of the other photogrammetry systems."
One of photogrammetry's major advantages for quality assurance in manufacturing is its
flexibility. Thanks to self-calibration, triangulation and resectioning (STARS),
acceptable dimensional data may be gathered with one or more cameras. And the work may be
done in either "online" or "offline" modes.
In the multi-camera or "on-line" method, exact positions of each point are
triangulated from photos taken with each separate camera. The speed of the triangulation
analysis in the multi-camera approach brings it to near real-time. And its accuracy and
repeatability are comparable to that of coordinate measuring machines (CMMs).
The single-camera method requires several pictures of the object taken from many
different positions. Depending on the size and complexity of the part, these photographs
can quickly generate several thousand points. That is sufficient for the most painstaking
analysis. This mode is referred to as off-line because data is triangulated and analyzed
after the photography is taken.
With both approaches, the point coordinates can be loaded into PowerINSPECT to perform
detailed analysis and provide reports of the product compared to the CATIA model.
PowerINSPECT enhances photogrammetry by generating both pictorial and graphical summaries
of the results, plus a Microsoft Excel spreadsheet containing all the numerical data.
Summarizes Mr Johanning, "The role of PowerINSPECT is to hold the original CAD file
in memory and run comparisons for each point of inspection, analyze any variances found,
and then generate a report."
The V-STARS acronym comes from Video Self-calibration, Triangulation And Resection.
Resection refers to Geodetic Services' ability to calculate camera position and aiming
angles--the system's orientation--from the digital images or photos. The Geodetic Services
self-calibration process is also derived from these measurements. With V-STARS, resection
and self-calibration can be combined and done automatically.
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