aerospace metrology


Laser Trackers Expand Dimensions Airframe manufacturers have used laser trackers for the last 20 years, and the technology has evolved signifi - cantly in that time. For example, O’Reilly points out that the FARO Vantage, the company’s latest release, offers a 25% reduction in size and weight over its predecessor. “This allows it to be stowed conveniently in a backpack and fi t into an airline overhead compartment,” he said. Laser tracker systems employ a laser beam mounted in a precision gimbal, to generate a refl ection off a spherically mounted retrorefl ector (SMR) to determine the distance to the SMR. These test points can then be referenced back to its CAD model. The Vantage provides test point measurement over wings or fuselages, with up to 160 m of working volume coverage with accuracies up to 18 µm at 5 m (per ASME B89.4.19).


While laser trackers are useful, especially for parts the


size of aircraft, they have limitations. They offer only three degrees-of-freedom (DOF) from the pedestal origin, since they can measure only areas in the line of sight of the laser. That is why the industry developed two-device, 6


DOF systems such as the FARO TrackArm system. FARO essentially married the Vantage laser tracker to its Edge, Prime, or Fusion portable arm contact CMMs by placing an SMR on them. “The TrackArm combines our FARO Vantage Laser Tracker into a 6 DOF probe—enabling the user to switch between the FaroArm and Laser Tracker to reach hidden points, around corners, and inside hole features,” said O’Reilly. All points measured are put in the same refer- ence frame. They can be referenced to CAD as well. The company advertises Tracker accuracy as 0.0014" (0.035 mm) with a range 230' (70.1 m) and arm accuracy 0.0005" to 1' (0.013 mm–0.3 m) for the combined system. For noncontact measurement of smaller parts and surfaces, FARO has the ScanArm HD device that uses a laser line probe to measure up to 560,000 points per second. Within a 1.8-m volumetric measuring range, its accuracy is 34 µm (per ASME B89.4.22). Using a blue line laser improves signal-to-noise ratio and a new crosshair feature for the LED rangefi nder improves usability.


Ease of use and speed are both qualities that Hexagon stressed in the development of its T-Scan 5 laser scanner that can work with laser trackers, like its new AT960.


Noncontact Accessories There are other issues for measuring with laser track- ers besides line-of-sight. “When you measure with an SMR there are challenges such as complex geometry or small holes that can’t be measured with the SMR itself,” said Joel Martin, laser tracker product manager of Hexa- gon Metrology (North Kingstown, RI). Although such complex mea- surements can be made, it requires both adapters and a high degree of operator skill. Hexagon’s answer was to develop a hand-held probe with a series of LED lights mea- sured by a special camera embed- ded in the laser tracker. Hexagon’s Absolute Tracker uses hand-held devices for its 6 DOF measure- ments, rather than a portable CMM arm. Its fi rst system was a contact probe and later a hand-held laser line scanner. The latest iteration, said Martin, is the Leica T-Scan 5, which boasts a typical accuracy of ±30 µm. It collects up 210,000


130 — Aerospace & Defense Manufacturing 2015


Photo courtesy Hexagon Metrology


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