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With 7DoF Innovation, Door to Precise Machine Control Opens Much Wider


T


hanks in part to a European Commission-funded program to build a “Megarob” (www.megarob.eu/) system with metrology-assisted machine control and


robotic guidance, we now have a prototype COTS (commer- cial off the shelf) robot on a ceiling-mounted, overhead crane that can be used to repair parts or tools and perform preci- sion manufacturing of large-scale components to accuracies found in small-part production, well outside the positioning capability of standard industrial robots. Eight European fi rms developed a robot for milling, drilling, deburring, grinding, polishing, riveting, screwing, welding, painting or quality dimensional control tasks. They achieved the project’s targeted accuracy of less than 0.015" (0.381 mm) over a 320' (97.5 m) part/assembly, which could lead to countless applications for large-scale manufacturers in the aerospace, marine and energy fi elds. Hexagon’s Leica Absolute Tracker AT960 is an integral part of the Megarob system. The portable laser tracker with 7DoF (Degrees of Freedom) capabilities precisely measures the end-effector of the robot in real-time and corrects its po- sition based on the part’s global coordinate system. Address- ing the issue of robot accuracy, this robot’s end-effector can now be driven at up to 1 kHz in real-time by the positioning accuracy of the laser tracker. To understand 7DoF, it helps to go back to the original laser Interferometer. In the '80s, a NIST project worked to add two additional degrees of freedom to a laser interfer- ometer. Although interferometers were very accurate, they were diffi cult to deploy because they could only measure a change in distance in a perfectly straight line. That NIST project led to the fi rst commercially available laser tracker, the Leica SMART 310.


Hexagon engineers challenged the idea of measuring only in 3D and wanted to expand beyond X, Y, Z coordinate data to include i, j, k (pitch, roll and yaw) rotational data. Thus 6DoF was born. The Leica Laser Tracker LTD800 added a photogrammetry camera (T-Cam) to the top of the tracker, as well as a series of LED targets on the measure- ment end effector. Behind the accuracy of this technology


Joel Martin


Product Manager Laser Trackers Hexagon Manufac- turing Intelligence North America


are the Leica T-Cam’s full-range lenses to keep the fi eld of view of the end effector both zoomed in and focused throughout the device’s measurement range. This technol- ogy expanded the use of trackers in new applications where 3D coordinates did not suffi ce.


Megarob and 7DoF laser tracking technology move industry closer to the “factory of the future.”


One such application is the use of a Leica T-Mac (Ma- chine Control sensor) on the end of a large robot. By using the 6DoF laser tracker to measure this device, a COTS robot can be used as a large-scale, high-precision measure- ment device. The touch trigger probe on a T-Mac registers its absolute location as the inspected part is touched. The measurements are generated by the tracker, which is similar to the digital scales used by a CMM with a touch probe. The measurement probe is then isolated from the positioning equipment (in this case a robot) to achieve laser tracker ac- curacies for large automated part inspections. Early 6DoF laser tracker automation project successes


spurred the same engineering team to fi nd a way to use the data from the T-Mac to drive the robot through a precision path. Latency time became the seventh degree of freedom, and 7DoF technology went commercial in March. The fact that the laser tracker and its portable sensor could measure at 1 kHz was not crucial. The critical issue was how long it took to calculate and communicate the measurement to an outside source. This factor defi ned the la- tency of the tracker’s communication. The lower the latency, the faster the robot could move while the laser tracker sent its next path-correction value. Megarob’s use of 7DoF tech set the stage for new con-


fi gurable robotic systems with multitasking capabilities. In theory, a manufacturer could standardize on one robot brand and model, and it could move inventory pallets in the AM and be used in milling operations in the PM.


35 — Aerospace & Defense Manufacturing 2016


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