ADVANCED MANUFACTURING NOW Modern Manufacturing Processes, Solutions & Strategies
Joel Martin Product Manager, Laser Trackers
Hexagon Manufacturing Intelligence North America
With 7DoF, door to precise machine control opens wider
T
hanks in part to a European Commission-funded program to build a “Megarob” (www.
megarob.eu/) system with metrol- ogy-assisted machine control and robotic guidance, we now have a prototype COTS (commercial off the shelf) robot on a ceiling-mounted, overhead crane that can be used to repair parts or tools and perform pre- cision manufacturing of large-scale components to accuracies found in small-part production, well outside the positioning capability of standard industrial robots. Eight European firms developed a
robot for milling, drilling, deburring, grinding, polishing, riveting, screwing, welding, painting or quality dimen- sional control tasks. They achieved the project’s targeted accuracy of less than 0.015" over a 320 ft. part/ assembly, which could lead to count- less applications for large-scale man- ufacturers in the aerospace, marine and energy fields. Hexagon’s Leica Absolute Tracker
AT960 is an integral part of the Megarob system. The portable laser tracker with 7DoF (Degrees of Free- dom) capabilities precisely mea- sures the end-effector of the robot in real-time and corrects its position based on the part’s global coordinate system. Addressing 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 origins of laser mea- surement. In the '80s, a NIST project worked to add two additional degrees
6
of freedom to a laser interferometer. Even though laser interferometers were very accurate, they were difficult to deploy because they could only measure a change in distance in a perfectly straight line.
touch trigger probe on a T-Mac is isolated from the uncertainty of the positioning equipment (the robot in this case), which allows it to achieve laser tracker accuracies for large au- tomated part inspections.
7DoF laser tracking technology and an EC-funded program to build a “Megarob” system move industry closer to the “factory of the future.”
That NIST project led to the first
commercially available laser tracker, the Leica SMART 310.
Heagon 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 measurement end effector. Behind the accuracy of this technology are the Leica T-Cam’s full-range lenses to keep the field of view of the end effector both zoomed in and fo- cused throughout the device’s entire measurement range. This technology expanded the use of trackers in new applications where 3D coordinates did not suffice.
One such application is the use of
a Leica T-Mac (Tracker-Machine Con- trol sensor) on the end of a large ro- bot. By using the 6DoF laser tracker to measure this device, a COTS robot can be used as a large-scale, high- precision measurement device. The
Early 6DoF laser tracker automa-
tion project successes spurred the same engineering team to look for a way to use the data from the T-Mac to drive the robot through a preci- sion path. Latency time became the seventh
degree of freedom, and 7DoF technol- ogy went commercial in March. With 7DoF, both the 1-kHz mea-
surement speed and how long it took to calculate and communicate the measurement to an outside source became critical. These factors de- fined the latency 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 technology
set the stage for new configurable robotic systems with multitasking capabilities. In theory, a manufacturer could
standardize on one robot brand and model, and these robots could be used to move inventory pallets in one shop floor area or used in mill- ing operations in another area of the shop floor.
Summer 2016
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