Thanks 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 (commercial off the shelf) robot on a ceiling-mounted, overhead crane that can be used to repair parts or tools and perform precision 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 dimensional control tasks. They achieved the project’s targeted accuracy of less than 0.015″ over a 320 ft. part/assembly, which could lead to countless applications for large-scale manufacturers 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 Freedom) capabilities precisely measures 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 measurement. In the ’80s, a NIST project worked to add two additional degrees 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.
That NIST project led to the first 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 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 focused 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 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 measurement device. The 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 automated part inspections.
Early 6DoF laser tracker automation 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 precision path.
Latency time became the seventh degree of freedom, and 7DoF technology went commercial in March.
With 7DoF, both the 1-kHz measurement speed and how long it took to calculate and communicate the measurement to an outside source became critical. These factors defined 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 milling operations in another area of the shop floor.
This article was first published in the Summer 2016 edition of Smart Manufacturing magazine.