Advanced manufacturers supplying large carbon fiber reinforced polymer (CFRP) parts to the aerospace industry face a pressing dilemma: How can they effectively reduce lead times as demand looms over this sector with extraordinary order backlogs?
Airbus alone commissioned roughly 1200 new aircraft in 2015 and is committed to delivering close to 7000 total aircraft in 2018. This industry needs a sea change.
With limited production capacity, CFRP manufacturers have to accelerate and reshape their operations with the use of emerging technologies. This new line of attack translates to effectively machining large structures through the use of automation and robotics as a viable alternative to the currently installed specialist machines. One area offering promise for driving productivity in the manufacturing process is automating the drilling and milling processes on large CFRP assemblies using COTS (commercial off the shelf) industrial robots.
A demonstration was conducted recently that featured a high-precision 7 Degrees of Freedom (7DoF) control of an industrial milling robot as a flexible and accurate solution for these escalating CFRP assembly challenges. This work is the culmination of a partnership between Hexagon Manufacturing Intelligence and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM.
The project intended to deliver a zero-defect production automated solution, which required a strong error-prevention system. The objective was achieved by integrating industrial robotics with processes that enable high precision and mobility of the tool along the work piece. The technologies applied during the exhibition consisted of an industrial robot with milling, drilling, a Leica Absolute Tracker AT960 with a Leica T-Mac (Tracker-Machine control sensor) probe, an EtherCAT bus system connected to a CNC-based machine controller, and computer hardware and software systems.
Large-scale production tasks require manufacturing intelligence solutions that can provide very accurate data about big assemblies. So large volume production applications like aerospace assembly are ideally suited for a portable measurement laser tracking system. A Leica laser tracker can measure distance, horizontal and vertical angles to a spherical mounted reflector—providing precise 3DoF measurements 1000 times a second.
Moreover, by combining conventional laser tracker measurements with photogrammetry, the device’s integrated mini-variozoom camera additionally determines roll, pitch and yaw angle on LED-equipped probes like the Leica T-Mac. This capability lets the system deliver precise 6DoF positioning and orientation information with accurate time stamps. For traditional tasks like part inspection, this data runs through the Leica Metrology Foundation interface, while 7DoF metrology-assisted manufacturing is supported by the addition of the real-time industrial Ethernet feature pack based on the EtherCAT standard.
The emergence of 7DoF machine control led to these recent advancements. The Leica laser tracker’s 7DoF capabilities lets it precisely measure the end-effector of a robot in real-time and corrects its position based on the part’s global coordinate system. This technology has addressed the issue of robot accuracy. A robot’s end-effector can now be driven at up to 1 kHz in real-time by the positioning accuracy of the laser tracker.
For this automated milling project, the use of 7DoF control reduced the mean positioning error of the robot of just 20 μm over the full test environment.
To prove the dynamic capabilities of the system, two parallel notches were milled in opposite directions. The width between the bars varied by about ±1 mm without realtime guidance to under ±0.250 mm with 7DoF control.
This marriage of automation, laser trackers and milling and drilling systems is proving precise control and agility can be harnessed to perform extremely tight tolerance tasks.