Advances in Aircraft Construction: Hexapod Provides High Dynamics 6D Error Compensation to Traditional Robot
Compensation kinematics improve contour machining for the Wing of Tomorrow, through milling, deburring, sealing, and drilling.
PI presented a new custom-designed hexapod at the International Aerospace Exhibition ILA Berlin in June 2024, as a partner in the LuFo VI-1: ADMAS (Advanced Machining and Sealing) research project.
As part of a joint project under the leadership of Airbus, PI developed a high-performance 6-DOF hexapod motion and positioning system (model H-900K101) for highly dynamic compensation kinematics in aircraft construction. The project's goal was to enhance the processing and sealing precision of carbon fiber-reinforced composite (CFRP) components in resin transfer molding (RTM) manufacturing processes, while also reducing manufacturing times and process costs.
Carbon fiber composites combine high stiffness with extremely low weight, helping increase energy efficiency in air travel and reduce CO2 emissions.
Dynamic Compensation of Process Forces
The compensation kinematics focus on improving the contour machining of single-aisle wing shells for the Wing of Tomorrow, made of CFRP, through milling and deburring, followed by sealing and drilling. The H-900K101 PI hexapod, in combination with the coarse motion of a traditional articulated robot, is used for highly dynamic, high-precision positioning of the machining and sealing tools.
“With the new H-900K101 hexapod, we have implemented a highly dynamic, multi-axis motion system that can accelerate a payload of up to 40kg in the XY plane at up to 1.2G," says Dr Christian Sander, Head of Technology Development Parallel Kinematics at PI’s headquarters in Karlsruhe, Germany. "The system is able to dynamically compensate for high-frequency path deviations under the influence of process forces during milling." The new design was an improvement on a previous dynamic hexapod with a lower center of mass, reduced operating height, and many other incremental improvements over previous designs.
Fine Positioning of Tools
The newly developed hexapod precision motion system is specifically designed to meet the boundary and environmental conditions of the application, making it particularly optimized for time-critical manufacturing processes with the tight tolerances required by the aerospace industry.
In addition to this high dynamic performance, the hexapod system offers fiber-optic data transmission between the hexapod controller and the driver electronics, enabling a very low latency time when using the EtherCAT® interface and IP54-sealed components. Fiber-optic data transmission allows the hexapod controller to be conveniently located in the control cabinet outside the processing cell without compromising the variety of interfaces while controlling the driver electronics over a distance of up to 100m (330ft). For the EtherCAT® interface, PI has achieved a deep integration of EtherCAT® functionality within its controller and driver architecture. As a result, PI's positioning system contributes only a minimal amount of dead time to the complex control loop for path error compensation between the edge-detection sensor system, the articulated robot, and the hexapod. The response time in the EtherCAT® control loop between the hexapod's position request and position response is a swift 1.5ms.
Condition Monitoring
The ADMAS LuFo project also investigated the use of acceleration sensors for wear detection on individual hexapod struts in the context of condition monitoring. The frequency spectrum of vibrations within each strut can be mapped to characteristic eigenfrequencies of drive components, providing an indicator of system changes and the need for service.
Hexapod Parallel-Kinematic Motion Systems
Hexapods are parallel-kinematic machines and offer motion in six degrees of freedom in a smaller envelope than traditional serial kinematic motion systems. For industrial applications, absolute position sensors for each hexapod strut provide safety and render referencing unnecessary. All coordinate transformations run on the motion controller and user-friendly software allows the execution of complex motion profiles. PI's precision hexapods are available in standard configurations for loads from 10lbs to 1,000lbs with repeatability down to ±0.06μm. Customized precision hexapods can support loads more than 4,000lbs.
With travel ranges from a few millimeters to several hundred millimeters, the machines enable resolution down to the nanometer range and velocities from 0.1mm/sec to 500mm/sec. PI hexapods are used worldwide in applications including automation, metrology, photonics and optics alignment, automotive, medical technology, astronomy, and research. To meet various requirements, they can be specified for environments such as IP54, laboratory, cleanroom, high vacuum, and ultrahigh vacuum.
Under the leadership of Airbus and the IFAM, PI has contributed with its four decades of hexapod design experience to this pioneering research. The project, publicly funded by the German Federal Ministry for Economic Affairs and Climate Protection and overseen by the DLR Project Management Agency for Aviation Research, was successfully completed at the end of April 2024.
The Fraunhofer IFAM institute for manufacturing technology, in Stade, Germany, was tasked with building the overall system demonstrator and addressing the research topics within the project and the individual partners. They were also taking on the role of the overarching project coordinator while applying their valuable application knowledge.
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