RoMaNi 2 project enhances industrial robot flexibility for composites, metals machining

The recently completed Lower Saxony LuFo project, “Robots Made in Lower Saxony 2” (RoMaNi 2) has reportedly closed the gap between industrial robots and machine tools, achieving a novel type of drivetrain that enable versatile, efficient machine of materials — from composites to metals —  with a production tolerance of up to 0.1 millimeter. R&D work was led by Fraunhofer IFAM (Stade, Germany) together with Broetje-Automation GmbH, Hexagon Aicon Etalon GmbH and Siemens AG, as well as associated partners Airbus Operations GmbH and A&T Service GmbH.

In the project, two robot kinematics designed for machining were examined in detail, optimized by control-side settings and metrological support for path-accurate processes and tested on a 1:1 scale using real-life aviation applications. As part of the RoMaNi 2 project, researchers at Fraunhofer IFAM were able to completely assemble the prototype kinematics with a new type of hybrid drive that had already been developed in the Flexmatik 4.1 project, put it into operation and test it in machining processes. RoMaNi 2 concluded with the high-precision milling of an Airbus vertical tail plane and a steel test item. 

Flexible serial industrial robots vs. stationary, high-precision machine tools

In recent years, novel manufacturing processes have led to further developments in the design freedom and structural integrity of near-net shape carbon fiber-reinforced polymer (CFRP) components in aircraft manufacturing. Due to economic and technical requirements, high-precision postprocessing of large components in the meter range with tolerances in the submillimeter range is usually carried out with large, gantry-designed machine tools. However, these machines can be considerably large depending on their kinematic structure, which often dictates the layout of the production facilities and limits their ability to react flexibly to changes in the production process.

An alternative is the industrial robot, including workspace expansion by means of a movable platform, the installation of several robots or the use of additional translatory axes such as linear axes. Compared to gantry systems or machine tools, this machine concept is much more space-saving and is not economically tied to individual large components. In addition, no special foundations are required, which facilitates the future adaption of production lines. 

In the project, path accuracy for large components (up to 7 meters) was improved by 0.15 millimeter.

Although industrial robots have already been successfully used in the machining of thin aerospace shell components, increasing their robustness to process forces and their ability to handle increasingly demanding machining tasks is the next step for the growing use of industrial robots in the production of large aerospace components.

Industrial robot dynamic behavior improvements

Automation and production technology experts from Fraunhofer IFAM  assembled and investigated the prototype of a serial robot. The in-house developed kinematics are fundamentally designed to meet the requirements of processes with high path accuracy. The aim was to achieve manufacturing tolerances of at least ±0.1 millimeter when machining large components.

This prototype robot, including the linear axis, was developed as part of the Flexmatik 4.1 project, which ended with the fabrication of all components. The assembly of the kinematics, commissioning, control-side optimization and further development, as well as intensive investigation of the robot kinematics, have now been successfully completed in the RoMaNi 2 project. Accuracy tests confirmed that the target of 0.1 millimeter was achieved.

A key element in improving the prototype robot’s dynamic behavior, in addition to structural optimization, was the use of an innovative drive concept in lower joint axes. By using an additional direct drive parallel to the conventional gear drive, partners note that a torque can be applied directly on the load side. This hybrid drive concept combines the possibility of compensating for undesirable effects of the gearbox and damping high-frequency excitations, while at the same time ensuring high energy efficiency in static and quasi-static load cases.

Kinematics are controlled by a Siemens Sinumerik One controller, avoiding the need for retraining on new control systems. In the RoMaNi 2 project, hybrid drive control components were further developed so that the drive’s full potential can be used in the industrial control system.

Wide application range

According to partners, the combination of serial articulated arm kinematics with a linear axis offers advantages over large gantry systems and other machining systems. First, the linear axis, with a smaller installation space and modular design, makes the system highly flexible. Second, the use of two preloaded rack and pinion drives compensates for reversal effects and achieves a sufficiently high drive stiffness of the linear axis carriage for path-accurate robot processes; the linear axis’ high structural stiffness does not really affect robot accuracy despite the large lever arms to the load application point.

In the project, path accuracy for large components (up to 7 meters) was improved by 0.15 millimeter. Fraunhofer IFAM researchers believe even higher accuracy can be achieved by compensating for other static influencing factors, such as temperature, rather than compensating for other dynamic effects.

The use of direct drives significantly improves reference tracking behavior and disturbance rejection of the serial robot kinematics at axis level. Direct mechanical transmission of the motor torques to the kinematics also enables increased jerk adjustment of all lower joint axes — this is reported to be 10-100 times higher than conventional robots with servo drives and therefore offers potential for increasing productivity. In addition, a significant increase in path accuracy can also be demonstrated at high path speeds. At a feed rate of 10 meters/minute, a path accuracy in the range of the previously recorded static accuracy can be demonstrated. Damping of the first eigenmodes — caused by the gear drives — also offers potential for improved disturbance rejection.

Next steps

Researchers at Fraunhofer IFAM, together with partners from industry, intend to further develop this technology until it is ready for series production. There is a wide range of applications for industrial robots with hybrid drives: In combination with a linear axis, the spectrum ranges from machining tasks such as fiber-reinforced composite structures and aluminum alloys, to the machining of harder materials, such as steel or titanium.