Hybrid drive for industrial robots - High path accuracy

Industrial robots have become an integral part of modern production processes. Their serial kinematic design ensures a high degree of flexibility while taking up little mounting space. Fraunhofer IFAM in Stade, Germany has now developed a novel solution for the dynamic improvement of these kinematics: the hybrid drive for industrial robots.

The hybrid drive combines the proven gear drive with an additional direct drive, allowing a torque to be applied directly on the load side. This combination enables targeted compensation of undesirable gearbox effects and effective damping of high-frequency excitations - with maximum energy efficiency. Both drives run in parallel and follow the identical position value. The direct drive controls the dynamic part, while the gear drive takes over the static and quasi-static loads.

This innovative drive control system ensures high energy efficiency and maximum precision: The direct drive is only actively powered during the acceleration and braking phases, which minimizes energy consumption. At the same time, the damping within the drivetrain is increased so that the resonance frequencies no longer result from the base axes, but from the structure and bearings of the system. This significantly reduces the transmission of vibrations, resulting in more stable and precise control of the drive and therefore the entire kinematics.

This optimized drive technology not only improves the dynamics and responsiveness of the system, but also its resistance to external disturbances such as process forces. Industrial robots with hybrid drives therefore achieve higher performance and precision - decisive factors for applications that require maximum path accuracy and stability.  

Improved path accuracy and constant behavior in the workspace  

The improved path accuracy of the robot through the drive system represents a major improvement. Path accuracy tests carried out using the ISO 9283 test track show a significant increase in path accuracy to less than 0.1 mm. This test track includes various geometric elements such as corners, circles and changes of direction, which are crucial for evaluating the performance of the system.

A key feature of this system is the avoidance of path errors at sharp geometric elements such as corners. The path error is also significantly reduced at reversal points and changes of direction. 

The additional torque from the direct drive compensates for the torsional spring torque of the gearbox, which takes on different characteristics in the acceleration and braking phases and can lead to deviations between the target and actual values. In addition, friction effects of the transmission drive are also compensated, which leads to a much more linear behavior of the entire system. These developments lead to a constant and homogeneous path behavior across the entire working space, so that precise manufacturing and automation processes in particular benefit.

Higher productivity and process robustness

The additional direct drive dampens the first eigenmodes of the kinematics caused by the gear drives. This allows the jerk setting in the robot controller in the respective axes to be increased by up to a factor of 100 compared to conventional robots with servo drives. This significantly increases the dynamics in the acceleration and braking phase. At the same time, the path accuracy remains constant while the feed rate increases. These properties offer enormous potential for increasing productivity and opening up new fields of application for robotics.

In addition, the direct mechanical transmission of the motor torques to the load side enables improved disturbance rejection. Highly dynamic disturbances, which cannot be compensated for with conventional gear drives due to their low-pass behavior, can now be effectively compensated for. As a result, the systems are able to compensate for sudden changes in process forces. This technology opens up new possibilities for precise and stable production, particularly when machining high-strength materials such as steel, which often cause increased excitation of the robot.

Robots with hybrid drives close the gap between standard industrial robots and machine tools

Robots with hybrid drives represent an advanced solution that successfully bridges the gap between standard industrial robots and machine tools. Compared to conventional industrial robots, robots with hybrid drives offer significantly improved path accuracy and higher dynamics, while at the same time exhibiting increased robustness and a pose-independent bhavior known from machine tools.  

The use of industrial robots with hybrid drives is proving to be forward-looking, as they can master the challenges of machining difficult-to-cut materials and at the same time significantly increase the efficiency of production processes. This synergy enables highly precise and efficient execution of complex processes with simultaneous flexibility and adaptability - two essential requirements of modern production technology. This opens up new potential in automation technology and Industry 4.0, e.g. in the automotive, energy, aviation and shipbuilding sectors.