Safe & Accurate at Speed with Tendons:

A Robot Arm for Exploring Dynamic Motion

Simon Guist, Jan Schneider, Hao Ma, Le Chen, Vincent Berenz, Julian Martus, Heiko Ott, Felix Grüninger, Michael Muehlebach, Jonathan Fiene, Bernhard Schölkopf and Dieter Büchler


Max Planck Institute for Intelligent Systems

Link

arXiv

GitHub

GitHub

Link

Motions Dataset

Link

URDF model file

Operating robots precisely and at high speeds has been a long-standing goal of robotics research. Balancing these competing demands is key to enabling the seamless collaboration of robots and humans and increasing task performance.  However, traditional motor-driven systems often fall short in this balancing act. Due to their rigid and often heavy design exacerbated by positioning the motors into the joints, faster motions of such robots transfer high forces at impact. To enable precise and safe dynamic motions, we introduce a four degree-of-freedom~(DoF) tendon-driven robot arm. Tendons allow placing the actuation at the base to reduce the robot's inertia, which we show significantly reduces peak collision forces compared to conventional motor-driven systems. Pairing our robot with pneumatic muscles allows generating high forces and highly accelerated motions, while benefiting from impact resilience through passive compliance. Since tendons are subject to additional friction and hence prone to tear, we validate the reliability of our robotic arm on various experiments, including long-term dynamic motions. We also demonstrate its ease of control by quantifying the nonlinearities of the system and the performance on a challenging dynamic table tennis task learned from scratch using reinforcement learning. 


Design Overview




Design of the tendon-driven robot arm (a).

The arm has a rotational and a swivel DoF within the first (e), (f), and second joint (c), (d). It features ball bearings, which are low in friction. Many parts are self-designed and 3D-printed, which

are shown colored in black. The four angle encoders are shown with a small green circuit board.

The Bowden tubes (b) feature an inner tube and outer support elements that help maintain constant tendon length.


Tasks


Table Tennis Smashing


In the HySR setup for the table tennis smashing task, we learn with a real robot and a simulated ball.


The robot’s initial position at the beginning of an episode is shown in (a). During the training, the robot learns a motion in which it first draws back to generate momentum before striking the ball (b). The racket reaches speeds of up to 12 m/s during this motion.


PAMY2, achieves significantly higher ball speeds than the previous design.

Despite the higher ball speeds, it is also more precise in terms of more frequent ball contacts and lower distance to the ball’s target location. Overall, PAMY2 reaches performance far superior to PAMY1, demonstrating the benefits of the improved design for highly dynamic and precise motions.