Gravity Compensation Strategy of Space Robotic Arm for On-Ground Testing Using Gyroscope-Type Coupling Interface and Cable-Driven Parallel Robot

Space robotic arms are designed to operate in microgravity environments, so their own weight makes them difficult to move on Earth's 1g surface. A cable-driven parallel robot (CDPR) can handle partially the arm's self-weight, enabling operation on the ground. The CDPR connects to a specific point on the robotic arm, where gravitational joint torques are reduced by applying the optimal tensile forces of the CDPR. To facilitate this, we newly designed a coupling interface (CI) that links the robotic arm to the CDPR and propose a control method for it. The CI is a gyroscopic mechanism that makes the cables' directions intersect at a single point, aligning the cables with the force direction the CDPR should exert. Additionally, by using the ideal cable lengths calculated via the CI instead of the actual lengths from the winch encoder, we were able to achieve robustness against environmental uncertainties. To demonstrate this, we compared the use of the ideal length and the actual length under environmental uncertainties, and showed that the use of the ideal length provides superior robustness.