Augmented Dynamic FEM Modeling of a Cable-Based Validation System for Space Manipulators

Kurzfassung

This thesis presents an augmented dynamic finite element formulation for the modeling of a cable-based suspension system used to validate robotic manipulators under controlled dynamic conditions. In particular, the work is motivated by the on-ground testing of the CAESAR space manipulator developed by DLR, where a cable-driven parallel robot (CDPR) "Motion Suspension System" (MSS) is employed to compensate gravitational loads and replicate zero-gravity operational conditions. Accurate modeling of cable dynamics, including nonlinear elasticity, variable cable length, and damping, remains an open challenge that limits the precision of such ground validation systems. The proposed formulation extends the classical Absolute Nodal Coordinate Formulation (ANCF). It introduces time-dependent shape functions to handle variable cable lengths. It also incorporates a nonlinear cubic stress-strain constitutive law, identified experimentally through loading-unloading tests on fiber rope cables. Finally, it includes a parametrized Rayleigh damping model, with coefficients that depend on both cable length and axial strain, determined from impulse-response experiments. The model is validated hierarchically across three configurations of increasing complexity: a single prestressed cable, a two-cable controlled system, and a complete four-cable parallel robot executing a prescribed circular trajectory. Results demonstrate spatial convergence against analytical references, coherent dynamic tension redistribution, and accurate prediction of end-effector natural frequencies, confirmed by comparison with experimental impulse-response measurements. The proposed framework provides a reliable tool for predicting cable vibration behavior in motion suspension systems, supporting improved control strategies for space manipulator ground qualification.