STABILITY ANALYSIS AND TESTING OF A DOCKING SIMULATOR

摘要

The European Proximity Operation Simulator (EPOS) aims, amongrnother objectives, at performing tests for verification and validation of therndocking phase of an on-orbit servicing mission. The simulator includesrntwo robots, equipped with very accurate position controllers, holdingrna docking interface, a probe element, and (virtual) satellites. A keyrnfeature of the simulator set-up is a feedback loop that is closed on thernreal force and torque sensed at the docking interface during the contactrnwith the probe and that is used to drive a free-floating bodies numericalrnsimulation. The high stiffness of the controlled robots causes the con-rntact dynamics to be quicker than the robots’ dynamics. This leads torninconsistencies in the docking simulation results and to potential insta-rnbility and damage of the closed-loop system. This work presents a novelrnmitigation strategy to the given challenge, accompanied with a stabilityrnanalysis and validating experiments. The high stiffness issue is addressedrnby combining a virtual stiffness and damping in the software with a realrnstiffness in the hardware, designing, thus, a “hybrid” docking simulator.rnNonlinear state-space modeling of the closed-loop system is performedrnfor the three dimensional case, where the tracking robots system is ap-rnproximated as a pure delay. A linearized model is developed for the tworndimensional case and a simplified expression is obtained by defining asrnstates the depth and the rate of penetration along the normal to therncontact surface. A stability analysis is then performed, easily extendingrnprevious results obtained in single-dimension, which provides stabilityrnregions as a function of the contact stiffness and damping, the trackingrnrobots time delay, and the satellites masses. The design of a hardwarerncompliance device is presented and the effective stiffness along the nor-rnmal to the contact surface is developed. The proposed hybrid contactrndynamics model and the accompanying analysis are envisioned to enablernsafe and flexible docking simulation. The proposed simulator shall en-rnable the emulation of a desired contact dynamics for any stiffness andrndamping characteristics within the stability region.