Modeling, Stability Analysis, and Testing of a Hybrid Docking Simulator

A hybrid docking simulator is a hardware-in-the-loop (HIL) simulator that includes a hardware element within a numerical simulation loop. One of the goals of performing a HIL simulation at the European Proximity Operation Simulator (EPOS) is the verification and validation of the docking phase in an on-orbit servicing mission. A key feature of the HIL docking simulator setup is a feedback loop that is closed on the real force sensed at the docking interface during the contact with the probe. This force signal is used as input to the numerical simulation of the free-floating bodies in contact. The resulting relative 3D trajectory serves as a position command for the two robots end-effectors holding the probe and the docking interface. The high stiffness of the robots causes the contact duration to be shorter than the time delay in the robots' dynamics. This can lead to inconsistencies in the simulation results, to instability of the closed-loop system, and eventually to damages in the HIL system. This work presents a novel mitigation strategy to the given challenge, accompanied with stability analysis and validating experiments. The high-stiffness compliance issue is addressed by combining virtual and real compliances in the software and hardware, respectively. The method is presented here for six degrees of freedom. A linear stability analysis is provided for a 2D case. Experimental results are presented for a translational linear motion and for a 3D motion. This hybrid contact dynamics model and the accompanying analysis is envisioned to provide a safe and flexible docking simulator tool. This tool shall allow reproduction of the desired impact dynamics for any stiffness and damping characteristics within a desired stability, and thus safe, domain of operation.