Modelling, stability analysis, and testing of a hybrid docking simulator

Abstract

A hardware-in-the-loop (HIL) simulation is a hybrid simulation 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 to simulate on-orbit servicing for verification and validation of the docking phase. The simulator essentially consists of two robots, with very accurate position controllers, holding a docking interface and a probe element, respectively. A key feature of the HIL docking simulator set-up is a feedback loop that is closed on the real force sensed at the docking interface during the contact with the probe, which is used in order to drive the free-floating bodies numerical simulation. The high stiffness of the robots causes the contact duration to be shorter than the (incompressible) time delay in the robots' dynamics. This leads to inconsistencies in the HIL docking simulation results and to potential instability and damage of the closed-loop 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 modified by combining virtual and real compliances in the software and hardware. The method is presented here for six degrees of freedom. Experimental results for the case of a translational motion along a single axis and the six degrees of freedom are presented, too. In addition, analysis is performed in order to characterize the stability region of the HIL docking simulator as a function of the contact force parameters, the time delay and the simulated satellites' masses. 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 the stability region.