Experimental Validation of a Planar Free-Floating Emulator via Model-free Force-Feedback Control

The ability to emulate free-floating conditions on Earth within a laboratory is critical to advancing the science of on-orbit space robotics. Robotic Platforms (RPs) that recreate on-orbit motion of Space Robots (SRs) through Hardware-In-the-Loop (HIL) simulation are currently used to fulfill this need. The authors recently proposed the use a force-feedback control scheme to emulate free-floating conditions. In this approach, forces and torques measured at the interface between a RP and a SR installed on the RP end-effector are driven to zero by controlling the RP motion. With no knowledge of the zero, first, and second mass moments of the SR, numerical results proved the viability of such an approach for a planar system. To perform an experimental validation of these results, a RP is developed that integrates a two-Degree-Of-Freedom (2-DOF) Cartesian manipulator with a six-axis Force-Torque (FT) transducer mounted on a rotational third joint. A planar 4-DOF manipulator is then used as SR, and it is mounted on the FT transducer. The SR is excited using sinusoidal inputs, and the two planar forces and the off-plane torque measured by the FT transducer are nullified using two force-feedback control strategies. Experimental results show that the proposed method is capable of minimizing the linear momentum error of the SR and thus represents a viable alternative to HIL simulation when no knowledge of the SR dynamic model is available.