ESA GNC Conference Papers Repository
Closed-loop validation of a free-floating guidance scheme for a space robot
Robotic systems are playing a key role in future space missions and need to increase constantly their autonomy and robustness to replace the astronauts in the years to come. Among these projects, the active debris removal can be performed by robotic means with the design of a spacecraft embedding a robotic arm to capture a non-cooperative debris. To that end, this paper presents the development methods of a guidance loop for such a system, called 'chaser', whose goal is to softly grasp a moving target point on a tumbling spacecraft, called 'target'. The modeling of the chaser is based on a classic rigid multi-body algorithm with a Newton-Euler approach . A free-floating control is assumed for the bus, considering that the Attitude Control System (ACS) is shut down during the capture. This hypothesis aims at avoiding any adverse motion from the base while the arm is moving toward the grasping point. Regarding the guidance, the free-floating behavior of the base in reaction to the arm motion implies a nonholonomic property due to the momentum conservation. This latter is exploited through the <i>Generalized Jacobian Matrix</i> concept in order to drive the end-effector along the capture trajectory . The main contribution of this paper is to validate the guidance loop and the modeling algorithms in a closed-loop scheme with <i>Hardware-in-the-Loop</i> (HIL), by using a camera to measure the relative distance between the chaser and the target. The test bench used for this validation is made of two industrial robotic arms in Thales Alenia Space labs, each one reproducing the kinematic behavior of either the end-effector of the chaser, or of the target point on the debris. The results are presented and commented in terms of capture accuracy and noise rejection on the feedback signal given by the camera measurement.