A Modified Proportional Navigation Algorithm for GEO Autonomous Robotic In-Orbit Servicing Mission

The growing need for a space debris mitigation strategy is increasing the importance of in-orbit servicing missions. In its commitment for a sustainable Space Economy, ESA is engaging collaborations with institutions and industrial partners in developing solutions for debris mitigation to reach the goal of Zero Debris by 2030. RISE is ESA first in-orbit servicing mission built, operated and co-founded by D-Orbit, with the primary aim of demonstrating the capability of extending GEO satellites lifetime. Activities such as refuelling, repairing, but also reusing functioning parts of non-operative satellites rely on space manipulator systems capable of autonomously performing different tasks. A crucial point is the motion planning of the manipulator when approaching the target. During this phase a free-floating mode (i.e. the chaser is not controlled during the arm motion) is often considered the most reliable choice. This operational mode increases the relevance of proper motion planning of the robotic arm because of the dynamic coupling between the manipulator and its base. Depending on mission tasks, different objectives might be relevant like optimization of power resources, minimization of time of manoeuvre and contact forces, collision avoidance. Designing robust guidance algorithms of a free-floating manipulator allowing for real-time trajectory-planning optimization with multiple objective functions is still a great challenge. Multi-objective genetic algorithms as well as Particle Swarm Optimization algorithms have been investigated, but their main limitation resides in their intrinsic complexity, hence computational cost already for 2 degree-of-freedom (DOF) manipulators. The idea of this paper is to apply simpler trajectory planning algorithms (usually employed in different applications) to steer the end-effector of the 7 DOF manipulator designed for RISE in-orbit servicing mission, from an initial position to the target’s launch adapter ring, where the grasp can be secured. As an example, classical Proportional Navigation guidance schemes commonly used for missile interception are robust, but they are applied only in the normal direction to the Line-Of-Sight (LOS), while during berthing manoeuvres, relative position and velocity along the LOS are crucial. In this paper, we propose a modified proportional navigation algorithm which enables smooth and impactless berthing of a free-floating space manipulator with a client GEO satellite. Different secondary objective functions are optimized according to their priority in the assigned servicing mission without invalidating the converge rate of the algorithm. Manipulator performance constraints (joint velocity and acceleration limits) are considered, and simulation results are presented to verify the performance of the algorithm developed.