NATIONAL TECHNICAL UNIVERSITY OF ATHENS DEPARTMENT OF MECHANICAL ENGINEERING CONTROL SYSTEMS LAB Georgios Rekleitis A Comparison of the Use of a Single Large vs a Number of Small Robots in On-Orbit Servicing Georgios Rekleitis Evangelos Papadopoulos May 17, 2013

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Motivation On-οrbit Servicing / OOS (construction, maintenance, docking, inspection, orbital debris handling/disposal) Currently done by astronaut EVA Robotic OOS can relieve astronauts from risky activities Successful OOS experiments: ETS VII, Orbital Express. 10/10/2015 2

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos OOS object handling can be done by a number of servicers with on-off thrusters and manipulators This results in improved performance in terms of accuracy and fuel consumption, compared to pure on-off control Is the performance better when the passive object is handled by a number of small servicers or by a single, large one? Problem Statement 10/10/2015 3

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Manipulation by free-flying servicers Space Robotic System Dynamics and Control One Large vs. Several Small Servicers Simulation Results Conclusions RoadMap 10/10/2015 4

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Single manipulator servicers. Point contact / firm grasp between end-effectors and object. Deactivated thrusters towards the object. Manipulation by Free-Flying Servicers 10/10/2015 5

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 10/10/ Space Robotic System Dynamics and Control- 1 (1)

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Passive object Model Based Control Constraints: (1) Friction constraints: For the point contact case (2) Manipulator repulsive force. Space Robotic System Dynamics and Control- 2 10/10/ (2)

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Constraints notwithstanding, the redundancy of the system leads to the absence of analytical solution. Constrained nonlinear optimization, as a force distribution method ̶ Design parameters: the components of the end-effector generalized forces Space Robotic System Dynamics and Control- 3 10/10/2015 8

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos i th – Servicer Controller: apply the required generalized force maintaining base desired position and attitude take into account workspace and collision avoidance requirements. Model Based Control Switching strategy for the on-off thrusters No need for asymptotic stability Space Robotic System Dynamics and Control- 4 10/10/2015 9

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos For the case of a number of small servicers: Thrusters facing the passive object are turned off. Additional manipulator force to push the servicer away from the passive object. The other servicer manipulator forces compensate, keeping total applied force on the object as in Eq. (2). One Large vs. Several Small Servicers /10/

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos For the case of a single, large servicer: No other servicers to compensate for additional manipulator force. 10/10/ One Large vs. Several Small Servicers - 2 Appropriate servicer base configuration. Simpler controller Force components opposing each other.

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 10/10/ One Large vs. Several Small Servicers - 3 New switching strategy for the single, large servicer case: Calculate the required continuous control force fi at the base frame. Examine if the force components exceed a preset threshold. Far fewer components opposing each other

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Other differences between the two cases: 1. Contact type between passive object and manipulator: Number of small servicers: Both point contact and firm grasp are feasible. Single, large servicer: Firm grasping is required. 10/10/ One Large vs. Several Small Servicers - 4

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 2. Servicer size limitations The single, large servicer case can be affected, whenever very large objects have to be handled. In the several small servicers case, we only need to add more servicers. 10/10/ One Large vs. Several Small Servicers - 5

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 3. Robustness in servicer performance failure In the several small servicers case, failure in the performance of one servicer may not be catastrophic ̶ The remaining servicers may be able to compensate In the single, large servicer case, there are no other robots to compensate for any performance failures. 10/10/ One Large vs. Several Small Servicers - 6

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 10/10/ Simulation Results Several small servicers case m 0 = 180 Kg, m i = 70 Kg, i = 1,2,3 f m = 20 N, f t = 10 N Single large servicer case m 0 = 180 Kg, m 1 = 210 Kg, f m = 60 N, f t = 25 N

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Simulation Results 10/10/ Thruster and reaction wheel forces/torques (both cases)

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos Three small servicers: More frequent but less powerful thrusting. More flexible in terms of gain tuning, servicer base position tuning and thrusting application. Single large servicer: More scarce but more powerful thrusting. Simulation Results 10/10/

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 10/10/ Manipulator reach for both cases Simulation Results

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos 10/10/ Conclusions Post-docking manipulation of a passive object in zero-g, utilizing manipulator equipped free-flyers, was studied. The case of passive object handling by a number of small servicers was compared to handling by a single large servicer. Several small servicers case was found superior, since: 1. It is more robust in terms of contact type requirements, size limitations and servicer performance failures. 2. It is more flexible in terms of gain tuning, servicer base position tuning and thrusting application 3. Superior performance in terms of fuel consumption, for the same object tracking errors.

National Technical University of Athens Department of Mechanical Engineering A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing G. Rekleitis & E. Papadopoulos THANK YOU! 10/10/