Bartlomiej Koszek, KuLeuven
Alvaro Roman Sanchez, KuLeuven
Bartlomiej Koszek, Design of an ADCS enabled by the development of a digital twin of CubeSat equipped with robotic manipulator, KuLeuven
In this research, the authors develop a digital twin of a small satellite equipped with a robotic manipulator to inform the design of an attitude determination and control system (ADCS) for this type of mission. The significance of on-orbit assembly, repair, and other servicing operations has increased in recent years, bringing with it a demand for satellite platforms capable of performing such operations. Robotic manipulators stand as one of the most appropriate solutions to in-orbit servicing since their flexibility and high degree of dexterity enable them to perform an extensive range of tasks. Historically, only large platforms have been equipped with robotic manipulators. However, the miniaturization of attitude control actuators has made it possible to mount these payloads on CubeSat-sized satellites. This could potentially reduce mission costs.
Current approaches to modeling these kinds of vehicles found in the literature frequently assume that the satellite’s mass and inertia vastly exceed those of the robotic arm. With this assumption in mind, the interaction dynamics between the satellite and the moving manipulator are negligible compared to external disturbances acting on the attitude of the vehicle. However, when scaling these systems down, the robotic manipulator’s mass and inertia become comparable to those of the satellite. The previous assumption is then no longer valid, and the dynamic coupling between becomes several orders of magnitude larger than external disturbances. This problem, which is specific to small satellites, creates additional complexity in the design of these systems as the selection of the attitude actuators and synthesis of control law for the ADCS are inherently coupled with tasks that the robotic manipulator will be expected to perform and the speed at which the robotic arm’s joints should ultimately move.
The goal of the research is to develop a digital twin of a small satellite equipped with a robotic manipulator that can simulate the multibody interaction of the manipulator and satellite body as well as the attitude control system for this vehicle. This digital twin is then used to find an optimal control strategy and to select adequate attitude actuators for a set of scenarios. Within this study three principal operational cases are evaluated:
1) Base case: Performance during manipulator movement along a predefined trajectory without grasping or other object interaction.
2) Satellite servicing case: Performance during the capture of a cooperative object with known inertial properties. The manipulator’s movement matches the velocity and orientation of another free-flying object.
3) Active debris removal case: Detumbling performance following the capture of a non-cooperative object with unknown inertial properties.
The digital twin encompasses the manipulator, satellite body, reaction wheels, a star tracker, an accelerometer, a gyroscope, and the chosen control law. The model of the control system is a quaternion-based closed loop incorporating a PID controller, which uses feedback from sensors such as star trackers and gyroscopes. This is augmented by a feedforward system that estimates the disturbance forces and torques applied by the base of the robotic arm on the satellite.
Two complementary simulation tools are utilized: a high-fidelity multibody simulation performed by Siemens NX Mechatronics Concept Designer and a low-fidelity analytical simulation of disturbances written in MATLAB. The low-fidelity simulation is used in the feedforward system mentioned earlier.
Some of the expected outcomes of this research include:
1) A dependency of optimal control gains on both the inertial characteristics and the pose of the robotic manipulator. This would suggest that the controller needs to change its control gains based on the manipulator’s trajectory. This could be accomplished using gain scheduling between optimal gains selected based on the robot pose.
2) An enhancement in attitude performance through the feedforward integration of analytically computed disturbance forces and torques.
3) An increased torque and angular momentum capacity requirement for this type of mission. The digital twin could be utilized to find the optimal reaction wheel parameters based on a control margin and robot joint speeds.
In-orbit servicing operations encompass a wide range of tasks, including active debris removal, satellite relocation, inspection, refueling, in-space manufacturing and de-manufacturing, and maintenance. The broader applicability of this research extends to guidance and advice for the design of in-orbit demonstration missions for spacecraft capable of carrying out these types of tasks while maintaining attitude control.