Coordinated control architecture of a space robot for capture and servicing operations

ESA-GNC 2021 Online 22-25 June 2021: “EROSS PROJECT – COORDINATED CONTROL ARCHITECTURE OF A SPACE ROBOT FOR CAPTURE AND SERVICING OPERATIONS” V. Dubanchet, P. Lopez Negro, D. Casu, A. Giovannini, G. Rekleitis, K. Nanos, I. S. Paraskevas, E. Papadopoulos


The on-going developments of On-Orbit Servicing operations are massively investigating the usage of space robotics to realise precise tasks in an autonomous way. Such technologies raise the issue of properly coordinating the motion of a robotic arm mounted on a floating platform in order to ensure the tracking and/or capture of a target object. The Guidance, Navigation and Control (GNC) architecture of such a system, often referred to as “space robot”, is presented in this paper, with an emphasis on the control algorithms and how these are being implemented using the platform and the robotic arm processing units to account for the capabilities and limitations of the space hardware at hand.

The work to be presented in this paper is related to the H2020 project “European Robotic Orbital Support Services” (EROSS), which receives funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 821904. The paper will be organised as follows: a first section will cover the EROSS servicing mission, with a particular focus on the design of the space robot used for this operation; then a second section will describe the overall GNC architecture with the platform and robot sharing, and will introduce the theoretical foundations of the coordinated control. A third section will provide simulation results of the reachable performances for the berthing with a collaborative spacecraft. Next, a short section will tackle the capabilities of the processing units being developed to run such controllers on-board, before providing conclusions and recommendations on the hardware implementation of these technologies.

The EROSS mission scenario focuses on the last steps of a traditional rendezvous in space between a Servicer spacecraft chasing a Client spacecraft to be serviced. After the orbit injection by the launcher, a first set of orbital manoeuvres puts the Servicer within the orbital plane of the Client, then a second set of manoeuvres brings it closer to the Client, either behind or above it [1]. The final forced motion allows the Servicer to perform a straight line relatively to the Client in order to capture it by the robotic arm. The coordinated control of the Servicer space robot is then designed to allow a smooth and safe deployment of the robotic arm while the Servicer platform is moving towards the Client, and also to perform the final capture with a Servicer platform floating, for safety purposes.

To that end, the Servicer vehicle is seen as a traditional platform when its embedded arm is stowed. During that time, the platform controller is based on traditional techniques developed within Thales Alenia Space to ensure the proper thruster pointing during the orbital manoeuvres, to maintain the solar panels illumination for power generation, while also keeping track of the Client spacecraft within the relative sensors field of view when the relative navigation starts. On the other hand, as soon as the robotic arm is deployed during the forced motion, the Servicer turns into a “space robot” [2-3-4] with multiple Degrees-of-Freedom (DoF) which moves and aligns the arm end-effector with a target point on the Client spacecraft.

Many techniques were developed in the past to control such a system. The National Technical University of Athens (NTUA) played a key role in these developments, and it is in charge of the robotic control in the EROSS project. Different GNC techniques are summarized in [5] for a space robot, with the main hypothesis of controlling or not the platform when the arm is moving. In the first case, the space robot is said to be “free flying” when the platform controller is actuating the thrusters or reaction wheels to maintain a given pointing while compensating for the disturbances coming from the robotic arm motion. From a practical point of view, this method induces vibrations from the platform to the robotic arm since its structure is very light and sensitive to any disturbance exciting its low frequency flexible modes. On the other hand, the platform controller can also be completely switched off to prevent the transmission of any residual vibration from the platform to the arm: the space robot is then said to be “free-floating”. This method also presents the additional advantage of saving fuel and power on the platform side, but at the expense of a reduced workspace for the end-effector of the robotic arm, and of the loss of direct position and attitude control of the Servicer base, which may rise safety issues at such close proximity. The proper trade-off must then be made to select the relative distance between the Servicer and Client platforms at which the robot switches from a free-flying to a free-floating mode in order to capture the Client with a maximum motion accuracy and robustness.

The control architecture designed for the EROSS mission will be evaluated and validated through numerical simulations using a high-fidelity simulator, by coupling traditional Attitude and Orbit Control System (AOCS) simulators from Thales Alenia Space with multi-body modelling tools to account for the dynamics of the space robot during the capture of the Client. The performance will be presented in terms of relative position and attitude tracking error and attitude between the Client capture point with respect to the end-effector.

In addition to this kinematic figure of merit, another performance index will be derived by investigating the complexity of such algorithms and by analysing their compatibility with the current and on-going developments of space processors. This last step is necessary to bridge the gap between the theoretical works available in the literature and their implementation on a real space robot for a short-term mission.

The resulting performances and trade-offs will eventually be reviewed to propose improvements and recommendations for future space robotics developments from an industrial and practical point of view.

[1] Fehse, “Automated Rendezvous and Docking of Spacecraft”, 2003.
[2] Flores-Abad et al., “A review of space robotics technologies for on-orbit servicing”, 2014.
[3] Yoshida and Wilcox, “Space Robots and Systems”, 2008.
[4] Yoshida, “Achievements in Space Robotics”, 2009.
[5] Moosavian and Papadopoulos, “Free-flying robots in space”, 2007. 


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The project has received funding from the European Union’s Horizon 2020 Research and Innovation  Programme under Grant Agreement No 821904