ESA GNC Conference Papers Repository
Combined control for active debris removal using a satellite equipped with a robot arm
Studies have shown that the population of large objects has become a problem in Low Earth Orbit (LEO). The danger of collisions is higher than ever before and important objects are at high risk of major damage. One possible solution for this problem is Active Debris Removal (ADR) using a robot arm mounted on a spacecraft (chaser). A gripper installed on the robot arm can be used to capture a debris part or inactive satellite (target) that endangers other satellites in the orbit. Once a connection between the chaser and target is established, the chaser can be used for the detumbling and to safely deorbit the target by transferring it to a disposal orbit. To analyze this approach, a GNC model in the loop (MIL) simulation tool for ADR with a robot arm was developed and extended to include high fidelity simulation models and a combined control setup for the chaser with its robot arm. The MIL tool, implemented using the Modelica modelling language, allows implementing detailed multi-physics models of the spacecraft within a LEO space environment. A realistic scenario based on the capturing of the inactive Envisat satellite was chosen for a simulation study. The parameters of all models as well as the sensor and actuator specification are based on the e.Deorbit study to allow for a realist design for the GNC development. The first version of the MIL simulation tool, developed within previous project cooperation between the DLR and ESA, was improved by upgrading the physical models of the chaser spacecraft with its robotic manipulator and the target satellite Envisat. The object-oriented design of the MIL tool allows to easily changing the modular components as well as parameters of the simulation. Flexible modes for the target satellite solar array and elastic modeling of the seven axis robot arm have been included and fuel sloshing effects for the chaser satellite are considered. In addition the thrusters of the chaser have been modeled in high fidelity. Every control thruster of the chaser is modelled individually. For the control of the thrusters a force and torque allocation has been designed. The thruster firing pulse sequencing is implemented using a PWPF control. The chaser sensor models have been upgraded to include a camera performance model. It provides the required relative pose information between the chaser and target with representative performance. The trajectory planning of the GNC considers the spinning and tumbling motion of Envisat and the seven axis kinematics of the robot arm. The inverse kinematics of this over-actuated system is solved using an online-optimization algorithm. The large inertia as well as the high rotation speed of the inactive Envisat satellite together with the limited thrust forces of the chaser satellite and limited torques for the long robot arm leads to a challenging control problem for both chaser satellite and robot arm. The chaser satellite together with its robot arm has to mimic the target motion in close proximity to the rotating and tumbling target while the robot arm is grasping the adapter ring of the target. After a successful grip, the system properties, as seen by the controller, change drastically. For a controlled detumbling the controller has to be able to handle the combined system of the chaser connected to the target with its robot arm. To achieve accurate and collision free trajectory following, a single controller for the full translation and rotation motion of the chaser satellite as well as the seven axis robot has been developed. Based on modern robust MIMO control design methods and a multi-stage optimization synthesis procedure, a controller has been designed which is able to control all states of the combined system of chaser and robot arm simultaneously during all relevant mission phases (combined control). The combined control is robust against sensor noise and parameter uncertainty. A Monte Carlo simulation study was performed to verify this robustness. The combined control was used in the MIL simulation tool, using high fidelity models, to enable the chaser satellite with its robot arm to successfully demonstrate the capturing and detumbling of Envisat in its low Earth orbit.