Energy-based Control for Simulation of Multi-body Dynamics using Robotic Facilities

Kurzfassung

The use of a spacecraft equipped with a robotic manipulator is recognised as a promising technology for on-orbit servicing missions. This can extend the lifetime of a defunct satellite or accomplish a safe de-orbiting manoeuvre, thus, mitigating the problem of space debris. Control algorithms for the spacecraft equipped with the manipulator need to be validated on ground prior to the space mission. Hence, a reliable simulator capable of reproducing the micro-gravity conditions on ground is required. To this end, a state-of-the-art robotic facility, namely OOS-Sim was developed at the German Aerospace Center for experimental validation of space robot control algorithms. The facility is composed of two admittance-controlled industrial robots equipped with force-torque sensors to simulate model-based satellite dynamics and one of the industrial robots is equipped with a light-weight robot. In such kind of robotic simulators, delays in the control loop and discretization effects of the signals can lead to an increase in energy, potentially rendering the system unstable. In this thesis, the aforementioned factors violating the energy conservation principle are identified, isolated and addressed through control strategies to ensure an energy-consistent dynamics simulation while preserving system stability. In particular, energy-observers are designed to monitor the activity in the system and passivity-based controllers are developed to correct the velocity or the force commanded to the robot. This approach is firstly implemented to compensate the unstable effects caused by the time-delays inherent in the control loop. Secondly, an explicit and passive discrete-integrator is designed to prevent the energy drift caused by the integration process of the model-based dynamics, which the robot motion relies on. Finally, a unified framework is designed to compensate both the time-delay and discrete-time integration effects. As a result, the robotic simulator is rendered stable and energy-consistent while simulating satellite dynamics as proved by experiments. Hence, the simulator serves as a reliable platform to validate control algorithms for the space robot. A further contribution of the thesis is the design and experimental validation of different torque-based controllers for the space manipulator. In particular, a torque-controller for the manipulator mounted on the non-actuated satellite-base is firstly designed. Later, driven by ESA space mission requirements, which impose a low frequency control of the base actuation, a torque-controller is designed for the regulation of the manipulator and of the actuated base. In particular, stability issues due to the multi-rate controllers of the manipulator and base are analysed from an energy perspective and a passivity-based stabilising controller is proposed. Finally, the control strategy is extended to the tracking case for the manipulator considering the multi-rate effects, which is a typical scenario for a space-robot performing servicing manoeuvres. The controllers are also validated through experiments on the reliable robotic simulator.