Marco De Stefano

The OOS-SIM: An on-ground simulation facility for on-orbit servicing robotic operations

On-orbit servicing involves a new class of space missions in which a servicer spacecraft is launched into the orbit of a target spacecraft, the client. The servicer navigates to the client with the intention of manipulating it, using a robotic arm. Within this framework, this work presents a new robotic experimental facility which was recently built at the DLR to support the development and experimental validation of such orbital servicing robots. The facility allows reproducing a close-proximity scenario under realistic three-dimensional orbital dynamics conditions. Its salient features are described here, to include a fully actuated macro-micro system with multiple sensing capabilities, and analyses on its performance including the amount of space environment volume that can be simulated.

Passivity of virtual free-floating dynamics rendered on robotic facilities

This paper describes a control strategy to achieve high fidelity dynamics simulation rendered on admittance controlled robotic facilities. It explores the reasons for an increasing energy found in the virtual dynamics of a free-floating satellite rendered on a six degree of freedom robot, which can lead the system to become unstable and proposes a method to cope with it. The proposed method identifies the sources of intrinsic instability provoked by time delays that are found in the computational loop of the rendered dynamics and counteracts their destabilizing effects using the passivity criteria. The performance of the system and the benefits of the method are shown in simulations and are verified experimentally.

An optimized passivity-based method for simulating satellite dynamics on a position controlled robot in presence of latencies

This paper introduces a performance oriented method for simulating stable free-floating satellite dynamics on a position controlled robot. Intrinsic latencies found in robot controllers, i.e. between input and output data, are known to produce stability issues and performance degradation. These issues are even more apparent during contact phases, where impact dynamics play a major role. The approach presented in this paper guarantees stability through passivity and preserves the performance through the use of an optimal damping. The energy produced by delays found in the closed loop system is monitored and dissipated when necessary. In order to implement the dynamics accurately, the damping process is formulated as an optimization problem. Thus, over-dissipation can be avoided and the system becomes less conservative. Performance and effectiveness of the method are shown in simulation and verified experimentally on a position controlled seven degrees of freedom Light Weight Robot equipped with a force-torque sensor at the end-effector.

Teleoperation for on-orbit servicing missions through the ASTRA geostationary satellite

Force-feedback teleoperation for on-orbit servicing tasks demands real-time communication requirements, latencies below one second and the presence of a skilled human operator to perform the on-orbit servicing tasks in real-time from an on-ground station. On the other hand, teleoperation is a technology that enjoys high TRLs, has evidenced benefits in other domains as nuclear or medical and has little dependency on optical sensors and image processing algorithms that need to operate in extreme illumination conditions. While all of these factors could be of high value in future on-orbit servicing missions, the following questions remain still to be answered: 1) How is the free floating dynamics and time delay affecting the control structure of the system? 2) Can current space communication infrastructures support real time control requirements established by the bilateral controller (i.e. force-feedback teleoperation)? 3) Can a skilled human operator perform on-orbit servicing tasks through the teleoperation system, probably affected by high latencies and force-feedback distortions? This paper presents initial answers to these questions based on results from a force-feedback teleoperation system that has been implemented using the ASTRA geostationary satellite and the DLR on-orbit servicing facility (OOS-SIM).

Design and Operational Elements of the Robotic Subsystem for the e.deorbit Debris Removal Mission

This paper presents a robotic capture concept that was developed as part of the e.deorbit study by ESA. The defective and tumbling satellite ENVISAT was chosen as a potential target to be captured, stabilized, and subsequently de-orbited in a controlled manner. A robotic capture concept was developed that is based on a chaser satellite equipped with a seven degrees-of-freedom dexterous robotic manipulator, holding a dedicated linear two-bracket gripper. The satellite is also equipped with a clamping mechanism for achieving a stiff fixation with the grasped target, following their combined satellite-stack de-tumbling and prior to the execution of the de-orbit maneuver. Driving elements of the robotic design, operations and control are described and analyzed. These include pre and post-capture operations, the task-specific kinematics of the manipulator, the intrinsic mechanical arm flexibility and its effect on the arms positioning accuracy, visual tracking, as well as the interaction between the manipulator controller and that of the chaser satellite. The kinematics analysis yielded robust reachability of the grasp point. The effects of intrinsic arm flexibility turned out to be noticeable but also effectively scalable through robot joint speed adaption throughout the maneuvers. During most of the critical robot arm operations, the internal robot joint torques are shown to be within the design limits. These limits are only reached for a limiting scenario of tumbling motion of ENVISAT, consisting of an initial pure spin of 5 deg/s about its unstable intermediate axis of inertia. The computer vision performance was found to be satisfactory with respect to positioning accuracy requirements. Further developments are necessary and are being pursued to meet the stringent mission-related robustness requirements. Overall, the analyses conducted in this study showed that the capture and de-orbiting of ENVISAT using the proposed robotic concept is feasible with respect to relevant mission requirements and for most of the operational scenarios considered. Future work aims at developing a combined chaser-robot system controller. This will include a visual servo to minimize the positioning errors during the contact phases of the mission (grasping and clamping). Further validation of the visual tracking in orbital lighting conditions will be pursued.

Reproducing physical dynamics with hardware-in-the-loop simulators: A passive and explicit discrete integrator

In this paper we present a passive and reliable explicit discrete integrator, which allows to preserve the energy and dynamic properties of a physical body rendered on a hardware-in-the-loop simulator. Starting from the standard Euler integrator, we identify the energy generation that results from the integration process. This energy makes the time discrete dynamics deviate from the ideal one, resulting in position drifts or stability issues. By exploiting the time domain passivity approach, the simulated dynamics is reshaped in order to preserve its physical energy properties. The proposed integration method allows precise simulation of virtual bodies on industrial robot facilities. The method has been validated in simulation and experimentally tested on the DLR OOS-SIM facility.

Dynamics and control of a free-floating space robot in presence of nonzero linear and angular momenta

Common control methods for free-floating robots assume zero initial linear and angular momenta, for which a reduced joint dynamics equivalent to that of a fixed-base robot can be obtained. On the other hand, a disturbance is induced in the system dynamics when the linear or angular momenta are not zero, leading to a deviation of the end effector. In this work the dynamics of the free-floating robot in presence of momentum is analyzed and a torque feedback control is proposed. An operational space formulation is considered to identify the disturbing Coriolis/centrifugal forces and to cancel them by feedback. A stability proof for the proposed controller is developed using a time-varying approach. The effectiveness of the control is shown in simulation for a seven degrees-of-freedom arm connected to a floating-base under the effect of linear and angular momenta considering model parameters uncertainties.