Robot Excitation Trajectories for Dynamic Parameter Estimation using Optimized B-Splines

In this paper we adressed the problem of finding exciting trajectories for the identification of manipulator link inertia parameters. This can be formulated as a constraint nonlinear optimization problem. The new approach in the presented method is the parameterization of the trajectories with optimized B-splines. Experiments are carried out on a 7 joint Light-Weight robot with torque sensoring in each joint. Thus, unmodeled joint friction and noisy motor current measurements must not be taken into account. The estimated dynamic model is verified on a different validation trajectory. The results show a clear improvement of the estimated dynamic model compared to a CAD-valued model

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 arm's 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

Parameter Identification of Free-Floating Robots with Flexible Appendages and Fuel Sloshing

n this paper we adressed the effects of flexibilities and liquid fuel sloshing on the on-orbit robotics-based dynamic parameter identification. For modelling the liquid fuel sloshing we combined the general free-floating robot dynamics with a mechanical pendulum equivalent model. For the dynamic parameter identification we extended our identification algorithm for rigid body systems to account for these two effects. The flexible and sloshing modes are excited only with the manipulator executing optimized trajectories. For the identification algorithm we make use of the robotic joint position and torque sensor data as well as of on board GNC sensor data. Numerical simulations showed that the two effects can have significant influence to the free-floating dynamics. Furthermore, we showed that the extended parameter identification algorithm improves the accuracy of the dynamic model

Modelling, Dynamic Analysis and Robotic Control Strategies for the Deorbiting Operations of the ESA Satellite Envisat

This master thesis was developed in the context of the e.Deorbit project. Firstly, the force for the deorbiting manoeuvre was analysed in order to find a profile that does not induce detachment between the Servicer module and Envisat when a clamping mechanism is not considered. Then, the proposed configurations with a four points clamping connection and a clamping to the payload adapter of Envisat were modelled and analysed. A special focus was posed on the flexibility of the 16 meter long solar panel of Envisat: this was considered divided in rigid sections and flexible joints. Additionally, the detumbling manoeuvre was analysed in order to validate the feasibility of this manoeuvre with the robotic arm grasped to the target. The main part of the work was to analyse a closed loop configuration: the aim was to reduce the oscillation of the solar panel during the deorbiting operations. In order to simulate that configuration, a the Loop Joint method by Featherstone was implemented and included in the SpaceDyn library. After testing the written algorithms, a damping control was simulated

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

Coupled Control of Chaser Platform and Robot Arm for the e.Deorbit Mission

The e.Deorbit mission is devoted to safely remove Envisat from its orbit by robotic capture means. The major challenges in the close range are the motion synchronisation between the Chaser and the Target satellite Envisat and the coupled control during capture employing the robot arm. This paper is devoted to the coupled control phase, during which the Chaser performs station keeping at the Capture Point, which is a point relative to the Target in the Target body frame, while the robot is grasping the Target. The robot arm has to place the end-effector at the Grasping Point, a well-defined position at the Target's launch adapter ring, while compensating the station keeping errors of the Chaser platform. The impedance controlled robot operates in operational space coordinates defining the pose of the robot end-effector with respect to the Grasping Point and also directly controls the robot joint configuration. The bandwidths of the two controllers considered in this study differ by more than two orders of magnitude, allowing independent control design of the two. The overall performance of the coupled control in terms of station keeping performance for the Chaser and positioning performance of the end-effector is demonstrated in Monte Carlo simulations

Solid Solutions of Grimm–Sommerfeld Analogous Nitride Semiconductors II-IV-N2 (II=Mg, Mn, Zn; IV=Si, Ge): Ammonothermal Synthesis and DFT Calculations

Grimm-Sommerfeld analogous II-IV-N-2 nitrides such as ZnSiN2, ZnGeN2, and MgGeN2 are promising semiconductor materials for substitution of commonly used (Al,Ga,In)N. Herein, the ammonothermal synthesis of solid solutions of II-IV-N-2 compounds (II=Mg, Mn, Zn; IV=Si, Ge) having the general formula ((II1-xIIxb)-I-a)-IV-N-2 with x approximate to 0.5 and ab initio DFT calculations of their electronic and optical properties are presented. The ammonothermal reactions were conducted in custom-built, high-temperature, high-pressure autoclaves by using the corresponding elements as starting materials. NaNH2 and KNH2 act as ammonobasic mineralizers that increase the solubility of the reactants in supercritical ammonia. Temperatures between 870 and 1070 K and pressures up to 200 MPa were chosen as reaction conditions. All solid solutions crystallize in wurtzite-type superstructures with space group Pna2(1) (no. 33), confirmed by powder XRD. The chemical compositions were analyzed by energy-dispersive X-ray spectroscopy. Diffuse reflectance spectroscopy was used for estimation of optical bandgaps of all compounds, which ranged from 2.6 to 3.5 eV (Ge compounds) and from 3.6 to 4.4 eV (Si compounds), and thus demonstrated bandgap tunability between the respective boundary phases. Experimental findings were corroborated by DFT calculations of the electronic structure of pseudorelaxed mixed-occupancy structures by using the KKR+CPA approach

GNC Architecture for the e.Deorbit Mission

The GNC architecture presented in this paper has been developed in the frame of e.Deorbit phase B1. The architecture is dedicated to approach and capture the uncooperative target satellite Envisat, comprising ascent from launch orbit to the target orbit, rendezvous with the target satellite, capture and stabilization of the coupled system and de-orbiting. The homing and closing trajectories are based on e/i separation allowing a passively safe approach until the proximity operations begin. The chaser has to synchronize its motion with the target due to its large dimensions. The safety monitoring concept is briefly discussed. The propellant budget and the GNC performance requirements are consolidated by Monte Carlo simulations