This paper exploits the robotic capabilities of an orbital manipulator equipped with an actuation module at its end-effector to perform close-proximity robotic operations. The proposed control strategy enables repositioning the system's center-of-mass by reconfiguring the manipulator configuration and using the end-effector-mounted thrusting mechanism to achieve displacement. The key advantage of the proposed method is that the plume impingement due to thruster firing of the servicer satellite in close-proximity operations towards the client is mitigated. This is achieved by regulating the internal motion of the manipulator such that the thrust firing does not occur near the space asset. The effectiveness of the controller is verified through a multibody dynamic simulation of an orbital manipulator.

The on-ground validation of orbital manipulators is a challenging task because the robot is designed for a gravity-free operational environment, but it is validated under the effect of gravity. As a consequence, joint torque limits can be easily reached in certain configurations when gravity is actively compensated by the joints. Hence, the workspace for on-ground testing is restricted. In this paper, an optimal strategy is proposed for achieving gravity compensation of an orbital manipulator arm on ground. The strategy minimizes the joint torques acting on the manipulator by solving an optimization problem and it computes the necessary forces to be tracked by an external carrier. Hence, full gravity compensation is achieved for the orbital manipulator. Experimental results validate the effectiveness of the method on the DLR CAESAR space robot, which uses a cable suspended system as external carrier to track the desired gravity compensation force, resulting from the proposed method.

At first glance, a floating-base robotic system is a kinematic chain, and its equations of motion are described by the inertia-coupled dynamics of its shape and movable base. However, the dynamics embody an additional structure due to the momentum evolution, which acts as a velocity constraint. In prior works of robot dynamics, matrix transformations of the dynamics revealed a block-diagonal inertia. However, the structure of the transformed matrix of Coriolis/Centrifugal (CC) terms was not examined, and is the primary contribution of this article. To this end, we simplify the CC terms from robot dynamics and derive the analogous terms from geometric mechanics. Using this interdisciplinary link, we derive a two-part structure of the CC matrix, in which each partition is iteratively computed using a self-evident velocity dependency. Through this CC matrix, we reveal a commutative property, the velocity dependencies of the skew-symmetry property, the invariance of the shape dynamics to the basis of momentum, and the curvature as a matrix operator. Finally, we show the application of the proposed CC matrix structure through controller design and locomotion analysis.

In this paper, we propose a detumbling strategy that stabilizes the motion of a tumbling client satellite using an orbital servicing manipulator, which is the goal of the post-grasp phase. One of the critical aspects in this phase is ensuring that excessive contact forces are not generated at the grasp interface. In addition, space mission requirements might demand a nominal manipulator configuration that is suitable for further manipulation/servicing activities. The proposed strategy allows the detumbling of the client motion while ensuring that the contact forces developed at the grasp interface do not violate a safety threshold. Further, it allows the reconfiguration of the manipulator arm by exploiting the full actuation capability of the manipulator-equipped servicing spacecraft. The controller guarantees joint task convergence in the nullspace of the manipulator's end-effector, and is also valid for kinematically singular configurations of the manipulator. It is further augmented using a quadratic programming based approach to optimally constrain the contact forces. Finally, simulation results for a post-grasp detumbling scenario are shown to validate the effectiveness of the proposed method.

This paper presents a novel telepresence system for enhancing aerial manipulation capabilities. It involves not only a haptic device, but also a virtual reality that provides a 3D visual feedback to a remotely-located teleoperator in real-time. We achieve this by utilizing onboard visual and inertial sensors, an object tracking algorithm and a pregenerated object database. As the virtual reality has to closely match the real remote scene, we propose an extension of a marker tracking algorithm with visual-inertial odometry. Both indoor and outdoor experiments show benefits of our proposed system in achieving advanced aerial manipulation tasks, namely grasping, placing, force exertion and peg-in-hole insertion.

In this paper, we propose three novel Hardware-in-the-loop simulation (HLS) methods for a fully-actuated orbital robot in the presence of external interactions using On-Ground Facility Manipulators (OGFM). In particular, a fixed-base and a vehicle-driven manipulator are considered in the analyses. The key idea is to describe the orbital robot's dynamics using the Lagrange-Poincaré(LP) equations, which reveal a block-diagonalized inertia. The resulting advantage is that noisy joint acceleration/torque measurements are avoided in the computation of the spacecraft motion due to manipulator interaction even while considering external forces. The proposed methods are a consequence of two facilitating theorems, which are proved herein. These theorems result in two actuation maps between the simulated orbital robot and the physical OGFM. The chief advantage of the proposed methods is physical consistency without level-set assumptions on the momentum map. We validate this through experiments on both types of OGFM in the presence of external forces. Finally, the effectiveness of our approach is validated through a HLS of a fully-actuated orbital robot while interacting with the environment.

We present an algorithm for passivity verification and enforcement of multivariate macromodels whose state-space matrices depend in closed form on a set of external or design parameters. Uniform passivity throughout the parameter space is a fundamental requirement of parameterized macromodels of physically passive structures, that must be guaranteed during model generation. Otherwise, numerical instabilities may occur, due to the ability of nonpassive models to generate energy. In this paper, we propose the first available algorithm that, starting from a generic parameter-dependent state-space model, identifies the regions in the frequency-parameter space where the model behaves locally as a nonpassive system. The approach we pursue is based on an adaptive sampling scheme in the parameter space, which iteratively constructs and perturbs the eigenvalue spectrum of suitable skew-Hamiltonian/Hamiltonian pencils, with the objective of identifying the regions where some of these eigenvalues become purely imaginary, thus pinpointing local passivity violations. The proposed scheme is able to detect all relevant violations. An outer iterative perturbation method is then applied to the model coefficients in order to remove such violations and achieve uniform passivity. Although a formal proof of global convergence is not available, the effectiveness of the proposed implementation of the passivity verification and enforcement schemes is demonstrated on several examples.

This article proposes a passivity-based approach for simulating satellite dynamics on a position-controlled robot equipped with a force-torque sensor. Time delays intrinsic in the computational loop and discrete-time integration degrade the behavior of the satellite dynamics reproduced by the robot. These factors can generate an energy-inconsistent simulation and can even render the system unstable. In this article, time delay and discrete-time integration effects are analyzed from an energetic perspective and compensated through a passivity-based control strategy to ensure a faithful and stable dynamic simulation with position-controlled robots. The benefit of the proposed strategy is validated by simulations and experiments on the On-Orbit Servicing Simulator (OOS-SIM), a robotic facility used for simulating free-floating dynamics.

In this work we present a novel approach which compensates the destabilising effects of the time delay intrinsic in the control loop of an admittance-controlled robot employed for satellite dynamics simulation. The method is based on an energy storing element, the tank, which is exploited by the controller to preserve the passivity of the system and to avoid instability. Furthermore, we compare the performance of the proposed method with existing energy-based approaches, namely time-domain-passivity and wave variable transformation. The performance comparison and robustness of the methods are analysed in a Montecarlo simulation and validated experimentally.

In this paper we address the problem of controlling a robotic system mounted on an actuated floating base for space applications. In particular, we investigate the stability issues due to the low rate of the base control unit. We propose a passivity-based stabilizing controller based on the time domain passivity approach. The controller uses a variable damper regulated by a designed energy observer. The effectiveness of the proposed strategy is validated on a base-manipulator multibody simulation.

This paper presents a passive compliance control for aerial manipulators to achieve stable environmental interactions. The main challenge is the absence of actuation along body-planar directions of the aerial vehicle which might be required during the interaction to preserve passivity. The controller proposed in this paper guarantees passivity of the manipulator through a proper choice of end-effector coordinates, and that of vehicle fuselage is guaranteed by exploiting time domain passivity technique. Simulation studies validate the proposed approach.

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.

This paper proposes a passive and explicit integrator for simulating a rotational rigid-body dynamics rendered by a robot. Considering the Euler integration method, active energy terms are identified. These sources of energy are due to the external torque and the coupled dynamics which can lead to a non-physical behavior of the simulated dynamics. The proposed method dissipates this energy using a variable damper regulated by an energy observer. The new algorithm guarantees not only passivity but also a consistent energetic integration. The integration method is sustained by simulations and tested on a real-time hardware-in-the-loop simulator.

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.

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.

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.