A Nonlinear Observer for Free-Floating Target Motion using only Pose Measurements

In this paper, we design a nonlinear observer to estimate the inertial pose and the velocity of a free-floating non-cooperative satellite (Target) using only relative pose measurements. In the context of control design for orbital robotic capture of such a non-cooperative Target, due to lack of navigational aids, only a relative pose estimate may be obtained from slow-sampled and noisy exteroceptive sensors. The velocity, however, cannot be measured directly. To address this problem, we develop a model-based observer which acts as an internal model for Target kinematics/dynamics and therefore, may act as a predictor during periods of no measurement. To this end, firstly, we formalize the estimation problem on the SE(3) Lie group with different state and measurement spaces. Secondly, we develop the kinematics and dynamics observer such that the overall observer error dynamics possesses a stability property. Finally, the proposed observer is validated through robust Monte-Carlo simulations and experiments on a robotic facility.Comment: 8 pages, 6 figure

Dynamics and Control of a Reconfigurable Multi-Arm Robot for In-Orbit Assembly

In this paper, a passivity-based controller is proposed for a reconfigurable multi-arm system, which is employed for in-orbit assembly. The reconfigurable system is composed of multiple anthropomorphic arms which can be connected to a torso or operate independently. The proposed control is applicable for all the morphologies of the multi-arm system while performing several operations required for the assembly task, such as walking on a telescope structure through dedicated mechanical interfaces. The constrained dynamics is exploited to derive a unified joint actuation control law for all the considered operations and morphologies. A Co-simulation framework was developed to simulate the operations for control prototyping, and simulation results prove the effectiveness of the proposed unified controller

A Compliant Partitioned Shared Control Strategy for an Orbital Robot

In this letter, a novel partitioned shared controller is proposed, which exploits a fully-actuated orbital robot to perform a primary end-effector task involving environmental interactions. This task is remotely performed using a bilateral teleoperation controller, while a secondary task is automatically controlled in situ for operational safety in a partitioned manner. In particular, the proposed method is derived as a modified 4-Channel teleoperation architecture. The orbital robot’s momentum and shape (joints) dynamics are exploited to benefit the controller design. Asymptotic stability and finite-gain L2-stability are proved in the absence and presence of external interactions, respectively. Furthermore, the proposed method is validated experimentally on a hardware-in-the-loop facility

EigenMPC: An Eigenmanifold-Inspired Model-Predictive Control Framework for Exciting Efficient Oscillations in Mechanical Systems

This paper proposes a Nonlinear Model-Predictive Control (NMPC) method capable of finding and converging to energy-efficient regular oscillations, which require no control action to be sustained. The approach builds up on the recently developed Eigenmanifold theory, which defines the sets of line-shaped oscillations of a robot as an invariant two-dimensional submanifold of its state space. By defining the control problem as a nonlinear program (NLP), the controller is able to deal with constraints in the state and control variables and be energy-efficient not only in its final trajectory but also during the convergence phase. An initial implementation of this approach is proposed, analyzed, and tested in simulation

Optimization of Multi-arm Robot Locomotion to Reduce Satellite Disturbances for In-orbit Assembly

Traditionally, manufacturing and assembly of space assets is performed on ground before sending them into orbit. However, this monolithic approach involves high launch costs due to increasing asset sizes, e.g., large telescopes for space observation. Alternatively, in-orbit assembly of space structures after launching the raw materials to orbit opens wider possibilities at a reduced cost. Mobile robotics, such as walking manipulators or multi-arm robots, are a critical component for this approach due to their mobility in orbit. However, unlike terrestrial assembly tasks, the continuous motion of the robot and materials, coupled with the change of inertial properties of the structure, results in a rotational deviation of the platform due to conservation of angular momentum in orbit. This might violate the tolerance limits of the platform antennas cone angle for communication with the ground stations. Although exploiting the attitude control system of the platform is a straightforward solution, it might lead to issues related to the associated actuators like reaction wheels saturation, high-frequency vibration, or high fuel consumption. To deal with this problem, in this paper we formulate the attitude disturbance problem as a minimization of the effects created by the gait of the walking manipulator. Investigating the dynamic coupling between the robot system and the space structure gives a deeper understanding of the spacecrafts behavior depending on the robot gaits. The paper proposes a controller that optimizes the forces that the robotic arm applies to the structure, hence minimizing the base rotation. As an application, we use a space structure composed of identical elements, namely the mirrors of a segmented telescope, endowed with standard interfaces to allow the robot locomotion. We show the effects of optimizing these interaction forces in various scenarios and positions on the structure through multiple dynamic simulations

Hybrid Planning to Minimize Platform Disturbances during In-orbit Assembly Tasks

In-orbit space assembly has been proposed as a method to overcome the obstacles for deployment of large spatial structures. To make such assemblies economically feasible, they must rely on robotic arms to perform the required manipulation actions. The operations with the robotic arm inevitably affect the attitude and orientation of the spacecraft. This influence is well understood for simple trajectories; however, assembly sequences for full structures require multiple repetitive motions, with and without load, which significantly affect the attitude and orbital control of the satellite. This paper analyzes such perturbations for a complex assembly task, the construction of the primary mirror for a space telescope, using a hybrid planner with two levels: a low level that considers individual motions of the robotic arm, and a high level that generates the overall assembly sequence while minimizing the perturbations created on the attitude control system. The method effectively minimizes perturbations during orbital assembly tasks, therefore minimizing fuel or energy consumption in the spacecraft

Whole-Body Teleoperation and Shared Control of Redundant Robots with Applications to Aerial Manipulation

This paper introduces a passivity-based control framework for multi-task time-delayed bilateral teleoperation and shared control of kinematically-redundant robots. The proposed method can be seen as extension of state-of-the art hierarchical whole-body control as it allows for some of the tasks to be commanded by a remotely-located human operator through a haptic device while the others are autonomously performed. The operator is able to switch among tasks at any time without compromising the stability of the system. To enforce the passivity of the communication channel as well as to dissipate the energy generated by the null-space projectors used to enforce the hierarchy among the tasks, the Time-Domain Passivity Approach (TDPA) is applied. The efficacy of the approach is demonstrated through its application to the DLR Suspended Aerial Manipulator (SAM) in a real telemanipulation scenario with variable time delay, jitter, and package loss

Design and Integration of a Multi-arm Installation Robot Demonstrator for orbital large Assembly

Space facilities for orbital exploitation and exploration missions are increasingly requiring larger structure to extend their capabilities. Dimensions of future scientific outposts, solar stations and telescopes undoubtedly matter to expand our horizons, power our planet or explore the universe. Due to the foreseen large structures for such applications, a single self-deploying piece contained in standard launcher fairings might become inadequate. Another approach is that large structures could be broken down into standard modules that will be built in-orbit. Assembling large structure in space is particularly challenging but the raise of key enablers as standard interconnects and advanced robotics opens a new horizon for such applications. It is assumed here that the large spacecraft structure and modules are equipped with standard interconnects (SI) that allow them to be mated to each other and to the robot system for manipulation/transport/installation, or to allow the robot system to move across them. This paper introduces the concept of a novel Multi-Arm Robot (MAR) dedicated to on-orbit large telescope assembly, its ground equivalent laboratory demonstrator design and preliminary hardware integration. The MAR is a modular robot composed of three robotic subsystems - a torso and two symmetrical 7-degree of freedom (DOF) anthropomorphic arms with non-spherical wrists - that are functionally independent and can be connected by the means of Standard Interconnects. The modular approach of the MAR reduces the complexity of the different robotic appendages and offers a set of robotic configuration that extends the range of possible operations and provides an intrinsic system redundancy that reduces the overall mission risk. To assess the MAR concept, a Technology Readiness Level (TRL) 4 ground demonstrator, has been designed to provide a framework that allows the multi-arm robot to execute its overall scope of operations in a ground laboratory environment. It comprises a testbed (dummy spacecraft structure, home base, storage area and mobile payloads) offering a space representative environment, a mission control center (computer, simulator and electrical/data support equipment) supervising the MAR's tasks, and a gravity compensation system (gantry crane and offloading system) for supporting the robot under 1-g

Demonstrator Design of a Modular Multi-arm Robot for On-orbit Large Telescope Assembly

The development of building blocks, and standard interconnects in particular, enables promising perspectives for the assembly of large structures on-orbit. By coupling these standard interconnects with dexterous arms, it is now possible to imagine orbital robots assembling, in-situ, modular structures to emancipate from launcher constraints. Such a mission scenario and related concept of operations are proposed within the ESA MIRROR project. It involves a modular multi-arm installation robot to address this challenge. This paper deals with the design of a fully representative breadboard for this innovative robot in order to prove its concept and abilities. This demonstrator features a ground equivalent robotic system, a testbed and necessary ground support equipments

Designing robust pose estimator for non-cooperative space targets for visual servoing during approach maneuvers

For on-orbit autonomous grasping of an uncooperative spacecraft using visual servo control of a robotic manipulator, it is imperative that the pose estimation algorithm provide accurate estimates of relative motion parameters from the noisy vision measurements. These non-uniformly sampled measurements have variable noise characteristics and represent a past state owing to the processing time. In this thesis, an event-driven and Out of Sequence Measurement (OOSM)-capable Extended Kalman Filter (EKF) observer with adaptive behavior is derived for estimating motion, inertial and geometric characteristics of an uncooperative Target spacecraft with an objective of grasping while using the measurements of the kind mentioned above. Observability and stability analyses have been presented with conclusions about target inertia and geometry that affect the estimation process. Special focus has been laid on the vision sensor’s noise and time-response characteristics to improve the estimator’s robustness and optimality. Robustness is analysed in terms of convergence, immunity towards outliers and adaptive behaviour in the face changing noise characteristics. The adaptive behaviour in the EKF is achieved using a Variational Bayesian (VB) approach and an assessment is presented for a step-change in noise characteristics. A nonlinear state-space model for relative dynamics between the OOS’s end-effector and the tumbling target have been derived which incorporate orbital dynamics and manipulator’s servoing motion. In order to avoid rank-deficiency variances for the attitude quaternion, a Multiplicative Extended Kalman Filter (MEKF) approach with a reduced state-vector is used. The small angular rotation and the Gibbs vector were used as candidates and an evaluation of both of these representations is provided. A Software In Loop (SIL) has been developed which allows fast prototyping of estimation/control algorithms for the grasping problem using an eye-in-hand topology for position-based servo control