Dynamics and control of robotic systems for on-orbit objects manipulation

Multi-body systems (MSs) are assemblies composed of multiple bodies (either rigid or structurally flexible) connected among each other by means of mechanical joints. In many engineering fields (such as aerospace, aeronautics, robotics, machinery, military weapons and bio-mechanics) a large number of systems (e.g. space robots, aircraft, terrestrial vehicles, industrial machinery, launching systems) can be included in this category. The dynamic characteristics and performance of such complex systems need to be accurately and rapidly analyzed and predicted. Taking this engineering background into consideration, a new branch of study, named as Multi-body Systems Dynamics (MSD), emerged in the 1960s and has become an important research and development area in modern mechanics; it mainly addresses the theoretical modeling, numerical analysis, design optimization and control for complex MSs. The research on dynamics modeling and numerical solving techniques for rigid multi-body systems has relatively matured and perfected through the developments over the past half century. However, for many engineering problems, the rigid multi-body system model cannot meet the requirements in terms of precision. It is then necessary to consider the coupling between the large rigid motions of the MS components and their elastic displacements; thus the study of the dynamics of flexible MSs has gained increasing relevance. The flexible MSD involves many theories and methods, such as continuum mechanics, computational mechanics and nonlinear dynamics, thus implying a higher requirement on the theoretical basis. Robotic on-orbit operations for servicing, repairing or de-orbiting existing satellites are among space mission concepts expected to have a relevant role in a close future. In particular, many studies have been focused on removing significant debris objects from their orbit. While mission designs involving tethers, nets, harpoons or glues are among options studied and analyzed by the scientific and industrial community, the debris removal by means of robotic manipulators seems to be the solution with the longest space experience. In fact, robotic manipulators are now a well-established technology in space applications as they are routinely used for handling and assembling large space modules and for reducing human extravehicular activities on the International Space Station. The operations are generally performed in a tele-operated approach, where the slow motion of the robotic manipulator is controlled by specialized operators on board of the space station or at the ground control center. Grasped objects are usually cooperative, meaning they are capable to re-orient themselves or have appropriate mechanisms for engagement with the end-effectors of the manipulator (i.e. its terminal parts). On the other hand, debris removal missions would target objects which are often non-controlled and lacking specific hooking points. Moreover, there would be a distinctive advantage in terms of cost and reliability to conduct this type of mission profile in a fully autonomous manner, as issues like obstacle avoidance could be more easily managed locally than from a far away control center. Space Manipulator Systems (SMSs) are satellites made of a base platform equipped with one or more robotic arms. A SMS is a floating system because its base is not fixed to the ground like in terrestrial manipulators; therefore, the motion of the robotic arms affects the attitude and position of the base platform and vice versa. This reciprocal influence is denoted as "dynamic coupling" and makes the dynamics modeling and motion planning of a space robot much more complicated than those of fixed-base manipulators. Indeed, SMSs are complex systems whose dynamics modeling requires appropriate theoretical and mathematical tools. The growing importance SMSs are acquiring is due to their operational ductility as they are able to perform complicated tasks such as repairing, refueling, re-orbiting spacecraft, assembling articulated space structures and cleaning up the increasing amount of space debris. SMSs have also been employed in several rendezvous and docking missions. They have also been the object of many studies which verified the possibility to extend the operational life of commercial and scientific satellites by using an automated servicing spacecraft dedicated to repair, refuel and/or manage their failures (e.g. DARPA's Orbital Express and JAXA's ETS VII). Furthermore, Active Debris Removal (ADR) via robotic systems is one of the main concerns governments and space agencies have been facing in the last years. As a result, the grasping and post-grasping operations on non-cooperative objects are still open research areas facing many technical challenges: the target object identification by means of passive or active optical techniques, the estimation of its kinematic state, the design of dexterous robotic manipulators and end-effectors, the multi-body dynamics analysis, the selection of approaching and grasping maneuvers and the post-grasping mission planning are the main open research challenges in this field. The missions involving the use of SMSs are usually characterized by the following typical phases: 1. Orbital approach; 2. Rendez-vous; 3. Robotic arm(s) deployment; 4. Pre-grasping; 5. Grasping and post-grasping operations. This thesis project will focus on the last three. The manuscript is structured as follows: Chapter 1 presents the derivation of a multi-body system dynamics equations further developing them to reach their Kane's formulation; Chapter 2 investigates two different approaches (Particle Swarm Optimization and Machine Learning) dealing with a space manipulator deployment maneuver; Chapter 3 addresses the design of a combined Impedance+PD controller capable of accomplishing the pre-grasping phase goals and Chapter 4 is dedicated to the dynamic modeling of the closed-loop kinematic chain formed by the manipulator and the grasped target object and to the synthesis of a Jacobian Transpose+PD controller for a post-grasping docking maneuver. Finally, the concluding remarks summarize the overall thesis contribution

Learning-based control of a spacecraft with sloshing propellant

One of the major needs of present and future spacecraft is to fulfil highly demanding pointing requirements when performing attitude manoeuvres without losing stringent control over their flexible parts. For long-duration missions, the propellant mass is a significant portion of the overall mass budget of the satellite. The interaction between the fluid and the tank walls can lead to instability problems and even to mission failure if not properly accounted for in the design phase and control synthesis. The combined liquid-structure dynamic coupling is usually extremely difficult to model for a space system. An equivalent mechanical system is then desirable to carry out a computationally-efficient simulation of the liquid behaviour inside the tank. In this paper, the 3-D model of a spacecraft equipped with flexible appendages and tanks containing liquid propellant is presented. A learning-based control strategy using on-orbit available data is designed to improve the attitude tracking precision for repetitive on-orbit manoeuvres, compensating for cyclic disturbances such as liquid fuel sloshing effects. A co-simulation procedure between MSC Adams and Simulink is then carried out to test the performance of the controller. The effectiveness of the proposed control strategy is analysed and discussed and conclusions are presented

Parametric analysis of a controlled deployable space manipulator used for capturing a non-cooperative satellite

In the near future robotic systems will be playing an increasingly important role in space applications such as repairing, refueling, re-orbiting spacecraft and cleaning up the increasing amount of space debris. Space Manipulator Systems (SMSs) are robotic systems made of a bus (which has its own actuators such as thrusters and reaction wheels) equipped with one or more deployable arms. The present paper focuses on the issue of maintaining a stable first contact between the arms terminal parts (i.e. the end-effectors) and a non-cooperative target satellite, before the actual grasp is performed. The selected approach is a modified version of the Impedance Control algorithm, in which the end-effector is controlled in order to make it behave like a mass-spring-damper system regardless of the reaction motion of the base, so to absorb the impact energy. The effects of non-modeled dynamics in control determination such as the structural flexibility of the manipulator and the target satellite are considered as well, and their impact on control effectiveness is analyzed. The performance of the proposed control architecture and a parametric analysis is studied by means of a co-simulation involving the MSC Adams multibody code (for describing the dynamics of the space robot and target) together with Simulink (for the determination of the control actions). The results show that the first contact phase of the grasping operation of a large satellite requires careful tuning of the control gains and a proper selection of the end-effector dimensions; otherwise, the large inertia characteristics of the target could lead to a failure with serious consequences. Both successful and underperforming cases are presented and commented in the paper

A small platform application for close Inspection of an out of control satellite

The field of space proximity operations such as on-orbit inspection, refueling, repair and debris removal has been recently exploring new innovations for what technology and application scenarios concern. For this scope, small orbiting platforms, despite their limited power and operative capacity, represent a low-cost solution for the majority of proximity missions. Regarding mission architecture, various researches have been carried out to analyze the feasibility of several practical applications in a proximity operation scenario. In this sense, GNC systems, which play a fundamental role for the successfulness of the mission, have undergone relevant improvements. Optical navigation has proved to be a reliable solution for the navigation system, optimal path planning algorithms have been augmented with secure and robust collision avoidance, while control strategies offer satisfying performance in terms of desired position and velocity tracking. The present research shows a possible and feasible application of a small platform (chaser) for the close inspection of an uncontrolled, un-cooperative, Earth-orbiting satellite (target). In an initial phase of the mission, the chaser will follow a closed relative orbit (Parking Orbit) around the target and use measurements acquired from a passive camera and a range sensor in order to estimate target’s relative position, velocity, orientation, angular motion and shape. By means of the acquired information, a set of relevant viewpoints for a close and clear inspection of the target is evaluated. As a second step, starting from the Parking Orbit, the chaser will perform a rendezvous maneuver to reach one of the viewpoints, selected as the optimal among all. The Approach Trajectory followed in this phase is computed to be optimal from a propellant consumption point of view and free of collisions with the target. The desired trajectory will be tracked by implementing an impedance control law. Once the viewpoint has been reached, the chaser will maintain the relative position and orientation while performing the inspection by means of a camera mounted on a robotic arm. At the end of this operation, the chaser will transfer to the subsequent viewpoint, following an optimized and safe path. The mission will be achieved once all the viewpoints have been visited. In the paper rigorous analytical formulations of the relative dynamics, estimate process, optimal path planning and control strategy will be presented. Additionally, the feasibility of the proposed architecture will be evaluated through numerical simulations on some test case scenarios

Performance analysis and gains tuning procedure for a controlled space manipulator used for non-cooperative target capture operations

In the near future robotic systems will be playing an increasingly important role in space applications such as repairing, refuelling, re-orbiting spacecraft and cleaning up the increasing amount of space debris. Space Manipulator Systems (SMSs) are robotic systems made of a bus (which has its own actuators such as thrusters and reaction wheels) equipped with one or more deployable arms. The present paper focuses on the issue of maintaining a stable first contact between the arms terminal parts (i.e. the end-effectors) and a non-cooperative target satellite, before the actual grasp is performed. The selected approach is a modified version of the Impedance Control algorithm, in which the end-effector is controlled in order to make it behave like a mass-spring-damper system regardless of the reaction motion of the base, so to absorb the impact energy. A very important aspect in the analysis of the control performance is the evaluation of the field of applicability of the controller itself. In the present work the influence of this issue on the effectiveness of the proposed control architecture will be analysed, together with the control gains tuning which allows for a robust achievement of the mission requirements. Several numerical results will be presented and discussed

A combined impedance-PD approach for controlling a dual-arm space manipulator in the capture of a non-cooperative target

In the near future robotic systems will be playing an increasingly important role in space applications such as repairing, refueling, re-orbiting spacecraft and cleaning up the increasing amount of space debris. Space Manipulator Systems (SMSs) are robotic systems made of a platform (which has its own actuators such as thrusters and reaction wheels) equipped with one or more deployable arms. The present paper focuses on the issue of maintaining a stable first contact between the arms terminal parts (i.e. the end-effectors) and a target satellite, before the actual grasp is performed. The selected approach is a modified version of the Impedance Control algorithm, in which the end-effector is controlled in order to make it behave like a mass-spring-damper system regardless of the reaction motion of the base, so to absorb the impact energy. The usual approach consists in considering a point mass target and one-dimensional contact dynamics; however, the contact between the chaser and the target could generate a perturbation on the attitude of the target. On account of this, in the present work a more realistic scenario, consisting in a 2D rigid target and a relevant 2D contact dynamics, is considered. A two-arm configuration of the SMS is modelled and its effectiveness analyzed. The performance of the proposed control architecture is evaluated by means of a co-simulation involving the MSC Adams multibody code (for describing the dynamics of the space robot and target) together with Simulink (for the determination of the control actions). The co-simulation is a particularly useful tool to implement robust control applied to detailed dynamic systems. Several numerical results complete the work

Optimal in-orbit operations of a multi-degree of freedom space manipulator

Space manipulators are complex systems made of a platform equipped with one or more deployable robotic arms. They will be playing a major role in future autonomous on-orbit missions such as building large space structures and servicing satellites which ran out of propellant or need repair. In the latter case, the robotic arm mounted on the floating base must be deployed to reach the target spacecraft. In this work a multibody approach is adopted to describe the full three-dimensional non-linear dynamics of a space manipulator. The latter presents a seven-degree of freedom arm. More specifically, it is formed by seven links: the first six are connected by means of revolute joints (with each rotation axis oriented at 90° with respect to the previous one) and the last two using a prismatic joint. A rotational motor is present at each revolute joint, while a translational actuator is placed along the prismatic joint axis. The present paper aims at studying optimal trajectories for the arm deployment and developing a suitable control strategy to achieve the mission goal. The multi-objective optimality criterion is based on the minimum time necessary to complete the maneuver, the minimum power consumption and the minimum disturbance on the manipulator base position and attitude which are certainly features of relevance in space applications. In addition, the final point for the end-effector position is considered to be moving with respect to the manipulator, thus increasing the task complexity level. The optimization will also take actuators saturation into account as well. The deployment maneuver is studied by means of an in-house developed Matlab code which allows for an easy parameterization with respect to the sensitive variables in the system dynamics and optimization procedure

A two-arm flexible space manipulator system for post-grasping manipulation operations of a passive target object

Space Manipulator Systems (SMSs) are complex systems made of a platform equipped with one or more deployable robotic arms and they will be playing a major role in future autonomous on-orbit missions. The latter may as well involve manipulation operations of the target object which needs to be moved from an initial configuration to a final one suitable for the specific task under consideration. In the present study, the mission being analyzed concerns the manipulation of a passive body by means of a flexible SMS after grasping operations have been performed. The dynamics model is derived in a three-dimensional context (non-linear formulation for the rigid-body motions and linear for the elastic dynamics). The SMS involved in the investigation has two actuated seven-degree of freedom arms. Furthermore, the modeling of the grasp constraint existing between the SMS end-effectors and the target object is a critical issue which is addressed in the present paper. A combination of Jacobian Transpose and Proportional-Derivative Control is synthesized to accomplish the desired mission goals. Numerical results proving the effectiveness of the proposed strategy are presented and discussed

A deep learning strategy for on-orbit servicing via space robotic manipulator

Autonomous robotic systems are currently being addressed as a critical element in the development of present and future on-orbit operations. Modern missions are calling for systems capable of reproducing human’s decision-making process thus enhancing their performance. Generally, space manipulators are mounted on a floating spacecraft in a microgravity environment, consequently leading to a mutual influence between the robotic arms and the platform dynamics, thus making the motion planning and control design more challenging than those of terrestrial robots. Another aspect to be considered is that space robots are designed as lightweight systems, resulting in a significant dynamic coupling between their rigid motion and structural elasticity. These effects involve critical issues in modelling their dynamics and designing a suitable controller. In this context, Deep Neural Network (DNN) architectures and the related Deep Learning (DL) techniques have widely proved to have powerful capability in solving data-driven nonlinear modelling problems and they can hence represent a viable solution for space activities. The present paper deals with the design of a DNN controller for a space manipulator system, which has to follow a specific path for a typical on-orbit servicing mission. The goal is to provide proper control inputs autonomously adapting to the given desired trajectory. Structural flexibility and joint friction features are implemented in the dynamic model as well