Modeling a Controlled-Floating Space Robot for In-Space Services: A Beginner’s Tutorial

Ground-based applications of robotics and autonomous systems (RASs) are fast advancing, and there is a growing appetite for developing cost-effective RAS solutions for in situ servicing, debris removal, manufacturing, and assembly missions. An orbital space robot, that is, a spacecraft mounted with one or more robotic manipulators, is an inevitable system for a range of future in-orbit services. However, various practical challenges make controlling a space robot extremely difficult compared with its terrestrial counterpart. The state of the art of modeling the kinematics and dynamics of a space robot, operating in the free-flying and free-floating modes, has been well studied by researchers. However, these two modes of operation have various shortcomings, which can be overcome by operating the space robot in the controlled-floating mode. This tutorial article aims to address the knowledge gap in modeling complex space robots operating in the controlled-floating mode and under perturbed conditions. The novel research contribution of this article is the refined dynamic model of a chaser space robot, derived with respect to the moving target while accounting for the internal perturbations due to constantly changing the center of mass, the inertial matrix, Coriolis, and centrifugal terms of the coupled system; it also accounts for the external environmental disturbances. The nonlinear model presented accurately represents the multibody coupled dynamics of a space robot, which is pivotal for precise pose control. Simulation results presented demonstrate the accuracy of the model for closed-loop control. In addition to the theoretical contributions in mathematical modeling, this article also offers a commercially viable solution for a wide range of in-orbit missions

Linear controllers for free-flying and controlled-floating space robots: a new perspective

Autonomous space robots are crucial for performing future in-orbit operations, including servicing of a spacecraft, assembly of large structures, maintenance of other space assets and active debris removal. Such orbital missions require servicer spacecraft equipped with one or more dexterous manipulators. However, unlike its terrestrial counterpart, the base of the robotic manipulator is not fixed in inertial space; instead, it is mounted on the base�spacecraft, which itself possess both translational and rotational motions. Additionally, the system will be subjected to extreme environmental perturbations, parametric uncertainties and system constraints due to the dynamic coupling between the manipulator and the base-spacecraft. This paper presents the dynamic model of the space robot and a three�stage control algorithm for this highly dynamic non-linear system. In this approach, feed�forward compensation and feed-forward linearization techniques are used to decouple and linearize the highly non-linear system respectively. This approach allows the use of the linear Proportional-Integral-Derivative (PID) controller and Linear Quadratic Regulator (LQR) in the final stages. Moreover, this paper covers a simulation-based trade-off analysis to determine both proposed linear controllers’ efficacy. This assessment considers precise trajectory tracking requirements whilst minimizing power consumption and improving robustness during the close-range operation with the target spacecraft

Downsizing an orbital space robot: A dynamic system based evaluation

Small space robots have the potential to revolutionise space exploration by facilitating the on-orbit assembly of infrastructure, in shorter time scales, at reduced costs. Their commercial appeal will be further improved if such a system is also capable of performing on-orbit servicing missions, in line with the current drive to limit space debris and prolong the lifetime of satellites already in orbit. Whilst there have been a limited number of successful demonstrations of technologies capable of these on-orbit operations, the systems remain large and bespoke. The recent surge in small satellite technologies is changing the economics of space and in the near future, downsizing a space robot might become be a viable option with a host of benefits. This industry wide shift means some of the technologies for use with a downsized space robot, such as power and communication subsystems, now exist. However, there are still dynamic and control issues that need to be overcome before a downsized space robot can be capable of undertaking useful missions. This paper first outlines these issues, before analyzing the effect of downsizing a system on its operational capability. Therefore presenting the smallest controllable system such that the benefits of a small space robot can be achieved with current technologies. The sizing of the base spacecraft and manipulator are addressed here. The design presented consists of a 3 link, 6 degrees of freedom robotic manipulator mounted on a 12U form factor satellite. The feasibility of this 12U space robot was evaluated in simulation and the in-depth results presented here support the hypothesis that a small space robot is a viable solution for in-orbit operations

Precise Motion Control of a Space Robot for In-Orbit Close Proximity Manoeuvres

Space robotic solutions are gaining importance for undertaking in-orbit operations such as maintenance and repair, assembly of large structures, manufacturing and debris removal. The modelling and control of space robots is highly challenging due to (i) the inherent nonlinearities in the system, (ii) the dynamic coupling between the arm and the spacecraft base and (iii) the complex structure of two coupled systems i.e. a six Degree of Freedom (DoF) spacecraft base and an n DoF robotic arm. In addition to the aforementioned challenges, performing a precise motion of a space robot in the presence of environmental disturbances whilst considering the changes in the mass of the spacecraft base due to fuel consumption, is very intricate. Taking into account the above-mentioned challenges, this research is aimed at developing new control methodologies for precise manoeuvring of a space robot to safely capture a target in-orbit. Performing such fine motion control requires high precision manoeuvres by a space robot capable of tracking the grasping point on the target without a priori knowledge of the path to follow, whilst avoiding collisions and singularities. This research introduces a new mode of operation for space robots, defined as the controlled-floating mode. It allows the base of the space robot to move, in a controlled manner, simultaneously and in coordination with the arm to help reach the grasping point through following optimal trajectories for both the arm and its base. Unlike the classical free-flying and free-floating modes of operation, the controlled- floating mode offers extra DoFs, redundancy and unlimited workspace to the robotic arm of the space robot. The space robot, when operated in this mode, is hereafter referred to as the Controlled Floating Space Robot (CFSR). To control the motion of the CFSR, a new adaptive combined nonlinear Hinf controller was designed; it takes into account both external disturbances and internal parametric uncertainties due to the changes in the mass of the spacecraft base. This controller guarantees robustness when compared to the traditional linear controllers, such as the Proportional-Integral-Derivative controller and the Linear Quadratic Regulator. Approaching the target when the grasping point is out of its reach or when the motion of the arm is restricted by singular configurations and obstacles, is a difficult task using the arm's n DoFs only. Hence, in this research, an optimal trajectory generator for both the arm and its base, using a Genetic Algorithm, was developed. This novel algorithm ensures that the selected path is free of singularities and obstacles whilst using minimal energy. This algorithm requires only the Cartesian location of the grasping point, to generate a path for the space robot without a priori knowledge of the desired path

Optimised collision-free trajectory and controller design for robotic manipulators

Path planning and collision avoidance are two crucial interconnected algorithms used to perform desired tasks for both fixed and moving base manipulators. In this paper, the collision-free trajectory generation algorithm presented for capturing a stationary target is a newer form of the state-of-the-art method using cycloids. It is further optimised to minimise the distance between the obstacle and the manipulator. A control algorithm using the Computed Torque Control to accurately track the pre-designed trajectory is discussed. The simulations results presented in this paper verify the efficacy and robustness of the controller based on the non-linear dynamical model of the robotic manipulator. The optimised collision-free trajectory resulted in smooth joint displacement, velocity and acceleration. Moreover, the algorithm presented can be applied to ground based and space based stationary or moving robotic manipulators

Collision-free optimal trajectory generation for a space robot using genetic algorithm

Future on-orbit servicing and assembly missions will require space robots capable of manoeuvring safely around their target. Several challenges arise when modelling, controlling and planning the motion of such systems, therefore, new methodologies are required. A safe approach towards the grasping point implies that the space robot must be able to use the additional degrees of freedom offered by the spacecraft base to aid the arm attain the target and avoid collisions and singularities. The controlled-floating space robot possesses this particularity of motion and will be utilised in this paper to design an optimal path generator. The path generator, based on a Genetic Algorithm, takes advantage of the dynamic coupling effect and the controlled motion of the spacecraft base to safely attain the target. It aims to minimise several objectives whilst satisfying multiple constraints. The key feature of this new path generator is that it requires only the Cartesian position of the point to grasp as an input, without prior knowledge a desired path. The results presented originate from the trajectory tracking using a nonlinear adaptiv

Downsizing an Orbital Space Robot: A Dynamic System Based Evaluation

Small space robots have the potential to revolutionise space exploration by facilitating the on-orbit assembly of infrastructure, in shorter time scales, at reduced costs. Their commercial appeal will be further improved if such a system is also capable of performing on-orbit servicing missions, in line with the current drive to limit space debris and prolong the lifetime of satellites already in orbit. Whilst there have been a limited number of successful demonstrations of technologies capable of these on-orbit operations, the systems remain large and bespoke. The recent surge in small satellite technologies is changing the economics of space and in the near future, downsizing a space robot might become be a viable option with a host of benets. This industry wide shift means some of the technologies for use with a downsized space robot, such as power and communication subsystems, now exist. However, there are still dynamic and control issues that need to be overcome before a downsized space robot can be capable of undertaking useful missions. This paper rst outlines these issues, before analyzing the effect of downsizing a system on its operational capability. Therefore presenting the smallest controllable system such that the benefits of a small space robot can be achieved with current technologies. The sizing of the base spacecraft and manipulator are addressed here. The design presented consists of a 3 link, 6 degrees of freedom robotic manipulator mounted on a 12U form factor satellite. The feasibility of this 12U space robot was evaluated in simulation and the in-depth results presented here support the hypothesis that a small space robot is a viable solution for in-orbit operations. Keywords: Small Satellite; Space Robot; In-orbit Assembly and Servicing; In-orbit operations; Free-Flying; Free-Floating