Free-flying Space Robot Research Articles

Energy-optimal adaptive artificial potential field method for path planning of free-flying space robots

Aiming at planning a collision-free path for a free-flying space robot (FFSR), an energy-optimal collision avoidance strategy is proposed using the adaptive artificial potential field (AAPF) approach. To address the goals non-reachable with obstacles nearby (GNRON) issues of artificial potential field (APF), an adaptive and secure path planning method is proposed by incorporating a smooth decay function into the repulsive potential field. Meanwhile, a time-varying shaping parameter of attractive potential field is optimized combined with the state-dependent Riccati equation technique to reduce the energy cost of the FFSR. In case the system gets trapped in local minimum, an escape force is elaborately designed to replace artificial potential force based on optimal theory, whose direction vector guides the FFSR to jump out of the local minimum with minimal energy. Finally, simulation results on a planar FFSR demonstrate the effectiveness of the proposed methodology and show improved performance compared with the existing artificial potential field (APF) in terms of escaping local minimum and reducing energy consumption.

Trajectory optimization and tracking control of free-flying space robots for capturing non-cooperative tumbling objects

This paper investigates the problem of capturing non-cooperative tumbling objects by free-flying space robots. To solve the two challenges of task constraints and base-manipulator coupling, a pseudospectral method based trajectory optimization and a reinforcement learning based tracking control are proposed for the free-flying space robots. Multiple constrains, including dynamics, field of view and obstacle avoidance, are considered in trajectory optimization. The adaptive segmented Radau pseudospectral method is used to discretize the energy-optimal trajectory problem into a nonlinear programming problem. By adaptively dividing the global time interval into multiple subintervals, higher-order interpolation polynomials are avoided. A reinforcement learning based parameter tuning method is proposed for the base controller to suppress the reaction torque caused by the manipulator. Numerical simulations and experiments on air-bearing testbed verify the effectiveness of our methods in terms of planning efficiency and tracking precision.

Design and Dynamic Control: A Free-Flying Space Robot Inspired by Water Striders.

This work designed a free-flying space robot (FFSR) that simulates the on-orbit assembly of large space telescopes, drawing inspiration from the flexible movement of water striders on water surfaces. Initially, we developed the system structure of the robot, including the corresponding air-floating ground simulation system. This system enables floating movement of the robot in a gravity-free environment through the utilization of planar air bearings. Subsequently, we established the kinematics and dynamics models for the FFSR. Following that, we propose a novel adaptive boundary layer fuzzy sliding mode control (ABLFSMC) method to achieve trajectory tracking control of the FFSR. The virtual angle and angular velocity are formulated to serve as references for the angle and angular velocity in the body coordinate system. Furthermore, a fuzzy logic system is employed to minimize the chattering effect of the sliding mode control. The global stability of the proposed controller is guaranteed through the Lyapunov stability theory. Finally, we validate the effectiveness of the proposed control method as well as the high trajectory tracking accuracy of the developed FFSR through simulation and experimental results, respectively. Overall, our findings present a crucial experimental platform and development opportunity for the ground-based validation of technologies concerning the on-orbit assembly of large space telescopes.

Integrated Dynamics of Space Rigid-Flex Combination System with Time-Varying Configuration

Dual numbers were applied to the dynamics of a rigid-flexible combination system (RFCS) with time-varying configuration in this paper. The six-dimensional spinor form of the motion of flexible modules, including the dual vector, dual momentum, dual inertia operator, dual coupling coefficient operator, and dual-modal coordinates, was derived using the dual numbers that could represent spiral motion in a compact form. On this basis, the integrated dynamic model of a rigid-flexible combination system with a time-varying configuration was proposed. And then, the relative dynamics equations between two rigid-flexible combination systems which both have time-varying configuration were provided. An on-orbit assembly mission of flexible modules transported and operated by free-flying space robots (FFSRs) is presented as an exemplary application of relative dynamics. Simulation results illustrate the complex coupling effects on the relative motion between two rigid-flex combination systems with time-varying configuration.

Development and evaluation of a space robot prototype equipped with a cable-driven manipulator

This research focuses on developing and evaluating a free-flying space robot equipped with a cable-driven manipulator and an adaptive gripper. First, a transmission joint comprising a pair of gears and pulleys is designed and actuated by motors located in the first link of the manipulator. Afterward, based on the kinematics and dynamics models of the robot, a mission distribution control scheme is proposed for the robot to drive its end-effector to achieve the desired position and attitude for capturing space targets. Furthermore, exploiting this strategy, considerable comparison simulations between two space robots equipped with a traditional manipulator and a cable-driven arm are conducted. Wherein, the goal position and attitude are selected through a uniform sampling method. For the same capture mission, energy consumptions of the base and manipulator as well as state errors of the base are taken as cost functions for performance evaluation. Finally, a planar target capture case study using the adaptive gripper and a spatial motion test with the help of an air-bearing simulator are carried out to demonstrate the efficiency of the presented cable-driven space robot.

Attenuation of Target Satellite's Rotational Motion by a Free-Flying Space Robot with Nonlinear Model Predictive Control

Attenuation of Target Satellite's Rotational Motion by a Free-Flying Space Robot with Nonlinear Model Predictive Control

A fault-tolerant and robust controller using model predictive path integral control for free-flying space robots.

The use of manipulators in space missions has become popular, as their applications can be extended to various space missions such as on-orbit servicing, assembly, and debris removal. Due to space reachability limitations, such robots must accomplish their tasks in space autonomously and under severe operating conditions such as the occurrence of faults or uncertainties. For robots and manipulators used in space missions, this paper provides a unique, robust control technique based on Model Predictive Path Integral Control (MPPI). The proposed algorithm, named Planner-Estimator MPPI (PE-MPPI), comprises a planner and an estimator. The planner controls a system, while the estimator modifies the system parameters in the case of parameter uncertainties. The performance of the proposed controller is investigated under parameter uncertainties and system component failure in the pre-capture phase of the debris removal mission. Simulation results confirm the superior performance of PE-MPPI against vanilla MPPI.

Testing Gecko-Inspired Adhesives With Astrobee Aboard the International Space Station: Readying the Technology for Space

Gecko-inspired adhesives can allow free-flying space robots to grasp and manipulate large items or anchor themselves on smooth surfaces. In this article, we report on the first tests conducted using gecko-inspired adhesives on a gripper attached to an Astrobee free-flying robot operating inside the International Space Station (ISS). We present results from on-ground testing as well as two on-orbit sessions conducted during early 2021. Recorded data demonstrated that an adhesive gripper for Astrobee could provide 3.15 N of force for manual perching. The adhesives functioned as anticipated, despite a lengthy storage period. The results also highlighted some considerations for future adhesive gripping in space with free-flying robots. We discuss these topics along with system design considerations for successful implementation aboard the ISS, raising the readiness of this technology for a space-grade environment.

Motion planning of a free-flying space robot system under end effector task constraints

This study focuses on the motion planning of a free-flying space robot system with non-conserved linear and angular momentums due to thrusts acting on its base. The system model is established at the kinematic level based on the momentum theorem, which takes into account the change in base mass. In particular, a motion planning strategy is proposed to perform trajectory tracking of the end effector for the free-flying space robot. The thrusts and torque acting on base satellite are not predefined, but are taken as system control inputs to be optimized, as are joint accelerations. Therefore, the synchronous motion planning of the base satellite and manipulator is formulated as an open-loop optimization problem, which is subject to physical constraints such as thrust and torque saturation, joint motion limitations, and collision avoidance, etc. Finally, the system under different task constraints of end effectors is numerically simulated to demonstrate the effectiveness of the proposed method.

Nonholonomic Reorientation of Free-Flying Space Robots Using Parallelogram Actuation in Joint Space

A robot with large-degree-of-freedom joints is a promising future space robot capable of adapting to various environments and can perform dexterous tasks with multiple manipulators. The attitude dynamics of a free-flying robot shows nonholonomy, which enables the robot to reorient its attitude by moving its body. However, previous studies were not generally able to handle the nonholonomy, especially for robots with large degrees of freedom of joints. In the present study, we analytically investigate a maneuver in which joints are actuated along a parallelogram trajectory in joint angle space and propose an analytical path modification method in joint angle space in order to achieve the target attitude. Parallelogram actuation guarantees that the body configuration is returned to the initial state after one set of maneuvers, and thus the attitude of the robot is independently reoriented to the target maintaining the body configuration. The analytical solution is provided by applying Magnus expansion to the kinematics equation of rotational matrices, and its Lie group structure contributes to concise mathematical expressions. In addition, the analytical solution does not cause numerical difficulties, such as combinatorial explosion, and so can fully make use of the reorientation ability of a robot with large degrees of freedom.

Force control of a space robot in on-orbit servicing operations

Force control provided by a small-scaled free-flying space robot on a floating or spinning target will be required for future on-orbit servicing missions, such as detumbling a target, performing an assembly or a maintenance operation. Regarding to different contact geometries for a space robot interacting with a spinning target, this paper addresses the contact model considering its multi-dimensional characteristics. A hybrid motion and force controller is developed to acquire desired contact forces and space robot configuration despite the floating feature of the system. On this basis, the control strategy to implement the operations of three typical phases, namely approaching phase, contact phase and post-contact phase, is proposed. The simulations have been performed to verify the control approaches and visualize the contact scenarios.

Approximate Analytical Solution for Attitude Motion of a Free-flying Space Robot and Analysis of Its Nonholonomic Properties

Approximate Analytical Solution for Attitude Motion of a Free-flying Space Robot and Analysis of Its Nonholonomic Properties

Guidance and Digital Attitude Control of Mini-satellites and Space Robots

Abstract The information mini-satellites (for communication, geodesy, radio- and optoelectronic Earth-observing et al.) and the free-flying space robots (used for servicing both the mini-satellites and manned orbital space stations as well as assembling the large space structures) have some principal problems with respect to their spatial attitude guidance and control. In the paper these problems are considered and obtained results are presented.

Adaptive trajectory tracking control of a free-flying space robot subject to input nonlinearities

The input nonlinearities widely exist in a large range of mechanical systems. Nonetheless, they were relatively less considered in the trajectory tracking control of a space robot in the previous studies. In this paper, an adaptive neural network (NN) control method is proposed for the fast and exact trajectory tracking control of an attitude-controlled free-flying space robot subject to input nonlinearities. The parametric uncertainties and external disturbances are also taken into the consideration. First, a model-based controller is designed to track the desired trajectory of the space robot within a framework of backstepping technique. Then, an adaptive NN controller is designed by using two NNs to compensate for the lumped uncertainties caused by parametric uncertainties and external disturbances and the input nonlinearities, respectively. Rigorous theoretical analysis for the semiglobal uniform ultimate boundedness of the whole closed-loop system is provided. The proposed adaptive NN controller is structurally simple and model-independent, which makes the controller affordable for practical applications. In addition, the proposed adaptive NN controller can guarantee the position and velocity tracking errors converge to the small neighborhoods about zero even in the presence of parametric uncertainties, external disturbances, and input nonlinearities. To the best of the authors’ knowledge, there are really limited existing controllers can achieve such excellent performance in the same conditions. Numerical simulations illustrate the effectiveness and superiority of the proposed control method.

On the Accuracy of Inertial Parameter Estimation of a Free-Flying Robot While Grasping an Object

When model-based controllers are used for carrying out precise tasks, the estimation of model parameters is key for a better trajectory tracking performance. We consider the scenario of a free-flying space robot with limited actuation that has grasped an object of unknown properties. The inertial parameters - mass, centre of mass and the inertia tensor - of the robot-object system are to be determined. In our previous works, we used a parameter estimation algorithm where truncated Fourier series were used to represent both the reference excitation trajectory and the executed one. That algorithm is the focus of this paper, along with a study of the factors that could contribute to a loss of accuracy in the obtained estimates. Simulation results with the Space CoBot free-flyer robot are used to find the causes of the latter: first, a modified version of the minimum condition number criterion is used to generate feasible excitatory trajectories. The results are compared with those given by the maximum information criterion to check for better excitation. Next, an appropriate number of harmonics has to be found for the Fourier series, which will be used to fit the measured data. This is done autonomously, based on the least squares residual. Finally, the relation between input saturation while executing the excitation trajectory and the errors in the resulting parameter estimates is studied.

A Robust and Adaptive Control Method for Flexible-Joint Manipulator Capturing a Tumbling Satellite

A space manipulator needs to exhibit high stiffness for tracking, and good compliance for capturing a tumbling satellite. However, the elasticity caused by harmonic drives challenges both the high-precision motion and the flexibility of the manipulator. In this paper, we propose a novel control method based on a robust torque-tracking controller to eliminate the unexpected vibration resulting from elasticity and reject the perturbation caused by nonlinear friction of the motor. In order to achieve the high-precision motion of the manipulator with uncertain dynamics, we combined the method with an improved adaptive method. The position-based impedance control, taking both the translational and rotational impedance into account, is added to achieve the soft-capture of a tumbling satellite. The stability of the proposed method has been strictly proven by the Lyapunov method. The space manipulator is mounted on a controlled chaser, forming a free-flying space robot. A loose coordinated control between the chaser and the manipulator is adopted in the task simulation. The simulations for verifying controller performance and the task simulation reveal that the manipulator has high-precision motion performance in the pre-capture phase, achieves soft-capture in the capturing phase and offers a reliable connection between the chaser and the target in the de-tumbling phase.

Hybrid Control of Space Robot in On-Orbit Screw-Driving Operation

Controlled force–torque exertion by a small-scaled free-flying space robot will be potentially required for future on-orbit assembly missions. This paper presents a hybrid controller to achieve the desired end-effector motion and contact forces required for driving a screw into a floating target. Attitude of the spacecraft base is controlled simultaneously. Postcapture control to stabilize the entire system is also addressed in this paper. Simulation results have demonstrated the effectiveness of the proposed control laws.

Maneuver and Active Vibration Suppression of Free-flying Space Robot

This work studies the maneuver control and vibration suppression of a flexible free-flying space robot using variable-speed control moment gyros as actuators. A novel flexible space manipulator is designed. The dynamics of the flexible multibody system is derived by using Kane method. Based on the singular perturbation approach, the dynamics of the flexible manipulator is decoupled into a slow subsystem and a fast subsystem. The slow subsystem is associated with the rigid motion dynamics, and the fast subsystem is related to the link flexible dynamics. A composite control strategy is proposed as a combination of two controllers for these subsystems. An adaptive sliding mode controller is designed for the slow subsystem, and an adaptive controller is designed for the fast subsystem. Uncertainty estimation can be achieved by the adaptive terms of the composite controller. A weighted robust pseudo-inverse steering law is proposed for the variable-speed control moment gyros. Numerical results demonstrate that the proposed composite controller is robust to parameter uncertainties and external disturbances.

Robust Adaptive Coordination Controller for a Spacecraft Equipped with a Robotic Manipulator

Free-flying robotic spacecraft play a significant role in the space industry today. Unlike ground-based robots, the manipulator motion in a space robot can cause undesirable disturbances to the spacecraft platform, causing its attitude to change, potentially disrupting communication and solar energy collection processes as a result. Thus, coordinated control of both the spacecraft attitude and the manipulator motion become essential for successful space operations. Though past research has developed dynamic models for spacecraft manipulators, the contribution of reaction wheels to the angular momentum of the entire system needs further consideration. This paper reformulates the dynamic equations of a free-flying space robot by taking the aspect of reaction wheels into account. A diagonalization method is introduced to decouple the highly nonlinear multi-input/multi-output system model. A novel adaptive variable structure control method is then applied to implement a robust coordination controller for the space robot subjected to system uncertainties. Simulations are carried out on a spacecraft model with a 3-degree-of-freedom manipulator mounted on it to demonstrate the robustness of the proposed approach. Comparisons with sliding mode control show that the new controller results in faster settling times, which leads to maintaining uninterrupted communication links and efficient solar energy harvesting.

Reactionless camera inspection with a free-flying space robot under reaction null-space motion control

The possibility of implementing reactionless motion control w.r.t. base orientation of a free-flying space robot in practical tasks is addressed. It is shown that such possibility depends strongly on the kinematic/dynamic design parameters as well as on the mission task. A successful implementation of a camera inspection task is reported. The presence of kinematic redundancy and the manipulator attachment position are shown to play important roles. More specifically, for a manipulator arm with a typical seven degree-of-freedom (DoF) kinematic structure, it is shown that two motion patterns, wrist reorientation and folding/unfolding of the arm, result in almost reactionless motion. The orientation pattern is adopted as the main task for camera inspection, while the remaining four DoFs are used to ensure complete reactionless motion and to minimize the position errors. Since the composition of these tasks introduces the so-called algorithmic singularities, two methods are suggested to alleviate the problem. Furthermore, it is shown that other types of singularities may also be introduced in case of an inappropriate choice of the manipulator attachment position. At the end, numerical analysis is performed to show that reactionless motion provides an advantage in terms of kinetic energy as well.