Free-flying Robot Research Articles

AstrobeeCD: Change detection in microgravity with free-flying robots

This work presents AstrobeeCD, a system for 3D scene change detection toward near-real-time environmental awareness of space outposts using the Astrobee free-flying robot in microgravity. Assistive free-flyer robots autonomously caring for future crewed space habitats must be able to detect day-to-day interior changes to track inventory, detect and diagnose faults, and monitor the outpost status. A set of image and depth data from one time step is used to reconstruct a 3D model of the environment. The 3D model is used as the basis for comparison for free-flyer environment surveys at future time steps, where an image-based change detection algorithm identifies inconsistencies against the 3D model. Change detection is demonstrated using real image and pose data collected by an Astrobee robot in a test environment on Earth at NASA Ames Research Center and from microgravity aboard the International Space Station. Change detection computation time and performance are quantitatively evaluated on the test data captured on Earth, and it identifies scene changes more quickly than a point cloud clustering-based algorithm applied to data from the same surveys.

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.

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.

Robust Attitude Controller for NASA Astrobee Robots Operating in the ISS

Free-flying robots have been recently developed to operate on-board the International Space Station (ISS) as semi-autonomous robotic assistants. NASA Astrobee robots are the new free-flyers proposed by NASA, and equipped with a 3 degree of freedom manipulator. This research aims to realize an accurate mathematical model of the robot attitude dynamics, and an attitude controller for the NASA Astrobee system. The robot has been modeled coupling the attitude dynamics with the manipulator motion, and robust attitude controllers are proposed to stabilize disturbances and configuration changes induced by the manipulator usage. First, a second-order twisting sliding mode controller is designed to achieve a trade-off among control law flexibility, robustness and accuracy. Among robust control strategies, sliding mode controllers are characterized as low complexity, and low computational cost control methods, making the system able to achieve reactivity and stability also when parameters are changed. The performance of the proposed control method are compared with a backstepping controller that include an iterative algorithm to compute an adaptive control, as function of disturbances and orientation error. The performance are evaluated and analyzed considering several scenarios in Matlab Simulink simulations, and combining Simulink and ROS Gazebo to run co-simulations with the NASA Astrobee simulator. Simulations with variable body mass, links masses, and different gripped objects have been run to test the controllers performance in off-design conditions.

Concept of the Ring Enclosing Docking System for Spherical Shaped Free-Flying Robot

Concept of the Ring Enclosing Docking System for Spherical Shaped Free-Flying Robot

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.

Active Fault-tolerant Gyromoment Control of Satellites and Free-flying Robots

Abstract The methods for modeling and detection of anomalous in the automatic control systems, and also practical methods for active analysis of anomalous situations, are presented. We develop algorithms for consecutive classification of onboard equipment failures and reconfiguration of control loop. The simulation results are presented for the attitude control systems of the information satellites and space robots.

自由飞行机器人气浮式模拟器设计

自由飞行机器人气浮式模拟器设计

Path Planning for Rigid Bodies Using Burs of Free C-Space

Abstract This paper presents a path planning algorithm designed for rigid, free-flying robots. The algorithm is based on recently proposed structure called bur of free C-space. The bur of free C-space was originally developed to improve the exploration of configuration spaces for articulated robots, which further enabled efficient and fast path planning. In this paper, we extend the notion of bur for configuration spaces of rigid body robots that move in 2D/3D spaces, i.e., for Lie groups SE(2) and SE(3). Such defined bur is then used within a suitable RRT-like algorithm. The numerical study shows measurable performance improvements with respect to classical path planning methods.

Handrail detection and pose estimation for a free-flying robot

We present a handrail detection and pose estimation algorithm for the free-flying Astrobee robots that will operate inside the International Space Station. The Astrobee will be equipped with a single time-of-flight depth sensor and a compliant perching arm to grab the International Space Station handrails. Autonomous perching enables a free-flying robot to minimize power consumption by holding its position without using propulsion. Astrobee is a small robot with many competing demands on its computing, power, and volume resources. Therefore, for perching, we were limited to using a single compact sensor and a lightweight detection algorithm. Moreover, the handrails on the International Space Station are surrounded by various instruments and cables, and the lighting conditions change significantly depending on the light sources, time, and robot location. The proposed algorithm uses a time-of-flight depth sensor for handrail perception under varying lighting conditions and utilizes the geometric characteristics of the handrails for robust detection and pose estimation. We demonstrate the robustness and accuracy of the algorithm in various environment scenarios.

Control of a Free-flying Robot at Preparation for Capturing a Passive Space Vehicle

Abstract Methods for guidance and control of a space robot-manipulator at its approach to a non-cooperative space vehicle on a sun-synchronous orbit in conditions of uncertainty and incompleteness of measurements are considered. An operational strategy to estimate the resources needed for onboard devices of the robots attitude and orbit control system is discussed.

Designing planning and control interfaces to support user collaboration with flying robots

Robots are becoming increasingly prevalent and are already providing assistance in a variety of activities, ranging from space exploration to domestic housework. Recent advances in the design of sensors, motors, and microelectromechanical systems have enabled the development of a new class of small aerial robots. These free-flying robots hold great promise in assisting humans by acting as mobile sensor platforms to collect data in areas that are difficult to access or infeasible to instrument. In this work, we explored the design of interfaces that support users in working with free-flying robots to accomplish tasks including inventory logistics and management, environmental data collection, and visual inspection. Extending prior work in control interfaces for ground robots, we conducted a formative study in order to identify key design requirements for free-flyer interfaces. We designed several realistic tasks for use in evaluating human–robot interaction within the context of indoor free-flyer operation. We implemented three prototype interfaces that each provide varying degrees of support in enabling remote users to work with a flying robot to plan, communicate goals, accomplish tasks, and respond to changes in a dynamic environment. An experimental evaluation of each interface found that the interface designed to support collaborative planning and replanning using an interactive timeline and three-dimensional spatial waypoints significantly improved users’ efficiency in accomplishing tasks, their ability to intervene in response to spontaneous changes in task demands, and their ratings of the robot as a teammate compared to interfaces that support low-level teleoperation or waypoint-based supervisory control. Our results demonstrate the utility of a data-driven design process and show the need for free-flyer interfaces to consider planning phases in addition to task execution. In addition, we demonstrate the importance of providing interface support for interrupting robot operations as unplanned events arise.

Adaptive Jacobian force/position tracking for space free-flying robots with prescribed transient performance

This paper presents a tracking control with guaranteed prescribed performance (PP) for space free-flying robots with uncertain kinematics (Jacobian matrix) and dynamics, uncertain normal force parameter, and bounded disturbances in a compliant contact with a planar surface. Given the orientation of the surface and a nonlinear model of the elastic force, a controller is designed requiring no information on the robot parameters and the disturbances. This controller will guarantee that the tracking errors satisfy PP indexes such as the maximum steady-state errors and overshoots, and the minimum convergence rates. Thus, contact maintenance can be ensured as prescribed. An approximation of the Jacobian is utilized in the presence of uncertain robot kinematics, and PP position/attitude tracking of the free-flying base is achieved in addition to the PP force/position tracking of the manipulator’s fingertip. The proposed controller is based on an error transformation technique, and a directly tunable gain for the transformed error feedback is introduced in the control to trade off between the tracking performance and control effort. Numerical simulations and comparisons demonstrate the effectiveness and superiority of the proposed controller.

Adaptive hybrid suppression control of space free-flying robots with flexible appendages

SUMMARYControl of rigid–flexible multi-body systems in space, during cooperative manipulation tasks, is studied in this paper. During such tasks, flexible members such as solar panels may vibrate. These vibrations in turn can lead to oscillatory disturbing forces on other subsystems, and consequently may produce significant errors in the position of operating end-effectors of cooperative arms. Therefore, to design and implement efficient model-based controllers for such complicated nonlinear systems, deriving an accurate dynamics model is required. On the other hand, due to practical limitations and real-time implementation, such models should demand fairly low computational complexity. In this paper, a precise dynamics model is derived by virtually partitioning the system into two rigid and flexible portions. These two portions will be assembled together to generate a proper model for controller design. Then, an adaptive hybrid suppression control (AHSC) algorithm is developed based on an appropriate variation rule of a virtual damping parameter. Finally, as a practical case study a space free-flying robot (SFFR) with flexible appendages is considered to move an object along a desired path through accurate force exertion by several cooperative end-effectors. This system includes a main rigid body equipped with thrusters, two solar panels, and two cooperative manipulators. The system also includes a third and fourth arm that act as a communication antenna and a photo capturing camera, respectively. The maneuver is deliberately planned such that flexible modes of solar panels get stimulated due to arms motion, while obtained results reveal the merits of proposed controller as will be discussed.

On Grasping a Tumbling Debris Object with a Free-Flying Robot

The grasping and stabilization of a tumbling, non-cooperative target satellite by means of a free-flying robot is a challenging control problem, which has been addressed in increasing degree of complexity since 20 years. A novel method for computing robot trajectories for grasping a tumbling target is presented. The problem is solved as a motion planning problem with nonlinear optimization. The resulting solution includes a first maneuver of the Servicer satellite which carries the robot arm, taking account of typical satellite control inputs. An analysis of the characteristics of the motion of a grasping point on a tumbling body is used to motivate this grasping method, which is argued to be useful for grasping targets of larger size.

Attitude Guidance and Control of Mini-satellites and Free-flying Robots

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

Inexact Restoration for Euler Discretization of Box-Constrained Optimal Control Problems

The Inexact Restoration method for Euler discretization of state and control constrained optimal control problems is studied. Convergence of the discretized (finite-dimensional optimization) problem to an approximate solution using the Inexact Restoration method and convergence of the approximate solution to a continuous-time solution of the original problem are established. It is proved that a sufficient condition for convergence of the Inexact Restoration method is guaranteed to hold for the constrained optimal control problem. Numerical experiments employing the modelling language AMPL and optimization software Ipopt are carried out to illustrate the robustness of the Inexact Restoration method by means of two computationally challenging optimal control problems, one involving a container crane and the other a free-flying robot. The experiments interestingly demonstrate that one might be better-off using Ipopt as part of the Inexact Restoration method (in its subproblems) rather than using Ipopt directly on its own.

Unified Modeling Approach of Kinematics, Dynamics and Control of a Free-Flying Space Robot Interacting with a Target Satellite

In this paper a unified control-oriented modeling approach is proposed to deal with the kinematics, linear and angular momentum, contact constraints and dynamics of a free-flying space robot interacting with a target satellite. This developed approach combines the dynamics of both systems in one structure along with holonomic and nonholonomic constraints in a single framework. Furthermore, this modeling allows consid-ering the generalized contact forces between the space robot end-effecter and the target satellite as internal forces rather than external forces. As a result of this approach, linear and angular momentum will form holonomic and nonholonomic constraints, respectively. Meanwhile, restricting the motion of the space robot end-effector on the surface of the target satellite will impose geometric constraints. The proposed momentum of the combined system under consideration is a generalization of the momentum model of a free-flying space robot. Based on this unified model, three reduced models are developed. The first reduced dynamics can be considered as a generalization of a free-flying robot without contact with a target satellite. In this re-duced model it is found that the Jacobian and inertia matrices can be considered as an extension of those of a free-flying space robot. Since control of the base attitude rather than its translation is preferred in certain cases, a second reduced model is obtained by eliminating the base linear motion dynamics. For the purpose of the controller development, a third reduced-order dynamical model is then obtained by finding a common solution of all constraints using the concept of orthogonal projection matrices. The objective of this approach is to design a controller to track motion trajectory while regulating the force interaction between the space robot and the target satellite. Many space missions can benefit from such a modeling system, for example, autonomous docking of satellites, rescuing satellites, and satellite servicing, where it is vital to limit the con-tact force during the robotic operation. Moreover, Inverse dynamics and adaptive inverse dynamics control-lers are designed to achieve the control objectives. Both controllers are found to be effective to meet the specifications and to overcome the un-actuation of the target satellite. Finally, simulation is demonstrated by to verify the analytical results.

Reduction of Control Inputs for Multiple Mobile Robots

AbstractThis paper is concerned with control problems of multiple mobile robots such as wheeled mobile robots defined on Se(2) or free-flying robots defined on SO(3). If each robot were supposed to be controlled independently, the total number of control inputs would be the number of individual inputs times the number of robots. In this paper, we show that it is possible to reduce the number of control inputs where the whole system remains controllable by imposing artificial kinematic constraints between the robots, e.g., two wheeled mobile robots can be controllable with only 2 or 3 inputs. We also discuss what happens if we try to make the multiple robots track the reference trajectories.

Novel reaction control techniques for redundant space manipulators: Theory and simulated microgravity tests

This paper presents two novel redundancy resolution schemes aimed at locally minimizing the reaction torque transferred to the spacecraft during manipulator manoeuvres. The subject is of particular interest in space robotics because reduced reactions result in reduced energy consumption and longer operating life of the attitude control system. The first presented solution is based on a weighted Jacobian pseudoinverse and is derived by using Lagrangian multipliers. The weight matrix is defined by means of the inertia matrix which appears in the spacecraft reaction torque dynamics. The second one is based on a least squares formulation of the minimization problem. In this formulation the linearity of the forward kinematics and of the reaction torque dynamics equations with respect to the joint accelerations is used. A closed-form solution is derived for both the presented methods, and their equivalence is proven analytically. Moreover, the proposed solutions, which are suitable for real-time implementation, are extended in order to take into account the physical limits of the manipulator joints directly inside the solution algorithms. A software simulator has been developed in order to simulate the performance of the presented solutions for the selected test cases. The proposed solutions have then been experimentally tested using a 3D free-flying robot previously tested in an ESA parabolic flight campaign. In the test campaign the 3D robot has been converted in a 2D robot thanks to its modularity in order to perform planar tests, in which the microgravity environment can be simulated without time constraints. Air-bearings are used to sustain the links weight, and a dynamometer is used to measure the reaction torque. The experimental validation of the presented inverse kinematics solutions, with an insight on the effect of joint flexibility on their performance, has been carried out, and the experimental results confirmed the good performance of the proposed methods. In particular, two test cases have been analyzed in order to validate and evaluate the performance of both the unconstrained solution and the solution which takes into account the robot physical limits.