Free-floating Robot Research Articles

Probing Hydrodynamic Fluctuation-Induced Forces with an Oscillating Robot.

We study the dynamics of an oscillating, free-floating robot that generates radially expanding gravity-capillary waves at a fluid surface. In open water, the device does not self-propel; near a rigid boundary, it can be attracted or repelled. Visualization of the wave field dynamics reveals that when near a boundary, a complex interference of generated and reflected waves induces a wave amplitude fluctuation asymmetry. Attraction increases as wave frequency increases or robot-boundary separation decreases. Theory on confined gravity-capillary wave radiation dynamics developed by Hocking in the 1980s captures the observed parameter dependence due to these "Hocking fields." The flexibility of the robophysical system allows detailed characterization and analysis of locally generated nonequilibrium fluctuation-induced forces [M. Kardar and R. Golestanian, Rev. Mod. Phys. 71, 1233 (1999)RMPHAT0034-686110.1103/RevModPhys.71.1233].

Impedance Control Using Selected Compliant Prismatic Joint in a Free-Floating Space Manipulator

The success of space missions like capture-and-deorbit or capture-and-service relies on the ability of the capturing satellite to establish a stable mechanical connection by its gripping tool with the object being intercepted. Most of the potential objects of capture missions are not equipped with dedicated docking ports; hence, the satellite robot intercepting them will have to provide the mechanical compliance necessary for the safe establishment of contact between the two structures. Articulated robotic arms with controlled mechanical impedance are one set of promising solutions for this challenge. In this study, the authors discuss how the mechanical impedance realized only along a single axis can be useful for facilitating the contact between the manipulator arm’s end effector of a free-floating robot and an uncooperative object in microgravity. By distinguishing a dominant direction in the final approach and contact establishment maneuver, the need for impedance control of six degrees of freedom may be relaxed, and a single prismatic joint with controlled impedance can be used at the end effector. Such architecture is simulated and compared with the full model-based six-degree-of-freedom Cartesian impedance control of a free-floating manipulator. Authors then discuss the limitations and possibilities of such architecture in a potential practical setting.

Study on operational space control of a redundant robot with un-actuated joints: experiments under actuation failure scenarios

This paper analyzes operational space dynamics for redundant robots with un-actuated joints and reveals their highly nonlinear dynamic impacts on operational space control (OSC) tasks. Unlike conventional OSC approaches that partly address the under-actuated system by introducing rigid grasping or contact constraints, we deal with the problem even without such physical constraints which have been overlooked, yet it includes a wide range of applications such as free-floating robots and manipulators with passive joints or unwanted actuation failure. In addition, as an intuitive application example of the drawn result, an OSC is formulated as an optimization problem to alleviate the dynamics disturbance stemmed from the un-actuated joints and to satisfy other inequality constraints. The dynamic analysis and the proposed control method are verified by a number of numerical simulations as well as physical experiments with a 7-degrees-of-freedom robotic arm. In particular, we consider joint actuation failure scenarios that can be occurred at certain joints of a torque-controlled robot and practical case studies are performed with an actual redundant robot arm.

Vision-Based Object Recognition and Precise Localization for Space Body Control

The space motion control is an important issue on space robot, rendezvous and docking, small satellite formation, and some on-orbit services. The motion control needs robust object detection and high-precision object localization. Among many sensing systems such as laser radar, inertia sensors, and GPS navigation, vision-based navigation is more adaptive to noncontact applications in the close distance and in high-dynamic environment. In this work, a vision-based system serving for a free-floating robot inside the spacecraft is introduced, and the method to measure space body 6-DOF position-attitude is presented. At first, the deep-learning method is applied for robust object detection in the complex background, and after the object is navigated at the close distance, the reference marker is used for more precise matching and edge detection. After the accurate coordinates are gotten in the image sequence, the object space position and attitude are calculated by the geometry method and used for fine control. The experimental results show that the recognition method based on deep-learning at a distance and marker matching in close range effectively eliminates the false target recognition and improves the precision of positioning at the same time. The testing result shows the recognition accuracy rate is 99.8% and the localization precision is far less than 1% in 1.5 meters. The high-speed camera and embedded electronic platform driven by GPU are applied for accelerating the image processing speed so that the system works at best by 70 frames per second. The contribution of this work is to introduce the deep-learning method for precision motion control and in the meanwhile ensure both the robustness and real time of the system. It aims at making such vision-based system more practicable in the real-space applications.

Experimental Validation of Pseudospectral-Based Optimal Trajectory Planning for Free-Floating Robots

This paper proposes the use of pseudospectral methods to solve the nonlinear trajectory planning problem for free-floating robots. Specifically, three different optimization tools are analyzed. Using each tool, simulations are performed, and it is shown that each solver is capable of finding a deployment trajectory that minimizes the final attitude of a free-floating robot. Each solution is then validated using Pontryagin’s minimum principle and Bellman’s principle of optimality, as well as by propagating the control torques using a numerical integrator and the dynamics model. Experimental validation is performed at Carleton University’s Spacecraft Robotics and Control Laboratory to further investigate the solutions obtained from each tool. Ultimately, it was determined that all solutions resulted in a reduced attitude disturbance at the end of the robotic deployment maneuver.

Tracking Control for the Grasping of a Tumbling Satellite With a Free-Floating Robot

In this letter, a novel method is presented for grasping a partially cooperative tumbling satellite with a free-floating robot. A reference trajectory, provided by an offline motion planner and adapted online to account for modeling uncertainties, is tracked with a visual servo for the approach phase and with a joint-space controller for the rigidization phase, which follows the grasp. An extended Kalman filter is implemented for providing robustness, predictions during measurement unavailability and a velocity estimate of the tumbling satellite as a feed-forward for control, throughout the grasping task. The visual servo is in cascade with an impedance controller to achieve tracking while being compliant. The method is successfully validated on the OOS-SIM facility, which is used for simulating gravity-free multibody dynamics with a novel momentum-based control method.

Space Robot End-Effector Tracking Control Complexities

Space robot end-effector tracking control is a highly complex task. This paper describes problems encountered while controlling the end-effectors of a multi-arm free-floating robot performing simple tasks. In addition, this paper conceptualizes noncoherent trajectories, which are much evident in free-floating robot tracking control task. It is shown here that the conventional inverse dynamics controller fails to track desired noncoherent trajectories resulted during the tracking.

Direct image-based visual servoing of free-floating space manipulators

This paper presents an image-based controller to perform the guidance of a free-floating robot manipulator. The manipulator has an eye-in-hand camera system, and is attached to a base satellite. The base is completely free and floating in space with no attitude control, and thus, freely reacting to the movements of the robot manipulator attached to it. The proposed image-based approach uses the system's kinematics and dynamics model, not only to achieve a desired location with respect to an observed object in space, but also to follow a desired trajectory with respect to the object. To do this, the paper presents an optimal control approach to guiding the free-floating satellite-mounted robot, using visual information and considering the optimization of the motor commands with respect to a specified metric along with chaos compensation. The proposed controller is applied to the visual control of a four-degree-of-freedom robot manipulator in different scenarios.

Variable Structure Control for Space Robots Based on Neural Networks

Problems of trajectory tracking for a class of free-floating robot manipulators with uncertainties are considered. Two neural network controls are designed. The first scheme consists of a PD feedback and a dynamic compensator which is an RBF neural network controller. The second scheme syncretizes neural networks with variable structures using a saturation function. Neutral networks are used to adaptively learn about and compensate for the unknown system. Approach errors are eliminated as disturbances by using the variable structure controller. The shortcomings of local networks are considered. The control is based on dividing aspects into three sections with classification and integration: state dimensional, neural network and variable structure separate control. When invalidations of the neutral network appeared, the controller was able to guarantee good robustness as well as the stability of the closed-loop system. The simulation results show that the methods presented are effective.

A Ground Experiment System of a Free-Floating Robot for Fine Manipulation

Robotic systems are expected to play an increasingly important role in future space activities with the development of space technology. One broad area of application is in the servicing, construction and maintenance of satellites and large space structures in orbit. Fine manipulation technology is very important for space robots to be able to perform these tasks, since it must ensure safe and reliable interaction with objects or the environment. In order to assure the task is accomplished successfully, ground experimentations are required in order to verify key planning and control algorithms before the space robot is launched. In this paper, based on the concept of a hybrid approach combining the mathematical model with the physical model, a ground experiment system is set up, which is composed of two industrial robots, global and hand-eye visual equipment, six-axis force/torquesensors, guide rail and four computers. Many control approaches of fine manipulation, such as compliance control, impedance control, hybrid force/position control, intelligent control and so on, can be verified using this system. As an example, a contour curves tracking experiment based on the compliance control strategy is performed. Experiment results show that the ground system is very useful for verifying the dexterous manipulation technology of space robots.

Editorial: special issue on space robotics

Outer space is a harsh environment, with extreme temperatures, vacuum, and intense cosmic radiation. The additional challenges ofmaneuvering inmicro gravity aswell as the requisite travel and communication across great distances make human access to space very difficult and limited. Thus, the application of intelligent robotics technology for outer space exploration is considered an important field. Robotics technology covers a large area of potential applications, from manipulation in the Earth’s orbit to locomotion on remote planets and from fully autonomous systems to human-inthe-loop systems for crew assistance. This special issue of the Springer Journal of Intelligent Service Robotics on Space Robotics seeks to present the latest developments in this exciting field of robotics research, in particular, addressing the issues of mathematical models for unique physics in both space free-floating robots and surface mobile robots, effective architectures for human crew assistance in complex mission scenarios, and specific implementation in hardware and software systems for ground-based verification. This special issue contains four peer-reviewed papers covering the exciting activities in the field of robotics applied in the orbital and planetary environments. In Nanos and Papadopoulos, mathematical models are investigated for a unique manipulator control on the use of free-floating space robots, where the presence of angular momentum influences on the behavior of the systems in orbital microgravity environment. In Ding, Deng, Gao, Nagatani and Yoshida, a

A Differentially Flat Open-Chain Space Robot with Arbitrarily Oriented Joint Axes and Two Momentum Wheels at the Base

The motion of a free-floating space robot is characterized by the principle of conservation of angular momentum. It is well known that these angular momentum equations are nonholonomic, i.e., are nonintegrable rate equations. If the base of the free-floating robot is partially actuated, it is difficult to determine joint trajectories that will result in point-to-point motion of the entire robot system in its configuration space. However, if the drift-less system associated with the angular momentum conservation equations is differentially flat, point-to-point maneuvers of the free-floating robot in its configuration space can be constructed by properly choosing trajectories in the differentially flat space. The primary advantages of this approach is that it avoids the use of nonlinear programming (NLP) to solve the nonintegrable rate equations, which at best can provide only approximate solutions. A currently open research problem is how to design a differentially flat space robot with under-actuated base. The contributions of this technical note are as follows: i) study systematically the structure of the nonholonomic rate constraint equations of a free-floating open-chain space robot with two momentum wheels at the base and arbitrarily oriented joint axes; ii) identify a set of sufficient conditions on the inertia distribution under which the system exhibits differential flatness; iii) exploit these design conditions for point-to-point trajectory planning and control of the space robot.

RETRACTED ARTICLE: Balance recovery control for biped robot based on reaction null space method

A biped walking robot should be able to keep balance even in the presence of disturbing forces. This paper presents a step strategy concept of biped walking robot that is stabilized by using reaction null space method. The called “step strategy” can be modeled by means of the reaction null space method that introduced earlier to tackle dynamic interaction problems of free-floating robots, or moving base robots in general. 6-DOF biped robot model simulations are used to confirm the validity.

Design of differentially flat planar space robots and their planning and control

The motion of free-floating space robots is characterized by non-holonomic constraints, i.e., non-integrable rate constraint equations. These constraints originate from the principles of conservation of angular momentum. It is well known that these rate constraints can also be written to form an input-affine drift-less control systems. Trajectory planning of these systems is extremely challenging and computation intensive since the motion must satisfy differential constraints. Under certain conditions, these drift-less control systems can be shown to be differentially flat. The property of flatness allows a computationally inexpensive way to plan trajectories for a dynamic system between two configurations as well as develop feedback controllers. The key contribution of this paper is to systematically study the non-holonomic rate constraints for free-floating planar open-chain robots and determine the design conditions under which the system exhibits differential flatness. A design is then proposed that can exploit the property effectively for trajectory planning and feedback control.

Path Planning of Free-Floating Robot in Cartesian Space Using Direct Kinematics

Dynamic singularities make it difficult to plan the Cartesian path of free-floating robot. In order to avoid its effect, the direct kinematic equations are used for path planning in the paper. Here, the joint position, rate and acceleration are bounded. Firstly, the joint trajectories are parameterized by polynomial or sinusoidal functions. And the two parametric functions are compared in details. It is the first contribution of the paper that polynomial functions can be used when the joint angles are limited(In the similar work of other researchers, only sinusoidla functions could be used). Secondly, the joint functions are normalized and the system of equations about the parameters is established by integrating the differential kinematics equations. Normalization is another contribution of the paper. After normalization, the boundary of the parameters is determined beforehand, and the general criterion to assign the initial guess of the unknown parameters is supplied. The criterion is independent on the planning conditions such as the total time tf. Finally, the parametes are solved by the iterative Newtonian method. Modification of tf may not result in the recalculation of the parameters. Simulation results verify the path planning method.

A Ground Experiment System of Free-floating Robot For Capturing Space Target

Technology for the capturing of space target is very important for on-orbit servicing. In order to assure the task is accomplished successfully, ground experimentations are required for the verification of the planning and control algorithms of space robotic system before it is launched. In this paper, an experiment concept is used, which is a hybrid approach, i.e. it combines the mathematical model with the physical model. The key issues of the concept are dynamic emulation and kinematic equivalence, in which the behaviors of the space robotic system are calculated by its dynamic equations. The motion of its end-effector and the space target is realized by two industrial robots. According to different observation spots, two modes of capturing process are emulated: one is observed from the inertial frame, the other is from the space base. Based on the concept proposed above, a ground experiment system is set up, which is composed of two industrial robots, a set of global visual system and five industrial computers. Using the system, algorithms of space robot of any geometry and mass properties can be tested. As an example, the autonomous trajectory planning algorithm is verified by the experiment of capturing a moving target. Moreover, a real-time 3D simulation system is developed to emulate the capturing process in 3D space. Numeric simulation and experiment results show that the ground system is effective in evaluating the planning and control algorithms of space robot.

Optimal motion planning for nonholonomic systems using genetic algorithm with wavelet approximation

An optimal motion planning scheme using genetic algorithm with wavelet approximation is proposed for nonholonomic systems. The motion planning of nonholonomic systems can be formulated as an optimal control of a driftfree system. A cost function is introduced to incorporate the control energy and the final state errors. The control inputs are determined to minimize the cost functional. By using the method of wavelet, the infinite-dimensional optimal control problem is truncated to a finite-dimensional one based on the wavelet bases. The genetic algorithm is employed to solve a feasible trajectory satisfying nonholonomic constraints. The proposed scheme is applied to a free-floating robot consisting of two one-link arms connected to a main base via revolute joints. The numerical results demonstrate that the genetic algorithm with the wavelet approximation is an effective approach to steer a nonholonomic system from its initial state to its final state.

State transformation processes for free-floating three-link planar robots

State transformation processes for free-floating three-link planar robots

Calibration of controls in steering nonholonomic systems

In this paper, the calibration of controls to effectively steer a nonholonomic system in a desired direction is described. The main ideas rely on selecting an optimal, with respect to motion in the desired direction, way of generating the vector field (local direction) via controls. The desired vector field is the only useful part in the Lie series of vector fields, while the remainder disturbs the desired vector field. After the selection of optimal motion, reduction of the remainder is performed via scaling some of the controls. These controls reduce the remainder more than the useful vector field. The ideas are illustrated with the generation of a [ X, Y] vector field at a given point of a state space for a free-floating robot. Results presented in this paper are important both for motion planning and the stabilization of nonholonomic systems.

Design and experiments on a free-floating planar robot for optimal chase and capture operations in space

This paper describes an autonomous free-floating robot system that was designed to investigate the behavior of free-floating robots that are involved in the capture of satellites in space. The robot is used as a test bed for algorithms that have been developed for efficient and autonomous capture of objects in space. The robot is a completely autonomous system running under a real-time operating system. It is equipped with two three-degree-of-freedom arms, a three-axis thruster system and a fast communications module. The robot works in conjunction with a host computer. The host computer is used to process the capture algorithms and the robot implements the results in real time. The entire system provides a test bed for algorithms developed for optimal capture of objects in space.