Ou Ma

On-Orbit Identification of Inertia Properties of Spacecraft Using a Robotic Arm

This paper presents a robotics-based method for on-orbit identification of inertia properties of spacecraft. The method makes use of an onboard robotic arm to change the inertia distribution of the spacecraft system. As a result of the inertia redistribution, the velocity of the spacecraft system will change correspondingly. Because the velocity change is measurable and the inertia redistribution of the robotic arm itself is precisely computable, the inertia parameters of the spacecraft body become the only unknown in the momentum equations and, hence, can be identified from the momentum equations of the spacecraft system. To treat the problem as a linear identification problem, it has to be solved in two steps. The first step is to identify the mass and mass center of the spacecraft; and the second step is to identify the inertia tensor of the spacecraft. The advantages of this method are 1) it does not consume fuel because a robotic subsystem is energized by solar power; 2) it requires measuring velocities only, but not accelerations and forces; and 3) it is not affected by internal forces, which are difficult to accurately measure. The paper investigates the sensitivity of the method with respect to different arm/spacecraft mass ratios, arm motion trajectories, and velocity-measurement errors.

Using advanced industrial robotics for spacecraft Rendezvous and Docking simulation

One of the most challenging and risky missions for spacecraft is to perform Rendezvous and Docking (RvD) autonomously in space. To ensure a safe and reliable operation, such a mission must be carefully designed and thoroughly verified before a real space mission can be launched. This paper describes a new, robotics-based, hardware-in-the-loop RvD simulation facility which uses two industrial robots to simulate the 6-DOF dynamic maneuvering of the docking satellites. The facility is capable of physically simulating the final approaching within 25-meter range and the entire docking/capturing process in a satellite on-orbit servicing mission. The paper briefly discusses the difficulties of using industrial robots for HIL contact dynamics simulation and how these problems are solved.

Passive Gravity Compensation Mechanisms: Technologies and Applications

Though passive gravity-force compensation or weight balancing technology has existed for centuries, novel system designs and new applications have been emerging in the past two decades. This survey paper reviews the recent patents and technical publications related to these new developments and applications. The paper first gives an overview of widely used passive gravity-force compensation techniques. It then presents the applications of these techniques in different areas with focus on the recent patents and publications. Finally, the likely trends of future development in this particular area are discussed.

Optimal approach to and alignment with a rotating rigid body for capture

This paper addresses a feed-forward optimal control problem for one rigid body to approach to and align with another arbitrarily rotating rigid body, with an application to the satellite rendezvous problem. In particular, we focus on the satellite rendezvous strategy of finding an optimal trajectory, and the required thrust force profiles, which will guide the chasing spacecraft to approach the tumbling satellite such that the two vehicles will eventually have no relative rotation and thus a subsequent docking or capture operation can be safely performed with a normal docking or capture mechanism. Our approach is to model the system using rigid-body dynamics and apply Pontryagin’s Maximum Principle for the optimal control. A planar problem is presented as a case study, in which together with the Maximum Principle, the Lie algebras associated with the system are used to examine the existence of singular extremals for the time-optimal control problem. Also, optimal trajectories and the corresponding set of control force/torque profiles are numerically generated for the time/fuel-consumption optimal control problem.

Dynamics analysis of a cable-driven parallel manipulator for hardware-in-the-loop dynamic simulation

This paper describes a preliminary study of the dynamics of a 6-DOF cable-driven parallel manipulator for a potential application in a ground-based hardware-in-the-loop simulator of microgravity dynamics and contact-dynamics of spacecraft or robotic systems. Two basic dynamics problems are studied. One is the inverse dynamics problem and the other is the rigidity and vibration problem. The study results support the feasibility of using such a cable-driven manipulator for hardware-in-the-loop simulation of contact dynamics

Control of a space robot for minimal attitude disturbance to the base satellite for capturing a tumbling satellite

The use of a space manipulator (robot) for capturing a tumbling object is a risky and challenging task, mainly because when the manipulator onboard a servicing satellite (base satellite) intercepts with an external object for capture, the resulting impulse will be transferred along the mechanical arm down to the servicing satellite causing disturbance to the attitude of the satellite. Such disturbance may destabilize the servicing satellite if the captured object is tumbling and the physical contact between the robot end-effector and the object is not controlled properly. Certainly, the risk may be mitigated with a force or impedance control capability of the manipulator. However, the implementation of force or impedance control usually requires the robot to have a joint torque sensing and control capability which is a very expensive requirement for a space manipulator. To date, there has never been a really flown space manipulator having a joint torque control capability. Further, even a force or impedance control capability becomes available, much development is still needed before safe capture of a tumbling object can be confidently tried in a real mission. This paper presents an optimal control strategy for a space manipulator to have minimal impact to the base satellite during a capturing operation. The idea is to first predict an optimal future time and motion state for capturing and then control the manipulator to reach the determined motion state such that, when the tip of the robot maneuvers to and intercepts with the tumbling object, a minimal attitude disturbance to the servicing satellite will occur. The proposed control strategy can be implemented regardless whether the manipulator has a joint torque control capability or not. Since the control acts before a physical contact happens, it will not affect but actually augment any existing force or impedance control capability of the manipulator. The proposed method is demonstrated using a simulation example.

Workspace Analysis of a 6-DOF Cable Robot for Hardware-in-the-Loop Dynamic Simulation

This paper describes the study of the force-closure workspace of a 6-DOF, cable-driven, parallel robot for the application in a hardware-in-the-loop dynamic simulator, which is used for simulating microgravity contact dynamics of spacecraft or robotic systems. The workspace under study is defined as the set of all end-effector poses satisfying force-closure condition. Force-closure also means that the inverse dynamics problem of the manipulator has a feasible solution. Since there is no limitation on the external wrench and the dynamic motion of the end-effector, such a workspace is the most desirable (or nonrestricted) workspace for the intended application simulating low-speed impact-contact dynamics. A systematic method of determining whether or not a given end-effectors pose is inside the workspace is proposed with mathematical proof. Based on this method, the shape, boundary, dimensions, and volume of the workspace of a 6-DOF cable robot are displayed and discussed

A New Active Body Weight Support System Capable of Virtually Offloading Partial Body Mass

This paper presents a novel active body weight support (BWS) method, which is capable of virtually offloading full or partial body mass for a potential application of enhancing treadmill-based locomotion rehabilitation. The mass-offloading capability is realized by actively compensating a desired amount of body weight and inertia force using an acceleration-feedback scheme. The method has been studied by dynamics simulations and a specially designed nonhuman experiment. In the simulation study, the human and the BWS device were modeled as a multibody dynamical system interacting dynamically with the treadmill. The ground reaction forces were recorded as the dynamic load on the person. A cam-slider-based experiment was designed and conducted to test the engineering feasibility of the mass-offloading capability. Both the simulation and experiment results demonstrated that the BWS system can compensate any desired amount of gravity force and inertia force and, therefore, has the effect of virtually reducing the mass of a person attached to the system.

Optimal Control for Spacecraft to Rendezvous with a Tumbling Satellite in a Close Range

One of the most challenging tasks for satellite on-orbit servicing is to rendezvous and capture a non-cooperative satellite such as a tumbling satellite. This paper presents an optimal control strategy for a servicing spacecraft to rendezvous (in close range) with a tumbling satellite. The strategy is to find an optimal trajectory which will guide the servicing spacecraft to approach the tumbling satellite such that the two vehicles will eventually have no relative rotation. Therefore, a subsequent docking or capture operation can be safely performed. Pontryagins maximum principle is applied in generation of the optimal approaching trajectory and the corresponding set of control force/torque profiles. A planar satellite chasing problem is presented as a case study, in which together with the maximum principle, the Lie algebras associated with the system are used to examine the existence of singular extremals for optimal control. Optimal trajectories for minimum fuel consumption are numerically simulated

Validation of A Satellite Docking Simulator using the SOSS Experimental Testbed

This paper describes an R&D project aimed at experimentally validating a generic contact-dynamics toolkit in a Simulink based satellite docking simulator. The simulation software was developed to support the design and verification of the space station robotic systems and several satellite on-orbit robotic servicing systems. The hardware experiment was performed using an air-bearing supported docking testbed specially designed for testing different docking and capture mechanisms. Simulation model parameters were separately identified from the hardware. The simulation results compare favourably with the measured experimental data, which demonstrates the validity of the simulator and increases the confidence of using the simulator for design of various robotics-based satellite servicing missions such as the shuttle-return-to- flight, orbital express, and the potential hubble telescope robotic repair missions