Orbit-Structure-Contact dynamics modeling and simulation for virtual validation of active debris removal by space robots

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.

Planar soft space robotic manipulators: Dynamic modeling and control

This paper studies the dynamics modeling and control simulation of space robot with the dynamic effects due to orbital mechanics. To a single-arm free-flying space robot, two dynamics models are built by Lagrangian formulation. One is a general model without the consideration of orbital mechanics, based on which a second model is derived by bringing in the generalized gravitation and generalized inertial force. The special form of this model with only the body coordinate considered can lead to the accurate dynamics model in satellite formation flying. Results of numerical control simulation show that the latter model give a more accurate modeling of space robot which is especially useful when the travel of system is of relatively long length.

The development of orbital space robotics in recent decades is aimed at meeting the demand for extravehicular missions, such as supply, maintenance, inspection, rescue and material transport. A space manipulator can be defined as one or more arms coupled to a floating base (satellite) for operation in orbit. The differential characteristic of these systems is the dynamic coupling existing between the base and the manipulators, which makes it difficult the independent analytical parameterization of the kinematics of its end-effectors. In this article, a mathematical modeling of the dynamics of the two-arm free-floating space robot structurally identical to the closed matrix format consolidated in the literature for single-arm robots is formulated. The dynamic coupling between the two arms and the base is resolved by the law of conservation of momentum, valid for isolated systems described in inertial frames. Due to the sharing of mathematical properties with the single-arm case, the proposed model can be validated in the task space through the Computed Torque approach. The modularity of the proposed model suggests an immediate extension to scenarios with multiple arms and floating base, which should present even more reliability and operational range than two-arm robots.

With the complexity of the dynamic model of free-floating space robots (FFSR), it is difficult to design the control system to capture targets. This paper presents a controller for FFSR to capture targets without the kinematic and dynamic model equations, where the agent learns a closed-loop control policy from state information only. At first, the process of the task is described as the reinforcement learning process without the dynamic models of the space robot. Then, we use the deep deterministic policy algorithm (DDPG) to train the policy for space manipulator motion planning. And we present a skill named pre-training in the training process to further import the learning efficiency. Finally, a 3 degrees of freedom space robot is modeled and simulated to demonstrate the validity of the controller.

In order to reveal dynamic coupling effect in a space robot's operation, the paper raises a method of using recursive formulation to establish the system dynamic model and designing operational procedure in a parameterized way. In the paper, a topological representation of a space robot operating system is established containing dynamic constraints. The weighted factors method is used to get a simplified spanning tree of the system. Finally, dynamic model of space robot operating on the surface of the spacecraft is formulated and numerical simulation is carried out. The obtained results show that space robot and spacecraft are strongly coupled during the operation. Continuous control may intense the dynamic coupling effect, while strong dynamic coupling decreases the operating efficiency. It is illustrated in this paper that dynamic coupling analysis of complex space operational tasks can be conveniently implemented using this method.

Trajectory tracking control of space robots in task space is of great importance to space missions, which require on-orbit manipulations. This paper focuses on position and attitude tracking control of a free-floating space robot in task space. Since neither the nonlinear terms and parametric uncertainties of the dynamic model, nor the external disturbances are known, an adaptive radial basis function network based nonsingular terminal sliding mode (RBF-NTSM) control method is presented. The proposed algorithm combines the nonlinear sliding manifold with the radial basis function to improve control performance. Moreover, in order to account for actuator physical constraints, a constrained adaptive RBF-NTSM, which employs a RBF network to compensate for the limited input is developed. The adaptive updating laws acquired by Lyapunov approach guarantee the global stability of the control system and suppress chattering problems. Two examples are provided using a six-link free-floating space robot. Simulation results clearly demonstrate that the proposed constrained adaptive RBF-NTSM control method performs high precision task based on incomplete dynamic model of the space robots. In addition, the control errors converge faster and the chattering is eliminated comparing to traditional sliding mode control.

Dynamics modelling and Hybrid Suppression Control of space robots performing cooperative object manipulation

It is well known that space robots play a significant role in the human space activities. Capturing a free floating object in space is one of important missions assigned to space robots and it has increasingly attracted researchers’ attention recently. In this paper, the dynamics and control of capturing a floating rigid body using a space robot are investigated. The dynamic model of the system is established by the single direction recursive construction method and the Jourdain’s velocity variation principle. The contact force between space robots and targets has been set up based on the Hertz contact theory. Then the influence on the whole system incurred by this contact force has been thoroughly analyzed through simulations of both central and eccentric impact. Our simulation result verified the impact on the whole system caused by the contact force, and it also indicated the extreme complexity of the dynamic response caused by impact during the capture process. The space robot base attitude has been controlled using the controller based on the nonlinear decoupling method. Our simulation results showed adequate effectiveness.

To ensure efficient transportation of large space structures in the future, it has been suggested that they be transported in an unassembled and compact form and then assembled inorbit using space robots. Forces generated during open contact operations have to be taken into careful consideration so that structural components can be successfully acquired and assembled by the robot. Unlike ground-based robots, the free-flying space robot base will deviate from its desired orientation when the spacecraft comes in contact with structural elements. This effect on spacecraft attitude may adversely affect communication, solar energy collection and in extreme cases the stability of the spacecraft. While thrusters can be used to provide regulation torques, in a scenario where the spacecraft is surrounded by structural components that are to be assembled, filing clusters is undesirable. Further, the force impulses generated by thrusters have poor resolution and hence are unsuitable for providing fine regulation. In this paper, the authors utilize reaction wheels (RWs) to provide accurate and smooth control torques without the need to have on-board fuel. By adopting the commonly used canonical-form dynamic model of a space robot as found in the literature, force tracking may be achieved, but a desired orientation of the spacecraft cannot be guaranteed. To solve this problem, the authors build dynamic equations of the space manipulator and the spacecraft base separately to form basis for separate contact-force control and attitude control of the platform. By taking a further step from previous literature which mainly try to maintain contact between the end-effector and the target, hybrid force/motion control is extended to a space robot in this paper in order to track a desired force trajectory and simultaneously maintain a desired relative position between the end-effector and the target to avoid inducing tumbling torques. A recently proposed adaptive variable structure control method is applied to design a robust attitude controller taking into account system uncertainties and external disturbances. A two-link planar space robot model is simulated to track a desired uni-axial contact force and regulate the spacecraft attitude during contact with a free-floating target. The simulation results have demonstrated better robustness and performance of the proposed controller, such as shorter settling time in the presence of disturbance and smaller force tracking error, in comparison to a conventional feedback linearisation attitude controller.

This paper presents a method to minimize the base attitude disturbance of a space robot during target capture. First, a general dynamic model of a free-floating space robot capturing a target is established using spatial operator Algebra, and a simple analytical formula for the base angular velocity change during the impact phase is obtained. Compared with the former models proposed in the literature, this model has a simpler form, a wider range of applications, and O(n) computation complexity. Second, based on the orthogonal projection matrix lemma, we propose the generalized mass Jacobian matrix (GMJM) and find that the base angular velocity change is a constant multiple of the component which the impact impulse projects to the column space of the GMJM. Third, a new concept, the base attitude disturbance ellipsoid (BADE), is proposed to express the relationship between the base attitude disturbance and the impact direction. The impact direction satisfying the minimum base attitude disturbance can be straightforwardly obtained from the BADE. In particular, for a planar space robot, we draw the useful conclusion that the impact direction unchanged base attitude must exist. Furthermore, the average axial length of the BADE is used as a measurement to illustrate the average base attitude disturbance under impact impulses from different directions. With this measurement, the desired pre-impact configuration with minimum average base attitude disturbance can be easily determined. The validity and the efficiency of this method are verified using a three-link planar space robot and a 7DOF space robot.

Dynamics simulation plays a key role in the design, verification, and operation planning of space manipulator systems because of the difficulties of ground-based physical tests with large, flexible robotic systems. Modeling of contact dynamics has increasingly become an essential step in dynamics simulation of space station robotic operations (such as the assembly and maintenance of the station). This role requires the modeling tool to have very high fidelity. This paper describes a research project aimed at experimentally validating a general contact dynamics simulation software developed by McDonald Dettwiler Space and Advanced Robotics Ltd (previously Spar Aerospace Ltd). The experimental tests were carried out in the robotics laboratory at the University of Victoria. The validation results demonstrated that the software is capable of predicting realistic contact behavior.

SUMMARYA dual-arm space robot has large potentials in on-orbit servicing. However, there exist multiple dynamic coupling effects between the two arms, each arm, and the base, bringing great challenges to the trajectory planning and dynamic control of the dual-arm space robotic system. In this paper, we propose a dynamic coupling modeling and analysis method for a dual-arm space robot. Firstly, according to the conservation principle of the linear and angular momentum, the dynamic coupling between the base and each manipulator is deduced. The dynamic coupling factor is then defined to evaluate the dynamic coupling degree. Secondly, the dynamic coupling equations between the two arms, each arm, and the base are deduced, respectively. The dynamic coupling factor is suitable not only for single-arm space robots but also for multi-arm space robot systems. Finally, the multiple coupling effects of the dual-arm space robotic system are analyzed in detail through typical cases. Simulation results verified the proposed method.

Dynamics analysis of space robot manipulator with joint clearance

This paper discusses a vibration suppression control method for a space robot with a rigid manipulator and flexible appendage. A suitable dynamic model that considers the coupling between the manipulator and flexible appendage was developed for the controller to accomplish the vibration suppression control of the flexible appendage. The flexible appendage was modeled using a virtual joint model, and the control method was developed on the basis of this model. Although this type of control requires feedback of the flexible appendage state, its direct measurement is generally difficult. Thus, an estimator of the flexible appendage state was constructed using a force/torque sensor attached between the base and flexible appendage. The control method was experimentally verified using an air-floating system.