Wenyi Qiang

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.

Autonomous Path Planning and Experiment Study of Free-floating Space Robot for Target Capturing

Space robotic systems are expected to play an increasingly important role in the future. The robotic on-orbital service, whose key is the capturing technology, becomes research hot in recent years. In this paper, the authors propose an autonomous path planning method for target capturing. The task is described in Cartesian space and it can drive the manipulator to approach the target along the closest path. Firstly, the target feature is extracted based on the measured information via the hand-eye camera, and the target pose (position and orientation) and velocities (linear velocity and angular velocity) are estimated using Kalman filtering technology. Then, a numerically feasible approach is presented to plan the manipulator motion and avoid the dynamic singularities, which are transformed into real-time kinematic singularities avoiding problem. Thirdly, the potential disturbance on the base due to the manipulator’s motion is estimated, and the joint rates are autonomously adjusted to reduce the disturbance if it is beyond the allowed bound. At last, a ground experiment system is set up based on the concept of dynamic emulation and kinematic equivalence. With the experiment system, the autonomous target capturing experiments are conducted. The experiment results validate the proposed algorithm.

A Chinese Small Intelligent Space Robotic System for On-Orbit Servicing

The Chinese experimental space system for on-orbit robotistic services (CESSORS) is to be developed by Shenzhen Space Technology Center. CESSORS consists of two satellites and a mounted robotic manipulator. In this paper, the design of the two satellites and the manipulator is described in detail, as well as the target detecting system and the ground teleoperation system. The planned missions include calibration of robot manipulator, teleoperation of the manipulator, coordinated control, on-orbit robotistic services, on-orbit target chasing and approaching, and flying around inspection. This paper introduces the components of CESSORS, and shows the on-orbit missions

Non-holonomic Path Planning of a Free-Floating Space Robotic System Using Genetic Algorithms

In this paper, the non-holonomic characteristic of a free-floating space robotic system is used to plan the path of the manipulator joints, by whose motion the base attitude and the manipulator joints attain the desired states. Here, we parameterize the joint trajectory using sinusoidal functions, whose arguments are high-order polynomials. Then, we define the cost function for optimization according to the constraint conditions and the accuracy of the space robot. Finally, genetic algorithms (GAs) are used to search for the solutions of the parameters. Compared with others, our approach has advantages as follows. (i) The motion of the manipulator and the disturbance on the base are practically constrained. (ii) The dynamic singularities cannot affect the algorithm since only the direct kinematic equations are utilized. (iii) The planned path is smooth and more applicable for the control of the manipulator. (iv) The convergence of the algorithm is not affected by the attitude singularity since the orientation error is represented by quaternion, which is globally singularity-free. The simulation results of the spacecraft with a 6-d.o.f. manipulator verify the performance and the validity of the proposed method.

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.

Inverse kinematics problem for 6-DOF space manipulator based on the theory of screws

Space manipulators will play a significant role in the maintenance and repair of space stations and satellites, and other space missions. The inverse kinematics is an important problem in the automatic control of space manipulator. The paper presents a novel method for solving the inverse kinematics problem of 6-DOF space manipulator based on the theory of screws. To solve the inverse kinematics problem, we formulate the kinematics equations of 6-DOF space manipulator and study a novel method. The method can give the analytic solutions of the inverse kinematics problem for 6-DOF space manipulator. The virtual prototyping technology is used to model the 6-dof mechanical arm system of Free-Flying space Robot (FFR) and simulate the space manipulator system, it shows the validity of the method which is presented in this paper. Compared with other methods, the method based on the theory of screws just establish two coordinates, is applicable to real-time control, allows one to directly choose the desired configuration from the multi-solutions and its geometry meaning is obvious.

Implicit surface reconstruction from 3D scattered points based on variational level set method

In this paper we propose a novel variational formulation for arbitrary surface reconstruction from 3D scattered points. An implicit surface is adopted for its more advantages, such as continuity and differentiability, easy estimation of points inside or outside the shape, and convenient implementation for these complicated set operations. The presented new energy functional in this paper considers more factors on properties of the scattered points (including distances and normal vectors), smoothness and the constraint of the signed distance function. The gradient flow which minimizes the total energy functional updates and drives the motion of the zero level set interface to the desired surface. The derived formulations were applied to certain 3D surface reconstruction with good results.

Trajectory Planning of Space Robot System for Target Berthing and Reorientation after Capturing

A typical servicing operation in space mainly includes three phases: capturing the target, berthing and docking the target, and repairing the target. The attitude of a satellite usually changes after the capturing, because the control system is turned off during this phase for safety reasons. In this paper, a method is proposed to achieve the berthing of the target and reorientating the satellite attitude at the same time, both by involving manipulator motion only. Firstly, the constraints on the manipulator and the objective function are defined according to the planning problem. Then the joint trajectory is parameterized by sinusoidal function, whose argument is the polynomial. Finally, genetic algorithm is used to search for the global optimal resolution of the parameters. When the parameters are found, each joint trajectory can be determined. This planned trajectory is smooth and more applicable for the control of the free-floating robotic system. Our proposed method is verified by simulation

Fuzzy Controller Design and Parameter Optimization

In this paper, a fuzzy controller with dual controller structure is proposed for solving the contradiction between steady performance and dynamic performance of usual fuzzy controllers. The structure of the obtained controller is simple and its algorithm is convenient, it has good robust and dynamic performance, and can effectively eliminate steady state deviations. For avoiding complex adjustment of parameters as asked for in the design of fuzzy controllers, and attain optimal control properties, a particle swarm optimization (PSO) algorithm has been made use of to optimize the parameters of a fuzzy controller during design. Numerical simulations based on typical controlled objects demonstrate the effectiveness and the adaptability of the algorithm, as well as the superiority of the designed controller

Improvement on Robots Positioning Accuracy Based on Genetic Algorithm

The paper analyzes the robot links positioning error sources and builds its error model of geometrical parameters. With the aid of the genetic algorithm (GA) that has the powerful global adaptive probabilistic search ability, 24 parameters of a 6-DOF robot are identified through simulation, which makes the robots position and orientation accuracy an great improvement. In the process of the robot calibration, stochastic measurement noises are considered. The simulation results show that with GA calibrating the robot is a kind of superior method, even if the robot links parameters are relative, GA still has search ability to find the optimum solution.