Due to the intense vibration during launching and rigorous orbital temperature environment,the kinematic parameters of space robot may be largely deviated from their nominal parameters.The disparity will cause the real pose(including position and orientation) of the end effector not to match the desired one,and further hinder the space robot from performing the scheduled mission.To improve pose accuracy of space robot,a new self-calibration method using the distance measurement provided by a laser-ranger fixed on the end-effector is proposed.A distance-measurement model of the space robot is built according to the distance from the starting point of the laser beam to the intersection point at the declining plane.Based on the model,the cost function about the pose error is derived.The kinematic calibration is transferred to a non-linear system optimization problem,which is solved by the improved differential evolution(DE) algorithm.A six-degree of freedom(6-DOF) robot is used as a practical simulation example,and the simulation results show:1) A significant improvement of pose accuracy of space robot can be obtained by distance measurement only;2) Search efficiency is increased by improved DE;3) More calibration configurations may make calibration results better.
To overcome the influence of on-orbit extreme temperature environment on the tool pose(position and orientation) accuracy of a space robot,a new self-calibration method based on a measurement camera(hand-eye vision) attached to its end-effector was presented.Using the relative pose errors between the two adjacent calibration positions of the space robot,the cost function of the calibration was built,which was different from the conventional calibration method.The particle swarm optimization algorithm(PSO) was used to optimize the function to realize the geometrical parameter identification of the space robot.The above calibration method was carried out through self-calibration simulation of a six-DOF space robot whose end-effector was equipped with hand-eye vision.The results showed that after calibration there was a significant improvement of tool pose accuracy in a set of independent reference positions,which verified the feasibility of the method.At the same time,because it was unnecessary for this method to know the transformation matrix from the robot base to the calibration plate,it reduced the complexity of calibration model and shortened the error propagation chain,which benefited to improve the calibration accuracy.
The kinematic error model of a 6-DOF space robot is deduced,and the cost function of kinematic parameter identification is built.With the aid of the genetic algorithm (GA) that has the powerful global adaptive probabilistic search ability,24 parameters of the robot are identified through simulation,which makes the pose (position and orientation) accuracy of the robot a great improvement.In the process of the calibration,stochastic measurement noises are considered.Lastly, generalization of the identified kinematic parameters in the whole workspace of the robot is discussed. The simulation results show that calibrating the robot with GA is very stable and not sensitive to measurement noise.Moreover,even if the robot's kinematic parameters are relative,GA still has strong search ability to find the optimum solution.
