Research On Self-calibration Method Of Space Manipulator For On-orbit Service

During the construction and operation of the space station,the space manipulator undertakes increasingly complex on-orbit service tasks,such as auxiliary cabin docking,satellite release and recycling as well as capture and installation of experimental load replacement.Ensuring end pose accuracy of space manipulator is the basis for successfully completing the above-mentioned on-orbit service tasks.However,kinematic parameters of space manipulator may change under compound influence of extreme launch conditions,accidental impact or collision,as well as wear and deformation caused by long-term service,finally leading to end positioning error.On-orbit self-calibration can correct kinematic parameters,which is an effective means to improve the end pose accuracy of the space manipulator.The absolute pose error model,commonly used for self-calibration,generally requires accurate measurement of the absolute end pose of manipulators,which depends on the determination of the coordinate transformation relationship between the end pose measurement system and the manipulator base.For the space manipulator,vibration and impact of launching,inner and outer pressure difference as well as thermal de-formation of the space capsule will change the transformation between the pose measurement system and the manipulator base,making it complicated,even hardly,to measure and calculate this transformation accurately.Therefore the absolute pose error model is no longer suitable for on-orbit self-calibration of space manipulators.In view of the above problems,this paper takes the 7-DOF manipulator of the space station as the object of research.The relative pose accuracy of the manipulators is described by introduce the concept of position distance error and angle distance error.Then by analysis of linear mapping between kinematic parameter error and relative pose error,a relative pose error modeling method of space manipulators is proposed.Based on this,the parameter independence criterion and measurement configuration optimization strategy of relative pose error model are studied.Finally,a set of theory and method for on-orbit self-calibration of space manipulator based on relative pose error model is established.The theoretical and applied research works of this paper are summarized as follows:(1)Influence factors analysis of end pose accuracy of space manipulator.Firstly A 7-DOF space manipulator is introduced as the object of research,and its kinematic model is established by MDH method.Then by establishing the concept of Accuracy of Rotation,an evaluation method of the relative pose accuracy of robot manipulators is put forward,avoiding the influence of coordinate transformation error between the measurement system and the base system on the evaluation of pose accuracy.Based on the evaluation method,experimental method for accuracy evaluation of space manipulator is designed as a foundation of simulation verification of following calibration theory.Finally,factors affecting the end pose accuracy of space manipulators and their distribution law are analyzed,and then mapping relation between absolute and relative pose accuracy is analyzed,indicating the feasibility of relative pose error modeling.(2)Relative pose error modeling of space manipulator for on-orbit self-calibration.Firstly,the influence of errors between the measurement system and the base system on the calibration accuracy of the absolute pose error model is analyzed by simulation,indicating the necessity of relative pose error modeling.Then in order to build the linear mapping relation between kinematic errors of space manipulator and its end relative pose errors,the relative distance error model is established by ignoring higher order nonlinear error.And then accuracy of the relative distance error model is proved to effective for calibration,with probability distribution and mathematical expectation of nonlinear error deduced and obtained.Afterwards,based on differential rotation and its corollaries,the linear mapping relation between kinematic errors and the relative rotation error of manipulator,term as relative rotation error model,is derived.Finally,a combination of the relative distance error model and relative rotation error model comes into a complete relative pose error model,making it possible to calibrate the space manipulator on orbit with its relative end pose errors.(3)Research on independent parameter criterion of relative pose error model of space manipulator.The influence of redundant parameters on calibration accuracy is analyzed,demonstrating and the necessity of parameter independence analysis.Then,according to the linear correlation of the Jacobian matrix,independence of the generalized error parameters is analyzed.The parameter independence criterion of the adjacent joints under different positional relations is established,so the unrecognizable parameters in the absolute pose error model are removed.Finally,based on parameter independence analysis of absolute pose error model,the parameter independence criterion of complete relative pose error model is established by analyzing the parameter independence of the relative distance error model and the relative rotation error model,respectively.The experimental results show that the robustness of on-orbit self-calibration accuracy to measurement noise is effectively improved after removing the unrecognizable parameters in the error models according to the above criteria.(4)Research on optimization strategy of on-orbit self-calibration measurement configuration of space manipulator.In order to ensure the on-orbit self-calibration accuracy under the premise of minimizing the measurement configuration,this paper proposes a measurement configuration optimization strategy based on the improved DETMAX algorithm and Particle Swarm Optimization algorithm.Two kinds of design method of measurement configurations are prosed to guarantee calibration accuracy with less measurement configurations.One is a modified DETMAX algorithm,improved by introducing constructing a better initial set and subdividing the configuration space.The other is the Particle swarm optimization,seeking for the optimal measurement configuration set in zero space for redundancy of the space manipulator.The experimental results show that compared with the original DETMAX method,the optimization strategy proposed in this paper can construct a set of measurement configurations with higher observability index,thus producing better calibration results.(5)Experiments on self-calibration method by a space manipulator.Based on the ground air-flotation platform and a 5-DOF manipulator,an end pose measuring device is designed,and a visual plan and control software of robot manipulator is developed.Through the design of experimental schemes and by collecting data with camera and laser tracker as well as analyzing results with MATLAB software,effectiveness of the method and theory of self-calibration proposed in the paper are proved.