Research On Parameter Calibration Of 6-DOF Parallel Mechanism Based On Orthogonal Displacement Measurement System

Segmented mirror support and pose adjusting mechanism of high stability is the key equipment of large aperture,space-borne optical systems in the future.It is an effective way to realize co-phasing of segmented mirror by using 6-DOF parallel mechanism.Machining and assembling errors are inevitably introduced into the 6-DOF parallel mechanism in the process of machining and assembling,which leads to some error between the actual structural parameters and the theoretical structural parameters,and makes the kinematic model inaccurate.Due to the structural parameter error,there will be some errors between the actual pose and the theoretical pose when the 6-DOF parallel mechanism moves according to the instructions.It is ineffective to adjust the segmented mirror by iterative method.Instead,it is effective to compensate the error and improve accuracy by parameter calibration.In this paper,the research is focused on the parameter calibration of 6-DOF parallel mechanism,which is of great significance to better the accuracy and efficiency of segmented mirror adjustment and improve the calibration method.Firstly,according to the closed-loop characteristic and the complete differential theory of parallel mechanism,the relationship between the pose error and the structural parameter error of parallel mechanism is deduced,and the pose error model of 6-DOF parallel mechanism is established.The influence of the structural parameter error on the absolute pose is quantitatively analyzed by numerical simulation.Based on the absolute pose error model,the relative pose error model is proposed for the first time.The influence of structural parameter error on relative pose is analyzed quantitatively by numerical simulation,and the law of influence structural parameter error on pose error is summarized,and the main factors affecting pose accuracy of parallel mechanism are obtained.Secondly,several methods of pose measurement based on absolute coordinate measurement system are analyzed,and the pose calculation methods of the above systems are described in detail.A pose measurement method based on orthogonal displacement measurement system is proposed,and the forward and inverse kinematics solutions of the system are described in detail.By means of simulation,the effects of system and random error on orthogonal displacement measurement system are quantitatively analyzed.The results lays a foundation for the subsequent calibration experiments.Then,the parameter identification model based on the least square algorithm is established.On this basis,the calibration algorithm and calibration accuracy evaluation index are described.The parameter identifiability of parallel mechanism is studied,and the sufficient condition which geometric parameter error of parallel mechanism can be identified is obtained,that is,the parameter error can be identified when the end experiences two rotational degrees of freedom.In order to verify the robustness and effectiveness of the calibration algorithm,the numerical simulation of the calibration algorithm is studied;the solution process of the calibration algorithm is discussed in detail,and through computer simulation,the robustness and effectiveness of the calibration algorithm are verified.Due to the importance of measurement configurations in parameter calibration,the optimization of measurement configurations is carried out based on Oasis,the relatively good measurement configurations is obtained.The optimization effect is verified through simulation.Finally,on the basis of the above research,the parameter calibration experiment based on the orthogonal displacement measurement system is carried out.The comparison of the pose error before and after calibration show that: the maximum position error is reduced by 41%~65%,the maximum attitude error is reduced by 60%~80%.The positioning accuracy of parallel mechanism is effectively improved.The validity and robustness of the calibration algorithm is verified.Meanwhile,the calibration effect of the orthogonal displacement measurement system is confirmed.