Research On Precision Adjustment Mechanism For Secondary Mirror Of Large Space Telescope

With the development of space technology, space telescope with large aperture and high-resolution imaging are playing an important role in many fields including civil use, commerce, military, and astronomy. For the large aperture telescope system, relative pose has strict requirements between primary mirror and secondary mirror. Based on the reasons for manufacture, assembly, gravity, thermal transmission and the change of material properties, the relative pose will change, which will bring down the image quality and lead to position drift. Therefore, the relative pose needs to be calibrated positively and the pose errors must be limited within the allowable range. Considering the consumption and the body mass, pose adjustment can realize by adjusting secondary mirror. And the accuracy of the telescope system is influenced by the accuracy of adjustment mechanism. So it will be of great importance for the large space telescope to research the techniques of the adjustment mechanism for secondary mirror. Based on all of this, the paper launches the research work.Firstly, a parallel manipulator based on Gough-Stewart platform is proposed, which is composed with offset joints and ball screw pairs. It is called 6-RRRPRR parallel manipulator. The inverse kinematics is analyzed based on Newton-Raphson algorithm and vector method. The parasitic motion which is caused by helical motion of the ball screw can be analyzed quantifiably and compensated. To verify the effectiveness of the proposed approach, simulations are performed with software package ADAMS. Finally, based on the inverse kinematics, the forward kinematics of 6-RRRPRR parallel manipulator is analyzed and verified.For the requirement of relative position between primary mirror and secondary mirror, the accuracy of adjustment mechanism needs to reach micrometer. In order to make the mechanism get high performance, optimization design of configuration parameters is performed. The optimization model based on kinematic accuracy and non-deformability is established and solved by genetic algorithm. Under the limitation, the objective function gets the best parameters.Workspace is an important requirement for the adjustment mechanism of secondary mirror. The workspace of the whole mechanism is affected by the workspace of universal joint, which is one of the parts of parallel manipulator. Therefore, the workspace of universal joints with cross-axis and offset axis is analyzed and compared. According to the limitation of stroke, interference and the angle of universal joint, the position workspace and orientation workspace can be solved. Then the limitation is analyzed for workspace.To verify the rationality of the analysis, the actuators with two generation are designed and tested for the requirement. Based on it, the adjustment mechanism is tested and the coupling repeatability is analyzed. Finally, the adjustment mechanism with load is tested on the workbench and the accuracy is analyzed. The results reveal that the designed mechanism can be satisfied with the requirement of positioning in micro class.