Study On The Key Techniques Of Magnetorheological Finishing For Large Aspheric Optics

Because the large aperture aspheric surfaces can effectively correct the aberrations, increase field of view, improve image quality, reduce the number of optical elements in optical system so as to decrease the weight and reduce the cost, the large aspherical optics have been widely used in large aperture optical system for the modern astronomical and earth observation. However, the modern optical system is stricter with the shape accuracy, the surface roughness, the sub-surface damage and so on. Due to the low deterministic process and the slow convergence efficiency, the traditional computer control optical surfacing(CCOS) and other methods can not meet the requirements of the manufacturing of the aspheric optics, especially the large aperture off-axis aspheric surfaces. As a new type of optical manufacturing technology, Magnetorheological Finishing(MRF) owning many advantages including the high deterministic process, the stable convergence efficiency, the controllable edge effect, the low sub-surface damage layer, the wide applicability for different optics, effectively fabricating optics of the large ratio of the diameter to thickness without ?footprint‘ and so forth, has a wide application prospect in manufacturing field of the aspheric optics.Based on the demands of the national project for the rapidly manufacturing the large aspheric optics with low cost, this thesis centers on the issue of manufacturing large aspheric optics with MRF and the related key techniques are studied in great detail. The MRF process for large aspheric optics is established and the visualized controllable algorithm software is accomplished. The 1.5m scale aspheric mirror was completed by MRF with high precision,short period and deterministic processing, and it laid the foundation of the manufacturing of large aspheric optics in our country.The main work in the thesis as follow:1. Study on MRF computer numerical controllable center. The design flow and parameters of permanent magnet field of MRF are presented, the design and optimization of the magnetic field is completed; The compact layout of the MRF modules are designed and integrated into the MRF160&MRF360 CNC center with the polishing wheel diameter of 160 mm and 360 mm respectively. The removal functions are tested.2. The dwell time algorithm of MRF for large optics. The Preston hypothesis for MRF is studied and the linear material removal model and the dwell time based on matrix operations are put forward; After the features of the dwell time algorithm of MRF for large optics are analyzed, the combined dwell time algorithm of the rapid non-negative SBB and the adaptive regularization is established.3. Restraining Mid-high Frequency Error and controlling Edge Effect. Two types of mid-high frequency of MRF process including Ring Effect and Tool Mask are studied separately. On the one hand, based on the conformal mapping theory and the biharmonic spline interpolation, the conformal extension method is proposed to extend any shape of optical surfaces and as a result to restrain Ring Effect. On the other hand, the integral model of the removal functions are built and discussed to explain the mechanism of tool mask errors,and the method to suppress and eliminate tool mask errors is put forward by analyzing the impact factors. Moreover, the edge effect of MRF is studied by testing the varied removal functions in CNC center and the edge effect is controlled in the dwell time algorithm by the variation regularities of the edge removal functions.4. The position and attitude control. It is raised that the direction of the polishing wheel radius should be along with the normal direction of the sphere which is closest to the aspheric surface and at the same time the polishing distance must be kept constant by CNC compensation according to the deviation between the aspheric surface and its closest spheric surface. What‘s more a variable removal function model is set up on the low degree of the freedom in the CNC machine. Also the virtual axis position and attitude control model is established though the optimization design of the permanent magnetic field5. The MRF process and the visualized control algorithm software. The completed MRF process is built up and MRF is introduced into the whole process of manufacturing large aspheric optics as one of the key fabricating methods. Moreover, based on the GUI in Matlab the visualized control software is obtained by the mixed programming technology of Matlab and C++. Finally, the high efficiency and high precision manufacturing process of the off-axis aspheric optics of 1500 mm magnitude is completed by the deterministic MRF process.