| The rapid development of space technology has continuously increased the demond for space-based optical observation systems.SiC material has become the best choice for large-scale optical mirrors because of their advantages of light weight and good stability.The traditional SiC mirrors are usually formed by lapping,which has a long processing cycle and low efficiency.It can no longer meet the current huge market demand.Therefore,the processing method of using ultra-precision grinding with high removal efficiency relapcing lapping to directly obtain a high-precision surface has become a recognized large-scale high-precision space optical mirror manufacturing process.In the entire process chain,ultra-precision grinding has the greatest influence on the surface and sub-surface quality of the optical mirror,and the processed mirror surface by ultra-precision grinding will directly determine the efficiency and feasibility of subsequent processing such as polishing.In order to ensure the surface precision and subsurface quality of large-aperture optical mirrors simultaneously,the ultra-precision grinding machines are required not only to have high stiffness,but also to fully ensure the compliance of grinding force control during grinding.Such mutual coupling and contradiction make ultra-precision grinding equipment for the large-scale SiC mirrors become one of the most complex and risky engineering problems.By reducing the number of moving axes,the system stiffness of the grinding machine can be greatly increased.Therefore,by replacing the traditional five-axis fixed point grinding with fewer-axis toric grinding has become one of the research hotspots in the field of ultra-precision optical grinding.Although many scholars have done a lot of research in related fields,the fewer-axis toric grinding has not been widely used in the ultra-precision grinding field for large-scale SiC mirrors.The main reasons include: firstly,the dynamic characteristics of large-scale optical grinders directly decides the surface accuracy of the mirror,and it requires the grinding machine to have extremely high system stiffness.Secondly,the complex and changeable contact conditions between toric wheel and workpiece have greatly increased the difficulty of modeling and controlling the grinding force,which has a great impact on the subsurface damage of the mirror.Therefore,aiming at the simultaneous control of surface accuracy and subsurface damage of SiC mirrors,the deep researches of the optimum design method of large-scale fewer-axis toric ultra-precision grinding optical grinder based on electromechanical coupling dynamics,the fewer-axis toric grinding process model and the position/force hybrid control method by adjusting the feed rate,are conducted in this paper.And the method was finally successfully applied to an independently developed 1.2-meter optical ultra-precision optical grinding machine.The main work and results of this article are summarized as follows: 1.A electromechanical coupling model based on state space theory is proposed to solve the problem of system dynamic characteristics optimization in the design of large-scale ultra-precision optical grinding machine.Firstly,the finite element method and rotational coordinate theory are used to establish the mechanical structure and control system dynamic model of the grinding machine,respectively.Secondly,based on the state space theory,an electromechanical coupling model of the ultra-precision optical grinder including the mechanical structure and the stiffness of the control system is established.On this basis,with the dynamic error and response time as the optimization goal,the system parameters were optimized using the constrained random direction search method,so as to optimize the dynamic characteristics of the grinding machine and ensure the surface accuracy requirement for large-scale optical mirror ultra-precision grinding.2.Considering the complex surface shape of the toric wheel and the nonlinearity of the parameters such as the wear of the grinding wheel and the change of the grinding depth caused by the change of the grinding force,based on the contact area model between the toric wheel and workpiece,the relationship between the actual grinding depth and the deformation of the large-scale optical grinder,the deformation and wear of toric wheel and the contour deviation are analyzed.Based on the transfer function between the grinding force,grinding depth,and feed speed,the grinding process model of fewer-axis toric wheel was established.And,the parameters such as specific grinding energy,toric wheel contour deviation,toric wheel wear stiffness and contact stiffness were identified through grinding experiments.Then,the simulations and grinding experiments of multiple sets of different feed speeds and grinding depth were conducted to verify the accuracy of the model.3.The grinding force control method by maintaining the grinding depth and changing the feed rate has been proposed,which not only guarantees the accuracy of the surface shape but also suppresses the grinding force fluctuation,thereby reducing the subsurface damage.Firstly,based on the fewer-axis toric grinding kinematics equation and the grinding process model,the trajectory planning and off-line force feed rate planning are carried out respectively.Second,based on the deviation between the target grinding force and the actual grinding force,the grinding force controller compensates for the pre-planned feed speed.Finally,a SiC mirror grinding force control experiment system was set up,and the control strategy of grinding force was successfully applied on the ultra-precision optical grinding machine.In summary,the successful application of the dynamic optimization of large-scale fewer-axis ultra-precision optical grinding machine and the position/ force hybrid control method not only validates the effectiveness of the surface and subsurface quality control for SiC mirror grinding,but also lays the foundation for the upgrade of large space optical mirror manufacturing technology in our country.And it makes important contributions to the development of critical space optics areas. |
