Research On Pneumatic Parallel Polishing Tool System

As the development of technology,The requirements of modern optical systems for imaging devices increase.Because of the aberration of the imaging,the traditional spherical mirror gradually fades out of the field of high precision optical imaging.The application of freeform surface effectively solves the aberration problem.The use of the compensating mirror is omitted,which simplifies the structure of the imaging system.So that the optical imaging system is now of higher precision,smaller size and better quality and has been widely used in space remote sensing,aerospace,optical precision instruments and other high-tech fields.The ability to produce high precision optical freeform mirror is an important standard to evaluate the optical manufacturing capability of a country.The polishing process is the key to the surface quality of the freeform mirror.The deterministic polishing technology based on modern computer has achieved the predictability of the polishing results,and greatly improved the processing accuracy and yield.In the deterministic polishing,the polishing pressure control between the polishing head and the workpiece is the main factor to determine the surface quality.In this paper,a set of pneumatic parallel polishing tool system is developed to solve the problem of the coupling between the polishing force and the contact position in the polishing process.and finally the polishing pressure control and position precision control are realizedThe new polishing tool system consists of four sets of parallel pneumatic rod unit,polishing spindle,pneumatic control platform and so on.It has three independent degrees of freedom,which can make the polishing head move along the normal direction of the contact point of the workpiece,and realize the active control of the polishing pressure.In CATIA V5R21,the 3D model of the tool system is designed,the part drawing and machining have been completed.Finally the system platform of pneumatic parallel polishing tool is set up.The overall control scheme of polishing system is established.According to the structural characteristics of the designed tool system,the actual polishing pressure detection algorithm is deduced.The polishing pressure error is transformed into the normal displacement compensation of the polishing contact point based on Hertz contact theory and position impedance control theory.The control strategy of polishing force control model is established.Using the homogeneous coordinate transformation method to get Inverse kinematics of the pneumatic rod and three-dimensional motion platform under the expected pressure of the machine polishing tool system.The workspace of parallel tool system is calculated and analyzed.The pneumatic servo position system is of strong nonlinearity.The approximate mathematical model of the system is obtained by the combination of theoretical analysis and off-line system identification.The simulation and experimental control program is designed in the Matlab/Simulink according to the mathematical model obtained.the parameters of PID controller are simulated and experimental tuning,The experimental results are analyzed and the traditional PID controller is optimized.Finally,the control system of pneumatic servo position system is obtained which position control accuracy is less than 0.1mm.The initial calibration of the force sensor and displacement sensor is carried out when the tool system is in the initial state.According to the control strategy of polishing force and the kinematics control of the tool system,the control program of the pneumatic parallel polishing tool system is designed.The polishing pressure control experiment is carried out which verifys the effectiveness of the pneumatic parallel polishing tool system in the design of the control pressure.The polishing experiments were carried out on the contact points,and the surface morphology changes before and after processing were analyzed and compared.