Study On Magnetorheological Fluid Torque Server Applied In Compliant Polishing Of Aspheric Parts

Currently, surface finishing process accounts for about 37% ~ 50% of the total manufacturing time, and relies mainly on the manual grinding and polishing of skilled technicians. Surface finishing process has become a key issue that restricts the development of manufacturing technology, especially in the manufacturing of molds and optical components. Therefore, it is of great importance to study and develop automatic finishing methods and techniques of high precision, high efficiency, and low cost.Aiming at the force-position coupling problem commonly existed in the surface finishing process, a new force-position decoupling control method was proposed in this paper. The method is based on a MRT (magnetorheological fluid torque server) which is used to control the polishing force independent of the position control, and an economical CNC lathe which is used to control the position and posture of polishing tool and support the polishing process. Thus, the coupling of force and position is successfully decoupled in the compliant polishing process, which provides a solid basis for the realization of the automatic polishing of high precision, high efficiency and low cost.Aiming at the need of design and application of the MRT, MRF (magnetorheological fluid) was tested and investigated, the concept of critical shear rate was defined, BINGHAM model was modified, and the output torque calculation model of the MRT was established. On the basis, the influence of rotating speed of the input shaft of the MRT on its output torque was analyzed, the concept of critical rotating speed was introduced. Analysis results indicate that the MRT should work above the critical rotating speed in order to provide a steady output torque. Furthermore, the magnetic circuit was numerical analyzed by means of ANSYS software, and the reasonableness of structural design of the MRT was verified.Aiming at the need of polishing force control, the dynamic model of polishing tool system was set up, polishing torque control system was numerically simulated, and the parameters of the PID controller for the polishing torque control were obtained. Finally, the experiment on torque control was carried out, reasonable control period were determined, and the ideal control effect was got in the unit step excitation.Aiming at the need for the control of the position and posture of the polishing tool, a method of path planning for the polishing tool was introduced. With the consideration of Preston equation, the influence of the planned path on polishing parameters were analyzed, and the result shows that the dwell time increases along with the decrease of the radius of curvature of parts under the condition of uniform feed. And mode selection of spindle speed was discussed as well.In order to meet the needs of compliant polishing of aspherical parts, the software for the control of the compliant polishing system was developed, a polishing torque planning method in time domain was proposed. Polishing experiments on paraboloid, ellipsoid, and flat surfaces were carried out respectively, and all the surface roughnesses of the parts after polishing reached nanoscale. This gives a strong proof that force-position decoupling theory and method proposed in this paper are feasible and effective.