Study On Automatic Control Method And System Of Water Content In Polishing Liquid Of Magnetorheological Polishing Process

Magnetorheological finishing (MRF), known for its great determinism, has been proved to be a promising approach for the high-precision/high-efficiency fabrication of large aperture optics. However, the material removal rate of MRF increases as a result of water loss in aqueous magnetorheological fluid during polishing. Thus failing its determinism and the efficiency and accuracy will be decreased therefore. To stabilize the material removal function of MRF for long periods of time, and guarantee the determinism of MRF, the methods and system for automatically controlling the water content of magnetorheological fluid were studied in this thesis.(1) The basic principles and key methods of the water content control system were analyzed. The method, based on mutual induction, was applied in the control system to measure the water content. And the flow rate at which water is replenished into the magnetorheological fluid vessel was controlled to maintain the water content. The principle of the measuring method was introduced and the model of the controlling process was established, laying foundations for further research.(2) Based on the principle of mutual induction measuring method, the apparatus for measuring water content of magnetorheological fluid was developed, applying differential conversion circuit. The measuring sensitivity was promoted, compared to induction measurement and mutual induction direct measurement. A0.03%variation of water mass fraction can be resolved according to experiments.(3) The control strategy of control system was studied. Based on the model of water content control process and control demand, a strategy, combined forward feed control, self-adjustable grey prediction, single neuron adaptive PID control and two degree of freedom control, was proposed and its effectiveness was demonstrated by MATLAB simulation.(4) The control system was realized and experimental studied. The water mass fraction variations did not exceed0.1%during10hours when the set point remained constant and there was few overshoot when adjust water content to a higher or lower level. The control system was adaptive to the different experimental situations and lays a support for the long time stabilization of MRF material removal rate.