Research On The Control System And Experimental Study Of Ultra-Precision Diamond Turning Of Microstructured Surfaces Based On FTS

Micro-structured surfaces which have particular micro-topological structures are widely used in military and civilian industries applications because of their special properties, such as optical, conglutinative, frictional characteristics. Precision diamond turning based on a fast tool servo as a micro-displacement module is a very popular micro-structured surface manufacturing technology. Nowadays, some developed countries have successfully fabricated precision micro-structured surfaces applying their precision equipment and numerical control multi-axis systems. However, we are on our first stage of researching the diamond turning of micro-structured surfaces, and there is still a large gap between our country and developed countries in the research of manufacturing equipment and fabricating quality. Based on this, a CNC system for machining of complex micro-structured surface is established and some relevant works of control and optimization during the process of micro-structured turning have been studied in this thesis.Multi-axis numerical control system is a precondition for obtaining non-symmetric micro-structured surfaces. In order to satisfy the real-time requirement, the fast tool servo system is taken as a universal machining axis so as to avoid the communication problems between the FTS and other machining axis caused by establishing a dependent control module for FTS. For non-symmetric micro-structured surface turning, the displacement of FTS is determined by the angle position of the spindle and the position of X-slide. In order to generate free form micro-structured surfaces, the displacement of FTS has to synchronize with the rotation of the spindle and the movement of the X-slide. By applying the UMAC's time-base trigger control method, the synchronization has been achieved and the non-symmetric micro-structured surfaces have been successfully fabricated. The numerical control system suited for the complex micro-structured surfaces turning and the auto programming system are also developed.Piezoelectric actuator is used as universal driving base of the FTS because of its high resolution and high stiffness. However, piezoelectric actuator exhibits hysteresis in their response to an applied electric field, which not only considerably degrades the dynamic performance of the system but also leads to inaccuracy of the FTS system. A hysteresis model using RBF neural networks of FTS is established based on expanded input space method, and closed loop control of FTS is applied using corresponding inverse model. However, it is known that neural networks can only be available for the approximation of the continuous systems with one-to-one or multiple-to-one mappings. It is unable to use the neural networks to directly identify the model of systems with multi-valued mapping such as FTS hysteresis. In this paper, a novel hysteresis operator is proposed to construct an expanded input space so as to transform the multi-valued mapping into a one-to-one mapping which enables neural networks to approximate the behavior of hysteresis of FTS. In order to achieve better tracking performance of FTS, which is treated as a universal machining axis, user defined servo algorithm is programmed to take the place of PID algorithm embedded in UMAC controller.Error compensation could be the one of the most effective and economic strategies to improve the quality of machined micro-structured surfaces. Error compensation method applying the least square support vector machine (LS-SVM) is proposed according to the particularity of micro-structured surfaces turning with varing depth of cut which is quite different from general face cutting. In order to reduce the influence of systematic error factors to the contour accuracy, a least square support vector regression model of micro-structured surfaces turning errors is established by analysing the contour errors of micro-structured surfaces machined with different cutting parameters. The compensation experiment has been excuted and indicated the effectiveness of the proposed compensation method.The machinging experiments were carried out with the established multi-axis numerical control system and Non-rotation symmetric surface structures—five bode and sinusoidal grid microstructured surfaces were successfully manufactured by the designed machining system. An evaluation technique based on the two-dimensional discrete Fourier transform (2D-DFT) of the sinusoidal grid interference microscope images was developed to evaluate the fabricated micro-structured surfaces effectively. According to the frequency components of the spectrum obtained by the 2D-DFT of the image, error sources were analysed and corresponding amendment strategies were proposed.