| The optical film is an important part in the backlight units of LCD. It is manufactured by the precision mold of light guide plate with optical micro structure, which divides into the micro-groove array, the micro-tapered array and the micro-lens array etc. The roller used to manufacture the light guide plate of LCD by the printing method needs to machine the above non-spherical array micro-structure. By controlling the fast tool servo (FTS) which is installed in the ultra-precision lathe, the sub-micrometer machining accuracy can be achieved based on the freeform surfaces characteristics of roller array micro-structure. And it can directly manufacture the required quality workpiece through the control of FTS to implement the precision linkage between the macro and the micro.FTS is mainly composed of a driver, a flexure hinge holder and a controller etc. The FTS driven by piezoelectric ceramic has advantages of high response frequency and high acceleration. Using the lever amplification mechanism, the buckling mechanism and the Scott-Russell mechanism can amplify the micro-displacement. The piezoelectric ceramics has high precision, fast speed and large output power etc. However, it also has defects such as creep, hysteresis and nonlinear, which are described by the Preisach model, Prandtl-Ishlinskii model, Duhem operator, generalized Maxwell model etc. The above characteristics bring some problems. The higher the frequency is, the smaller the micro-displacement is. And during the moving process, the bigger the micro-displacement of displacement amplification mechanism is, the greater the coupling displacement being vertical with the main motion. The discrete numerical method of Preisach model can not reflect the erasure characteristics, and the generalized Maxwell model brings a lot of calculation because the solution is too complicated.The bridge flexure hinge with displacement amplification function is designed, what can make FTS obtain the larger displacement and effectively eliminate the coupling of vertical axial movement by the symmetry at high frequencies. According to dynamic model, the optimal topology parameters of FTS are determined in the moving process. After constructing the ECNLP model, which explains the relationship among voltage, pressure and displacement, the pre-compensation algorithm is put forward combined with analysis of multi-source error. At last, it is proved the effectiveness of the work by the experiments, which includes the stiffness test, the machining test of different array micro-structure.1. Design the new structure of FTS. Based on principle of the rod inclined more than 45 degree, the displacements in both ends are significant different, the amplification mechanism of micro-displacement is designed.The mathematical model of magnification is established to solve the inconsistency between higher frequency and shorter displacement of FTS driven by the piezoelectric ceramic. The symmetrical bridge flexure hinge is designed to overcome the coupling displacement being vertical with the main motion during the moving process. The kinematics equation of equivalent planar mechanism is established, and the dynamic equations of bridge flexure hinge are built to obtain the dynamic model in the generalized coordinates. By the Cartesian method, the whole mathematical dynamics model of FTS in the absolute coordinates is developed. After considering the attribute of each part of FTS, such as quality, materials etc., and the topological structure of bridge flexure hinge is optimized to make the structure of FTS better.2. Explores the influence rule of dynamic characteristics on the structure parameters. The hinge form characteristics of bridge flexure hinge are analyzed, and then the results show the influence rule of different structure size parameters on the dynamic characteristics, such as the stiffness, natural frequency etc. By comparing analysis of elastic modulus, Poisson and density for flexure hinge material, the most suitable method is determined. By establishing the multi-body dynamics simulation model of FTS, the influence rules on the dynamic characteristics are represented, and also the rigidity, the natural frequency, the corresponding sensitive degree.3. Establish the control strategy of FTS. By analyzing the main characteristics of piezoelectric ceramic, the piezoelectric ceramic type is reasonably selected. The models of mechanical properties, electrical properties, mechanical electrical performance on the capacitor, power, electric field, displacement, pressure, voltage are established. And the mathematical relationships between the strain and the electric field intensity, between the pressure and the voltage, between the pressure and the micro-displacement are set up. With ECNLP dynamic model, it builds the relationship among the inertial force, the elastic force and the voltage for the bridge flexure hinge.On this basis, the full-scale voltage is divided into 15 equal segments; the experimental data is measured under the different voltage by linear interpolation method. The error between the theory voltage of ideal output and the simulation voltage is obtained through PID control theory and Preisach model. These can provide some theory basis for the error compensation strategy.4. Construction of multi-source error compensation algorithm. It analyzes the error sources of tool tip displacement for FTS, which includes the manufacturing errors of sensitive size for the bridge flexure hinge, the structure size of FTS due to the change of environmental temperature. And then the relationship is established between these errors and the tool tip displacement error. According to the relationship among the voltage, the force, the displacement, it proposes the strategy to compensate the angle error and the width error in the manufacturing process. And it also obtains the displacement error based on the temperature error model of RBF. The influence of inertial forces, elastic force, manufacturing error and temperature change on the displacement of piezoelectric ceramic is considered. Using PID control model and hysteresis nonlinear model of FTS, it puts forward the error pre-compensation to compensate the driving voltage of piezoelectric ceramic by the voltage in advance, thus it can reduce the effect of displacement accuracy on multi source error.Finally, after introducing the experimental platform of FTS and measuring tools, It proves FTS has a 3.5 times amplification displacement, which can overcome the coupling motion being vertical with main motion in the process. it explains the hysteresis nonlinear degree of FTS by the experiment. Compensating the driving voltage of piezoelectric ceramic, and it proves that the control compensation algorithm is effective for this new FTS by comparing the results before compensation to after compensation. Machining the different array icro-structure, it proved that FTS has the function of larger displacement and can machine the deeper array micro-structure. The dimensional precision can obtain sub-micron after comparing the surface roughness and the cutting precision. |
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