Studies On The Key Techniques Of Improving The Contour Precision Of Large-scale Crankshaft In Tangential Point Tracing Grinding

Now days the major countries having the developed machine tool industry make the high-speed, high-precision and complex machine tools as their development goals. The production capacity and technical level of such machine tools have become an important symbol to measure manufacture level of countries. Therefore China has already attached strategic importance to the machine tool industry and launched National Science and Technology Major Projects to produce high-end digitally controlled machine tools and key manufacturing equipments since 2009. This dissertation is derived from and supported by one of the National Science and Technology Major Projects which aims to research and develop the large-scale tangential point tracing grinding machine for crankshaft.This dissertation focuses on the key techniques of contour control for the large-scale crankshaft in its tangential point tracing grinding, including the crankshaft grinding movement model, the movement trajectory approximation and interpolation method, the torsion deflection's influence on the crank pin's shape and position accuracy, the grinding carriage feeding servo system with the characteristic of high precision and fast dynamic response and the movement trajectory error control method.Based on the tangential point tracing grinding motion model for crankshaft, two grinding control methods are researched by which the constant rotational speed of crankshaft and the constant equivalent grinding thickness are realized respectively. Aiming at the characteristics of large-scale crankshaft grinding, the control method based on the optimization of crankshaft's motion law is proposed through combining the features of the above two methods. The first step of this method is to construct the curve to describe the motion of crankshaft and build up its optimization model according to the influence factors on crank pin grinding precision. And then the adaptive multi-bit mutation genetic algorithm is adopted to solve the model for the optimal parameters of the constructed motion curve. The simulation analysis demonstrate when the crankshaft's movement is controlled according to the optimal motion law, the variety of equivalent grinding thickness during grinding process is reduced as well as the demand of the dynamic performance of servo system. And these results show the new control method is beneficial to the machining precision of large-scale workpiece and more suitable for tangential point tracing grinding control of the large-scale crankshaft.The approximation effects of equal-error beeline approaching method, spline curve approaching method and the dynamic sectioned polynomial approaching method for crank pin tangential point tracing grinding motion trajectory curve are comparatively studied. And then the analyses on the theoretical error of tangential point tracing grinding motion trajectory produced by point by point comparison interpolation, time-Sharing Interpolation and cubic spline interpolation are carried out, respectively. The analyses display that the jerk of grinding motion keeps constant in the interpolation cycle using cubic spline interpolation method, which satisfies the demand on interpolation speed and precision of crank pin tangential point tracing grinding movement. Furthermore, using different curve approaching methods and path interpolation methods, the experimental studies for the effects on contour precision and surface quality of crank pin are conducted. And the experimental results demonstrate that the combination of dynamic sectioned polynomial approaching and cubic spline interpolation method can obtain the best contour precision and surface quality of crank pin.On one hand, the big crank radius and the heavy quality cause the large-scale crankshaft to have great moment of inertia. Therefore a great driving moment is needed to rotate the crankshaft in grinding. On the other hand, the span and length to diameter ratio of the large-scale crankshaft are big and the structure and the distribution of crank pins are complex, which makes the torsional stiffness of crankshaft insufficient. Thus it is easier for crankshaft to generate biggish torsional deflection which finally results in the shape and position errors of crank pin in tangential point tracing grinding. Therefore based on the analyses of force and torsional deformation of crankshaft under the different crankshaft driving modes, including only driven by the headstock and synchronized driven by both the headstock and the tailstock, the relationship between torsional deformation and the rotation angle error of crankshaft in tangential point tracing grinding is discussed. And then the torsional deformation's influences on the parallelism and contour precision of the crank pin are also deeply researched.Grinding carriage feeding servos system is the critical component of tangential point tracing grinder, which plays an important role in ensuring the contour precision of crank pin. The detail studies are conducted on the grinding carriage feeding servo system with the characteristic of high precision and fast dynamic response. In the study, the direct driving pattern using the linear electric motor is compared with the traditional driving pattern using electric rotating machinery and ball-screw, which shows that the former driving pattern is more suitable for the large-scale tangential point tracing grinder. On the basis of the servo stiffness model of grinding carriage direct-driving servo system, the factors which influence the feed system servo stiffness are deeply analyzed to ensure the enough servo stiffness of grinding carriage direct-driving servo system. And then the compound feed-forward P/PI position control is proposed to reduce the following error of the grinding carriage direct-driving servos system. The results of relevant tests on the experimental tangential point tracing grinder validate the efficiency of this control method in enhancing the position tracking performance of grinding carriage direct-driving servo system.Finally the cross-coupled control method is introduced into the Trajectory Control of crankshaft tangential point tracing grinding movement. Aiming at the nonlinear feature of crankshaft tangential point tracing grinding movement trajectory, the cross-coupled control with variable parameters for different linear segments and the cross-coupled control based on feedrate fuzzy control are proposed and their effects on crank pin grinding movement trajectory control are discussed, respectively. Both the simulation analysis and the relevant experimental result indicate that cross-coupled control has a positive effect on improving the contour precision of crank pin in tangential point tracing grinding.