| As a necessary and key functional component in the five-axis CNC machine tools, the A-axismilling head has complex structure, a large number of parts, compact transmission chain andother characteristics, which is difficult to drive, control and guarantee the stiffness. Comparedto the machine tool body, design, manufacture and assembly of the A-axis are more difficult.Therefore, parameters level of the A-axis has an important influence on the processing quality,which is crucial for enhancing the processing capacity of the machine tool. Research focusesof the A-axis milling head can be summarized as improving the transmission and positioningprecision, increasing the drive torque and improving the milling head stiffness. Currently, allthe performance indicators have been improved. However, successful case that taking all theindicators into consideration is few. Therefore, analyzing systematically and improving theprecision and torque of the A-axis are particularly important. Usually, increasing the anglerange, improving the response speed, improving the reliability of milling head in complicatedenvironment are also significant in the design process.Due to the heavy load, high speed, large impact, large cutting force, serious deformation andother issues in the machining process of the efficient and powerful milling machine for bliskmanufacturing, in order to strengthen the shear resistance in the milling process, guarantee themilling process of the efficient and powerful milling machine for blisk manufacturing carriedout smoothly, designing and manufacturing of the A-axis with high power, high torque, highprecision and high rigidity is necessary. Based on the National Science and Technology MajorProject of the Ministry of Science and Technology of China, development and application ofthe efficient and powerful milling machine for aero-engine blisk manufacturing (Project No.2013ZX04001081), nonlinear dynamic characteristics and high precision control technologyof the A-axis with high power, high torque, high precision and high rigidity for milling thetitanium alloys, high-temperature alloys and other difficult to machine materials are studieddeeply and thoroughly, the main work and achievements are as follows.To satisfy the high power, high torque, high precision and high rigidity of the A-axis, at first,from the layout, drive mode, locking systems and anti-backlash solution, mechanical structureof the A-axis is designed. Secondly, according to the architecture, IPC, motion controller,servo drive system, motorized spindle, angle encoder and etc., control system of the A-axis isdesigned. Finally, combining the mechanical structure and the control system, overall design of the A-axis is accomplished in detail.In the research of flexible nonlinear dynamics, firstly, deformation amount of the engagementteeth is calculated based on the circular plate theory and the finite element theory respectively,form factor and its variation pattern of the deformation are derived by comparing those twocalculation results. Secondly, flexible nonlinear dynamics model of the A-axis are established,nonlinear friction and combined elastic coefficient are identified and calculated, and influenceof the parameters variation on dynamics of the A-axis are analyzed. Finally, deformation andfriction in the A-axis are compensated by designing the sliding mode controller (SMC) basedon the extended state observer (ESO).In the research of dynamic characteristics of the A-axis under parameter perturbation and loaddisturbance, first of all, relationship among the starting torque, the load torque, the movementdirection and the system parameters are studied, the mathematical nature of self-locking in theworm drive is revealed and the dynamics equations of the A-axis with nonlinear parametersand uncertain load torque are established. Secondly, two adaptive sliding mode controllers aredesigned based on the bipolar sigmoid function and hyperbolic tangent function respectively.Adaptive laws of the parameters in these two controllers are designed based on the Lyapunovtheory, therefore, stability of the closed-loop control system can be guaranteed. Thirdly, robustcontrol algorithm (LQSMC) is designed based on the linear quadratic optimal control (LQC)and the SMC. Based on the state space expression of the A-axis and LQC, this method canimprove the state space model and define a new sliding surface by introducing the stateestimation based on Kalman filter and control input, and the improved control algorithm isclose to the performance of LQC and can suppress the chattering of SMC effectively. Finally,two integral SMC are designed according to different dynamic characteristics of the static anddynamic friction, and input chattering in the traditional SMC can be suppressed effectively byintroducing the integral SMC.For the nonlinear function and uncertain load in the dynamics model of the A-axis, universalapproximation of the fuzzy system is adopted to approximate the external disturbance and themodel information of controlled object, and then, an adaptive fuzzy sliding mode controller isdesigned based on the proportional and integral control law. Finally, integrated control systemof the A-axis is constructed based on the designed six SMC, which further improved thecontrol precision and robustness of the A-axis system. |
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