Research On Online Vibration Control Of Machine Tools Based On CNC System

With the improvement of NC machining efficiency,precision control and surface quality requirements in aerospace and other industries,the machine tool vibration has a very important influence on the processing performance.Even a microvibration in the cutting process will make the workpiece size or surface roughness out-of-tolerance.With the development of intelligence in modern CNC machine,it is an trend for CNC system to realize real-time and on-line vibration controlling directly by system itself.Taking the NC milling vibration control as the goal,proceeding from the two aspects of driving residual vibration and milling vibration,an on-line vibration control technology based on CNC system werethoroughly studied in this thesis.The work includes a look-ahead algorithm with adjustable real-time feed rate,a curve fitting method of curvature pre-smoothing,a feedrate planning method of Jerk continuous,a curve interpolation algorithm based on minimization of feedrate fluctuation,a control method of hybrid dynamic vibration absorber and a CNC system with online vibration control function.The high speed driving of each axis is an important source of machine tools vibration in CNC machining.Generally the motion algorithm based on existed S-shaped acceleration and deceleration is complicated,and its maximum feed rate in one interpolation line can not be real-time adjusted,a look-ahead algorithm with adjustable real-time feed rate is proposed to meet the needs of real-time vibration reduction and speed smoothing.The maximum transfer feedrate of adjacent segments based on the acceleration constraint is first calculated in the algorithm,and the feedrate and acceleration could be adjusted adaptively according to the principle of acceleration following in the algorithm,so that the whole line segment interpolation process has high flexibility and rapitidy,and the real-time vibration suppression is improved by adjusting the motion parameters in the CNC system.Curve fitting and interpolation for micro segments is an important way to realize vibration reduction in five axis CNC machining.The existed researches often overlooked the curvature fluctuation brought about by the precision of original cutter location points,and therefore the the following curve interpolation feedrate fluctuates greatly,which unfavorable to the high-speed and steady machining.By analyzing the geometric characteristics of the original cutter path,a method of curvature pre-smoothing under the constraint of fitting error is proposed in this paper,and then a weighted and constrained least squares NURBS curve fitting method is proposed,the cutting path of machining contour is greatly restored,the impact is reduced which caused by the discontinuous rotation of the rotating shaft and the rapid rotation in the high curvature region of the tool path,a smooth tool path is created for stability control of the curve interpolation feedrate.Based on the double NURBS curves five axis interpolation based on RTCP(Rotational Tool Center Point)function,considering the limit drive capacity of the machine feed axis,limit cutting parameters of tool center and the integrated contour error control,the maximum feed speed in each interpolation cycle in curve interpolation is calculated.According to the existing S shape acceleration and deceleration feedrate planning with jerk mutation problem,a S shapeacceleration and deceleration methods with jerk continuous is proposed to improve the velocity planning,to further reduce the flexible impact of curve interpolation.The interpolation feedrate fluctuating caused by truncation error in conventional double NURBS curves interpolation is discussed,and a NURBS secant interpolation algorithm based on minimization of feedrate fluctuation is proposed.The machine drive axis movement mutation,machine tool vibration and workpiece overcut caused by the high curvature area in NC high-speed machining are avoided effectively,the smooth processing is achieved.The spindle rotating in high speed machining is another important factor which caused the vibration of the machine tools.At present,there are many researches on the vibration control of non rotating tools such as turning and boring,but there is little research on the milling vibration of high speed rotating tools.For the existing vibration control methods,which can not adapt to the complex machining conditions,by adding an additional active control function on the passive vibration absorber,an active control system of hybrid dynamic vibration absorber is designed.Taking the displacements and velocities as feedback signals,based on high speed communication with CNC system,a relevant control algorithm is adopted to control the milling cutter directly by actuators,so as to control the spindle vibration.A dynamics model of cutter and spindle vibration of two degree of freedoms(DOFs)is established,the equations of system states are derived,and the vibration control model is simulated by using Linear-Quadratic-Gaussian(LQG)control methods.then the experiments are carried out in milling processing to control the spindle vibration in a CNC engraving and milling machine.Finally,the active system is effectively combined with the CNC system developed.For the related theories proposed in this paper,some of them are simulated by computer to verify the accuracy and feasibility.Based on the analysis of the characteristics and development trend of modern CNC system,a motion control card and NC system software with vibration suppression module is designed in detail.At last,the running test and cutting test of typical parts are verified by experiment.The results show that by the proposed look-ahead algorithm with adjustable real-time feed rate,the tool path fitting optimization algorithm and NURBS curve interpolation feedrate control method based on minimization of feedrate fluctuation,and then with active vibration control based on the spindle hybrid dynamic vibration absorber,the smooth feedrate of NC machining could be achieved,machine movement impact and cutting vibration could be reduced,thereby improving the surface quality of parts and the processing accuracy.The algorithm proposed in this paper can effectively reduce the vibration of five axis CNC milling.The work presented in this thesis may be worthily used to study on the online vibration control of machine tools based on CNC System and the realization of the intelligence system.