Research Of Nonlinear Dynamics And Reliability Of Numerical Control Feed System And Main Kinematic Joints

Feed system is the control and motion actuator of NC machine tool.Its dynamic characteristics directly affect the quality of workpiece and determine the performance and reliability of NC machine tool.Considering the effects of installation error,dimension error,elastic deformation of connectors,dynamic cutting force and other factors,the vibration of feed system is inevitable in the machining process.Therefore,it is important to analyze the dynamic characteristics of feed system and main kinematic joints for restraining the vibration and improving the accuracy and reliability of of NC machine tools.The main research studies and achievements are following:(1)Considering the coupling effects between wear and contact characteristics of linear guide,an improved wear prediction model is proposed.Based on the Hertz contact theory and Archard wear model,the wear evolution is explained to study the dynamic performance of linear guide in the long-age service.Meanwhile,a new contact stiffness and the piecewisenonlinear dynamic model are established to analysis the dynamic behaviors of linear guide system.Moreover,some affecting factors in wear rates such as running distance and vertical load are discussed.To analyze the effects of wear on the dynamics of system,some basic properties such as frequency response,Lyapunov exponents,time histories,phase plane diagram and Poincare mapping are analyzed with numerical simulations.(2)Two multi-degree-of-freedom models are developed to investigate the vibration characteristics of feed system with nonlinear kinematic joints.Linear guide,ball screw,and ball bearing are the main kinematic joints that connect and support the work table of a feed system.Considering the contact behaviors of rolling balls,contact force is a piecewise smooth function of displacements of the system.In terms of modeling,geometry and material parameters are determined according to the experimental feed system and the model is verified by a dynamic experiment.The motion equations of the system are derived to evaluate the effects of force excitation and screw-nut position on the vibration amplitude and stable state of the vibratory system.In order to better investigate the effects of excitation amplitude,excitation angle,installation distance and height of workpiece on the dynamic vibration,frequency-amplitude curves,3-D frequency spectrums,time history,frequency domain,phase diagram,and Poincare section are employed to present the vibration properties.(3)In order to investigate the dynamic vibration of gantry-type machine tool system in the machining process,a typical 3-freedom parallel feeding mechanism is studied.Through analysis of relative displacements and deflections between each axis of feed system,the nonlinear interface forces of main kinematic joints are obtained.Moreover,using the halfpower bandwidth method,the damping ratio is estimated according to the frequency test.Based on the multi-body kinematics theory and rigid-body assumption,multi-degree-of-freedom equations of gantry-type machine tool system is established in relative coordinates.According to the principle of vector addition,the vibration errors of gantry-type machine tool system are obtained,which are caused by dynamic cutting force.Finally,the effects of excitation frequency,mass and position worktable,and initial deformation of kinematic joints on dynamic characteristic and stability of the system are studied.(4)The dynamic reliability and sensitivity analyses of feed system and linear guide with random geometric parameters are carried out,respectively.To analyze the dynamic performance of linear guide system,nonlinear vibration equation is established by employed a stiffness model,which takes the uncertainty of the geometric parameters into account.The failure mode of the linear guide system is defined based on the walking parallelism tolerance.The statistical fourth moment method is introduced to obtain the first four moments of system response and state function.The Edgeworth series is applied to approximately determine the probability distribution function of the state function indicating the positioning precision of the linear guide.The state function of gantry-type machine tool system is implicit and nonlinear.Then,a method,which combines Kriging model and active learning function,is more applicable to analyze the machining nonlinear vibration reliability and sensitivity.Monte Carlo simulations are performed as comparisons to verify the validity of the methods.Finally,the influence of random geometric parameters on dynamic characteristics and positioning precision is carried out to provide theoretical basis and reference for the design of feed system.