Hongzhao Liu

Modeling and Analysis of the Crossfeed Servo System of a Heavy-Duty Lathe with Friction

The performance of feed servo system has an important effect on the path tracking and positioning accuracy of computer numerical control (CNC) machine tools. The nonlinearity of friction can lead to the occurrences of unstable behavior of the feed servo system, such as stick-slip motion and oscillation. The crossfeed servo system of a heavy-duty lathe with closed loop control subjected to friction is studied in this article. The multi-degree of freedom (MDOF) mechanical and mathematical models of the crossfeed servo system are established. In the models, the torsional stiffness of shaft, meshing stiffness of gears, ballscrew-nut pair contacting stiffness, axial stiffness of the bearing, and the ballscrew and the kinematical friction force between up and down guideways are considered. The weakest link of the mechanical transmission system of the feed system is found by simulations. In the field test, the critical stick slip feed velocity at different feed position is measured by the Renishaw XL laser interferometer. And the relation curve of the critical stick slip velocity versus the axial stiffness of the ballscrew is obtained. The change tendency of the simulation results and that of the experimental results matches well. The research conclusions can provide a theoretical support for the optimal design and performance prediction of the feed servo system of heavy-duty lathes.

New Methods for Nonlinear Damping Identification of Damping Alloy

This paper presents two methods for the identification of nonlinear internal damping of alloy. One is the moving autoregressive model (MARM) method, and the other is the time-varying autoregressive model (TVARM) method. These procedures have been successfully implemented on two numerical examples. Comparison between simulation results demonstrates that the computation accuracy of the TVARM method is higher than that of the MARM method. In the experiments, the internal damping properties of the alloy Al-33Zn-6Si are researched, employing the rectangle beam with a nonuniform stress field and the trapezoid beam with a quasi-uniform stress field, respectively. Experimental results show that the internal damping of the alloy increases with the increasing strain and appears a nonlinear behavior. Moreover, the damping values of the trapezoid beam are relatively higher than those of the rectangle beam. Compared to the MARM method, the TVARM method can give a better description of nonlinear damping because the relation curve of loss factor versus strain obtained by the TVARM method is smoother than that obtained by the MARM method.

Research on the Identification Methods of Friction in Kinematical Joints of Mechanical Systems

This paper describes two approaches for the simultaneous identification of the coulomb and viscous parameters in kinematical joints. One is a time-domain method (TDM) and the other is a frequency-domain method (FDM). Simulation shows that both of the two methods have good performances in identifying friction at high SNR (90dB). But at low SNR (20dB), the estimation accuracy of the frequency-domain method is higher than that of the time-domain method. A field experiment employing a linkage mechanism driven by motor is also carried out. The experimental results obtained by the two approaches are almost identical under different experiment conditions. It has been concluded that the presented identification methods of friction in kinematical joints are correct and applicable.Copyright

Adaptive Friction Compensation of Servo Mechanisms With Nonlinear Viscous Friction

In this paper, a SDOF system with nonlinear viscous friction under harmonic excitation is studied. A nonlinear viscous friction model is built as an exponential expansion of the relative slip velocity. According to the adaptive control theory, an adaptive controller for the friction compensation of the SDOF system with nonlinear viscous friction is put forward by using the model-based parameterization techniques. The good performance of the proposed control scheme is illustrated through the computer simulation.© 2005 ASME

A New Method for the Identification of Coulomb and Viscous Friction

In this paper, a new method for simultaneously estimating Coulomb and viscous friction parameters from the free vibration of a damped oscillator is put forward. In the method, the nonlinear vibration equation with Coulomb and viscous friction is transformed into a linear procedure with equivalent viscous damping which is a function of velocity. The proposed method works well with both the displacement and velocity response data, while the case of zero velocity is not considered. From the displacement decaying curve, the equivalent viscous damping ratios are obtained by means of the local exponential fitting method, and different velocities corresponding to the equivalent viscous damping ratios are also obtained from the velocity decaying curve. Then, according to the relationship between the equivalent viscous damping ratio and the velocity, the Coulomb friction and the viscous damping are achieved using the least square method. The validity and accuracy of the proposed method are demonstrated through good simulation results.Copyright

Research on the Identification of Nonlinear Internal Damping and Kinematical Joint Damping of Mechanical Systems

A simultaneous identification method of the nonlinear internal damping of alloy material and the Coulomb friction in the kinematical joints in a mechanical system is put forward in this paper. The free vibration differential equation of a vibration system with these two kinds of damping is established. According to the free vibration response signal, the nonlinear internal damping characteristic curve and the Coulomb friction are got by using the moving rectangle window method. Several types of the nonlinear damping models are investigated in simulations. The validity and accuracy of the proposed method are illustrated by the good simulation results, in which the computing accuracy of the nonlinear internal damping is higher than that of the Coulomb damping in kinematical joints.Copyright

Modal Parameter Identification of Time-Varying Systems Using the Time-Varying Multivariate Autoregressive Model

A new time-varying multivariate autoregressive (TVMAR) model method for modal parameter identification of linear time-varying (TV) systems with multi-output is introduced. Besides, a modified recursive least square method based on the traditional one is presented to determine the coefficient matrices of the TVMAR model. In the proposed method, multi-dimensional nonstationary response signals of the vibrating system can be processed simultaneously. Not only the TV modal frequency and damping ratio of the system, but also the changing behavior of the mode shape in the course of vibration are identified by the proposed procedure. Numerical simulations, in which a three-degree-of-freedom system with TV stiffness is respectively subjected to impulse excitation and white noise excitation, are presented. The validity and accuracy of the method are demonstrated by the good simulation results.Copyright

Computing of Nonlinear Damping Using the Moving Autoregressive Model Method

Strictly speaking, internal damping of alloy materials is a function of temperature, frequency, strain and strain time rate and so on. Most of the previous papers with regard to damping computing only give a volumetric average when the alloy material is subjected to alternative stress. They cannot accurately describe the natural characteristic of damping. In this paper, the moving autoregressive model method (MARM) is presented to research the relationship between loss factor, strain and frequency of the alloys (Al-33Zn-6Si and Zn-27Al-1Cu). The experimental results show that the loss factor of alloy increases with the increasing strain, and increases with the increasing frequency in low-frequency region (below 400Hz). The damping appears strong nonlinear behavior. The three-dimension graph of the loss factor versus strain and frequency provides useful information for the optimum design of machine parts made from damping alloy.Copyright

Research on Nonlinear Damping Characteristics of Damping Alloy With Uniform Stress Field

For the high damping metal material like damping alloy, the damping capacity usually changes with the strain amplitude and frequency nonlinearly. First, to extract the pattern of the internal damping versus strain, two time-domain calculation methods are presented in this paper. One is the moving exponent method (MEM for short) based on FFT (MEM+FFT) and the other is the moving autoregressive model method (MARM). The computing accuracy of the two methods has been compared through numerical simulations. The nonlinear relation curve of loss factor versus strain is achieved by the impulse excitation experiment employing uniform stress field. Then, to extract the pattern of the internal damping versus vibrating frequency, the sine sweep-frequency excitation experiment based on the half-power bandwidth method is carried out. The resulting curve indicates that the internal damping is also a nonlinear function of frequency.© 2004 ASME