Precision Motion Control Of Liner Motors With Nonlinearity And Flexibility Characteristic Analysis And Compensation

Modern mechatronic systems, such as advanced machine tools, microelectronic and semicon-ductor manufacturing equipment, optical inspection systems and dispensing processes systems often require high-speed and high-accuracy linear movement. Direct-drive linear motors eliminate gear related mechanical transmission problems such as backlash, large friction and inertial loads, and structural flexibility, and thus, have the potential of achieving higher speed and higher accuracy. But to realize its high-speed/high-accuracy potential, some control issues have to be solved:sig-nificant model uncertainties, parameter uncertainties and external disturbances; various nonlinear dynamics; high-frequency flexible modes. Thus, this dissertation focuses on major nonlinearities and flexibilities of linear motor driven systems. Physical modeling of cogging force, dynamic fric-tion and nonlinear electromagnetic field effect are developed and validated by the system identifica-tions in time domain. These novel nonlinear dynamical models capture the physical characteristics more accurately and also consider their complexity for compensation control design. The high performance adaptive robust control (ARC) technique is integrated with the effective nonlinearity compensations. Theoretically, these proposed control algorithms guarantee excellent transient and steady-state performance. And comparative experimental results also show the further improved tracking performance of the effective nonlinearity compensations. The physical cause of major high-frequency dynamics is identified. The corresponding mathematical model is then built, and verified by the system identification in frequency domain. With the knowledge of high-frequency dynamics, the optimal tuning guidelines of control gains are developed to maximize the tracking performance of the previously proposed control algorithms. To further increase the achievable close-loop bandwidth, the high-frequency dynamics neglected in the existing research are then ex-plicitly taken into consideration in the design of controllers. Specifically, the simple pole/zero can-celation technique is first incorporated into the control design to attenuate the major flexible mode effect. A novel μ-synthesis based adaptive robust control strategy is then developed. The proposed control algorithm uses adaptive model compensation having accurate on-line parameter estimation to effectively deal with various nonlinearity effect and to transform the difficult trajectory tracking control problem into a robust stabilization problem. The well-developed μ-synthesis based linear robust control technique is then employed to deal with the robust control issue associated with the high-frequency dynamics explicitly to achieve higher close-loop bandwidth and better disturbance rejection in the feedback control loop.The dissertation consists of the following six chapters:In Chapter1, the research background and history of precision motion control of linear motor driven systems are detailed. Specifically, the control issues associated with the high-speed/high-accuracy movement of linear motor drive systems are first pointed out, followed by a comprehen-sive literature survey of linear motor driven systems, including existing control methods dealing with nonlinearities such as cogging force, friction and nonlinear electromagnetic field effect and high-frequency flexible modes. A brief summary of the dissertation’s contributions and significance is subsequently given.In Chapter2, the hardware equipment and their specifications used in the experiments are introduced. Rigid-body dynamics of linear motor drive systems are presented with a focus on better modeling of major nonlinearities inherited to the linear motor drive systems. Specifically, an aperiodic cogging force model is built by using B-spline functions, which captures both periodic and aperiodic characteristics of cogging force. The modified LuGre dynamic friction model is proposed to solve the passive problem and the observer instability problem of the existing ones. A third order polynomial model is developed to precisely describe the nonlinear electromagnetic field effect as well. All the proposed models not only provide a more accurate description of the nonlinearity under study, but also can be easily used in the subsequent controller design for more effective on-line model compensation. The system identifications in time domain are then carried out, validating the effectiveness of all proposed models.In Chapter3, based on the rigid-body dynamics of linear motor drive systems, the concepts and various implementations of adaptive robust control (ARC) strategy are introduced. Theoretically, the presented ARC can achieve a guaranteed transient and steady-state performance in the pres-ence of both parametric uncertainties and model uncertainties, as well as zero steady-state tracking error when subjected to parametric uncertainties only. The adaptive robust control technique is subsequently integrated with more effective compensations of various nonlinearities, including the adaptive robust control with aperiodic cogging force compensation, the adaptive robust control with dynamic friction compensation using modified LuGre model, the adaptive robust control with elec-tromagnetic nonlinearity compensation, and the adaptive robust control with integrated compen-sation of all major nonlinearities of linear motor drive systems. Comparative experimental results show the effectiveness of the proposed nonlinearity compensations and the excellent performance of the proposed control algorithms. In Chapter4, physical causes of the high-frequency dynamics of linear motor drive systems are analyzed. The stage rotation is found to be the cause of the major flexible mode of linear motor driven systems. The corresponding mathematical model is then built to capture this ef-fect and system identifications in frequency domain are carried out to verify the effectiveness of the proposed model on the high-frequency dynamics of linear motors. With the knowledge of these high-frequency dynamics, the relationship among high-frequency dynamics, control gains and close-loop bandwidth is established, and the optimal tuning guidelines on control gains are de-veloped to maximize the tracking performance of the rigid-body dynamics based control algorithms of linear motors in implementation.In Chapter5, to further increase the achievable close-loop bandwidth, the high-frequency dy-namics are explicitly taken into consideration in the design of controllers. Specifically, the simple pole/zero cancelation of known high-frequency dynamics is first incorporated to attenuate the ma-jor flexible mode effect so that the upper bound on the achievable close-loop bandwidth of the rigid-body dynamics based control algorithms can be raised. With the knowledge of nonlinear rigid-body dynamics and the structure of high-frequency flexible modes, a novel μ-synthesis based adaptive robust control algorithm is then proposed. Its adaptive feedforward loop has accurate on-line parameter estimation and effective nonlinearity compensation, which makes it possible to convert the difficult tracking control problem into a robust stabilization problem. By treating ma-jor high-frequency dynamics as a part of the nominal model, the μ-synthesis feedback loop of the proposed novel ARC strategy can achieve higher close-loop bandwidth and better disturbance re-jection. Comparative experiments are conducted and the results show the better performance of the proposed control algorithms over existing ones.In Chapter6, the research work of this dissertation is summarized. Major innovations are highlighted, and some future research directions are discussed.