Study On Structural Dynamics Characteristics Of A Lorentz Planar Motor

Lorentz planar motor is a novel permanent magnet planar driving system propelled by linear Lorentz force, which consists of three Lorentz linear motors (Y1,X and Y2). It can achieve 3 degree of freedom positioning, i.e. translation along the X and Y directions and rotation about Z axis, Rz. The positioning stage is drived directly by Lorentz planar motor. Some microvibrations from external or internal excitations will impact the positioning accuracy of the stage. Therefore, dynamic characteristic and normal vibration of Lorentz planar motor should be deeply researched. In the thesis, the Lorentz planar motor is our research object. Our research method is theoretical analysis, numerical simulation and physical experiment. Some theories of structure dynamics, flexible mechanism and electromagnetic field are adopted. The normal electromagnetic parameters are identified using 3D finite element method and modal testing. The 3D electromagnetic finite element model, structural dynamic model and normal vibration model of Lorentz planar motor are achieved by numerical computing and physical experiment. Researches are focusing on the dynamic characteristic analysis and normal vibration transmission of the Lorentz planar motor. Contents and results as follows,Firstly, the 3D magnetic field in the airgap generated by permanent magnets and coils is derived by using the equivalent surface current method. The relations of sizes and positions of magnets and coils, surface magnetization current density and magnetic field of airgap are obtained. Magnetic flux linkage equations and mathematical model of Lorentz planar motor are established. Based on calculation and analysis of Lorentz forces, an electromagnetic dynamic model, which is ready for motion control in X, Y and Rz directions, is established. The normal vibration transmission between the stator and mover is illustrated. Normal vibration models of the stator and mover are built separately, which is used for obtaining the relations among structure deflections, electromagnetic forces and currents.Secondly, the normal electromagnetic stiffness is identified using 3D finite element method and modal testing. The electromagnetic damping ratio is effectively identified by modal testings. According to the actual connections and constraints of Lorentz planar motor, structural dynamic finite element model of the planar motor system is established by adopting manually hexahedral mesh and loading equivalent normal electromagnetic springs. Based on structural finite element model, dynamic computation procedures are compiled and the structure dynamic characteristics are in-depth computed, including the normal frequencies, frequency response and transient response.Then, in order to reveal some important parameters influencing on dynamic characteristics of Lorentz planar motor, we focus on analyzing the excitation current, Z-displacement and velocity in motion directions, which is a solid foundation for the motion control strategy and stuctural optimization of the Lorentz planar motor.Finally, a testing system of Lorentz planar motor has been set up, and the magnetic field in airgap and motor's thrust constant by experiments are achieved. The distribution of airgap's magnetic field by experiments verifies that 3D magnetic field simulation results are in good agreement with experimental results. The normal frequencies, model shape, frequency domain response and time domain response are obtained by modal testings using LMS system. Modal testing results verify that dynamic parameter identifications are accurate and structural finite element model is reasonable.In this dissertation, the dynamic characteristics of Lorentz planar motor are revealed, and the phenomenon of vibration transmission between stator and mover is clarified, and the methods of parameter identification of electromagnetic dynamics are proposed. These simulation models and methods will provide theoretical guidelines for mechanical structure design, optimization and control strategies, as well as the conclusions have been effective used in the research on the ultra-precision positioning stage.