In this paper, a robust adaptive control scheme is proposed to reduce the effect of harmonic disturbances in feed drive systems. Harmonic forces are one of the most common disturbances in machine tools. They are generated, for instance, due the external cutting forces in milling or internally by the servo structure itself such as torque ripples of the motor and gear mechanism. If these sinusoidal disturbances could not be rejected by the feedback control, tracking performance of the drive system would be degraded. Hence, an adaptive torque ripple compensation technique is developed in this research, which can track the amplitude and phase of the sinusoidal disturbance forces synchronized with the servo position. A suppression signal is then generated to reject the specific frequency of the disturbances. In conjunction with the ripple compensator, a robust feedback controller is designed employing the variable structure control framework. Both, controller stability and the adaptation convergence are proven using the Lyapunov theory. The proposed controller is applied to various servo systems and its performance is verified.
In laser beam machining, the machining speed and quality are very sensitive to the flow of the assist gas and the position of the laser beam focal point. In order to increase the cutting speed and quality, and reduce the assist gas consumption, the real-time control of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control) is one of possible solutions. Furthermore, in order to increase the machining speed and the aspect ratio of machined hole in laser piercing, the high speed adjustment of the laser beam focal point is an effective solution. To satisfy the above requirements, a six-degree-of-freedom (6-DOF) controlled magnetic-levitated (maglev) lens drive actuator was developed. A lens holder was actuated by four novel electro-magnetic driving units. Each driving unit can generate both repulsive and attractive forces in radial and axial directions by utilizing a modified Halbach array on the lens holder and air core coils on the base. Radial displacement of the lens up to ±1mm with a tracking error of 1 micron and a bandwidth of more than 150Hz was achieved; and a ±5mm stroke of the lens with a tracking error of less than 3 microns and a bandwidth of more than 100Hz was also obtained in the axial direction.