Development of a maglev lens driving actuator for off-axis control and adjustment of the focal point in laser beam machining

In laser beam machining, the machining speed and quality are very sensitive to the flow of the assist gas and the position of the focal point of the laser beam. In order to reduce the consumption of assist gas and improve the cutting speed and capability to remove molten material, and, in addition, enable the focal point to be adjusted in order to drill high aspect-ratio holes, a six-degree-of-freedom (6-DOF) controlled magnetic-levitated (maglev) lens driving actuator was proposed and fabricated. A novel air core coil type electro-magnetic driving unit was designed to actuate the lens holder. The driving unit can generate both repulsive and attractive forces by utilizing a modified Halbach array on the driven target and the air core coils. The air core coil windings allowed a spatial-cross-arrangement of the components, enabling the actuator to be sufficiently compact for it to be accommodated in the machining head of a laser beam machining tool. The proposed actuator can drive the lens to achieve real-time control of the axial position of the focal point and of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control). A range of ±5 mm with a tracking error of less than 12 μm and a bandwidth of more than 100 Hz was achieved in the axial direction, and a radial displacement up to ±1 mm with a tracking error of 1 μm and a bandwidth of more than 150 Hz was also obtained. Furthermore, under half sine wave pulse acceleration with a peak of 40 m/s2 and duration of 10 ms in the radial direction, sufficient positional accuracy of the lens holder could be maintained.