Design and precision tracking control of a high-bandwidth fast steering mirror for laser beam machining

Abstract: In this article, we report on a high-bandwidth large-aperture fast steering mirror (FSM) actuated by piezoelectric actuators (PEAs) for laser beam machining and a control strategy to achieve high bandwidth and precision tracking control. To simultaneously achieve high bandwidth and a large aperture, we used a low-density and high-rigidity 50-mm round mirror to ensure sufficient stiffness and to minimize the moment of inertia. Also, the overall structure of the moving part in the FSM was simplified to further reduce the moment of inertia. Based on modal analysis, and taking into account the conflicting requirements for the natural frequency and the stroke, the thickness of the base plate used to provide the preload force to the PEAs and to transmit motion to the mirror was determined. Moreover, to achieve high bandwidth and precision tracking control, we propose an integrated control solution, in which an inner digital charge control (DCC) loop is used to eliminate hysteresis, an outer displacement loop with a design based on pole-zero cancellation and loop shaping is introduced to eliminate resonance, and feedforward compensation is applied to reduce the phase lag. The feasibility of an FSM with this control solution was validated by experiments. A bandwidth of 10 kHz with a resolution of less than 0.3 μrad can be achieved for the FSM. Moreover, precision tracking with a maximum radius error of less than 2.2 μrad below 2 kHz for a circular trajectory at a radius of 25 μrad can be realized.