Development of a 2-DOF Rotary Ultrasonic Actuator for Reflector Steering

Free-space optical communication systems typically require reflector steering mechanisms (RSM) with large angular travel, compact design, and microradian-level resolution. However, conventional electromagnetic motors, are generally unable to satisfy all these actuating requirements simultaneously due to inherent technical limitations. Herein, this study presents a RSM driven by a two-degree-of-freedom (2-DOF) rotary ultrasonic actuator (RUA). The proposed compact RUA combines a second-order longitudinal vibration mode and two orthogonal fifth-order bending vibration modes to enable continuous 360° rotational motion while achieving $\boldsymbol{\mu}$ rad-level displacement resolution through sinusoidal pulse excitation mode. The working principles are validated through theoretical and finite element analysis. A prototype was fabricated with dimensions of 82.4 × 9.2 × 9.2 mm³ and a mass of 29.4 g. Experimental results demonstrate maximum 2-DOF rotational speeds of 121.19 rpm ( $\boldsymbol{\theta}$ _X ) and 42.79 rpm ( $\boldsymbol{\theta}$ _Y ) under a 518.7 g load, with minimum step displacements of 27.46 and 18.50 $\boldsymbol{\mu}$ rad, respectively. Furthermore, the 2-DOF rotational capability of the RSM is utilized to generate programmable trajectories, including geometric shapes and alphanumeric characters. The experimental results suggest that the proposed RSM represents a potential candidate for lightweight, high-precision optical steering applications in free-space optical communication systems.