Precise Beam Steering Using a Hemispherical Gimbal for the Cube1G Laser Communication Terminal

Abstract

Free-space optical communications on CubeSats demands compact and precise pointing systems that minimize dependence on satellite body attitude. This paper presents the design and validation of a gimbaled-prism coarse pointing assembly for the Cube1G mission, enabling hemispherical pointing on CubeSat scale. The mechanism uses direct-drive brushless DC motors with absolute encoders for azimuth (356 deg) and elevation (80 deg) control, integrated with a compact launch-lock system. A hierarchical control strategy combines a cascaded servo loop for current, velocity, and position control with an s-curve profiler for smooth ephemerisbased tracking. During acquisition, a pre-deformed spiral scan compensates for the gimbals spherical coordinates, and once the beacon is detected, a quadrant photodiode-based PI loop closes the optical feedback. Experimental validation shows the emission of low mechanical vibration (dominant at 100 Hz) during a full rotation and stable closed-loop performance. The coarse pointing assembly maintains open loop pointing within the field of view for accelerations up to 1 rev s−2 and velocities up to 0.1 rev s−1. For the closed-loop tracking phase, a total optical mean pointing error of 11.1 µrad is maintained.