An Innovative Spherical Airbearing-Based Testbed for Testing 3D Attitude Motion of CubeSats

Testing free rotation of a satellite in all the three axes in a ground-based lab environment is difficult but often required for development, testing and validation of a satellite’s attitude determination and control strategies for proximity and robotic operations required by many In-Space or On-Orbit Assembly, Servicing and Manufacturing (ISAM or OSAM) and space logistics missions. This paper describes an innovative design of a spherical air bearing-based testbed along with an automated balance alignment method. This method enables alignment of the unknown center of mass with the air-bearing’s center of rotation. Unlike many other spherical air-bearing-based testbeds, this one uses only three actuators to quickly align the floating platform, including the tested satellite, across all the three rotational axes without needing to know the mass center of the tested satellite. A two-step alignment control strategy has been developed to achieve this desired balance alignment. Step one uses the three compensator arms to align the mass center in the horizontal plane based on the measurements from an inertial measurement unit. The second step further adjusts the three weights (which in turns moves the mass center up or down along the vertical axis) to align the mass center with the rotational center of the spherical air bearing. This step is challenging because the final location of the mass center must be precisely at an unstable equilibrium location between the static stability and static instability of the floating platform. To achieve this, an active perturbation is introduced in the control method to help determine whether the platform is in a stable or unstable state, so that a proper sequential control action can be taken. This “twitch and drift” approach will be repeated until the required marginal equilibrium is reached and then, the actuators are locked in place and the system is ready for attitude motion testing of the attached satellite. A prototype of the testbed has been designed and built. The test data obtained has shown the desired function and performance. This paper describes the design of the testbed, the balance alignment control method, and the test results.