Two-Stage Automatic Mass Balancing and Attitude Control for 3-DoF Spacecraft Attitude Simulators

Abstract

Three-degree-of-freedom (3-DoF) air-bearing simulators are widely used for ground-based validation of spacecraft attitude determination and control systems. However, residual gravity torques caused by center-of-rotation to center-of-gravity (CR-CG) offsets can degrade simulation fidelity, particularly during coupled three-axis maneuvers. This paper presents a sequential coarse-to-fine automatic mass balancing framework, referred to as the Robust Automatic Mass Balancing Operator (RAMBO), designed to reduce residual CR-CG offsets under mechanical non-orthogonality and cross-axis coupling while maintaining a negative vertical offset for pendulum stability. The framework sequentially applies an active coarse stage (CRAMBO) for rapid offset reduction and a fine stage (FRAMBO) for precise CR-CG offset estimation using free-response motion data collected after platform reinitialization. The final FRAMBO estimate is subsequently used to construct an attitude-dependent gravity-torque feedforward term within a conventional quaternion proportional-derivative (PD) controller for the hardware attitude-control experiment. Hardware experiments demonstrated that the dynamic-equivalent CR-CG offset estimate converged to approximately [0.08, 0.12, -27.62] μm. Based on the onboard VN-100 estimates, the resulting hardware configuration maintained steady-state attitude errors within ±0.5° and angular velocity errors within ±0.4°/s for roll and ±0.2°/s for pitch and yaw during a coupled diagonal maneuver. A secondary IMU comparison further provided a roll/pitch consistency check rather than an independent ground-truth validation. These results demonstrate the hardware implementation of the two-stage RAMBO procedure and the subsequent use of its final estimate in safety-constrained three-axis attitude-control testing.