Nonlinear control of dual-spin spacecraft during despin through precession phase lock

When a dual-spin spacecraft is placed on orbit, thrusters bring the rotor and platform to an all spun condition. Then, during despin an internal torque motor aligned with the main bearing is activated to increase the rotor rate and decrease the platform rate, until the platform rate is near zero. We focus on the capture dynamics of precession phase lock, a phenomenon that can prevent successful despin. A spacecraft model used to illustrate precession phase lock contains an axially symmetric, dynamically unbalanced rotor. During precession phase lock, the unbalanced rotor becomes synchronized with the inertial free precession of the spacecraft. If the despin motor torque is constant and small relative to unbalance size, then precession phase lock causes a large secular deviation of the bearing axis from its desired direction. One common despin strategy to avoid this is to apply constant torque at the maximum capability of a relatively large motor. An alternative based on nonlinear closed-loop feedback control of despin motor torque, exploiting a priori knowledge of states favorable for despin through precession phase lock, is developed. Computer simulations demonstrate that this strategy can lead to successful despin when constant torque cannot-even with relatively limited motor torque.