Dynamic Near-Optimal Control Allocation for Spacecraft Attitude Control Using a Hybrid Configuration of Actuators

This paper proposes a novel dynamic near-optimal control allocation scheme with combination of a saturated baseline controller for spacecraft attitude control using single-gimbal control moment gyros (CMGs) and reaction wheels. First, a saturated controller is proposed to stabilize the nominal system in the presence of actuation mismatch. Aided by a state-dependent variable, a dynamic control allocator is then proposed that allows for smooth switching between two actuation sets. Unlike the previous static constraint optimization formulations, the control allocation augments its performance function with penalty terms in order to enforce individual input constraints and configuration singularity avoidance. Moreover, this dynamic control allocation is implemented with an online update law, which has a modest computational complexity compared to its numerical optimization counterpart. The closed-loop boundedness is guaranteed by a constructive Lyapunov-design method. Simulation results demonstrate that during the attitude maneuvers, the input saturation constraint and the active avoidance of CMG singularities are enforced with a relatively small computational cost.

I. Introduction

In modern space applications, orbital spacecraft is expected to achieve fast maneuver and fine pointing for various operational missions. A hybrid actuator setting combining reaction wheels and single-gimbal control moment gyros (SGCMGs) is a promising actuation configuration to accomplish such a demanding mission [1], [2]. The reaction wheels (RWs) are widely used in aerospace attitude systems for their accurate response to control commands. In comparison, an SMCMG can generate an order of magnitude of reaction torque compared to RW, but with a relatively low torque resolution. Other actuation configuration concepts are also discussed in the literature, such as the well-studied double-gimbal CMGs [3], variable speed CMGs [4], newly developed double-gimbal variable speed CMGs [5], and tilted wheels [6], all of which can provide three-axis torque actuation with relatively small energy consumption and less singularity issues. However, generally speaking, the mechanisms of these actuation concepts are relatively complex, and many of them are still in the experimental phase or may require mission-tailed design. In contrast, both RWs and SGCMGs are low cost and off-the-shelf actuators. With this combination, RWs provide a hardware solution to SGCMGs singularities. Combinations of SGCMGs and thrusters are also of interest, but thrusters have to use nonrenewable fuel to place gimbals in favorable positions.