Design of an Adaptive Singularity-free Control Moment Gyroscope (ASCMG) actuator for agile and precise attitude control of cubesat

Abstract

Design considerations for agile, precise and reliable attitude control of micro-spacecraft using Adaptive Singularity-free Control Moment Gyroscope (ASCMG) actuators are presented here. A complete dynamics model of a spacecraft with an ASCMG is derived using the principles of variational mechanics, relaxing some assumptions made in prior literature on Control Moment Gyroscopes (CMG). The dynamics so obtained shows the complex nonlinear coupling between the internal degrees of freedom associated with an ASCMG and the spacecraft base body's attitude motion. By default, the general ASCMG model is equivalent to that of a Variable Speed Control Moment Gyroscope without symmetrical rotor and gimbal, and can operate as a CMG by spinning the rotor at constant speed. This dynamics model is then extended to include the effects of multiple ASCMGs placed in the spacecraft bus, and sufficient conditions for non-singular ASCMG cluster configurations are obtained to operate the cluster in CMG mode. The adverse effects of the simplifying assumptions that lead to the standard CMG design, and how they lead to CMG singularities, are described. A bare minimum hardware prototype of an ASCMG using low cost COTS components, is shown. A control scheme for agile and precise attitude pointing control of a cubesat using a finite number of ASCMGs in the absence of external torques, is presented. A Geometric Variational Integration scheme is obtained for this multibody spacecraft for numerical and micro-controller implementation.