Control torque generation of a CMG-based small satellite with MTGAC system: a trade-off study

Abstract

Abstract. In this paper, the gimbal angle compensation method using magnetic control law has been adopted for a small satellite operating in low earth orbit under disturbance toques influence. Three light weight magnetic torquers have been used to generate the magnetic compensation torque to bring diverge gimbals at preferable angle. The magnetic control torque required to compensate the gimbal angle is based on the gimbal error rate which depends on the gimbal angle converging time. A simulation study has been performed without and with the MTGAC system to investigate the amount of generated control torque as a trade-off between the power consumption, attitude control performance and CMG dynamic performance. Numerical simulations show that the satellite with the MTGAC system generates more control torques which leads to the additional power requirement but in return results in a favorable attitude control performance and gimbal angle management. IntroductionControl moment gyroscope (CMG) is a momentum exchange device that controls satellite attitude based on the conservation of angular momentum of the satellite system [1]. It has a torque amplification characteristic which makes it as a perfect actuator for fast slew maneuver and precise attitude pointing control over other actuators such as momentum wheels and reaction wheels. These features increase the operational envelop of the satellites and subsequently increase the return of mission data and reduce payload complexity [2,3]. Control torque is generated by rotating the CMG's gimbal at certain gimbal angle rate. This causes the direction of the flywheel's angular momentum vector to change and results in a generation of control torque, orthogonal to both flywheel's spin and gimbal axes [4]. There are three types of CMG i.e. single gimbal CMG (SGCMG), double gimbal CMG (DGCMG) and variable speed CMG (VSCMG). Among these three, SGCMG is less complex and cheaper than VSCMG and it has more significant torque amplification over DGCMG [5][6][7]. However, SGCMG has drawback in term of singularity where at certain gimbal angle orientations, the control torque cannot be generated by the CMG cluster thus resulting in a loss of satellite attitude controllability. The singularity problems have lead towards the development of singularity avoidance or steering laws that can avoid or escape the CMG cluster from singular states [8][9][10][11][12].Another concern about the CMG system is the gimbal angle offset due to disturbance torques. The constant disturbance torques cause the gimbals to drift away from preferred or reference angles. This situation reduces the repeatability of slew maneuvers and can also drive the CMG cluster into saturation singularity state which most of the steering laws fail to avoid or escape. Since the CMG