Magnetic Attitude Control of Satellites Using Coarse Pulse-Width-Modulation of Magnetorquers

摘要

Magnetorquers enable energy efficient attitude control of microsatellites in low Earth orbits. The challenging problem for the magnetic attitude control is the limitations on their component capability, and high-frequency Pulse-Width-Modulation (PWM) actuations are difficult to implement. Moreover since the magnetorquers generate control torques into a plane orthogonal to the geomagnetic field, three-axis attitude control using only the magnetorquers is challenging. In this context, this study deals with the magnetic attitude control of satellites using only magnetorquers that are driven with coarse PWM. The coarse PWM actuations in this study mean that the magnetorquers are successively actuated by on-state and then off-state in each actuation cycle of a few seconds. The difficulties of the coarse PWM actuations stem from the followings: 1) Conventional techniques for feedback controllers are hard to be applied because the magnitude of the magnetic moment is constant. 2) The dynamics of the satellite has nonlinear terms in control torques due to coarse PWM actuations. First, the dynamics of the satellite with on-off inputs are formulated as a discrete system in order to deal with the coarse PWM actuations of the magnetorquers. Then, to tackle with the nonlinearity of the control torques, averaging method for general case is used for reducing to the problem that is dynamic quantizer is applicable. As a preliminary study, dynamic quantizer is designed for satellites in sun-synchronous orbits and is verified for the coarse input system.