Magnetic spacecraft attitude stabilization with two torquers

Magnetic control has been used successfully for momentum management and attitude control when pointing precision requirements are loose. Typically, such systems are constructed using configurations of magnetic coils oriented in a way to provide command authority by varying the magnetic moment about all three body axes. Although magnetic control actuators and related systems are typically robust and not as prone to failure when compared to reaction wheels or thrusters, it is of interest to see if coarse 3-axis attitude pointing can be achieved with the loss of control about one of the 3 axes of magnetic moment actuation. These types of cases could occur during the normal life of the spacecraft or could also occur intentionally on very small satellites where space is limited, so any control modes that could prolong the life of a damaged satellite or intentionally underactuated satellite would be useful for some missions. This paper demonstrates the feasibility of 3-axis pointing of spacecraft using only two axes of magnetic moment actuation using linear time varying analysis to provide sufficient conditions for controllability. The paper then develops an LQR-based controller for these two torquer cases. Finally, the paper concludes with simulations that indicate it is feasible to control with only two axes of magnetic moment, although with the expected increased time to achieve targeted attitudes and reduced disturbance rejection.