Stable controller design of a six-degree-of-freedom magnetically suspended fine-steering mirror

Presently, there is a new generation of fine-steering mirrors (FSM) being developed using magnetic suspension. They possess the high bandwidth and line-of-sight stabilization characteristics desirable from an FSM, and eliminate the singlepoint failure concern of the flexure suspension system. These devices, while also providing active focus and collimation control, introduce an interesting servo control problem. Misalignments between the center-of-gravity (CG), actuator, and sensor operating axes present nontrivial balancing requirements. This paper discusses a technique developed by Ball Aerospace Systems Group (BASG) to establish servo control of a magnetically suspended FSM (MSFSM). The process is dependant on the mirror's mass property matrix and two unknown coordinate transformations. The first is a CG-to-actuator (decoupling) transformation defined by statically balancing the actuator forces. The second is a sensor-to-CG transformation characterized by injecting noise into the system and compensating for off-axis cross coupling. Using this technique, BASG has developed a 5-inch MSFSM having 600 hertz closed 1oop bandwidth in the pointing axes and 50 hertz in the suspension axes.