Navy

Improved satellite attitude control with control moment gyroscopes

System determines and implements spacecraft maneuvers while maintaining operation within torque and other physical or operational limits

Communications Sensors

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A typical spacecraft is equipped with an array of three or more control moment gyroscopes (CMGs) to facilitate attitude control of the vehicle. Many spacecraft systems are used in a manner that demands expeditious reorientation.

For instance, it is essential in many satellite missions to perform attitude maneuvers as rapidly as possible, where the capacity of commercial Earth-observing satellites can be improved by maneuvering more quickly to reduce the time needed to slew between image regions. By decreasing the slew time, it is possible for the satellite to acquire additional images and maximize revenue.

CMG momentum control systems create a unique challenge for controlling the attitude of a spacecraft. In particular, unlike an array of reaction wheels, whose torque capability is fixed concerning the vehicle frame, the torque capability of a CMG array varies continuously with the gimbal angles. Consequently, ensuring accurate torque production on the spacecraft requires proper configuration of the CMGs relative to one another to avoid gimbal configurations that result in singular states wherein torque cannot be produced in a specific direction.

CMGs also have gimbal rate and input torque constraints that can be violated during the operation of the spacecraft, particularly if the spacecraft angular rates exceed predetermined limits, potentially leading to a loss of control of the spacecraft. Because of these complexities, devising a mechanism to ensure the predictability of shortest-time and other desired reorientations or maneuvers is a critical aspect of the maneuver design problem for CMG spacecraft.

Addressing these issues, researchers at the Naval Postgraduate School have devised an approach for the design and implementation of shortest-time re-orientations and other maneuvers for CMG spacecraft with consistent internal states. In this method, a control law or steering law is incorporated as a path constraint or as a dynamics constraint into the formulation of a control problem. The solution provides a guidance command trajectory for maneuvering a spacecraft via CMGs to guide the spacecraft in closed loop fashion from an initial state to a desired final state.

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