Effects of flexibility on a momentum-stabilised communication-satellite attitude-control system

Transfer functions are derived for the coupled roll-yaw-axes control system of a momentum-stabilised satellite. First-mode approximation to the flexural response of movable solar paddles is included in the dynamic equations. The performance of the control system for varying degrees of flexibility is studied using root-locus techniques. The presence of a single flexural mode in the system transfer functions causes an additional pole-zero pair to appear on, or just to the left of, the imaginary axis in the root-locus diagrams. Instability, recognised by root-loci crossing into the right half s plane does not occur. However, flexible modes could be excited by disturbing forces, and, depending on the amplitude of the ensuing motions, would seriously degrade the attitude performance of the satellite.