For submersed vehicles, roll excursions are inevitable as they are caused by induced
propeller torque, wave disturbances and banking motion during turns. Assuming no
active roll stabilization, attitude control systems must tolerate nonzero roll motion
even though the control system is often designed as two single-axis controllers that
separately stabilize pitch and yaw. This is especially true for small AUV systems that
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4.1 Introduction 81
might posses a relatively small righting moment due to limited vertical separation
between center of mass and center of buoyancy.