ON COURSE-STABILITY AND CONTROL DELAY

The typical objective in a course-changing manoeuver is to perform the manoeuver quickly with minimum overshoot. This often requires rapid and large rudder movements, which are inevitably limited by finite actuator power. Without proper precaution, controller integral wind-up will occur which may lead to stability problems for the system. A reference conditioning technique is proposed in this paper that generates a new reference signal such that the rudder command is always consistent with the actual rudder input. This ensures a quiet, course-changing manoeuver is achieved; hence, the rudder saturation and slew rate limitation are accommodated without otherwise compromising the performance. The internal model control method adopted in this paper is very attractive in the design of the autopilot, since it highlights inherent performance limitations and allows a coherent treatment of control strategies that might otherwise appear as uncorrelated.