Analysis of Feedforward/Feedback Control Design for Underwater Gliders Based on Slowly Varying Systems Theory

Abstract

Underwater gliders are highly efficient, winged autonomous underwater vehicles that propel themselves by modifying their buoyancy and their center of mass. The center of mass is controlled by a set of servo-actuators which move one or more internal masses relative to the vehicle’s frame. Underwater glid ers are so efficient because they spend most of their time in stable, steady motion, expending control energy only when changing their equilibrium state. Motion control thus reduces to varying the parameters (buoyancy and center of mass) that affect the state of steady motion. These parameters are conventionally controlled through feedback, in response to measured errors in the state of motion, but one may also incorporate a feedforward component to speed convergence and improve performance. This paper describes the analysis of a feedforward/feedback control system for an underwater glider using slowly varying systems theory. The results provide (conservative) bounds on the rate at which the reference command (the desired state of motion) may be varied while still guaranteeing stability of the closed-loop system.