The influence of thruster dynamics on underwater vehicle behavior and their incorporation into control system design

TLDR

A nonlinear parametric model of a torque‑controlled thruster is developed and experimentally validated, and three compensation strategies—linear lead, nonlinear cancellation, and adaptive sliding—are evaluated using a hybrid simulation of an instrumented thruster coupled to a real‑time vehicle model. The model shows the thruster behaves as a sluggish nonlinear filter with response speed dependent on thrust level, introduces bandwidth constraints and limit cycles, and the compensators improve performance: the linear lead network is easy to implement but limited in range, the nonlinear cancellation works across the range but requires accurate modeling, and the adaptive sliding controller works across the range and compensates for uncertainties.

Abstract

A nonlinear parametric model of a torque-controlled thruster is developed and experimentally confirmed. The model shows that the thruster behaves like a sluggish nonlinear filter, where the speed of response depends on the commanded thrust level. A quasi-linear analysis which utilizes describing functions shows that the dynamics of the thruster produce a strong bandwidth constraint and a limit cycle, which are both commonly seen in practice. Three forms of compensation are tested, utilizing a hybrid simulation combining an instrumented thruster with a real-time mathematical vehicle model. The first compensator, a linear lead network, is easy to implement and greatly improves performance over the uncompensated system, but does not perform uniformly over the entire operating range. The second compensator, which attempts to cancel the nonlinear effect of the thruster, is effective over the entire operating range but depends on an accurate thruster model. The final compensator, an adaptive sliding controller, is effective over the entire operating range and can compensate for uncertainties or the degradation of the thruster.