Dorsal fin control of a non-minimum phase undersea vehicle under surface waves

This paper treats the inverse control of a small undersea vehicle using biologically-inspired dorsal fins. It is assumed that the vehicle is under the action of a free surface wave. The transfer function which relates the input force produced by the dorsal fins and the output variable (depth) is nonminimum phase. Since inverse controllers cannot control nonminimum phase systems, a modified controlled output variable is derived for the design of control systems. Then an inverse control law is derived for reference depth trajectory tracking. Although, the inverse controller accomplishes good depth trajectory control, oscillations in responses are observed in the presence of sinusoidal waves. Based on the internal model principle, a servo-compensator is designed for attenuating these oscillations. Simulation results are presented to show that in the closed-loop system, including the inverse controller and the servo-compensator, good depth control and regulation of pitch angle are accomplished in spite of the presence of surface waves and uncertainty in the system parameters