Experimental comparison of PID vs. PID plus nonlinear controller for subsea robots

This paper presents a new approach for designing simple nonlinear robust controllers for underwater vehicles. The paper presents several in-water experiments performed on the VORTEX vehicle developed by IFREMER. We first introduce some general modeling considerations of underwater vehicles, then we present the VORTEX dynamic model and some of the special features of the VORTEX vehicle that are important for control. Among these, low sampling rates for sensor and actuator nonlinearities are considered. The main aim of this paper is to experimentally investigate the benefits of adding an easy-to-tune nonlinear control loop to the actual linear compensator in order to improve the stability and the disturbance rejection properties of the closed-loop system. The advantage of this method is two-fold. First the additional nonlinear loop does not modify the original linear (PID) regulator. Second the design of this additional loop does not rely on the system model and is simple to tune. The results presented in this paper were obtained using the VORTEX vehicle both in simulation and during real experiments; they demonstrate the advantages of using a PID with this nonlinear loop over a simple PID control.