Dynamics and Control of a Buoyancy-Driven Underwater Vehicle for Estimating and Tracking the Scattering Layer

Abstract

This paper presents the design of a control strategy for an autonomous buoyancy-driven underwater vehicle, called the Driftcam, tasked with estimating and tracking the pelagic acoustic-scattering layer at depths of up to 700 meters. This reference-tracking control design is an alternative to the onboard control currently implemented on the Driftcam, as the existing control architecture is not designed for time-varying setpoint tracking. Tracking a known reference is presented using state-feedback control for continuous- and discrete-time command inputs. The known reference signal is chosen to represent the pelagic scattering layer vertical migration dynamics, modeled as a simple harmonic oscillator and estimated with a dynamic observer. We define a measurement function for the depth-varying density of organisms observable in the scattering layer. Time-varying reference tracking control is presented using output feedback for continuous- and discrete-time command inputs, where the reference is the online position estimate of the scattering layer. We also present an offboard trajectory motion planning algorithm using the existing onboard controller to emulate the response of the proposed reference tracking strategy. Experimental results from laboratory and field testing of the trajectory motion planner demonstrate that the buoyancy engine can feasibly achieve the desired behavior of the reference-tracking control strategy.