Theoretical and Experimental Investigations on the Design of a Hybrid Depth Controller for a Standalone Variable Buoyancy System—vBuoy

Design of controllers for underwater vehicles is challenging due to their nonlinear dynamics, time-varying model parameters, and environmental disturbances, which are difficult to measure or estimate. Conventional linear controllers sometimes fail to handle these issues effectively and hence it is necessary to design special controllers that are robust under such circumstances. Variable buoyancy (VB) engines are used in many underwater vehicles and standalone buoyancy modules are being developed for multiple underwater applications. Design and analysis of a hybrid depth controller for a single degree of freedom, standalone VB module, vBuoy, is presented in this paper. The design and mathematical model of the vBuoy is presented along with its open-loop performance analysis. A hybrid controller, which captures the best characteristics of a proportional–integral–derivative controller, a linear quadratic regulator, and a sliding mode controller, is designed for the depth control of the module. Based on the desired transient and steady-state behavior of the system, a supervisory controller is used to switch between the conventional controllers. The comparison of simulation results between the proposed hybrid controller and the conventional controllers shows a significant improvement in the closed-loop performance. The performance is evaluated using the parameters such as rise time, percentage overshoot, settling time, and root mean square error. The same has been implemented in an experimental vBuoy prototype to verify the performance of the hybrid controller and also to validate the robustness of the controller. Based on the simulation and experimental results, it was observed that the hybrid controller improves the trajectory tracking performance by 28%–33%.

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