Cascaded switching supervisory control of nonlinear underwater robotic vehicle for pipeline tracking
Chin, Cheng Siong
Date of Issue2008
School of Mechanical and Aerospace Engineering
In recent years, remotely operated vehicles (ROV) have experienced tremendous growth in underwater pipeline inspections where human diving is impractical. However, the ROV stabilization and pipeline tracking applications present several difficulties due to its inherent nonlinear state coupling, model uncertainties due to hydrodynamic forces, and in particular, and the vehicle possessing fewer actuators than the available degrees of freedom i.e. being underactuated. Most existing methods solve the problem in two-dimensional plane, very often by ignoring the nonlinear coupled terms and hydrodynamic uncertainties in the control system design, and treating stabilization and tracking separately. This research project thus aims to develop a robust cascaded switched control and simulation software for the simultaneous stabilization and position tracking for an underactuated Research Robotic Centre (RRC) ROV. This is achieved through a combination of approaches. By considering only a few degrees of freedom at a time during the horizontal and vertical plane motions, and taking advantage of the vehicle inherent self-regulation in two directions during station-keeping mode, the problem of insufficient actuators can be avoided. A cascade structure and a proportional controller with nonlinear dynamics for states decoupling are implemented to control the velocities and positions simultaneously. To improve on the robustness and to handle the different motions in the desired planes, multiple controllers together with a supervisory controller are proposed. The supervisor is used to orchestrate the outer to inner loop switching based on the inner loop “energy” and the position tracking error, and the controllers switching using the ROV reference input requirements. Applying a dwell time during these switching results in a lower control effort and improves the asymptotic stability about equilibrium sub-manifold. Analysis and simulation of the proposed scheme showed that the system is stable and is able to achieve the desired goals. The advantages of the cascaded control scheme are its simple structure and its improved robustness.
DRNTU::Engineering::Electrical and electronic engineering::Control and instrumentation::Control engineering