Disturbance Preview for Nonlinear Model Predictive Trajectory Tracking of Underwater Vehicles in Wave Dominated Environments

Abstract

Operating in the near-vicinity of marine energy devices poses significant challenges to the control of underwater vehicles, predominantly due to the presence of large magnitude wave disturbances causing hazardous state perturbations. Approaches to tackle this problem have varied, but one promising solution is to adopt predictive control methods. Given the predictable nature of ocean waves, the potential exists to incorporate disturbance estimations directly within the plant model; this requires inclusion of a wave predictor to provide online preview information. To this end, this paper presents a Nonlinear Model Predictive Controller with an integrated Deterministic Sea Wave Predictor for trajectory tracking of underwater vehicles. State information is obtained through an Extended Kalman Filter, forming a complete closed-loop strategy and facilitating online wave load estimations. The strategy is compared to a similar feed-forward disturbance mitigation scheme, showing mean performance improvements of 51% in positional error and 44.5% in attitude error. The preliminary results presented here provide strong evidence of the proposed method's high potential to effectively mitigate disturbances, facilitating accurate tracking performance even in the presence of high wave loading.

Figures (8)

• Figure 1: Schematic representation of the proposed control strategy; data is accumulated by the measuring device, subsequently passing disturbance preview information to the vehicle controller to mitigate state perturbations.
• Figure 2: The BlueROV2 Heavy, displaying notations in conjunction with the earth-fixed and body-fixed co-ordinate frames.
• Figure 3: Block diagram representation of the proposed predictive control scheme with disturbance preview information incorporated.
• Figure 4: (a) The wave frequency spectrum with frequency and amplitude thresholds applied to bound the components considered during wave reconstruction and (b) the space-time diagram formulated according to the extreme components within these bounded limits.
• Figure 5: Trajectory tracking spatial performance of each controller for case W1, showing the NMPC successfully suppressing the cyclical wave-induced loads for the majority of the mission, even at low operational depth.