Model Identification of a Small Omnidirectional Aquatic Surface Vehicle: a Practical Implementation

Abstract

This work presents a practical method of obtaining a dynamic system model for small omnidirectional aquatic vehicles. The models produced can be used to improve vehicle localisation, aid in the design or tuning of control systems and facilitate the development of simulated environments. The use of a dynamic model for onboard real-time velocity prediction is of particular importance for aquatic vehicles because, unlike ground vehicles, fast and direct measurement of velocity using encoders is not possible. Previous work on model identification of aquatic vehicles has focused on large vessels that are typically underactuated and have low controllability in the sway direction. In this paper it is demonstrated that the procedure for identifying the model coefficients can be performed quickly, without specialist equipment and using only onboard sensors. This is of key importance because the dynamic model coefficients will change with the payload. Two different thrust allocation schemes are tested, one of which is a known method and another is proposed here. Validation tests are performed and the models are shown to be suitable for their intended applications. Significant reduction in model error is demonstrated using the novel thrust allocation method that is designed to avoid deadbands in the thruster responses.

Model Identification of a Small Fully-Actuated Aquatic Surface Vehicle Using a Long Short-Term Memory Neural Network

“…In this work, two different system models of MallARD are compared in terms of their performance, one using a classical parametric approach based on a theoretical model and the other using modern recurrent neural networks. The parametric model and method of identifying parameters has been previously covered in-depth in a previous article by the authors [8]. A key problem that was identified in the author's previous work is the choice of appropriate equation terms for inclusion in the model.…”

“…The thruster layout of MallARD is presented in Figure 2. Throughout this work the always forward (AF) thrust allocation scheme presented in [8] is used. This scheme eliminates the problems associated with deadbands by ensuring that all thrusters are always spinning clockwise and never change direction.…”

“…Movement that involves inputs in more than one degree of freedom is treated as a linear combination of the thrusters inputs for each of the degrees of freedom. Full detail of the AF thrust allocation scheme is described in [8].…”

“…The force inputs to the system are F u , F v , F r with X F , Y F , N F being the associated linear force coefficients. More detailed description of the choice of parameters can be found in [8]. In this paper some of the coupling terms presented in [8] are omitted, because they have negligible effect on the measured accuracy of the model.…”

“…More detailed description of the choice of parameters can be found in [8]. In this paper some of the coupling terms presented in [8] are omitted, because they have negligible effect on the measured accuracy of the model.…”