A remark on "Nonlinear output feedback control of underwater vehicle propellers using feedback form estimated axial flow velocity"'

For original paper see T.I.Fossen and M.Blanke, ibid., vol.25, pp.241-55 (2000). In the work presented by Fossen and Blanke, a nonlinear observer for estimation of propeller axial flow velocity for UUVs was introduced. The proof of the convergence behavior of the observer was carried out with a Lyapunov analysis. In this technical communication, such an analysis was also briefly compared with the so-called contraction analysis. This communication goes further into the details of the comparison by proving - through a link between contracting and globally exponentially stable (GES) systems - that the observer is both contracting and GES, showing thus that contraction analysis might be regarded as an interesting alternative to Lyapunov-based designs. Furthermore, some qualitative advantages gained when using contraction analysis are given.     

Formation Control of Marine Surface Craft: A Lagrangian Approach

This paper presents a method for formation control of marine surface craft inspired by Lagrangian mechanics. The desired formation configuration and response of the marine surface craft are given as a set of constraint functions. The functions are treated according to constraints in analytical mechanics. Thus, constraint forces arise and feedback from the constraint functions keeps the formation assembled. Since the constraint functions are designed for a desired effect, the forces can be interpreted as control laws. Examples of constraint functions that maintain a formation are presented. Furthermore, the same approach has been applied with no major modification to position control purposes for a single vessel. An extension to underactuated vessels is given. Simulations with nonlinear models of tugboats illustrate the proposed method versatility and its robustness with respect to environmental disturbances and time delays     

Underwater Vehicle Navigation Using Diffusion-Based Trajectory Observers

This paper addresses the issue of estimating underwater vehicle trajectories using gyro-Doppler (body-fixed velocities) and acoustic positioning signals (earth-fixed positions). The approach consists of diffusion-based observers processing a whole trajectory segment at a time, allowing the consideration of important practical problems such as different information update rates, outages, and outliers in a very simple framework. Results of contraction theory are used to prove that the observers are convergent, i.e., stable in the incremental sense. Simulation and experimental results are presented to illustrate the potential of application of the method.