Model Structures and Identification for Fully Embedded Thrusters: 360-Degrees-Steerable Steering-Grid and Four-Channel Thrusters

Abstract

The European Watertruck + project introduced a new fleet of self-propelled inland cargo barges to the European waters, in order to induce more sustainable freight transport in the European hinterland. An augmentation of the automation level of this fleet could further advance their competitiveness and potentially pave the way for unmanned inland cargo vessels. The motion control of such a vessel forms a key component in this envisaged automation chain and benefits from the knowledge of the capabilities of the propulsion system, which here envelops a 360-degrees-steerable steering-grid thruster in conjunction with a 360-degrees-steerable four-channel thruster. Therefore, this study details the mechanical design of both thrusters and lists their experimental towing-tank data. Furthermore, two different modelling methods are offered, one theoretically based and one using a multilayer neural network. A model structure comparison, based on a bias-variance trade-off, verifies the adequacy of the theoretical model which got expended with an angle-dependent thrust deduction coefficient. In addition, several multilayer feedforward neural network architectures exemplify their inherent capability to model the complex, nonlinear, flow phenomena inside the thrusters. These identified model structures can additionally improve thrust allocation algorithms and offer better plant models to study more advanced control strategies.

Decoupled Hydrodynamic Models and Their Outdoor Identification for an Unmanned Inland Cargo Vessel with Embedded Fully Rotatable Thrusters

“…The bollard pull forces of both thrusters can be found in Figure A1, and more design details together with their main discrepancies with conventional actuation systems in [22]. Since the bow of the Cogge houses the steering-grid and the stern the four-channel thruster, α i b and α i s will denote their internal angles in this study, and n b and n s their propeller speeds.…”

“…This self made straightener was not installed during the bollard pull tests from Figure A1. b-e) were adapted or reproduced from [22] with permission from MDPI, 2020.…”

“…In the spectrum of the more conventional propulsion systems, tunnel thrusters [37][38][39] and azimuth thrusters [40][41][42] might appear to hold the largest similarities with the non-conventional thrusters of this study [22]. The modelling literature for these tunnel and azimuth thrusters does remain scarce and often uses the open-water propeller characteristics model of [43] as the starting point.…”

“…In [22], the authors demonstrated that T(ν = 0) = [1 − t(α i )]T nn n 2 can serve as a robust theoretical thrust force model for the speed-independent part of ( 16), by studying the data from Figure A1. Note that t and w denote thrust differences between fully open-water tests (only a propeller) and tests with a propeller attached to a hull.…”

“…. These boundaries aim to: (i) take into account the data uncertainty of both thrusters [22], (ii) allow some slack, given that higher rotational speeds were used compared to the towing tank data and the steering grid was now inside a hull, see Figure A1, and (iii) investigate the installed flow straightener, see [3], in the stern thruster which was not installed during the towing tank experiments. Finally, the T 0,b nν , T 0,b nnu , T 0,s nν , T 0,s nnu had an upper limit of zero, and an lower limit which would induce a thrust deduction the size of the available thrust which was taken to be 85% of the initial guess.…”