Asymmetric Steering Hydrodynamics Identification of a Differential Drive Unmanned Surface Vessel
Abstract This paper identifies the asymmetric steering characteristics of a Wave Adaptive Modular Vessel (WAM-V) deployed as an Unmanned Surface Vessel (USV). Differentially steered propellers create a virtual rudder movement without explicitly inducing lateral rudder forces. However, a rotating propeller will generate a small lateral force, depending on its rotational direction and speed, also known as propeller walk. The WAM-V USV uses two similar propellers to manoeuvre, hence the propeller walk effects are doubled. Consequentially, the vessel has asymmetric turning characteristics which result in different steering behaviours when turning port or starboard. Heading measurements and virtual rudder movements suffice for identifying these turning characteristics at a certain speed. To do so, a first order Nomoto model was chosen as identification model. Three varieties of this model were identified: one for turning port, one for turning starboard, and one that averages the two aforementioned cases. These offline identified Nomoto models can serve multiple objectives. They can be used for simulation purposes, which themselves can be used to test control algorithms offline. Moreover, the coefficients of the Nomoto model itself can be used to tune a Proportional Integral Derivative (PID) controller. Finally, the Nomoto models can also be used as a feed forward term in control algorithms.