This paper outlines a practical and cost-effective approach to the modeling of servo motor marine thrusters for underwater robot applications. This procedure only considers variables that are easily and confidently controlled and measured. For an underwater robot with multiple thrusters, the servo motor controllers must first be tuned to match one another as far as the reference input gains, offsets, and current limits to establish a decent open-loop baseline. Next, performing steady state experiments with the thrusters in air approximates the friction load (equal to the motor torque) as a function of the shaft speed. The final experiment measures thrust as a function of the hydrodynamic torque load, which is in turn a function of the accurately measurable electric current and shaft speed. The derived nonlinear friction function actually serves to linearize this current to thrust relationship. The total set of experimentally derived functions then provides a simple thruster model for the high-level vehicle motion controller.
The increasing demand of undersea applications has promoted the evolution of the autonomous underwater vehicle (AUV). Fine motion control of such a vehicle requires accurate modeling of its thrusters. It is common to derive any number of single or multiple state mathematical models based on the lift and drag physics at and around the thruster propeller. This necessitates the experimental measurement of physical attributes such as the shaft speed and torque, developed thrust, and the water flow velocity at the propeller face. This last variable can be problematic and is a cause for concern. Not all research groups have access to the facilities necessary to measure such flow rates, and even those that do would only be able to achieve limited and questionable accuracy due to the nature of turbulent fluid flow.