Abstract
This paper presents an empirical modeling of a 2-Degree-of-Freedom (DoF) azimuth thruster using the signal compression method. The thruster has a gimbal mechanism with two servo motors and generates thrust in arbitrary directions. This mechanism can reduce the number of thrusters in an underwater robot and contribute to compact design. When an underwater robot is controlled with azimuth thrusters, the influence from the rotational motion of the thruster has to be considered, and a dynamic model of the azimuth thruster is needed. It is difficult to derive an analytical model because the system model depends on complicated fluid dynamics. In this study, empirical models of force and moment for rotational motion were derived for practical use through frequency analysis. A signal compression method can effectively extract the system model in the frequency domain from just the mechanically constrained frequency response. Experiments were carried out using a force/torque sensor that was connected to a cantilever in a water tank. The system model was analyzed with Bode plots, and the model coefficients were derived through curve fitting. The derived model was verified by a validation experiment.
Highlights
A typical mechanism for the propulsion of underwater robots is a thruster with a propeller
The dynamic model of a tilting thruster is essential for designing a controller for underwater robots
Through the inverse fast Fourier transform (IFFT), the expanded pseudo-impulse signal can be obtained in the time domain as shown in Figure 6b, and it is applied to the system
Summary
A typical mechanism for the propulsion of underwater robots is a thruster with a propeller. The dynamic model of a tilting thruster is essential for designing a controller for underwater robots. The thrust is bent and dispersed during the rotational motion of the azimuth thruster, and a reaction moment occurs. A signal compression method was applied to derive an empirical model of the azimuth thruster. The system consists of a thruster and two servo motors and has a gimbal mechanism for 2-DoF rotational motion. The longitudinal thrust that is generated by the thruster and the tilting angles of the two servo motors are system inputs, and the actual thrust force and reaction torque are outputs of the system model. The measured forces and torques from the sensor are converted to the values of the system origin through the adjoint matrix of the frame transformation
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