  Dynamic modeling has been capturing attention for its fundamentality in
precise locomotion analyses and control of underwater robots. However, the
existing researches have mainly focused on investigating two-dimensional motion
of underwater robots, and little attention has been paid to three-dimensional
dynamic modeling, which is just what we focus on. In this article, a
three-dimensional dynamic model of an active-tail-actuated robotic fish with a
barycentre regulating mechanism is built by combining Newton's second law for
linear motion and Euler's equation for angular motion. The model parameters are
determined by three-dimensional computer-aided design (CAD) software
SolidWorks, HyperFlow-based computational fluid dynamics (CFD) simulation, and
grey-box model estimation method. Both kinematic experiments with a prototype
and numerical simulations are applied to validate the accuracy of the dynamic
model mutually. Based on the dynamic model, multiple three-dimensional motions,
including rectilinear motion, turning motion, gliding motion, and spiral
motion, are analyzed. The experimental and simulation results demonstrate the
effectiveness of the proposed model in evaluating the trajectory, attitude, and
motion parameters, including the velocity, turning radius, angular velocity,
etc., of the robotic fish.
