Vessel Dynamics Simulator

Work at RWTH IRT -1

Overview

A modern C++/Eigen/Boost implementation of a seven‑state, three‑input vessel model. It integrates hull, wind, and current hydrodynamics with live keyboard steering so you can feel how surge (τ₁), sway (τ₂) and yaw (τ₃) interact in real time.

  • Physics – nonlinear surge–sway–yaw equations with parameter scaling, Froude‑number lookup tables, and wind‑tunnel coefficients.
  • Integrators – choose Heun (default), RK4, or Euler at runtime.
  • UI – a minimal ncurses‑style keyboard handler lets you bump forces up/down (Shift reverses, Alt cycles step sizes).
  • Build – CMake‑based; links Eigen & Boost only.

The core update routine scales, unscales, and fuses:

  1. The main function vessel_model_ident – computes $\dot x$ from current state, control, current & wind.
  2. Integrator – Heun / RK4 / Euler selected via integrator_scheme.
  3. Keyboard thread – atomically adjusts the control input vector tau3_in without blocking the simulation.

All major matrices (mass, added mass, transformation, coriolis) are pre‑allocated as Eigen::Matrix<double, …> to keep runtime allocations at zero.

Equations of Motion

The 3‑DOF body‑fixed dynamics are expressed as

\[\mathbf{M}\dot{\boldsymbol\nu} + \mathbf{C}(\boldsymbol\nu)\,\boldsymbol\nu + \mathbf{D}(\boldsymbol\nu)\,\boldsymbol\nu = \boldsymbol\tau_{\text{hull}} + \boldsymbol\tau_{\text{wind}} + \boldsymbol\tau_{\text{control}} + \boldsymbol\tau_{\text{res}},\]

where

  • $\boldsymbol\nu = \begin{bmatrix}u & v & r\end{bmatrix}^\top$ is the velocity vector (surge, sway, yaw‑rate);
  • $\mathbf{M} = \operatorname{diag}(m_x, m_y, I_z)$ is the rigid‑body mass matrix;
  • $\mathbf{C}(\boldsymbol\nu)$ is the Coriolis/centripetal matrix;
  • $\mathbf{D}(\boldsymbol\nu)$ represents nonlinear hydrodynamic damping;
  • $\boldsymbol\tau_{*}$ denote the force/moment contributions implemented in the code.

The earth‑fixed pose update is

\[\dot{\boldsymbol\eta} = \mathbf{R}(\psi)\,\boldsymbol\nu, \qquad \boldsymbol\eta = \begin{bmatrix}x & y & \psi\end{bmatrix}^\top,\]

with rotation matrix

\[\mathbf{R}(\psi)=\begin{bmatrix} \cos\psi & -\sin\psi & 0\\ \sin\psi & \phantom{-}\cos\psi & 0\\ 0 & 0 & 1 \end{bmatrix}.\]

(Extending to 6‑DOF is straightforward—add heave, roll, and pitch states and expand $\mathbf{M}$, $\mathbf{C}$, and $\mathbf{D}$ accordingly.)

Other Work (FernBin Project)

Simulation of vessel dynamics with surrounding traffic.
Actual FernBin project vessels.

  • A ROS 2 package was implemented for publishing encrypted AIS messages tailored to the FernBin vessel type. It takes the simulated navigation states from the dynamics core and broadcasts them to ROS2 topics. The node auto‑discovers the simulator via ROS 2 parameters and starts publishing immediately.

  • Implemented Gilbert–Johnson–Keerthi (GJK) collision detection modules for convex shapes, integrating robust support function routines into the simulation pipeline. Combined with the Expanding Polytope Algorithm (EPA), it provides real‑time penetration depth calculations and collision response hooks.

Need the code? Browse the repository on GitHub.

Pull requests & issues welcome!