OpenMBD - Multibody Dynamics Simulator

Capabilities

What OpenMBD can do

A general-purpose rigid body simulator designed for biomechanics research, sports science, and mechanical systems analysis.

⚙

Constrained Multibody Dynamics

Full articulated rigid body simulation with revolute, spherical, and free joints.

PHYSICS ENGINE

◎

Ellipsoid Contact Model

Bodies are represented as collections of ellipsoids. Penalty-based contact forces with configurable stiffness curves handle contact and collisions without discontinuities.

CONTACT

🦴

Physiological Joint Limits

AAOS-sourced range-of-motion constraints for hips, knees, ankles, shoulders, elbows, wrists, and spine.

BIOMECHANICS

⬡

Visual Model Editor

A standalone HTML5 model editor lets you inspect and modify body geometry, joint transforms, inertia, and contact force curves without running Python.

TOOLING

📊

Rich Data Export

Automatic CSV export of kinematics, contact force history, and a plain-text simulation summary after every run.

ANALYSIS

🎬

GIF Animation Export

Every simulation automatically captures frame-by-frame renders and stitches them into a GIF on completion.

OUTPUT

Getting Started

Up and running in minutes

OpenMBD runs on Python 3.8+ with standard scientific libraries. No compilation, no build system.

Clone the repository

Download the full project from GitHub including all model files and the HTML model editor.

git clone https://github.com/GTBiomech/OpenMBD cd openmbd

Install dependencies

OpenMBD has exactly three third-party dependencies. NumPy is used for physics, matrix, and quaternion math. Matplotlib drives the 3D viewport, live rendering, and the analysis tab plots. Pillow creates GIF export after each run.

pip install numpy matplotlib pillow

Launch the simulator

Run the main entry point. The GUI opens with the Setup tab where you choose a model, set initial conditions, and configure physics parameters.

python openmbd_simulator.py

Run a simulation

Select a model file (e.g. openmbd_male.json), set an initial position and velocity, then click Finalize → Run. Results auto-export as CSV and GIF.

Explore the Model Editor

Open openmbd_model_editor.html directly in your browser. No Python needed — edit body geometry, joint transforms, and contact force curves, then export a new JSON.

# Or just double-click openmbd_model_editor.html open openmbd_model_editor.html

📁 Project structure — openmbd/

▶openmbd_simulator.pyentry point

◈physics_engine.pycore integrator

◈physics_rigid_body.pybody state

◈physics_constraints.pyjoint constraints

◈physics_joint_limits.pyROM limits

◈physics_utils.pyquaternion math

◈models_parser.pyJSON → engine

◈models_multibody.pykinematics

◈models_config.pysim config

◈geometry_ellipsoid.py

◈geometry_mesh_generator.py

◈ui_main_window.py

◈ui_setup_tab.py

◈ui_run_tab.py

◈ui_analysis_tab.py

◈ui_controls.py

⬡openmbd_model_editor.htmlbrowser tool

▣openmbd_male.json

▣openmbd_female.json

▣openmbd_car.json

▣openmbd_bicycle.json

Model Format

The OpenMBD JSON schema

Every model is a self-contained JSON file. Bodies, joints, geometry, and contact properties are all defined in one place. Readable and version-controllable.

bodiesobjectrequired

"{body_name}"object

massnumberkgreq

inertia[6]Ixx Iyy Izz Ixy Iyz Ixz kg·m²

center_of_mass[3]local xyz, m

ellipsoidsarray

namestring

dimensions[3]semi-axes a b c, m

local_position[3]offset from body origin

local_orientation[4×4]transform matrix

force_curve[[d,F]…]penetration → force (N)

jointsarrayrequired

namestring

typestring"fixed" | "revolute" | "spherical" | "free"

parentstringbody name or "GROUND"

childstringbody name

T1[4×4]parent anchor transform

T2[4×4]child anchor transform

T2_inv[4×4]inverse of T2

{
  "bodies": {
    "pelvis": {
      "mass": 11.22,
      "inertia": [0.0955, 0.0594, 0.0914, 0, 0, 0],
      "center_of_mass": [0, 0, 0.006],
      "ellipsoids": [
        {
          "name": "pelvis_main",
          "dimensions": [0.14, 0.10, 0.09],
          "local_position": [0, 0, 0],
          "local_orientation": /* 4×4 matrix */,
          "force_curve":
            [[0, 0], [0.01, 10000]]
        }
      ]
    }
  },
  "joints": [
    {
      "name": "hipL",
      "type": "spherical",
      "parent": "pelvis",
      "child": "thighL",
      "T1": /* parent anchor 4×4 */,
      "T2": /* child anchor 4×4 */,
      "T2_inv": /* inverse of T2 */
    }
  ]
}

Outputs

What you get after every run

OpenMBD auto-exports three files the moment a simulation completes. Everything is timestamped so runs never overwrite each other.

📈

Output CSV

Full time-series state for every rigid body in the model, sampled at the recording interval.

Time

PosX / PosY / PosZ — world position

QuatW/X/Y/Z — orientation

VelX/Y/Z — linear velocity

AngVelX/Y/Z — angular velocity

AccX/Y/Z — linear acceleration

AngAccX/Y/Z — angular acceleration

ForceX/Y/Z — accumulated force

TorqueX/Y/Z — accumulated torque

📋

Summary Text File

Human-readable run report with all simulation parameters and statistics.

Duration — total sim time (s)

Time step dt

Friction coefficient

Contact damping

Total contacts recorded

Joint constraints

Model paths & ICs

🎬

Animated GIF

Frame-by-frame capture of the 3D viewport stitched into a shareable animation. Generated automatically with Pillow — no extra steps needed.

RGBA capture — direct from renderer

Loop — infinite by default

Timestamped filename

What OpenMBD can do

Up and running in minutes

The OpenMBD JSON schema

What you get after every run

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