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JSim

Deterministic 3D physics simulation for FRC robots

JSim gives your robot code a physics world it can step through each control loop iteration. Bodies collide, bounce, and respond to motor forces using impulse-based rigid-body dynamics, all from a clean Java API built on WPILib geometry types.

What JSim does

  • Rigid-body physics — spheres, boxes, and infinite planes with realistic collision response (Baumgarte + Sequential Impulse solver)
  • Force generators — built-in gravity, linear drag, Magnus lift for spinning game pieces, and per-body actuator inputs from motor outputs
  • WPILib-native API — all poses and velocities use Pose3d, Translation3d, Rotation3d, and WPILib units
  • Deterministic — fixed 20 ms timestep, raw-double internal state; two runs with the same inputs always produce the same results
  • Self-contained example — a complete swerve-drive robot in examples/java/simple_robot/ that you can compile and simulate out of the box

Quick start

1. Add the vendordep

In VS Code WPILib or from the command line, install from:

https://jsim.dev/JSim.json

Or add the JSON URL to your .wpilib/vendordeps/ folder manually. See Getting Started for details.

2. Create a world

// In your subsystem or Robot class:
SimWorld simWorld = new SimWorld();          // 50 Hz fixed timestep

// Static floor (infinite half-space facing +Z)
simWorld.addBody(new SimBodyBuilder("Floor")
    .planeCollider(0, 0, 1, 0)
    .material(Material.CARPET));

// Robot body — tagged as Blue alliance station 1
SimBody robot = simWorld.addBody(new SimBodyBuilder("Robot")
    .robotId(RobotId.BLUE_1)
    .position(0, 0, 0.1)
    .mass(54)                                // kg
    .boxCollider(0.3, 0.3, 0.15)            // half-extents in metres
    .noGravity()
    .fixedRotation()
    .material(Material.CARPET));

3. Step each loop

@Override
public void simulationPeriodic() {
    simWorld.step();                         // advance by 20 ms
    Pose3d pose = robot.getPose();
    field.setRobotPose(pose.toPose2d());
}
Resource URL
Javadoc jsim.dev/api/javadocs
GitHub github.com/Yet-Another-Software-Suite/JSim
Maven jsim.dev/maven/releases
Vendordep JSON jsim.dev/JSim.json

Example project

The examples/java/simple_robot/ directory is a complete, buildable FRC swerve-drive robot that integrates JSim for desktop simulation. Open it directly in VS Code with the WPILib extension.

See Swerve Drive Simulation for a guided walkthrough.