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By default, MuJoCo contact parameters are tuned for collision, not grasping. Objects will slide out of grippers when lifted unless you increase friction and tighten the contact solver.

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Required Contact Parameters

Add these to your SceneObject definitions: 
const config: SceneConfig = {
  src: "https://raw.githubusercontent.com/google-deepmind/mujoco_menagerie/main/franka_emika_panda/",
  sceneFile: "scene.xml",
  sceneObjects: [{
    name: "cube",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.4, 0, 0.025],
    rgba: [0.9, 0.2, 0.15, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  }],
};

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What Each Parameter Does

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friction: '1.5 0.3 0.1'

MuJoCo friction has three components: [sliding, torsional, rolling]. High sliding friction (1.5+) prevents the object from slipping through the gripper fingers.

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condim: 4

Contact dimensionality. The default (3) uses a friction pyramid, but condim: 4 enables full elliptic friction cone with torsional friction — critical for maintaining grip when the object is rotated or lifted.

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solref: '0.01 1'

Solver reference parameters [timeconst, dampratio]. A short time constant (0.01) makes contacts stiff — the object doesn’t slowly push through the gripper.

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solimp: '0.95 0.99 0.001 0.5 2'

Solver impedance [dmin, dmax, width, midpoint, power]. Tight values (0.95–0.99) mean the constraint is enforced aggressively with minimal penetration.

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mass: 0.05

Keep mass low. Heavy objects need proportionally stronger grip forces, which may exceed what position-controlled grippers can provide.

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freejoint: true

Required for the object to move freely in the scene. Without this, the object is welded to the world body.

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Without These Parameters

If you omit condim: 4 and friction settings:
  • The gripper can close around the object
  • Lifting causes the object to slide out immediately
  • The object may tunnel through the gripper fingers

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Full Example

const graspableObjects: SceneObject[] = [
  {
    name: "red_cube",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.4, 0.0, 0.025],
    rgba: [0.9, 0.2, 0.15, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  },
  {
    name: "green_cube",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.35, 0.1, 0.025],
    rgba: [0.15, 0.8, 0.3, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  },
];

const config: SceneConfig = {
  src: "https://raw.githubusercontent.com/google-deepmind/mujoco_menagerie/main/franka_emika_panda/",
  sceneFile: "scene.xml",
  gripperActuatorName: "gripper",
  sceneObjects: graspableObjects,
};

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Tips

  • Smaller objects are easier to grasp — they fit better between gripper fingers
  • Lower mass reduces the force needed to hold the object
  • Spheres are harder to grasp than cubes — cubes have flat faces that create better contact patches
  • Use <ContactMarkers /> or <Debug showContacts /> to visualize contacts and verify the gripper is making good contact