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README.md

ANYmal C Description (MJCF)

Requires MuJoCo 2.2.2 or later.

Overview

This package contains a simplified robot description (MJCF) of the ANYmal C robot developed by ANYbotics. It is derived from the publicly available URDF description.

Colab

  • [MJX] Use first-order policy gradients to make the ANYmal C locomote Open In Colab

URDF → MJCF derivation steps

  1. Converted the DAE mesh files to OBJ format using Blender.
  2. Processed .obj files with obj2mjcf.
  3. Added <mujoco> <compiler discardvisual="false"/> </mujoco> to the URDF's <robot> clause in order to preserve visual geometries.
  4. Loaded the URDF into MuJoCo and saved a corresponding MJCF.
  5. Added a <freejoint/> to the base, and a tracking light.
  6. Manually edited the MJCF to extract common properties into the <default> section.
  7. Added <exclude> clauses to prevent collisions between the base and the thighs.
  8. Added position-controlled actuators, roughly corresponding to the motor spec here.
  9. Added joint damping to correspond to D gains of the same spec.
  10. Added joint frictionloss to correspond to torque resolution of the same spec.
  11. Softened the contacts of the feet to approximate the effect of rubber.
  12. Added scene.xml which includes the robot, with a textured groundplane, skybox, and haze.

MJX

A version of the ANYmal C model for use in MJX is available in scene_mjx.xml with the following changes:

  • The solver iterations and ls_iterations were modified for performance.
  • frictionloss was removed.
  • A custom init_qpos was added.
  • The friction cone was changed from elliptic to pyramidal.
  • All contacts other than the sphere geoms on the feet were turned off. Contact dimensionality was changed from 6 to 3.
  • Pairwise contacts between feet were turned off to speed up simulation.
  • The compiler option was changed to <option iterations="1" solver="Newton"/>, with <flag eulerdamp="disable"/>.
  • Some decorative geoms were removed from the torso, to speed up rendering.

Hint: Call mujoco.MjModel.from_xml_path on scene.xml or scene_mjx.xml, not on the Anymal files!

License

This model is released under a BSD-3-Clause License.

Acknowledgments

The MJX model of the ANYmal C was contributed by Andrew Luo.