MuJoCo

Introduction

MuJoCo is a standard physics simulator, coming with an extensive description format.

• Frames will be added as site tags in the MuJoCo XML file.

• Actuators will be created for all actuated joints (see below).

• When kinematic loops are present, they will be enforced using equality constraints.

  • If the loop is achieved using a fixed connector, a weld constraint will be added.

  • If a ball joint is used, a connect constraint will be added

  • If a revolute joint is used, two connect constraints will be used

• Additionally to the robot.xml file, a scene.xml file will be produced, adding floor and lighting useful for testing purpose.

config.json entries (MuJoCo)

Here is an example of complete config.json file, with details below:

{
    "url": "document-url",
    "output_format": "mujoco",
    // ...
    // General import options (see config.json documentation)
    // ...

    // Additional XML file to be included in the URDF (default: "")
    "additional_xml": "my_custom_file.xml",

    // Override joint properties (default: {})
    "joint_properties": {
        // Default properties for all joints
        "*": {
            "actuated": true,
            "forcerange": 10.0,
            "frictionloss": 0.5,
            "limits": [0.5, 1.2]
            // ...
        },
        // Set the properties for a specific joint
        "joint_name": {
            "forcerange": 20.0,
            "frictionloss": 0.1
            // ...
        }
    },

    // Override geometry properties (default: {})
    "geom_properties": {
        // Set properties for specific links using pattern matching
        "foot": {
            "collision": {
                "name": "left_foot_collision",
                "friction": "1.2 0.005 0.0001"
            }
        },
        // Wildcard patterns are supported
        "leg_*": {
            "collision": {
                "solimp": "0.9 0.95 0.001",
                "solref": "0.02 1"
            },
            "visual": {
                "rgba": "1 0 0 1"
            }
        }
    },

    // Override equality attributes
    "equalities": {
        "closing_branch*": {
            "solref": "0.002 1",
            "solimp": "0.99 0.999 0.0005 0.5 2"
        }
    }
}

joint_properties (default: {})

Allow to specify the properties of the joints produced in the URDF output. The key should be joint names. The special default key will set default values for each joints.

Possible values are:

• actuated: (default: true) whether an actuator should be associated to this joint,

• class: a class="..." to be added to the joint (and actuator)

• type (default: position) defines the actuator that will be produced

• range (default: true): if true, the joint limits are reflected on the joint range attribute

• limits: Override the joint limits, should be a list of two values (min, max)

• The following are reflected as <joint ...> attributes:

  • frictionloss

  • damping

  • armature

  • stiffness

• The following are reflected as actuator (<position ...> or other) attributes:

  • kp, kv and dampratio gains

  • forcerange

geom_properties (default: {})

Allow to specify the properties of the geometries (collision and visual) produced in the MuJoCo output. The key should be part names with support for wildcard pattern matching.

Properties can be specified separately for visual and collision geometries, or applied to both if not nested.

Wildcard patterns (*, ?, [seq]) are supported for matching part names. When multiple patterns match, properties are merged in order with later matches overriding earlier ones.

All properties are added as XML attributes to the <geom ...> tag. Common MuJoCo geom attributes include:

• name: Override the geometry name

• friction: Friction coefficients (e.g., "1.2 0.005 0.0001")

• solimp: Solver impedance parameters (e.g., "0.9 0.95 0.001")

• solref: Solver reference parameters (e.g., "0.02 1")

• contype: Contact type bitmask (e.g., "1")

• conaffinity: Contact affinity bitmask (e.g., "1")

• rgba: Color and transparency (e.g., "1 0 0 1")

equalities (default: {})

This entry allows to override the equality attributes of the MuJoCo XML file. The key should be the equality name (which might contains wildcards *), and the value should be a dictionary of attributes.

This can be used to adjust the solref and solimp attributes of the equality constraints.

additional_xml (default: “”)

If you want to include additional XML in the URDF, you can specify the path to the file here. This file will be included in the produced XML.

Note

Alternatively, additional_xml can be a list of files