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.

  • Else, a connect constraint will be added.

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

config.json entries (URDF)

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

{
    "document_id": "document-id",
    "output_format": "urdf",
    // ...
    // General import options (see config.json documentation)
    // ...

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

    // Whether the collisions should be used in visual as well (default: false)
    "draw_collisions": true,
    // Use only pure shapes (when available) for collisions
    "collisions_no_mesh": true,
    // Whether to add a free joint to the root link (default: true)
    "freejoint": false,
    // Additional XML file to be included in the URDF (default: "")
    "additional_xml": "my_custom_file.xml",
}

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, kd and ki gains

  • forcerange

draw_collisions (default: false)

If this flag is set to true, the same items that are used for collisions will be used for visuals as well. If you have pure shape approximations, this is useful for debugging purposes.

collisions_no_mesh (default: false)

If this flag is set to true, only pure shapes will be used for collisions, and not the mesh. This is useful for performance reasons.

freejoint (default: true)

If this flag is set to false, a free joint will not be added to the root link. This will result in a fixed robot.

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 at the end of the URDF file.