MuJoCo Data Class Variables Documentation

This page holds the general information about some Mujoco variables used in ARIEL. This info is, in general, the same as what can be found on the Mujoco documentation. But some things have been added for better understanding.

Matrix-Related Variables

• bind: Class variable for binding operations. can be used to get data in each step of the simulation. Example:

geoms = world.spec.worldbody.find_all(mujoco.mjtObj.mjOBJ_GEOM)
to_track = [data.bind(geom) for geom in geoms if "core" in geom.name]

• M: Inertia matrix

Actuator States

• act: Actuator activation states

• act_dot: Time-derivative of actuator activations

• actuator_force: Forces applied by actuators

• actuator_length: Current length of actuators

• actuator_velocity: Velocities of actuators

Spatial Properties

• cam_xmat, cam_xpos: Camera orientation matrix and position

• cfrc_ext, cfrc_int: External and internal contact forces

• ctrl: Control signals

Energy and State

• energy: Potential and kinetic energy

Geometric Properties

• geom_xmat, geom_xpos: Geometry orientation matrices and positions

Mapping Arrays

Motion Capture

• mocap_pos, mocap_quat: Motion capture positions and quaternions

State Variables

• qpos: Generalized position state vector.
  Contains the positions of the core and all hinge joints, in the order they are defined in the XML.

  • Core (free joint):

    • [x, y, z] : 3D position of the core body frame

    • [qw, qx, qy, qz] : Orientation quaternion of the core

  • Hinge joints (bricks): [angle] for each hinge, representing the rotation angle in radians

  Actuator inputs and states are not stored here (see data.ctrl and data.act instead).

• qvel: Velocity state vector. Holds the current velocity of each actuator.

• qacc: Acceleration state vector

Forces

• qfrc_actuator: Actuator forces

• qfrc_applied: Externally applied forces

• qfrc_bias: Bias forces

• qfrc_constraint: Constraint forces

• qfrc_passive: Passive forces

• qfrc_spring: Spring forces

Sensors and Sites

• sensordata: Sensor measurements

• site_xmat, site_xpos: Site orientation matrices and positions

Spatial Transforms

• xfrc_applied: Applied Cartesian forces

• xmat, xpos: Orientation matrices and positions

• xquat: Quaternion orientations