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MuJoCo XML Reference: Actuator and Position Elements

Introduction​

The MuJoCo (Multi-Joint dynamics with Contact) physics engine utilizes an XML-based language for modeling physical scenes. This document provides a detailed reference to the elements and attributes used within the MJCF (MuJoCo Format) XML schema, focusing on actuators and position elements.

XML Schema​

MJCF files are structured around XML elements and attributes, with no use of text content within elements.

Actuator Elements​

Actuators in MuJoCo are responsible for generating forces on joints or other elements. They are defined within the <actuator> tag and can be customized with various attributes to simulate different behaviors.

General Actuator Attributes​

  • name (optional): A unique identifier for the actuator.
  • class (optional): A class to apply default settings from.
  • group (default "0"): An integer group identifier for the actuator.

Position Actuator (<position>)​

The <position> actuator is a specialized actuator that simulates a position servo with an optional first-order filter. It has the following specific attributes in addition to the general actuator attributes:

  • kp (default "1"): The position feedback gain.
  • kv (default "0"): The damping coefficient applied by the actuator.
  • dampratio (default "0"): The damping ratio, an alternative to kv using units of 2kp*sqrt(m) where m is the mass at the reference configuration.

Actuator Dynamics​

  • dyntype: The type of activation dynamics, such as none, integrator, filter, filterexact, muscle, or user.
  • gaintype: Determines the output of the force generation mechanism, with options like fixed, affine, muscle, or user.
  • biastype: Specifies how the bias term of the force generation mechanism is calculated.

Transmission Types​

Actuators can be attached to joints, tendons, or other elements through various transmission types, influencing how the actuator's force is applied.

XML Example​

<mujoco>
  <actuator>
    <position name="positionActuator1" kp="100" kv="10" />
  </actuator>
</mujoco>