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URDF and Humanoid Modeling

Humanoid Robotics Fundamentals​

Humanoid robotics focuses on the unique challenges of building and controlling bipedal, human-form robots capable of operating in human-centric environments.

Kinematics and Dynamics​

These are the mathematical foundations for understanding how a robot moves and maintains stability.

  • Forward Kinematics: Calculating the position and orientation of the end-effector (e.g., the hand) given the angles of all the joints.
  • Inverse Kinematics (IK): Calculating the required joint angles to place the end-effector at a desired spatial location. Essential for arm control and grasping.
  • Balance Control: The system that ensures the robot does not fall over while moving or performing tasks.

Bipedal Locomotion​

Walking is one of the most challenging problems in robotics, requiring constant dynamic control.

  • Gait Cycles: The repetitive sequence of leg and joint movements required for stable walking.
  • CoM (Center of Mass) Control: The robot must continuously adjust the location of its Center of Mass to stay balanced.
  • ZMP (Zero Moment Point): The point on the ground around which the robot can pivot without falling. Stable locomotion requires keeping the ZMP within the support polygon.

Manipulation​

The ability to interact with the world through hands and arms.

  • Arm Control: Executing the IK solution smoothly and under torque limits.
  • Grasping: The strategy for firmly and safely picking up an object, dependent on object properties (size, weight, fragility).
  • Hand Design: Understanding the trade-offs between simple grippers (robust) and multi-fingered hands (dexterous).

Human-Robot Interaction (HRI)​

As robots enter human spaces, safety and clear communication become paramount.

  • Safety: Implementing immediate stop mechanisms and compliant control to prevent injury during contact.
  • Gestures: Using the robot's body language (head, arms) to signal intent or confirm understanding.
  • Communication: Effective exchange of information, including responding to commands, confirming task completion, and reporting failures.

The URDF Specification​

The Unified Robot Description Format (URDF) is an XML specification that defines a robot's physical structure: its links (rigid bodies), joints (connections between links), and their geometric and inertial properties.

<?xml version="1.0"?>
<robot name="simple_humanoid">
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="10.0"/>
      <inertia ixx="0.1" iyy="0.1" izz="0.1"
               ixy="0" ixz="0" iyz="0"/>
    </inertial>
  </link>

  <link name="right_upper_arm">
    <visual>
      <geometry>
        <cylinder radius="0.04" length="0.3"/>
      </geometry>
    </visual>
  </link>

  <joint name="right_shoulder" type="revolute">
    <parent link="torso"/>
    <child link="right_upper_arm"/>
    <origin xyz="0.15 0 0.2" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1.0"/>
  </joint>
</robot>

Key elements:

  • Links: rigid bodies with visual geometry, collision geometry, and inertial properties
  • Joints: connections between links with type (revolute, prismatic, fixed), axis, and limits
  • Origin: spatial relationship between parent and child links

Visualizing the Robot​

# Install URDF visualization tools
sudo apt install ros-humble-urdf-tutorial

# Launch the robot state publisher and RViz
ros2 launch urdf_tutorial display.launch.py model:=simple_humanoid.urdf

RViz renders the URDF as an interactive 3D model. You can move joint sliders to see how the humanoid articulates.