URDF Processor User Guide

This guide covers the URDF Processor module in ManipulaPy, which converts URDF (Unified Robot Description Format) files into SerialManipulator and ManipulatorDynamics objects for analytical robotics computations.

Introduction

The URDF Processor bridges the gap between URDF robot descriptions and ManipulaPy’s analytical framework. It automatically extracts kinematic and dynamic parameters from URDF files and creates the necessary objects for robotics analysis.

Key Features: - Automatic parameter extraction from URDF files - Kinematic chain analysis and screw axis computation - Inertial property extraction for dynamics - Joint limit handling with PyBullet integration - Conversion to SerialManipulator and ManipulatorDynamics objects

Basic Usage

Simple URDF Loading

from ManipulaPy.urdf_processor import URDFToSerialManipulator

# Load URDF and create objects
processor = URDFToSerialManipulator("path/to/robot.urdf")

# Access the created objects
robot = processor.serial_manipulator      # SerialManipulator instance
dynamics = processor.dynamics             # ManipulatorDynamics instance

# Use the robot for computations
import numpy as np
theta = np.array([0.1, 0.3, -0.2])
T = robot.forward_kinematics(theta)

print(f"End-effector position: {T[:3, 3]}")

Using Built-in Models

from ManipulaPy.ManipulaPy_data.xarm import urdf_file as xarm_urdf

# Load built-in xArm robot
processor = URDFToSerialManipulator(xarm_urdf)
robot = processor.serial_manipulator

print(f"Robot has {len(robot.joint_limits)} joints")

URDFToSerialManipulator Class

Class Constructor

URDFToSerialManipulator(urdf_name, use_pybullet_limits=True)

Parameters: - urdf_name (str): Path to the URDF file - use_pybullet_limits (bool): Extract joint limits from PyBullet simulation

Attributes: - serial_manipulator: SerialManipulator object for kinematics - dynamics: ManipulatorDynamics object for dynamics - robot_data: Dictionary containing extracted parameters - urdf_name: Path to the loaded URDF file - robot: Loaded URDF object from urchin library

Extracted Parameters

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Extracted Parameters
~~~~~~~~~~~~~~~~~~~

The robot_data dictionary contains:

processor = URDFToSerialManipulator("robot.urdf")
data = processor.robot_data

print(f"Degrees of freedom: {data['actuated_joints_num']}")
print(f"Home configuration shape: {data['M'].shape}")          # (4, 4)
print(f"Space screw axes shape: {data['Slist'].shape}")        # (6, n)
print(f"Body screw axes shape: {data['Blist'].shape}")         # (6, n)
print(f"Number of inertia matrices: {len(data['Glist'])}")     # n links

Core Methods

load_urdf()

Extracts kinematic and dynamic parameters from the URDF file:

def parameter_extraction_example():
    processor = URDFToSerialManipulator("robot.urdf")
    data = processor.robot_data

    # Access screw axes
    Slist = data["Slist"]  # Shape: (6, n_joints)
    for i in range(Slist.shape[1]):
        omega = Slist[:3, i]  # Angular velocity part
        v = Slist[3:, i]      # Linear velocity part
        print(f"Joint {i+1}: ω={omega}, v={v}")

    # Access inertial properties
    Glist = data["Glist"]  # List of (6, 6) spatial inertia matrices
    for i, G in enumerate(Glist):
        mass = G[3, 3]  # Mass (assuming diagonal)
        print(f"Link {i+1} mass: {mass:.3f} kg")

    # Home configuration
    M = data["M"]  # (4, 4) homogeneous transformation
    print(f"Home position: {M[:3, 3]}")

initialize_serial_manipulator()

Creates the SerialManipulator object:

# The processor automatically calls this during initialization
processor = URDFToSerialManipulator("robot.urdf")
robot = processor.serial_manipulator

# Access SerialManipulator properties
print(f"Joint limits: {robot.joint_limits}")
print(f"Screw axes shape: {robot.S_list.shape}")
print(f"Home configuration:\n{robot.M_list}")

initialize_manipulator_dynamics()

Creates the ManipulatorDynamics object:

processor = URDFToSerialManipulator("robot.urdf")
dynamics = processor.dynamics

# Use dynamics for computations
theta = np.array([0.1, 0.3, -0.2])
theta_dot = np.array([0.5, -0.3, 0.8])

M = dynamics.mass_matrix(theta)
c = dynamics.velocity_quadratic_forces(theta, theta_dot)
g = dynamics.gravity_forces(theta, [0, 0, -9.81])

print(f"Mass matrix shape: {M.shape}")
print(f"Coriolis forces: {c}")
print(f"Gravity forces: {g}")

Joint Limit Handling

PyBullet Integration

When use_pybullet_limits=True, the processor extracts joint limits from PyBullet:

# With PyBullet limits (default)
processor_pyb = URDFToSerialManipulator("robot.urdf", use_pybullet_limits=True)

# Without PyBullet limits (uses default ±π)
processor_default = URDFToSerialManipulator("robot.urdf", use_pybullet_limits=False)

# Compare limits
pyb_limits = processor_pyb.serial_manipulator.joint_limits
default_limits = processor_default.serial_manipulator.joint_limits

for i, (pyb, default) in enumerate(zip(pyb_limits, default_limits)):
    print(f"Joint {i+1}:")
    print(f"  PyBullet: [{np.degrees(pyb[0]):6.1f}, {np.degrees(pyb[1]):6.1f}] deg")
    print(f"  Default:  [{np.degrees(default[0]):6.1f}, {np.degrees(default[1]):6.1f}] deg")

Custom Joint Limits

processor = URDFToSerialManipulator("robot.urdf")
robot = processor.serial_manipulator

# Set custom limits
custom_limits = [
    (-np.pi, np.pi),        # Joint 1: full rotation
    (-np.pi/2, np.pi/2),    # Joint 2: ±90°
    (-np.pi/3, np.pi/3),    # Joint 3: ±60°
]

robot.joint_limits = custom_limits[:len(robot.joint_limits)]

Utility Methods

Static Methods

# Extract position from transformation matrix
T = np.eye(4)
T[:3, 3] = [1, 2, 3]
pos = URDFToSerialManipulator.transform_to_xyz(T)
print(f"Position: {pos}")  # [1, 2, 3]

# Find link by name
processor = URDFToSerialManipulator("robot.urdf")
link = URDFToSerialManipulator.get_link(processor.robot, "link_name")

# Convert joint axes to screw axes
joint_axes = np.array([[0, 0, 1], [0, 1, 0]]).T      # 2 joints
joint_positions = np.array([[0, 0, 0], [0, 0, 0.5]]).T
Slist = URDFToSerialManipulator.w_p_to_slist(joint_axes.T, joint_positions.T, 2)
print(f"Screw axes shape: {Slist.shape}")  # (6, 2)

Visualization Methods

processor = URDFToSerialManipulator("robot.urdf")

# Visualize robot using urchin (matplotlib)
processor.visualize_robot()

# Visualize trajectory animation
n_joints = len(processor.serial_manipulator.joint_limits)
trajectory = np.random.uniform(-0.5, 0.5, (50, n_joints))

processor.visualize_trajectory(
    cfg_trajectory=trajectory,
    loop_time=3.0,
    use_collision=False
)

# Get joint information
joint_info = processor.print_joint_info()
print(f"Number of joints: {joint_info['num_joints']}")
print(f"Joint names: {joint_info['joint_names']}")

Working Example

Complete Robot Setup

def complete_robot_setup():
    """Complete example of setting up a robot from URDF."""

    # Load URDF
    processor = URDFToSerialManipulator("robot.urdf")
    robot = processor.serial_manipulator
    dynamics = processor.dynamics

    print("Robot Setup Complete:")
    print(f"- DOF: {len(robot.joint_limits)}")
    print(f"- Joint limits: {robot.joint_limits}")

    # Test forward kinematics
    theta = np.zeros(len(robot.joint_limits))
    T_home = robot.forward_kinematics(theta)
    print(f"- Home position: {T_home[:3, 3]}")

    # Test inverse kinematics
    target = np.eye(4)
    target[:3, 3] = [0.3, 0.2, 0.4]

    solution, success, iterations = robot.iterative_inverse_kinematics(
        target, theta, max_iterations=500
    )

    print(f"- IK test: {'Success' if success else 'Failed'} ({iterations} iter)")

    # Test dynamics
    theta_test = np.array([0.1, 0.3, -0.2])[:len(robot.joint_limits)]
    M = dynamics.mass_matrix(theta_test)
    print(f"- Mass matrix condition: {np.linalg.cond(M):.2e}")

    return processor

# Run complete setup
processor = complete_robot_setup()

Kinematics and Dynamics Usage

def kinematics_dynamics_example():
    """Example using both kinematics and dynamics."""

    processor = URDFToSerialManipulator("robot.urdf")
    robot = processor.serial_manipulator
    dynamics = processor.dynamics

    # Define robot state
    n_joints = len(robot.joint_limits)
    theta = np.random.uniform(-0.5, 0.5, n_joints)
    theta_dot = np.random.uniform(-1.0, 1.0, n_joints)
    theta_ddot = np.random.uniform(-2.0, 2.0, n_joints)

    # Kinematics
    T = robot.forward_kinematics(theta)
    J = robot.jacobian(theta)
    V_ee = robot.end_effector_velocity(theta, theta_dot)

    print("Kinematics Results:")
    print(f"- End-effector position: {T[:3, 3]}")
    print(f"- Jacobian shape: {J.shape}")
    print(f"- End-effector velocity: {V_ee}")

    # Dynamics
    M = dynamics.mass_matrix(theta)
    c = dynamics.velocity_quadratic_forces(theta, theta_dot)
    g = dynamics.gravity_forces(theta, [0, 0, -9.81])

    # Inverse dynamics
    tau = dynamics.inverse_dynamics(
        theta, theta_dot, theta_ddot, [0, 0, -9.81], np.zeros(6)
    )

    # Forward dynamics
    theta_ddot_computed = dynamics.forward_dynamics(
        theta, theta_dot, tau, [0, 0, -9.81], np.zeros(6)
    )

    print("\nDynamics Results:")
    print(f"- Mass matrix determinant: {np.linalg.det(M):.6f}")
    print(f"- Required torques: {tau}")
    print(f"- Verification error: {np.linalg.norm(theta_ddot - theta_ddot_computed):.6f}")

    return robot, dynamics

# Run example
robot, dynamics = kinematics_dynamics_example()

Error Handling

Common Issues and Solutions

def robust_urdf_loading(urdf_path):
    """Robust URDF loading with error handling."""

    try:
        # Attempt to load URDF
        processor = URDFToSerialManipulator(urdf_path)

        # Validate basic properties
        robot = processor.serial_manipulator
        dynamics = processor.dynamics

        # Check if robot has reasonable properties
        if len(robot.joint_limits) == 0:
            raise ValueError("No actuated joints found in URDF")

        # Test basic computation
        theta = np.zeros(len(robot.joint_limits))
        T = robot.forward_kinematics(theta)
        M = dynamics.mass_matrix(theta)

        # Check for numerical issues
        if not np.all(np.isfinite(T)):
            raise ValueError("Forward kinematics produces invalid results")

        if np.linalg.cond(M) > 1e12:
            print("Warning: Mass matrix is poorly conditioned")

        print(f"✅ Successfully loaded robot with {len(robot.joint_limits)} joints")
        return processor

    except FileNotFoundError:
        print(f"❌ URDF file not found: {urdf_path}")
        print("   Check file path and permissions")

    except Exception as e:
        print(f"❌ Error loading URDF: {e}")
        print("   Possible solutions:")
        print("   - Validate URDF syntax")
        print("   - Check for missing mesh files")
        print("   - Verify joint and link definitions")

    return None

# Example usage
processor = robust_urdf_loading("robot.urdf")

Best Practices

URDF File Requirements

For optimal results, ensure your URDF file has:

1. Proper inertial properties for all links

2. Realistic joint limits defined

3. Consistent coordinate frames throughout the chain

4. Valid joint axis definitions (unit vectors)

5. Accessible mesh files (if using complex geometries)

Performance Tips

# Cache the processor for repeated use
_urdf_cache = {}

def get_robot_processor(urdf_path):
    """Get cached processor or create new one."""
    if urdf_path not in _urdf_cache:
        _urdf_cache[urdf_path] = URDFToSerialManipulator(urdf_path)
    return _urdf_cache[urdf_path]

# Use the cached version
processor = get_robot_processor("robot.urdf")

Validation Checklist

Before using a processed URDF:

def validate_processor(processor):
    """Quick validation of URDF processor results."""

    robot = processor.serial_manipulator
    dynamics = processor.dynamics

    # Check 1: Forward kinematics at home
    theta_home = np.zeros(len(robot.joint_limits))
    T_home = robot.forward_kinematics(theta_home)
    print(f"✓ Home position: {T_home[:3, 3]}")

    # Check 2: Mass matrix properties
    M = dynamics.mass_matrix(theta_home)
    is_symmetric = np.allclose(M, M.T)
    is_positive_def = np.all(np.linalg.eigvals(M) > 0)
    print(f"✓ Mass matrix: symmetric={is_symmetric}, pos_def={is_positive_def}")

    # Check 3: Joint limits are reasonable
    reasonable_limits = all(
        abs(limit[1] - limit[0]) > 0.1 for limit in robot.joint_limits
    )
    print(f"✓ Joint limits: reasonable={reasonable_limits}")

    return is_symmetric and is_positive_def and reasonable_limits

# Validate before use
is_valid = validate_processor(processor)

Summary

The URDF Processor provides seamless conversion from URDF robot descriptions to ManipulaPy’s analytical framework:

Key Components:

• URDFToSerialManipulator class: Main interface for URDF processing

• Automatic parameter extraction: Kinematic and dynamic properties

• Joint limit handling: PyBullet integration for realistic limits

• Object creation: SerialManipulator and ManipulatorDynamics instances

*Typical Workflow:

1. Load URDF file with URDFToSerialManipulator(urdf_path)

2. Access serial_manipulator for kinematics computations

3. Access dynamics for dynamics computations

4. Use standard ManipulaPy methods for analysis and control

Best Practices: - Validate URDF files before processing - Use PyBullet limits for realistic joint constraints - Cache processors for repeated use - Check extracted parameters for consistency

The URDF Processor enables you to leverage existing robot models while benefiting from ManipulaPy’s analytical capabilities for advanced robotics applications.