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URDF description

The Unified Robot Description Format (URDF) is an XML format for representing a robot model. It has been developed by the Robotic Operating System project (ROS).

Kraby URDF description is available at https://github.com/erdnaxe/kraby/blob/master/gym_kraby/data/hexapod.urdf.

Editing and building the URDF with Jinja templating

To simplify URDF editing and to avoid input errors, the project uses Jinja2¹ templates to generate the URDF. You may edit files under gym_kraby/data/src/ then execute gym_kraby/data/generate_urdf.py.

Computing 3d-printed part inertia

Meshlab is able to compute a inertia tensor from a STL file with the “Compute Geometric Measures” filter. Nevertheless the STL is in millimeter and Meshlab implies a density of 1. The following Python script is able to take MeshLab output and print the corresponding inertia URDF tag.

mass = float(input("Object Mass is : "))
volume = float(input("Mesh Volume is : "))
print("Inertia Tensor is :")
j = [list(map(float, input("  ").strip().split(" "))) for _ in range(3)]

for i in range(3):
    for k in range(3):
        # Change density and convert millimeter to meter
        j[i][k] *= mass/volume*0.000001

print(f"""\n<inertia ixx="{j[0][0]:.16f}" ixy="{j[0][1]:.16f}" ixz="{j[0][2]:.16f}" iyy="{j[1][1]:.16f}" iyz="{j[1][2]:.16f}" izz="{j[2][2]:.16f}" />""")

For example, this is the input and output to computer a foot inertia.

% python3 compute_inertia.py
Object Mass is : 0.0105
Mesh Volume is : 15253.618164
Inertia Tensor is :
  8171479.500000 -1091.582153 858.932007
  -1091.582153 6636559.000000 29447.435547
  858.932007 29447.435547 2481984.750000    

<inertia ixx="0.0000056249300217" ixy="-0.0000000007514029"
          ixz="0.0000000005912555" iyy="0.0000045683501941"
          iyz="0.0000000202704742" izz="0.0000017085021793" />

For more information, see http://gazebosim.org/tutorials?tut=inertia&cat=build_robot.

PyBullet integration

Now that our robot is fully described in URDF, we may load it in BulletPhysics using PyBullet Python bindings.

import pybullet as p

p.connect(p.GUI)  # Open new physic server with GUI
p.setGravity(0, 0, -9.81)  # No, we are still on earth
p.setAdditionalSearchPath(getDataPath())  # Add pybullet_data
p.loadURDF("plane.urdf")  # Load a ground
p.loadURDF("hexapod.urdf")  # Load the full robot

You may then use p.setJointMotorControlArray to control motors.