MuJoCo and XML

Modeling via XML

Basic Example

import mujoco
import mujoco.viewer
import time 

# Load a model from a file (won't do this here to avoid external file calls)
# model = mujoco.MjModel.from_xml_path('path/to/your/model.xml')

# Create a simple pendulum model
model = mujoco.MjModel.from_xml_string("""
<mujoco>
  
  <worldbody>
    <light name="light1" pos="0.3 0.3 1"/>
    <geom name="ground" type="plane" pos=" 0 0 -1.2" size="2 2 0.1" rgba="0.5 0.5 0.5 1"/>
    
    <body name="pendulum" pos="0 0 0">
      <joint name="pivot" type="hinge" pos="0 0 0" axis="0 1 0" damping="0.1"/>
      <geom name="rod" type="capsule" fromto="0 0 0 0 0 0.6" size="0.05" rgba="0.5 0.5 0.5 1"/>
      <geom name="mass" type="sphere" pos="0 0 0.6" size="0.07" rgba="0.8 0.2 0.2 1"/>
    </body>
  </worldbody>
</mujoco>
""")

data = mujoco.MjData(model)
data.qpos[0]=0.01

# Create a viewer
viewer = mujoco.viewer.launch(model, data, 
        show_left_ui=False,
        show_right_ui=False,
)

Run python3 code/basic_mujoco.py

Basic Example

import mujoco
import mujoco.viewer
import time 

# Load a model from a file (won't do this here to avoid external file calls)
# model = mujoco.MjModel.from_xml_path('path/to/your/model.xml')

# Create a simple pendulum model
model = mujoco.MjModel.from_xml_string("""
<mujoco>
  
  <worldbody>
    <light name="light1" pos="0.3 0.3 1"/>
    <geom name="ground" type="plane" pos=" 0 0 -1.2" size="2 2 0.1" rgba="0.5 0.5 0.5 1"/>
    
    <body name="pendulum" pos="0 0 0">
      <joint name="pivot" type="hinge" pos="0 0 0" axis="0 1 0" damping="0.1"/>
      <geom name="rod" type="capsule" fromto="0 0 0 0 0 0.6" size="0.05" rgba="0.5 0.5 0.5 1"/>
      <geom name="mass" type="sphere" pos="0 0 0.6" size="0.07" rgba="0.8 0.2 0.2 1"/>
    </body>
  </worldbody>
</mujoco>
""")

data = mujoco.MjData(model)
data.qpos[0]=0.01

# Create a viewer
viewer = mujoco.viewer.launch(model, data, 
        show_left_ui=False,
        show_right_ui=False,
)

Basic Example with Loading

import mujoco
import mujoco.viewer
import time 

# Load a model from a file (won't do this here to avoid external file calls)
# model = mujoco.MjModel.from_xml_path('path/to/your/model.xml')

# Create a simple pendulum model
model = mujoco.MjModel.from_xml_string("""
<mujoco>
  
  <worldbody>
    <light name="light1" pos="0.3 0.3 1"/>
    <geom name="ground" type="plane" pos=" 0 0 -1.2" size="2 2 0.1" rgba="0.5 0.5 0.5 1"/>
    
    <body name="pendulum" pos="0 0 0">
      <joint name="pivot" type="hinge" pos="0 0 0" axis="0 1 0" damping="0.1"/>
      <geom name="rod" type="capsule" fromto="0 0 0 0 0 0.6" size="0.05" rgba="0.5 0.5 0.5 1"/>
      <geom name="mass" type="sphere" pos="0 0 0.6" size="0.07" rgba="0.8 0.2 0.2 1"/>
    </body>
  </worldbody>
</mujoco>
""")

data = mujoco.MjData(model)
data.qpos[0]=0.01

# Create a viewer
viewer = mujoco.viewer.launch(model, data, 
        show_left_ui=False,
        show_right_ui=False,
)

code/basic_mujoco.py

import mujoco
import mujoco.viewer
import time 

# Load a model from a file (won't do this here to avoid external file calls)
model = mujoco.MjModel.from_xml_path('../models/simple_pendulum_act.xml')

data = mujoco.MjData(model)
data.qpos[0]=0.01

# Create a viewer
viewer = mujoco.viewer.launch(model, data, 
        show_left_ui=False,
        show_right_ui=False,
)

code/mujoco_load_xml.py

Interactive

python3 -m mujoco.viewer

and then drag and drop xml files from models/

Block Example

<mujoco>
<option gravity="0 0 -9.81"/>
    <worldbody>
    <light diffuse=".5 .5 .5" pos="0 0 3" dir="0 0 -1"/>
        <geom type="plane" size="1 1 0.1" rgba=".9 0 0 1"/>
        <body pos="0 0 1">
            <joint type="free"/>
            <geom type="box" size=".1 .2 .3" rgba="0 .9 0 1"/>
        </body>
    </worldbody>
</mujoco>

Blocks Example

<mujoco>
<option gravity="0 0 -9.81"/>
    <worldbody>
    <light diffuse=".5 .5 .5" pos="0 0 3" dir="0 0 -1"/>
        <geom type="plane" size="1 1 0.1" rgba=".9 0 0 1"/>
        <body pos="0 0 1">
            <joint type="free"/>
            <geom type="box" size=".1 .2 .3" rgba="0 .9 0 1"/>
        </body>
        <body pos="0.1 0 1.5" euler="0 90 0">
            <joint type="free"/>
            <geom type="box" size=".1 .2 .3" rgba="0 0 0.9 1"/>
        </body>
        <body pos="0.1 0 2.0">
            <joint type="free"/>
            <geom type="sphere" size=".1" rgba="0.9 0.9 0 1"/>
        </body>
    </worldbody>
</mujoco>

Actuated Example

import mujoco
import mujoco.viewer
import time 

# Load a model from a file (won't do this here to avoid external file calls)
model = mujoco.MjModel.from_xml_path('../models/simple_pendulum_act.xml')


dt: float = 0.002
model.opt.timestep = dt
data = mujoco.MjData(model)
data.qpos[0]=0.01

# Create a viewer
viewer = mujoco.viewer.launch_passive(model, data,
        show_left_ui=False,
        show_right_ui=False,
      )
try:
    while viewer.is_running():
        step_start = time.time()
        # Step the simulation
        data.ctrl[0] = -100*data.qpos[0] - 20*data.qvel[0]
        print(data.qpos[0])
        mujoco.mj_step(model, data)
        # Update the viewer
        viewer.sync()
        # Add a small delay to control frame rate
        time_until_next_step = dt - (time.time() - step_start)
        if time_until_next_step > 0:
            time.sleep(time_until_next_step)
except KeyboardInterrupt:
    print("Viewer closed by user")
finally:
    viewer.close()

code/mujoco_load_xml_control.py

Extras

Models

MuJoCo menagerie

Simulation

MuJoCo Playground MJ Control