Physics engines

Physics engines

The description of the physics engine's parameters is one of the most import parts in the world description in Gazebo. A <world> can only have one physics element.

It can use the following engines:

• ODE (ode)
• Bullet (bullet)
• Simbody (simbody)
• DART (dart)

and a specific SDF block is available to describe each engine.

# Import the element creator
from pcg_gazebo.parsers.sdf import create_sdf_element

# Create first the global physics block
physics = create_sdf_element('physics')
print(physics)

<physics default="1" name="default_physics" type="ode">
  <max_contacts>20</max_contacts>
  <real_time_factor>1</real_time_factor>
  <max_step_size>0.001</max_step_size>
  <real_time_update_rate>1000</real_time_update_rate>
</physics>

# The physics engine's configuration modes are named after the 
# engine being used, the default being `ode`
physics.reset(mode='ode', with_optional_elements=True)
print(physics)

<physics default="1" name="default_physics" type="ode">
  <max_contacts>20</max_contacts>
  <real_time_update_rate>1000</real_time_update_rate>
  <real_time_factor>1</real_time_factor>
  <max_step_size>0.001</max_step_size>
  <ode>
    <solver>
      <use_dynamic_moi_scaling>0</use_dynamic_moi_scaling>
      <sor>1.3</sor>
      <friction_model>pyramid_model</friction_model>
      <type>quick</type>
      <iters>50</iters>
      <min_step_size>0.0001</min_step_size>
      <precon_iters>0</precon_iters>
    </solver>
    <constraints>
      <cfm>0</cfm>
      <contact_max_correcting_vel>100</contact_max_correcting_vel>
      <contact_surface_layer>0.001</contact_surface_layer>
      <erp>0.2</erp>
    </constraints>
  </ode>
</physics>

physics.reset(mode='bullet', with_optional_elements=True)
print(physics)

<physics default="1" name="default_physics" type="bullet">
  <max_contacts>20</max_contacts>
  <bullet>
    <solver>
      <type>quick</type>
      <iters>50</iters>
      <sor>1.3</sor>
      <min_step_size>0.0001</min_step_size>
    </solver>
    <constraints>
      <split_impulse>1</split_impulse>
      <cfm>0</cfm>
      <erp>0.2</erp>
      <split_impulse_penetration_threshold>-0.01</split_impulse_penetration_threshold>
      <contact_surface_layer>0.001</contact_surface_layer>
    </constraints>
  </bullet>
  <real_time_factor>1</real_time_factor>
  <max_step_size>0.001</max_step_size>
  <real_time_update_rate>1000</real_time_update_rate>
</physics>

physics.reset(mode='simbody', with_optional_elements=True)
print(physics)

<physics default="1" name="default_physics" type="simbody">
  <simbody>
    <accuracy>0.001</accuracy>
    <contact>
      <plastic_impact_velocity>0.5</plastic_impact_velocity>
      <viscous_friction>0</viscous_friction>
      <dissipation>100</dissipation>
      <stiffness>100000000.0</stiffness>
      <plastic_coef_restitution>0.5</plastic_coef_restitution>
      <static_friction>0.9</static_friction>
      <override_stiction_transition_velocity>0.001</override_stiction_transition_velocity>
      <override_impact_capture_velocity>0.001</override_impact_capture_velocity>
      <dynamic_friction>0.9</dynamic_friction>
    </contact>
    <max_transient_velocity>0.01</max_transient_velocity>
    <min_step_size>0.0001</min_step_size>
  </simbody>
  <max_contacts>20</max_contacts>
  <real_time_factor>1</real_time_factor>
  <max_step_size>0.001</max_step_size>
  <real_time_update_rate>1000</real_time_update_rate>
</physics>