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Surface friction

Gazebo default physics engine (DART) use Coulomb friction model

friction force = coefficient * how hard you press

In Gazebo / SDF, mu and mu2 are friction coefficients in two perpendicular directions on the contact surface.

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      ↑ normal (N)
      |
------•------→ direction 1 (mu)
      |
      ↓ direction 2 (mu2)

for example for wheels we set

  • forward/backward direction → you want low resistance to rolling
  • sideways direction → you want high resistance to slipping
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<mu>0.1</mu>   <!-- rolling direction -->
<mu2>1.0</mu2> <!-- sideways grip -->

DART support

The friction mu, mu2 support by dart engine event that is declare under ode tag

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<friction>
  <ode>
    <mu>1</mu>
    <mu2>0</mu2>
  </ode>
</friction>

Demo:

run xacro and load the world
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xacro friction.sdf.xacro > /tmp/my_world.sdf && gz sim /tmp/my_world.sdf

alt text

  • Cube1 Default friction. sdformat states that if no friction is set, mu1 and mu2 are set to 1 (high friction). Therefore, this cube shouldn't move.
  • Cube2 mu=1, mu2=0. The ramp is pointed down the X axis. Therefore, I expect that with a high mu along that axis, the cube will not move
  • Cube3 mu=0, mu2=1. The ramp is pointed down the X axis. Therefore, I expect that with no friction along this direction, the cube will move down the ramp.
  • Cube4 mu=1, mu2=0, fdir1=1 0 0 (pointed down the ramp). Since the mu value is aligned with the ramp, I expect the block to stay still.

  • Cube5 mu=1, mu2=0, fdir1=0 1 0 (pointed toward the sides of the ramp). Since the mu value is perpendicular to the ramp, I expect the block to move.

  • source code

Reference