Question

Hello! I'm trying to understand how the parameters that I set in <ode><friction> and <ode><contacts><max_vel> influence the simulation.

I build a basic gripper and I wanted to set the maximum friction on the gripper surface, so I put the following parameters in my SDF:

`

   <surface>
      <friction>
        <ode>
          <mu>1</mu>
          <mu2>1</mu2>
        </ode>
      </friction>
    <contact>
        <ode>
          <max_vel>0.0</max_vel>
        </ode>
      </contact>
    </surface>

` The questions are the following:

What exactly represent mu an mu2? I have read in the ODE manual that they are Coulomb friction parameters, so I would expected that mu is the static friction parameter and mu2 the dynamic one, but in the manual and in the SDF documentation there are some references to different directions...
Why there still subsist a relative motion between my gripper and the object?

Asked by Formigola on 2015-08-20 14:28:50 UTC

Answer 1

You can read about mu and mu2 on the ODE user guide. The difference is not about static vs dynamic, but about direction: mu acts on direction fdir1 and mu2 acts perpendicular to that and to the contact normal.

If fdir1 is not specified in the SDF, "automatically compute friction direction 1 to be perpendicular to the contact normal (in which case its resulting orientation is unpredictable)."

If you want contact that never slips, you must set mu to infinity (<mu>Inf</mu>) as explained in the same link.

max_vel, as explained in the Gazebo API, is not about surfaces sliding against each other (like friction), but about interpenetration: "Maximum interpenetration error correction velocity. If set to 0, two objects interpenetrating each other will not be pushed apart."

Asked by chapulina on 2015-08-21 11:08:41 UTC

ODE friction parameters and max_vel parameter in SDF format.

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