robot sliding (and rotating)

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Hi, I'm trying to simulate a humanoid robot. To try to tune the PID coefficients, I put the feet as being kinematic, and I put a range between 0 and 0 (so they are fixed) for all joints excepted the ankles for witch I was tunning the PID. Up to that point all went right.

Then I decided to remove the kinematic tag for the foots to see if my robot falls or not. The result was quiet unexpected : the robot didn't fall (that's good, it isn't supposed to if my tunings are good enough), but it started to slide slowly (about 1cm/s) and to rotate around the vertical axis).

Do you have any idea where it comes from and how to correct it?

• I noticed that if either I fix the ankle joints, or I if I put the feet as kinematic, it doesn't occurs.
• I tried with different friction coefficients (0.6 (the realistic one), 1 and 1000), but it didn't seemed to change anything.
• The robot doesn't seem to slow down
• I I replace ODE by Bullet, the sliding is far less important, but the rotation about the vertical axis is faster. With simbody the play button seems to have no effect, and gzserver stops almost immediately when I try with DART.

I'm using gazebo 8.1.0 and Xubuntu 16.04

The files:

• world.sdf : the world
• model.sdf : the SDF file containing my robot (included by world.sdf)
• plugin.cpp : the plugin doing the control part (it's the only one I use)

Thank you very much in advance

Félix