Robot slips/slides/vibrates over ground plane

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Hi guys,

i have this rather annoying problem that my robot ( 4 legged walking robot ) vibrates over the ground plane. As a result it has no friction and every simulation of a step is useless. The force peaks are over 100N within a fraction of a second. Do you know any way to damp this force? I can't figure out how these force peaks occur. For the movement of the joints I use a plugin see code below. I would be grateful for any idea.

this->jointsFL[joint]->SetParam("fmax", 0, this->50.0);
this->jointsFL[joint]->SetParam("vel", 0, ((double)this->targetVelFL[joint]));

What I have tried until now: -Interta System checked (twice) -friction of the links increased -damping of the joints increased - And the Tags: <min_depth>0.01</min_depth> <max_vel>0</max_vel>