How to get stable foot contacts when a biped robot is standing still?

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I'm simulating a biped robot in gazebo 1.7 but the foot contacts showed by the simulator are intermittent also when the robot is standing still.

DRCSim Atlas simulation shows constant and stable contacts on the feet when the robot is walking. How that is achieved?

Is it because the Atlas joint controllers, link contacts and solver parameters are better tuned than mine or there is something I'm missing?

There are some additional parameters to tune to get a better interaction with the ground?

I'm using the same contact model as the Atlas model for the foot body:

<gazebo reference="r_foot"> <kp>6000000.0</kp> <kd>1.0</kd> <mu1>0.9</mu1> <mu2>0.9</mu2> <fdir1>1 0 0</fdir1> <maxvel>1.0</maxvel> <mindepth>0.003</mindepth> </gazebo>

If I simulate only Atlas foot, without controllers because there are no joints to control, the intermittent behavior appears. Can someone explain why this is happening?

Pictures of the Atlas robot with stable contacts: atlas-contacts

Only one foot with intermittent contacts: foot-contacts1 foot-contact2