Weird behaviour on contact - bodies diffusing into each other

[achuwilson]

Hi, I am having a weird observation. I designed a simple 2 finger gripper (URDF model with STL meshes) and attached it to the simulated Kuka robot and is trying to pick the checkered sphere given with pybullet_data. I close the gripper by position control, by commanding the fingers to move to the centre position.

When the fingers of the gripper is having a mass of 0.1,(100 grams, which is reasonable), the sphere diffuses into the fingers on grasping as in the following picture.

If I reduce the mass of the finger in URDF to 0.0025 ( 2.5 grams, which is unreasonable), it works fine.

What is the reason for this?
Is there any other fix to this apart from modifying the mass of the finger to a non-realistic value?

[erwincoumans]

It is likely a mass ratio issue with the pgs constraint solver. Did you try a smaller time step and more solver iterations (and small residual threshold)? Or direct solver?
We can only give feedback with small self-contained pybullet examples attached as zip.

If you cannot share, could you modify panda grasp?

https://github.com/bulletphysics/bullet3/tree/master/examples/pybullet/gym/pybullet_robots/panda