Contact Forces Unrealistic


I’m using Gazebo 11 with DART 6 to simulate robotic grasping.

My problem is that my contact forces are very unrealistic in multi-contact scenarios. However, I need precise contact forces to do slip prediction in my project.

Have a look at the below screenshots: I am computing the angle between the contact force and the contact normal. The normals are very precise, however the forces go wild: If I have a single contact the angle is about 30-40 degrees, which sounds about right. In the second screenshot, when I have contact on multiple links, you can see that the angle is very large (sometimes up to 110 degrees). I’ve plotted the contact normal in blue and force in red on the right.

Physically, an angle between force and normal above 90 degrees would mean that the contact separates, but my grasp seems to be nice and stable (i.e. no oscillations visible). I guess “microscopically” the object oscillates in the hand and this is why my forces point in such weird directions. The problem happens less with boxes and spheres.

Any ideas on how to make the force readings more realistic?