DART Integration in Gazebo 11

[trns1997]

Hey guys again,
I've been trying to fix up DART soft bodies in gazebo, and was wondering if I could get your opinions on the topic. I have already addressed the issue in the gazebo repo (gazebosim/gazebo-classic#2255 (comment)). But this time I tried to do a more comprehensive test where I compare the results from the DART tutorial and the integration in gazebo.

The Test:

1. Box Size: 0.1 * 0.1 * 0.06
2. Total Mass: 0.1736 kg
3. SoftBody Mass: 0.044000 kg
4. Drop height: 0.4 m

I basically replicated the mass and box size values from the DART tutorial into gazebo so the tests are consistent between the 2 simulations.

Here are my results:

So visually they seem similar, but the graph plots seem slightly different, I have a couple guesses to why:

1. The collision detector I used in gazebo is FCL and in the dart tutorial I am not sure (you guys would know better). I tried using the DART collision detector but does not work, apparently it does not support SoftMeshShapes.
2. The way I log. The log variable updates more often in the gazebo plugin than that in the dart tutorial, I wasn't sure how to do it in the dart tutorial soo I just logged the variable displayTimer method.

At steady state both force fz make sense and are pretty similar in both simulation instances with mean of 1.7011 N and 1.703016 N for gazebo and dart sim test respectively. Just not sure if that giant peak in the case of the gazebo simulation makes sense. Let me know what you guys think or have any suggestion.