Drift of models on an inclined plane

[scpeters]

I have observed vehicles in gazebo drift when supposedly at rest on a planar surface. Even on an inclined plane, they should be still if the static friction coefficient is sufficiently large and the wheels have the brakes applied. I have observed this specifically with ODE, and an initial test indicates that bullet doesn't have this problem.

To demonstrate this, I have added the following models:

• triball_lumped: a single link with three collision spheres
• triball_fixed: a chassis link connected to three sphere links with fixed joints
• triball_revolute: a chassis link connected to three sphere links with revolute joints that have joint friction

I also added an example world triball_drift.world with a lateral gravity component that mimics an inclined plane. With ODE, the triball_lumped model does not drift, the triball_fixed model drifts some, and the triball_revolute model drifts more.

@rosebudflyaway do you have any ideas why ODE might have this drift problem?

[scpeters]

This shows the model with revolute joints closest to the camera, the fixed joint model in the middle, and the lumped model farthest away.

[scpeters]

With 50 iterations and grav = [1, 0, -9.8], the model with revolute joints drifts at about 4 mm/minute. With grav = [5, 0, -9.8], it drifts at about 20 mm/minute. Increasing iterations to 200, it drops from 20 mm/minute to 3e-6/minute. That's good news for this simple case, but increasing solver iterations didn't seem to help with a more complex model I have been investigating.