What could be the cause of mjWARN_BADQACC?

[XinyuWang2]

Hi, I am having a weird behavior in the mujoco simulator. The scenario is described below:

1. Two objects(box) are combined through a weld constraint. The smaller one is inside another.
2. The smaller one can be controlled by slide actuators.

Ideally, they should move together, slowly because of a small action, which does happen in most of the other scenarios. However, for this rare one, all of a sudden, they move so abruptly.

These are xyz locations of the smaller box in 5 consecutive physics.steps.
[-0.54017543 -0.06785287 0.04978686] -> (activate constaint and keep it for later steps) -> step() -> [-0.54133 -0.0676916 0.04982307] -> (movement between this and last step is indeed small), step() -> [0.85014079 0.50555359 1.88797865] <------ magic happens from here !! [-16.61497857 -17.21111811 1.36517479] [1049.15584802 1045.70554427 5.04508734]

Actually, I have set joint range limit and the working space is surrounded by walls and a ceiling(the space is rather small, about 2x2x2). So ideally they should at least stay inside this space.

Probably this is related to setting constraints. I found a previous post that says weird behavior might happen when objects are very small. But it didn't change anything when I change from one 0.05, one 0.01 box to one 0.05, one 0.03 box.

[yuvaltassa]

We can't reproduce without a model.

I suspect the two objects are contacting each other and the huge contact forces are causing divergence? Activate contact force visualisation ('F' in simulate) to diagnose.

[Balint-H]

Just in case, but have you disabled collisions between the two objects?

[yuvaltassa]

Thanks @Balint-H,

Yes, that's what I meant.

[XinyuWang2]

Yes, having contacts is expected. I think the main reason is that during the constraint activation, quaternion for relpose is [1, 0, 0, 0] and this correction step would lead to bad acc in some rare cases (e.g. there are so many rotations to make an object quaternion to [1, 0, 0, 0]). Fortunately, I later solved my issue by setting relpose by changing eq_data in the runtime.

I also make a model that can reproduce the error. Thanks! minimul_case.txt