Hi, I'm trying to use Mujoco to clamp an object (box) with two vertical capsules. The clamping is achieved by two positional sliding actuators. The phenomena I observed is that after I clamp the box tightly and lift it above the table, it will slowly slide downward. I printed out the constrained force (qfrc_constraint) for the box and it equals to the gravity force it's receiving. So I'm wondering what's the potential cause of this and how can I prevent it? Attached is a screenshot of my experiment. Thanks! Wenhao
This is an unavoidable side effect of the soft contact model that MuJoCo is based on. You cannot eliminate the effect completely, but you can reduce it by making the contacts harder, which is done by increasing solimp[0,1] and/or reducing solref[0]. Setting impratio to a larger value can also help. Using the elliptic solvers (instead of the pyramidal solvers) tends to reduce this effect as well. Increasing friction also helps of course, but only up to a point, and not as much as one might expect.