I'm using the striker environment and trying to control the position of the hand directly (instead of using actuators). I used the "weld" equality to attach a box on the hand ("attached.png"), and then moved the box using mocap. However, the hand is not always strictly attached to the box. In "nonattached.png", the robot arm is actually capable of moving to the correct position/orientation, but it is not following the mocap box. I've decreased timestep but it doesn't seem to help. Any ideas? Thanks!
Equality constraints in MuJoCo are soft. If there is another force fighting against the constraint, it can be detached. You can adjust the strength of all constraints with the solref and solimp parameters; see documentation for details. Changing the time step is not doing anything because what you are seeing is not an integration error; it is happening by design.