Position control of a joint

[dan.sandberg]

I’m trying to do simple position control of a prismatic joint. If I do:

from omni.isaac.dynamic_control import _dynamic_control

dc = _dynamic_control.acquire_dynamic_control_interface()
articulation = dc.get_articulation("body")
dc.wake_up_articulation(articulation)
dof_ptr = dc.find_articulation_dof(articulation, "prismatic_joint")
dc.set_dof_position(dof_ptr, 1)

This works fine, but the joint position is influenced by gravity and so is not maintained.

If I do:

dc.set_dof_position_target(dof_ptr, 1)

The joint doesn’t move, presumably because I have not added a drive or controller. If I add a “Linear Drive” to the prismatic joint in the GUI, and then do the set_dof_position_target, the joint still doesn’t move.

What am I missing here? I just want to give the joint a target position and have it maintain that.

Thanks!

[dan.sandberg]

Not sure if this is relevant, but I’m seeing these warnings when I use UsdPhysics.ArticulationRootAPI.Apply(prim) to set the articulation root to the cuboid that has a fixed joint to the world.

The “/presenter/slider” mentioned is the cuboid that has the prismatic joint and which slides around when doing set_position.

2023-09-21 11:23:39 [6,564,651ms] [Warning] [omni.physx.tensors.plugin] Cannot assign transform to non-root articulation link at '/presenter/slider'
2023-09-21 11:23:39 [6,564,651ms] [Warning] [omni.physx.tensors.plugin] Cannot assign velocities to rigid body at '/presenter/slider'

Update: Seems like the velocity error is caused by having kinematicsEnabled set to False. Not sure how to enable it though – these didn’t work:

set_prim_property('/presenter/slider', 'kinematicsEnabled', True)
set_prim_property('/presenter/slider', 'physics:kinematicsEnabled', True)

Update: This turns on the kinematics:

from omni.isaac.core.utils.prims import get_prim_at_path, get_prim_property, set_prim_property
from pxr import UsdPhysics
prim = get_prim_at_path('/presenter/slider')
rigidAPI = UsdPhysics.RigidBodyAPI(prim)
attr = rigidAPI.GetKinematicEnabledAttr()
attr.Set(True)

Update: Turning on the kinematics now gives me an error that the body must be non-kinematic :(. So I have no idea what the story is here…

Update: The issue was the stiffness needed to be set to some high value like 100,000. Also the joint origins were not aligned. Now it works.