[Question] Initial Joint Position

[Viru97]

Question

I am trying to spawn UR10e arm and Inspire hand in the interactive scene and then create joint between them because i want to keep separate articulation for arm and hand.
There is a problem in default joint values of Hand 'R_thumb_MCP_joint1' and 'R_thumb_MCP_joint2'. default value of these joints are 0.0, which are out of lower limit of these joints.
So, i am doing

 init_state=ArticulationCfg.InitialStateCfg( pos=(1,1,1),
            joint_pos={
                'R_thumb_MCP_joint1': 0.3,
                'R_thumb_MCP_joint2': 1.4
            }
        ),

Works and spawn arm and hand, but when i try to add joint between them

`def fixed_joint():
stage = omni.usd.get_context().get_stage()
for i in range(args_cli.num_envs):
joint_path = f"/World/envs/env_{i}/Hand/inspire_hand_r/root_joint"
joint_prim = stage.GetPrimAtPath(joint_path)
print(f'Joint Prim : {joint_prim}')
# Verify that joint exists and its a PhysicsFixedJoint
if not joint_prim or not UsdPhysics.FixedJoint(joint_prim).GetPrim().IsValid():
raise ValueError(f'Joint at {joint_path} does not exist or is nor a PhysicsFixedJoints')

joint= UsdPhysics.FixedJoint(joint_prim)
joint.GetBody0Rel().SetTargets(['/World/envs/env_0/Arm/flange'])
joint.GetLocalPos0Attr().Set(Gf.Vec3f(0.0,0.0,0.0))
joint.GetLocalRot0Attr().Set(Gf.Quatf(1.0,0.0,0.0,0.0))
joint.GetCollisionEnabledAttr().Set(False)

# Apply ArticulationRootAPI to maintain separate articulations
if not UsdPhysics.ArticulationRootAPI(joint_prim): # Because original hand file has articulation root applied to the root joint
    UsdPhysics.ArticulationRootAPI.Apply(stage.GetPrimAtPath("/World/envs/env_0/Hand/inspire_hand_r/root_joint"))

`

Traceback (most recent call last):
  File "/home/admin/Snap_Assembly/scenes/training_scene.py", line 372, in <module>
    main()
  File "/home/admin/Snap_Assembly/scenes/training_scene.py", line 362, in main
    sim.reset()
  File "/home/admin/IsaacLab/source/isaaclab/isaaclab/sim/simulation_context.py", line 536, in reset
    self.render()
  File "/home/admin/IsaacLab/source/isaaclab/isaaclab/sim/simulation_context.py", line 591, in render
    raise exception_to_raise
  File "/home/admin/IsaacLab/source/isaaclab/isaaclab/assets/asset_base.py", line 291, in _initialize_callback
    self._initialize_impl()
  File "/home/admin/IsaacLab/source/isaaclab/isaaclab/assets/articulation/articulation.py", line 1214, in _initialize_impl
    self._validate_cfg()
  File "/home/admin/IsaacLab/source/isaaclab/isaaclab/assets/articulation/articulation.py", line 1511, in _validate_cfg
    raise ValueError(msg)
ValueError: The following joints have default positions out of the limits: 
	- 'R_thumb_MCP_joint1': 0.000 not in [0.200, 1.300]
	- 'R_thumb_MCP_joint2': 0.000 not in [1.300, 1.800]

It no longer take the given joint_pos. If i am printing hand articulation then it prints (check joint_pos values):

ArticulationCfg(class_type=<class 'isaaclab.assets.articulation.articulation.Articulation'>, prim_path='/World/envs/env_./Hand', spawn=UsdFileCfg(func=<function spawn_from_usd at 0x7707dc1b9bd0>, visible=True, semantic_tags=None, copy_from_source=True, mass_props=None, deformable_props=None, rigid_props=RigidBodyPropertiesCfg(rigid_body_enabled=None, kinematic_enabled=None, disable_gravity=True, linear_damping=None, angular_damping=None, max_linear_velocity=None, max_angular_velocity=None, max_depenetration_velocity=5.0, max_contact_impulse=None, enable_gyroscopic_forces=None, retain_accelerations=None, solver_position_iteration_count=None, solver_velocity_iteration_count=None, sleep_threshold=None, stabilization_threshold=None), collision_props=None, activate_contact_sensors=False, scale=None, articulation_props=None, fixed_tendons_props=None, joint_drive_props=None, visual_material_path='material', visual_material=None, usd_path='/home/admin/Snap_Assembly/CAD_Models/USD_Models/Hand_flattened_joint_modified.usd', variants=None), init_state=ArticulationCfg.InitialStateCfg(pos=(1, 1, 1), rot=(1.0, 0.0, 0.0, 0.0), lin_vel=(0.0, 0.0, 0.0), ang_vel=(0.0, 0.0, 0.0), joint_pos={'R_thumb_MCP_joint1': 0.3, 'R_thumb_MCP_joint2': 1.4}, joint_vel={'.': 0.0}), collision_group=0, debug_vis=False, articulation_root_prim_path=None, soft_joint_pos_limit_factor=1.0, actuators={'hand': ImplicitActuatorCfg(class_type=<class 'isaaclab.actuators.actuator_pd.ImplicitActuator'>, joint_names_expr=['.*'], effort_limit=0.5, velocity_limit=100.0, effort_limit_sim=None, velocity_limit_sim=None, stiffness=0.6, damping=0.001, armature=None, friction=0.01)})

[RandomOakForest]

Thank you for posting this. Here's a summary to consider.

Core Issue

The validation error occurs because:

1. Isaac Lab's _validate_cfg() checks USD-defined default joint positions before applying your InitialStateCfg
2. Your hand's USD file has invalid defaults (0.0) for R_thumb_MCP_joint* joints
3. The physics engine validates against USD defaults before your code applies overrides

Possible Solution Approach

1. Modify the USD File (Recommended)

from pxr import UsdPhysics, Gf

stage = omni.usd.get_context().get_stage()
hand_path = "/World/envs/env_0/Hand/inspire_hand_r"

# Set valid default positions in USD
thumb_joints = ["R_thumb_MCP_joint1", "R_thumb_MCP_joint2"]
for joint in thumb_joints:
    joint_prim = stage.GetPrimAtPath(f"{hand_path}/{joint}")
    attr = joint_prim.GetAttribute("state:angular:position")
    attr.Set(0.3 if "joint1" in joint else 1.4)  # Valid defaults

2. Programmatic Override (Alternative)

def override_joint_defaults():
    stage = omni.usd.get_context().get_stage()
    for i in range(args_cli.num_envs):
        for joint_name in ["R_thumb_MCP_joint1", "R_thumb_MCP_joint2"]:
            prim_path = f"/World/envs/env_{i}/Hand/inspire_hand_r/{joint_name}"
            joint_prim = stage.GetPrimAtPath(prim_path)
            if joint_prim.IsValid():
                attr = joint_prim.GetAttribute("state:angular:position")
                attr.Set(0.3 if "joint1" in joint_name else 1.4)

# Call BEFORE sim.reset()
override_joint_defaults()

Fixed Joint Setup Correction

def fixed_joint():
    stage = omni.usd.get_context().get_stage()
    for i in range(args_cli.num_envs):
        joint_path = f"/World/envs/env_{i}/Hand/inspire_hand_r/root_joint"
        joint_prim = stage.GetPrimAtPath(joint_path)
        
        # Create fixed joint properties
        fixed_joint = UsdPhysics.FixedJoint.Define(stage, joint_path)
        fixed_joint.GetBody0Rel().SetTargets([f"/World/envs/env_{i}/Arm/flange"])
        fixed_joint.GetLocalPos0Attr().Set(Gf.Vec3f(0,0,0))
        fixed_joint.GetLocalRot0Attr().Set(Gf.Quatf(1,0,0,0))
        
        # Apply articulation root to hand ONLY
        root_api = UsdPhysics.ArticulationRootAPI.Apply(
            stage.GetPrimAtPath(f"/World/envs/env_{i}/Hand")
        )

Key Fixes

1. USD Modification Required
   Isaac's validation uses USD defaults before your config. Either:
   • Pre-edit the hand USD file to set valid defaults
   • Runtime modification via override_joint_defaults() (call before sim.reset())
2. Articulation Root Placement
   Apply ArticulationRootAPI to the hand's root prim (/Hand), not its joint.
3. Joint Attachment
   Verify attachment points exist:

# Check flange exists before attaching
flange_prim = stage.GetPrimAtPath("/World/envs/env_0/Arm/flange")
if not flange_prim.IsValid():
    print("ERROR: Arm flange not found!")

[Viru97]

Thank you for the reply.
I am trying to follow 1st method, but getting this error.

Traceback (most recent call last): File "/home/admin/Snap_Assembly/Testing_Dir/joint_modification.py", line 109, in <module> main() File "/home/admin/Snap_Assembly/Testing_Dir/joint_modification.py", line 104, in main run_simulator(sim, scene) File "/home/admin/Snap_Assembly/Testing_Dir/joint_modification.py", line 80, in run_simulator attr.Set(0.3 if "joint1" in joint else 1.4) # Valid defaults pxr.Tf.ErrorException: Error in 'pxrInternal_v0_22__pxrReserved__::UsdStage::_SetValueImpl' at line 6174 in file /builds/omniverse/usd-ci/USD/pxr/usd/usd/stage.cpp : 'Empty typeName for </World/envs/env_0/Hand/joints/R_thumb_MCP_joint1.state:angular:position>'

I launched issac lab, loaded the hand and then executed your code.
`def run_simulator(sim: sim_utils.SimulationContext, scene: InteractiveScene):
#
stage = omni.usd.get_context().get_stage()
hand_path = "/World/envs/env_0/Hand/joints"

# Set valid default positions in USD
thumb_joints = ["R_thumb_MCP_joint1", "R_thumb_MCP_joint2"]
for joint in thumb_joints:
    joint_prim = stage.GetPrimAtPath(f"{hand_path}/{joint}")
    attr = joint_prim.GetAttribute("state:angular:position")
    attr.Set(0.3 if "joint1" in joint else 1.4)  # Valid defaults

while simulation_app.is_running():
    scene.write_data_to_sim()
    sim.step()`

Sorry if I was not clear enough earlier but
init_state=ArticulationCfg.InitialStateCfg( joint_pos={ 'R_thumb_MCP_joint1': 0.3, 'R_thumb_MCP_joint2': 1.4 } ),
this works perfectly problem arise when i try to create joint.

If it helps, here is the snippet of .usd file of inspire_hand

` def PhysicsRevoluteJoint "R_thumb_MCP_joint1" (
            prepend apiSchemas = ["PhysicsJointStateAPI:angular", "PhysxJointAPI", "PhysicsDriveAPI:angular"]
        )
        {
            float drive:angular:physics:damping = 0.0003441867
            float drive:angular:physics:maxForce = 50
            float drive:angular:physics:stiffness = 0.8604667
            float drive:angular:physics:targetPosition = 0
            uniform token drive:angular:physics:type = "force"
            uniform token physics:axis = "Y"
            rel physics:body0 = </inspire_hand_r/world>
            rel physics:body1 = </inspire_hand_r/R_thumb_proximal_base>
            float physics:breakForce = 3.4028235e38
            float physics:breakTorque = 3.4028235e38
            float physics:JointEquivalentInertia = 0.0013767468
            point3f physics:localPos0 = (0.401367, 0.48863596, 0.73173)
            point3f physics:localPos1 = (0, 0, 0)
            quatf physics:localRot0 = (-0.000001847744, 0.99001336, 2.5331974e-7, 0.14097384)
            quatf physics:localRot1 = (0, 1, 0, 0)
            float physics:lowerLimit = 11.459155
            float physics:upperLimit = 74.484505
            float physxJoint:maxJointVelocity = 57.295776
        }`

[RandomOakForest]

Thank you for following up. I will move this post to our Discussions for the team to follow up.