F/T Sensor (force/torque sensor)

[arambarricalvoj]

Hello everyone,

I have recently added a force/torque sensor to my cobot .usd file in Isaac Sim. Thus, I can read the forces applied to it through forces = db.state.robot.get_measured_joint_forces() (checked https://docs.isaacsim.omniverse.nvidia.com/5.1.0/py/source/extensions/isaacsim.core.nodes/docs/ogn/OgnIsaacArticulationState.html and https://docs.isaacsim.omniverse.nvidia.com/5.1.0/sensors/isaacsim_sensors_physics_articulation_force.html).

The Python script I wrote is as follows:

import omni.graph.core as og
from omni.isaac.core.articulations import Articulation
from omni.isaac.core.utils.prims import get_prim_at_path

def setup(db: og.Database):
    # Crear el articulation del robot
    db.state.robot = Articulation("/World/xarm6")

    # Joint del sensor
    # sensor_joint_prim_path = "/UF_ROBOT/TorqSensor/ft_sensor_root_joint"
    sensor_joint_prim_path = "/World/JoinedTorqueSensor/FixedJoint_TorqueSensor_Link6"
    sensor_joint_prim = get_prim_at_path(sensor_joint_prim_path)
    sensor_joint_name = sensor_joint_prim.GetName()

    # Metadata del articulation
    metadata = db.state.robot._articulation_view._metadata
    joint_indices = metadata.joint_indices
    joint_names = metadata.joint_names

    # Índice interno de la joint
    sensor_joint_index = joint_indices[sensor_joint_name]

    print("Joint indices: ", joint_indices)
    print("Joint names: ", joint_names)
    print("sensor_joint_prim_path: ", sensor_joint_prim_path)
    print("sensor_joint_name: ", sensor_joint_name)
    print("sensor_joint_index: ", sensor_joint_index)

    # Fila real en get_measured_joint_forces()
    db.state.row_index = sensor_joint_index + 1

    db.log_info(f"F/T sensor joint: {sensor_joint_name}, row index: {db.state.row_index}")

    # db.state.initialized = True

def compute(db: og.Database):
    # if not db.state.initialized:
    #    setup(db)
    
    # Leer fuerzas cada frame
    forces = db.state.robot.get_measured_joint_forces()
    fx, fy, fz, tx, ty, tz = forces[db.state.row_index]

    # Output double[6]
    #db.outputs.Forces = [float(fx), float(fy), float(fz),
    #                     float(tx), float(ty), float(tz)]

    db.outputs.Fx = fx
    db.outputs.Fy = fy
    db.outputs.Fz = fz
    db.outputs.Tx = tx
    db.outputs.Ty = ty
    db.outputs.Tz = tz

    return True


def cleanup(db: og.Database):
    db.log_info("F/T Script Node cleaned up")

def cleanup(db: og.Database):
    db.state.robot = None
    db.state.row_index = None
    # db.state.initialized = False
    db.log_info("F/T Script Node cleaned up")

And that Python script is run inside an OmniGraph as shown in the following screenshot:

However, after the simulation runs for the first time, I have to click on “Reload script” within the “Script node” block for it to work again. Otherwise, the warning 2026-03-02 07:30:47 [Warning] [isaacsim.core.prims.impl.articulation] Physics Simulation View is not created yet in order to use get_measured_joint_forces appears, and when I read the values, I get the last values read from the first simulation run and not those corresponding to the current run.

Has anyone else encountered this situation, and how did you resolve it? Having to click on reload script every time the execution is launched is tedious, considering that there may be several action graphs where you have to click.

[PeterL-NV]

Hi, the warning appears because your script tries to access physics data before the physics simulation view is fully initialized after a timeline stop/play cycle.

You can try following options to fix it:

1. Defer Execution Until Physics Ready
   Use the OnPhysicsStep event instead of OnPlaybackTick in your action graph

2. Check Physics Initialization
   Add a validation check in your script node before accessing physics data:

import omni.isaac.core.utils.prims as prim_utils
from isaacsim.core.prims import Articulation

def compute(db):
    # Get your articulation
    articulation = Articulation(prim_path="/your/articulation")
    
    # CHECK: Is physics handle valid before accessing?
    if not articulation.is_physics_handle_valid():
        # Physics not ready yet - skip this frame
        return
    
    # CHECK: Is articulation view initialized?
    if articulation._articulation_view is None:
        return
        
    # Now safe to access physics data
    try:
        joint_forces = articulation.get_measured_joint_forces()
        # ... your logic here
    except Exception as e:
        # Handle any remaining edge cases
        return

[arambarricalvoj]

Hi Peter,

Thank you for your reply. I had to set my OmniGraph to On-Demand to use OnPhysicsStep and tried the following snippet of code:

import omni.isaac.core.utils.prims as prim_utils
from isaacsim.core.prims import Articulation
from omni.isaac.core.utils.prims import get_prim_at_path

def compute(db: og.Database):
    # Get your articulation
    articulation = Articulation(prim_path="/UF_ROBOT/ft_sensor/FixedJoint")
    
    # CHECK: Is physics handle valid before accessing?
    if not articulation.is_physics_handle_valid():
        # Physics not ready yet - skip this frame
        return
    
    # CHECK: Is articulation view initialized?
    if articulation._articulation_view is None:
        return
        
    # Now safe to access physics data
    try:
        joint_forces = articulation.get_measured_joint_forces()
        fx, fy, fz, tx, ty, tz = joint_forces[db.state.row_index]
        db.outputs.Fx = fx
        db.outputs.Fy = fy
        db.outputs.Fz = fz
        db.outputs.Tx = tx
        db.outputs.Ty = ty
        db.outputs.Tz = tz
    except Exception as e:
        # Handle any remaining edge cases
        return

But it arises the following error:

2026-03-09T07:57:06Z [321,282ms] [Error] [omni.kit.app._impl] [py stderr]: Exception ignored in: <function Articulation.__del__ at 0x794f049b1ee0>
2026-03-09T07:57:06Z [321,282ms] [Error] [omni.kit.app._impl] [py stderr]: Traceback (most recent call last):
2026-03-09T07:57:06Z [321,282ms] [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/articulation.py", line 184, in __del__
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:     XFormPrim.__del__(self)
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/prim.py", line 62, in __del__
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:     self.destroy()
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/prim.py", line 65, in destroy
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:     for callback_id in self._callbacks:
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]:                        ^^^^^^^^^^^^^^^
2026-03-09T07:57:06Z [321,283ms] [Error] [omni.kit.app._impl] [py stderr]: AttributeError: 'Articulation' object has no attribute '_callbacks'

I'm really lost since I'm unable to find any documentation or tutorial.

[arambarricalvoj]

Furthermore, I have read in the links above that the OmniGraph “Articulation State” block only works for non-fixed joints, and my torque sensor is obviously on a fixed joint.

Is there any other way?

[PeterL-NV]

Hi, The error occurs because you're creating a new Articulation object every frame in your OmniGraph script node's compute() function. Each time compute() runs, a new Articulation is created and the old one is garbage collected, triggering the destructor which tries to access _callbacks that were never initialized.

Can you try following fix:

def compute(db: og.Database):
    # Initialize articulation ONCE and store in db.state
    if not hasattr(db.state, 'articulation'):
        db.state.articulation = Articulation(prim_path="/UF_ROBOT/ft_sensor/FixedJoint")
        db.state.articulation.initialize()  #Important: call initialize
    
    articulation = db.state.articulation

[arambarricalvoj]

Hi, Thanks for your support, but I'm still unable to get it.

Following your advice my current snippet of code is:

import omni.isaac.core.utils.prims as prim_utils
from isaacsim.core.prims import Articulation

def compute(db: og.Database):
    # Get your articulation
    # Initialize articulation ONCE and store in db.state
    if not hasattr(db.state, 'articulation'):
        db.state.articulation = Articulation(prim_path="/UF_ROBOT/ft_sensor/FixedJoint")
        db.state.articulation.initialize()  #Important: call initialize
    
    articulation = db.state.articulation
    
    # CHECK: Is physics handle valid before accessing?
    if not articulation.is_physics_handle_valid():
        # Physics not ready yet - skip this frame
        return
    
    # CHECK: Is articulation view initialized?
    if articulation._articulation_view is None:
        return
        
    # Now safe to access physics data
    try:
        joint_forces = articulation.get_measured_joint_forces()
        fx, fy, fz, tx, ty, tz = forces[db.state.row_index]

        # Output double[6]
        #db.outputs.Forces = [float(fx), float(fy), float(fz),
        #                     float(tx), float(ty), float(tz)]

        db.outputs.Fx = fx
        db.outputs.Fy = fy
        db.outputs.Fz = fz
        db.outputs.Tx = tx
        db.outputs.Ty = ty
        db.outputs.Tz = tz

        return True
    except Exception as e:
        # Handle any remaining edge cases
        return

But the output error is the same:

2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]: Exception ignored in: <function Articulation.__del__ at 0x7bdf97a3e340>
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]: Traceback (most recent call last):
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/articulation.py", line 184, in __del__
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:     XFormPrim.__del__(self)
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/prim.py", line 62, in __del__
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:     self.destroy()
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:   File "/isaac-sim/exts/isaacsim.core.prims/isaacsim/core/prims/impl/prim.py", line 65, in destroy
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:     for callback_id in self._callbacks:
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]:                        ^^^^^^^^^^^^^^^
2026-03-18 07:29:00  [Error] [omni.kit.app._impl] [py stderr]: AttributeError: 'Articulation' object has no attribute '_callbacks'

Starting next week, I'll try to keep experimenting with different methods; I'm busy this week.

[orinceal]

I did something like this following the logic from the documentation for getting the measured joint forces and using "builtins" (recommended by Google Gemini). Maybe you can try using the SingleArticulation instead. I run with the ScriptEditor instead of building the action graph, (where on_physics_step) calls the get_measured_joint_forces() function. Maybe I'll try your way with the ActionGraph at some point. From your first error though, what I found is that "Simulation View" needs to open an existing file or create a new stage and cannot use the current open stage.

import builtins
import omni.physx
from pxr import UsdPhysics

# refresh the physics subscription
if hasattr(builtins, "phys_sub") and builtins.phys_sub is not None:
    builtins.phys_sub.unsubscribe()
    builtins.phys_sub = None

builtins.arti_view = None

# my_world = World.instance()
timeline = omni.timeline.get_timeline_interface()
ROBOT_PATH = "/World/robot"
SENSOR_JOINT_NAME = "sensor_joint"
JOINT_PATH = "/World/robot/joints/" + SENSOR_JOINT_NAME

# ----- SIMULATION LOGIC
def initialize_articulation():
     stage = get_current_stage()
     builtins.arti_view = SingleArticulation(ROBOT_PATH)
     if not builtins.arti_view._articulation_view._is_initialized:
          return False

     joint_names = builtins.arti_view._articulation_view.joint_names
     if SENSOR_JOINT_NAME in joint_names:
          joint = UsdPhysics.Joint.Get(stage, JOINT_PATH)
          body_1_path = joint.GetBody1Rel().GetTargets()[0]
          body_1_name = stage.GetPrimAtPath(body_1_path).GetName()
          builtins.sensor_frame = body_1_name
          print(f"body_1_name: {body_1_name}")
          print(f"body_1_path: {body_1_path}")
          builtins.child_link_index = builtins.arti_view._articulation_view.get_link_index(body_1_name)
          return True
     print(f"Error: Joint {SENSOR_JOINT_NAME} not found!")
     return False

def on_physics_step(step_size):
     if builtins.arti_view is None:
          if not initialize_articulation():
               return # wait for next step
     
     sensor_forces = builtins.arti_view.get_measured_joint_forces()
     if sensor_forces is not None:
       # output / publish 
     	
# subscribe to the physics event     
builtins.phys_sub = omni.physx.get_physx_interface().subscribe_physics_step_events(on_physics_step)

[K-Wince]

Hi, I was able to get it to work using an onSimulation ActionGraph. This was achieved by checking if the physics handle was valid during the compute() method and reinitializing if not, as well as destroying the Articulation on cleanup. I hope this works for you! This is on IsaacSim 6.0.0, as I'm not sure what version you are using.

import omni.usd
from isaacsim.core.prims import Articulation

def setup(db: og.Database):
    # Get the robot path
    robot = db.inputs.robot_prim_path

    # Create an articulation view
    db.state.arti_view = Articulation(robot)
    db.state.arti_view.initialize()

    # Get the joint index for the force-torque sensor and store it in the internal state
    # Add one to the index to use with get measured joint forces, which expects a 1-based index
    db.state.joint_link_id = db.state.arti_view.get_joint_index("{joint_name_here}") + 1

def compute(db: og.Database):
    # Check if physics handle is valid, else reinitialize
    if not db.state.arti_view.is_physics_handle_valid():
        db.state.arti_view.initialize()

    # Get the measure joint forces and torques from the articulation view
    joint_forces = db.state.arti_view.get_measured_joint_forces(joint_indices=[db.state.joint_link_id])
    
    db.outputs.joint_forces = joint_forces[0][0][:3]
    db.outputs.joint_torques = joint_forces[0][0][3:6]


def cleanup(db: og.Database):
    db.state.arti_view.destroy()
    pass