Articulation Joint Sensors#

Articulation sensors allow reading the active and passive components of the joint forces. To read articulation joint forces you can use Articulation or ArticulationView APIs. See Robot Simulation Snippets for more details about the Articulation and the ArticulationView classes. Specifically,

  • get_applied_joint_efforts API will return a tensor that specifies the efforts set by the user through the set_joint_efforts.

  • get_measured_joint_forces API will return a tensor that specifies 6-dimensional spatial forces per joints for all articulations (total overall joint forces). To mimic force-torque sensors, this API can be used to retrieve forces from a fixed joint.

  • get_measured_joint_efforts API will return a tensor which specifies the active components (the projection of the joint forces on the motion direction) of the joint forces for all the joints and articulations.

Note

In an articulation tree, each link can have a single parent link. The joint forces reported by get_measured_joint_forces and get_measured_joint_efforts APIs correspond to the forces, torques, or efforts exerted by the joint connecting the child link to the parent link. In short, the forces reported by these API denote the link incoming joints forces.

GUI#

Script Editor#

This section describes how to add and customize the articulation sensor through the Script Editor, opened from Window > Script Editor.

import asyncio

import omni
from isaacsim.core.api import World
from isaacsim.core.prims import SingleArticulation
from isaacsim.core.utils.stage import (
    add_reference_to_stage,
    create_new_stage_async,
    get_current_stage,
)
from isaacsim.storage.native import get_assets_root_path
from pxr import UsdPhysics


async def joint_force():
    World.clear_instance()
    await create_new_stage_async()
    my_world = World(stage_units_in_meters=1.0, backend="torch", device="cpu")
    await my_world.initialize_simulation_context_async()
    await omni.kit.app.get_app().next_update_async()
    assets_root_path = get_assets_root_path()
    asset_path = assets_root_path + "/Isaac/Robots/IsaacSim/Ant/ant.usd"
    add_reference_to_stage(usd_path=asset_path, prim_path="/World/Ant")
    await omni.kit.app.get_app().next_update_async()
    my_world.scene.add_default_ground_plane()
    arti_view = SingleArticulation("/World/Ant/torso")
    my_world.scene.add(arti_view)
    await my_world.reset_async(soft=False)
    stage = get_current_stage()

    sensor_joint_forces = arti_view.get_measured_joint_forces()
    sensor_actuation_efforts = arti_view.get_measured_joint_efforts()
    # Iterates through the joint names in the articulation, retrieves information about the joints and their associated links,
    # and creates a mapping between joint names and their corresponding link indices.
    joint_link_id = dict()
    for joint_name in arti_view._articulation_view.joint_names:
        joint_path = "/World/Ant/joints/" + joint_name
        joint = UsdPhysics.Joint.Get(stage, joint_path)
        body_1_path = joint.GetBody1Rel().GetTargets()[0]
        body_1_name = stage.GetPrimAtPath(body_1_path).GetName()
        child_link_index = arti_view._articulation_view.get_link_index(body_1_name)
        joint_link_id[joint_name] = child_link_index

    print("joint link IDs", joint_link_id)
    print(sensor_joint_forces[joint_link_id["front_left_leg"]])
    print(sensor_actuation_efforts[joint_link_id["front_left_leg"]])


asyncio.ensure_future(joint_force())
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