Joint Wrench Sensor

A JointWrenchSensor reports the incoming reaction wrench at each selected articulation body’s parent joint. It exposes force [N] and torque [N·m] separately, with entries ordered by body_names.

Wrench convention

The incoming_joint_frame convention expresses the wrench in the child-side joint frame at the child-side joint anchor. This matches the placement of a six-axis force/torque sensor mounted at the joint. Backend implementations convert their native solver output to this common convention.

Configure the sensor

Set prim_path to the articulation root. Reported body coverage depends on the physics backend:

• PhysX and OVPhysX report every articulation link, including the root link.

• Newton reports the child link of each non-free, non-fixed joint. It therefore excludes the root link and any links connected through free or fixed joints.

Use body_names or find_bodies() instead of assuming that different backends expose the same number or order of entries:

    # robot
    robot = ANT_CFG.replace(prim_path="{ENV_REGEX_NS}/Robot")

    # sensors
    joint_wrench = JointWrenchSensorCfg(prim_path="{ENV_REGEX_NS}/Robot")

Manager-based environments can select a body subset through SceneEntityCfg and use body_incoming_wrench() as an observation term:

        feet_body_forces = ObsTerm(
            func=mdp.body_incoming_wrench,
            scale=0.1,
            params={
                "sensor_cfg": SceneEntityCfg(
                    "joint_wrench",
                    body_names=["front_left_foot", "front_right_foot", "left_back_foot", "right_back_foot"],
                )
            },
        )

Read the data

For E environments and B reported bodies, force.torch and torque.torch each have shape (E, B, 3). Both buffers are None before simulation initialization.

joint_wrench = scene["joint_wrench"]
foot_ids, _ = joint_wrench.find_bodies([".*foot"])

force = joint_wrench.data.force.torch[:, foot_ids]
torque = joint_wrench.data.torque.torch[:, foot_ids]
wrench = torch.cat((force, torque), dim=-1)

The composed wrench has shape (E, num_selected_bodies, 6) with force components followed by torque components. B and the entry ordering can differ between backends.