physics-simulation

Physics Simulation in Isaac Sim

Targets Isaac Sim 6.0+ / Kit 110. Both backends share UsdPhysics.*; backend-specific behavior is called out per section.

Backend selection (Kit 110)

isaacsim.core.simulation_manager registers physics engines and picks the active one. default_engine in its extension.toml is "physx", but the isaacsim.physics.newton extension defaults auto_switch_on_startup = true, so any app that enables isaacsim.physics.newton (the standard isaacsim.exp.full.kit does) ends up with Newton active at startup.

from isaacsim.core.simulation_manager import SimulationManager

SimulationManager.switch_physics_engine("newton")   # or "physx"
print(SimulationManager.get_active_physics_engine())

# Inspect what is available
from isaacsim.physics.newton import get_available_physics_engines, get_active_physics_engine
print(get_available_physics_engines())

Force the engine explicitly when launching:

--/exts/isaacsim.core.simulation_manager/default_engine=newton     # or =physx
--/exts/isaacsim.physics.newton/auto_switch_on_startup=false        # opt out of auto-switch

Newton config classes (extension Python API; not surfaced in the user-guide RST yet — see the extension's API docs page or docs/isaacsim/physics/newton_physics.rst):

Both Newton and PhysX consume the standard UsdPhysics.Scene + PhysxSchema.PhysxSceneAPI; many PhysxSchema.* attributes are still honored under Newton, plus Newton reads its solver config via omni.usd.schema.newton.

Stack & reading order

1. This skill: scene config, per-prim setup, contact materials, drives, sensors, readback, backend selection.
2. usd-articulation: multi-link articulations + Robot Schema overlay.
3. urdf-mjcf-to-usd-conversion: importer config (RL vs teleop drives).
4. isaac-sim-troubleshooting: when physics misbehaves on Kit 110.

Mechanism recipes (impact, feeders, dominoes, tops, cradles, pendulum waves, escapements) live in Worked Examples.

---

Part 1 — Scene-Level Configuration

PhysicsScene Setup

from pxr import Usd, UsdGeom, UsdPhysics, PhysxSchema, Gf

ps = UsdPhysics.Scene.Define(stage, "/World/PhysicsScene")
ps.CreateGravityDirectionAttr().Set(Gf.Vec3f(0, 0, -1))
ps.CreateGravityMagnitudeAttr().Set(9.81)

px = PhysxSchema.PhysxSceneAPI.Apply(ps.GetPrim())
px.CreateTimeStepsPerSecondAttr().Set(240)   # see Hz table below
px.CreateEnableCCDAttr().Set(True)
px.CreateEnableStabilizationAttr().Set(True)
px.CreateSolverTypeAttr().Set("TGS")          # TGS or PGS; TGS preferred for articulations

Physics Hz Selection

Rule of thumb: physics timestep must be > 4× the highest frequency in the system (vibration, spin, contact-stiffness mode).

Solver Iteration Counts (per-body)

Set on PhysxRigidBodyAPI per body that needs it. Higher = more accurate, slower.

pxrb = PhysxSchema.PhysxRigidBodyAPI.Apply(prim)
pxrb.CreateSolverPositionIterationCountAttr().Set(32)
pxrb.CreateSolverVelocityIterationCountAttr().Set(8)
pxrb.CreateEnableCCDAttr().Set(True)

When to Disable Stabilization

EnableStabilizationAttr is on by default and helps stacks settle. It destroys angular momentum on free-spinning bodies. Disable it for:

• Spinning tops, gyros, flywheels
• Pendulum mechanisms (clock escapements, pendulum waves)
• Anything whose correctness depends on conserved angular velocity

px.CreateEnableStabilizationAttr().Set(False)

---

Part 2 — Per-Prim Physics Setup

RigidBody / Collision / Static / Kinematic

from pxr import UsdPhysics, Gf

def setup_dynamic_body(stage, prim_path, mass_kg=1.0, com_offset=None):
    """Movable, simulated body. RigidBodyAPI + CollisionAPI on the same prim."""
    prim = stage.GetPrimAtPath(prim_path)
    UsdPhysics.RigidBodyAPI.Apply(prim)
    mass_api = UsdPhysics.MassAPI.Apply(prim)
    mass_api.CreateMassAttr().Set(mass_kg)
    if com_offset:
        mass_api.CreateCenterOfMassAttr().Set(Gf.Vec3f(*com_offset))
    UsdPhysics.CollisionAPI.Apply(prim)
    return prim

def setup_static_collider(stage, prim_path):
    """Immovable terrain, walls, fixed obstacles."""
    prim = stage.GetPrimAtPath(prim_path)
    UsdPhysics.CollisionAPI.Apply(prim)
    return prim

def setup_kinematic_body(stage, prim_path):
    """Scripted motion: conveyors, elevators, vibrating bowls, escape wheels."""
    prim = stage.GetPrimAtPath(prim_path)
    UsdPhysics.RigidBodyAPI.Apply(prim)
    UsdPhysics.RigidBodyAPI(prim).CreateKinematicEnabledAttr().Set(True)
    UsdPhysics.CollisionAPI.Apply(prim)
    return prim

Rule: RigidBodyAPI + CollisionAPI on the same prim. Splitting them across parent/child causes intermittent collision failures.

Static Colliders with Scale — Translate-First Pattern

Scaling a Cube prim with CollisionAPI applied directly causes PhysX to use the wrong collision bounds (objects fall through ground). Use a parent xform for position, a child mesh for scale:

# CORRECT
xf = UsdGeom.Xform.Define(stage, "/World/Ground")
UsdGeom.Xformable(xf.GetPrim()).AddTranslateOp().Set(Gf.Vec3d(0, 0, -0.05))
mesh = UsdGeom.Cube.Define(stage, "/World/Ground/Mesh")
mesh.CreateSizeAttr().Set(1.0)
UsdGeom.Xformable(mesh.GetPrim()).AddScaleOp().Set(Gf.Vec3f(50.0, 50.0, 0.1))
UsdPhysics.CollisionAPI.Apply(mesh.GetPrim())

Cube.size=1.0 means the cube has half-extents of 0.5, not 1.0. Use size=2.0 when you want "the scale op equals the half-extent."

Mass & Inertia

mass_api = UsdPhysics.MassAPI.Apply(prim)
mass_api.CreateMassAttr().Set(0.25)                                    # kg
mass_api.CreateCenterOfMassAttr().Set(Gf.Vec3f(0, 0, 0.05))            # local
mass_api.CreateDiagonalInertiaAttr().Set(Gf.Vec3f(1e-4, 1e-4, 2e-4))   # kg·m²

For URDF-imported robots, prefer import_inertia_tensor: true in config.yaml over auto-computed geometric inertia (see urdf-mjcf-to-usd-conversion).

---

Part 3 — Contact Materials

def create_contact_material(stage, mat_path, static_friction=0.5,
                            dynamic_friction=0.4, restitution=0.1):
    prim = stage.DefinePrim(mat_path)
    mat = UsdPhysics.MaterialAPI.Apply(prim)
    mat.CreateStaticFrictionAttr().Set(static_friction)
    mat.CreateDynamicFrictionAttr().Set(dynamic_friction)
    mat.CreateRestitutionAttr().Set(restitution)
    return mat

Reference Values

For chains of stiff contacts (Newton's cradle, escapements), set restitutionCombineMode=max on PhysxMaterialAPI so the highest restitution wins at each contact.

---

Part 4 — Joint Drives

joint = stage.GetPrimAtPath("/World/Robot/joint_arm")
drive = UsdPhysics.DriveAPI.Apply(joint, "angular")    # "angular" | "linear"
drive.CreateTypeAttr().Set("force")                    # "force" | "acceleration"
drive.CreateStiffnessAttr().Set(1000.0)                # Kp (Nm/rad for angular)
drive.CreateDampingAttr().Set(100.0)                   # Kd (Nm·s/rad)
drive.CreateMaxForceAttr().Set(500.0)                  # torque/force limit
drive.CreateTargetPositionAttr().Set(0.0)              # target (deg or m)

For RL training, the agent commands torques directly. Set drive_type to none and stiffness/damping to 0 in config.yaml (see urdf-mjcf-to-usd-conversion). Active PD drives fight the RL agent.

For revolute pendulum joints (clock escapements, pendulum waves), set joint friction to 0:

joint_api = PhysxSchema.PhysxJointAPI.Apply(joint)
joint_api.CreateJointFrictionAttr().Set(0.0)

---

Part 5 — Backend Selection (Newton vs PhysX)

Quick Choice

Newton Solvers

Newton vs PhysX Differences

Newton + Torch — Critical Init Order

Never import torch before Newton physics settles — CUDA context conflict hangs Kit. Defer torch imports until after timeline.play() + settle loop. Use map_location="cpu" for policy inference if VRAM is tight.

Newton-Specific Configuration (Isaac Lab)

# config.yaml for URDF→USD conversion (Isaac Lab)
make_instanceable: true    # CRITICAL for RL parallel envs
fix_base: false            # true for fixed-base arm; false for mobile/legged

See urdf-mjcf-to-usd-conversion for the full schema.

---

Part 6 — Physics Sensors

The current namespace is isaacsim.sensors.experimental.physics (authoring + runtime classes paired). The legacy isaacsim.sensors.physics import path still works but is deprecated for new code.

Migration: see Migrating from isaacsim.sensors.physics to isaacsim.sensors.experimental.physics for the concept mapping and updated scripts.

Contact

from isaacsim.sensors.experimental.physics import Contact, ContactSensor
import isaacsim.core.experimental.utils.app as app_utils

contact = Contact.create(
    path="/World/Robot/foot/contact",
    min_threshold=0.0, max_threshold=1e6, radius=-1,   # -1 = use collision shape
)
sensor = ContactSensor(contact)
app_utils.play(commit=True)            # required before get_data()
reading = sensor.get_data()            # ContactSensorReading

IMU

from isaacsim.sensors.experimental.physics import IMU, IMUSensor

imu = IMU.create(path="/World/Robot/imu", tick_rate=200.0)
sensor = IMUSensor(imu, annotators=["linear_acceleration", "angular_velocity", "orientation"])
app_utils.play(commit=True)
frame = sensor.get_data()                # returns IMUSensorReading

Effort / joint state

Runtime-only classes from the same module; no separate authoring type — they wrap an existing joint by path.

from isaacsim.sensors.experimental.physics import EffortSensor, JointStateSensor

effort = EffortSensor("/World/Robot/joint_arm_1")
joint  = JointStateSensor("/World/Robot/joint_arm_1")
app_utils.play(commit=True)
reading = effort.get_data()              # EffortSensorReading
state   = joint.get_data()               # JointStateSensorReading

Raycast (scene query)

import omni.physics.tensors as physics_tensors

sim_view = physics_tensors.create_simulation_view("cuda:0")
hit = sim_view.raycast_closest(origin, direction, max_dist)

For higher-fidelity sensor simulation (LiDAR scan patterns, multi-ray, vendor sensor models, depth/radar/acoustic), see isaac-sim-sensor and isaac-camera. They use the modern isaacsim.sensors.experimental.rtx and .physics namespaces.

---

Part 7 — Physics-to-USD Readback (CRITICAL)

The most common silent bug: reading authored USD transforms instead of simulated state.

Why XformCache Is Wrong During Sim

updateToUsd=True writes physics state to Fabric, not the USD stage layer. XformCache reads the USD layer. Result: it always returns initial poses.

RigidPrim / GeomPrim pattern (Kit 110)

from isaacsim.core.experimental.prims import RigidPrim
import isaacsim.core.experimental.utils.app as app_utils

rp = RigidPrim(paths="/World/Dice/Die_*")
app_utils.play(commit=True)
pos_wp, quat_wp = rp.get_world_poses()   # warp arrays
positions  = pos_wp.numpy()              # (N, 3)
quaternions = quat_wp.numpy()            # (N, 4) [w, x, y, z]

Articulation pattern (Kit 110)

from isaacsim.core.experimental.prims import Articulation
import omni.timeline

robot = Articulation("/World/Robot")
omni.timeline.get_timeline_interface().play()        # required for tensor data
pos_wp, quat_wp = robot.get_world_poses()
dof_positions  = robot.get_dof_positions().numpy()    # (N, num_dofs)
dof_velocities = robot.get_dof_velocities().numpy()
J = robot.get_jacobian_matrices().numpy()             # for IK

Legacy isaacsim.core.api.articulations.Articulation / isaacsim.core.api.prims.RigidPrim still load but are superseded by the isaacsim.core.experimental.* stack.

Migration: for the broader omni.isaac.* → isaacsim.* renaming map, see Renaming Extensions. For the experimental Articulation / RigidPrim APIs see the Python scripting index.

Quaternion Convention

USD/Isaac uses [w, x, y, z]; scipy uses [x, y, z, w]. Convert:

from scipy.spatial.transform import Rotation
r = Rotation.from_quat([quat[1], quat[2], quat[3], quat[0]])
euler = r.as_euler('xyz', degrees=True)

---

Part 8 — Common Gotchas

1. CollisionAPI alone = static collider; RigidBodyAPI + CollisionAPI = dynamic.
2. Same-prim requirement: both APIs must be on the same prim.
3. Kinematic bodies: use CreateKinematicEnabledAttr().Set(True), not enable/disable on RigidBodyAPI.
4. Cube.size=1.0 = half-extent 0.5. Use size=2.0 if you want scale ops to equal half-extents.
5. physics:velocity USD attributes are ignored by PhysX at runtime. Use RigidPrim.set_linear_velocities() after timeline.play().
6. physics:angularVelocity is in DEGREES/second, not rad/s. Convert with math.degrees().
7. SimulationContext.step(render=True) is the only reliable physics-with-render advance. app.update() does not sync physics.
8. Experimental sensors need app_utils.play(commit=True) (or timeline.play()) before get_data(); do not call initialize() from the legacy World flow.
9. get_rigid_body_state() does not exist in Isaac Sim 5.1+; use RigidPrim.get_world_poses() from isaacsim.core.experimental.prims.
10. PhysX cannot resolve sequential momentum transfer in same-island contact chains (see Worked Example 4: Newton's Cradle).
11. Tunneling at high spin rates: compound colliders fail above ~50 rad/s after 5–6s. Use simpler convex hulls or higher physics Hz.

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Worked Examples (impact, vibratory feeder, gyro, cradle, escapement)

See examples.md for details.