Cannot spawn particles usd at right position and the particles are not falling down.

[gyeongjun9]

I made my water particles usd by USDComposer tool with particle samplers and physics material api. And here's the result simulation of the water particles usd I made.

Screencast.from.04-01-2025.11.16.21.PM.webm

And after that, I spawned this water usd file using AssetBaseCfg. Here's the code that I wrote

class SceneCfg(InteractiveSceneCfg):

# ground plane
ground = AssetBaseCfg(prim_path="/World/GroundPlane", spawn=sim_utils.GroundPlaneCfg())

# lights
dome_light = AssetBaseCfg(prim_path="/World/Light", spawn=sim_utils.DomeLightCfg(intensity=3000.0, color=(0.75, 0.75, 0.75)))

# articulation
robot: ArticulationCfg = FRANKA_PANDA_CFG.replace(prim_path="{ENV_REGEX_NS}/Robot")
""" What is the usage of 'robot : ' ? """

# water usd
water_cup = AssetBaseCfg(prim_path="{ENV_REGEX_NS}/Water",
                         spawn=sim_utils.UsdFileCfg(usd_path="omniverse://localhost/Projects/Water_Env/water.usd",
                                                    rigid_props=sim_utils.RigidBodyPropertiesCfg(),
                                                    mass_props=sim_utils.MassPropertiesCfg(mass=100.0),
                                                    collision_props=sim_utils.CollisionPropertiesCfg(),
                                                    ),
                         init_state=AssetBaseCfg.InitialStateCfg(pos=(0, 0, 0))
                         )

# cube & contact sensor
cube_1 = RigidObjectCfg(
    prim_path="{ENV_REGEX_NS}/Cube_1",
    spawn=sim_utils.CuboidCfg(
        size=(0.5, 0.5, 0.1),
        rigid_props=sim_utils.RigidBodyPropertiesCfg(),
        mass_props=sim_utils.MassPropertiesCfg(mass=100.0),
        collision_props=sim_utils.CollisionPropertiesCfg(),
        physics_material=sim_utils.RigidBodyMaterialCfg(static_friction=1.0),
        visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 1.0, 0.0), metallic=0.2),
        activate_contact_sensors=True
    ),
    init_state=RigidObjectCfg.InitialStateCfg(pos=(0.5, 0.5, 0.05)),
)

cube_2 = RigidObjectCfg(
    prim_path="{ENV_REGEX_NS}/Cube_2",
    spawn=sim_utils.CuboidCfg(
        size=(0.2, 0.2, 0.1),
        rigid_props=sim_utils.RigidBodyPropertiesCfg(),
        mass_props=sim_utils.MassPropertiesCfg(mass=50.0),
        collision_props=sim_utils.CollisionPropertiesCfg(),
        physics_material=sim_utils.RigidBodyMaterialCfg(static_friction=1.0),
        visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 1.0, 0.0), metallic=0.2),
        activate_contact_sensors=True
    ),
    init_state=RigidObjectCfg.InitialStateCfg(pos=(0.5, 0.5, 0.5)),
)

contact_forces_1 = ContactSensorCfg(
    prim_path="{ENV_REGEX_NS}/Cube_1",
    update_period=0.0,
    history_length=6,
    debug_vis=True,
    filter_prim_paths_expr=["{ENV_REGEX_NS}/Water_Cup"],
)

def run_simulator(sim: sim_utils.SimulationContext, scene: InteractiveScene):
"""Run the simulator"""
# Define simulation stepping (not understood it yet..)
sim_dt = sim.get_physics_dt()
sim_time = 0.0
count = 0

# Simulate physics
while simulation_app.is_running():
    # Reset
    if count % 500 == 0:
        # reset counter
        count = 0
        # reset the scene entities
        # root state
        # we offset the root state by the origin since the states are wrriten in simulation world frame
        # if this is not done, then the robots will be spawned at the (0,0,0) of the simulation world
        root_state = scene["robot"].data.default_root_state.clone()
        root_state[:, :3] += scene.env_origins
        scene["robot"].write_root_pose_to_sim(root_state[:, :7])
        scene["robot"].write_root_velocity_to_sim(root_state[:, 7:])
        # set joint positions with some noise
        joint_pos, joint_vel = (
            scene["robot"].data.default_joint_pos.clone(),
            scene["robot"].data.default_joint_vel.clone(),
        )
        joint_pos += torch.rand_like(joint_pos) * 0.1
        scene["robot"].write_joint_state_to_sim(joint_pos, joint_vel)
        # clear internal buffers
        scene.reset()
        print("[INFO]: Resetting robot state...")
    # Apply default actions to the robot
    # -- generate actions/commands
    targets = scene["robot"].data.default_joint_pos
    # -- apply action to the robot
    scene["robot"].set_joint_position_target(targets)
    # -- write data to sim
    scene.write_data_to_sim()
    # perform step
    sim.step()
    # update sim-time
    sim_time += sim_dt
    count += 1
    # update buffers
    scene.update(sim_dt)

def main():
"""Main function."""

# Initialize the simulation context
sim_cfg = sim_utils.SimulationCfg(dt=0.005, device=args_cli.device)
sim = sim_utils.SimulationContext(sim_cfg)
# Set main camera
sim.set_camera_view(eye=[3.5, 3.5, 3.5], target=[0.0, 0.0, 0.0])
# design scene. scene instantiation
scene_cfg = SceneCfg(num_envs=args_cli.num_envs, env_spacing=2.0)
scene = InteractiveScene(scene_cfg)
# Play the simulator
sim.reset()
# Now we are ready!
print("[INFO]: Setup complete...")
# Run the simulator
run_simulator(sim, scene)

if name == "main":
# run the main function
main()
# close sim app
simulation_app.close()

I ran this python file using IsaacLab python and the result was like this.

Screencast.from.04-01-2025.11.24.12.PM.webm

Before I ask some questions, let me explain how I made these particles usd file.

1. made a cylinder mesh.
2. add - physics - particle sampler on the cylinder mesh.
3. Create physics material - PBD particle material - and applied as physics material
4. Tuned some particle system properties

The thing that I want to ask is two things.

1. Why are these particles not falling down?
2. Why are these particles spawned at odd position? When I made the water usd file, I definitely made the particles at (0,0,z) position. And spawned the usd file at (0,0,0).

I would be so happy if some of you give me some feedback. Thank you.

[RandomOakForest]

Thank you for posting this. We haven't yet understood why this would be happening. Could you try running only 1 environment? Maybe simplifying the simulation to just the robot and bucket of particles, without the cubes, could help.