Does Isaac Sim provide realistic physics simulation?

Omniverse Isaac Sim
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1

I write a simple example, but a force that cannot satisfy the force balance was read.

Describe the bug

I set a ground plane and two cube. The first cube has size as (1.0, 1.0, 1.0) and init position as (0,0,1.5). The second cube has size as (0.8, 0.8, 0.8) and init position as (0,0,3.0). Then I let the simulation run.
Finally, when they all landed, the force to first cube is wrong.

Code

# %%
from omni.isaac.lab.app import AppLauncher

app_launcher = AppLauncher()
simulation = app_launcher.app

# %%
import omni.isaac.lab.sim as sim_utils

from omni.isaac.lab.assets import AssetBaseCfg, RigidObjectCfg
from omni.isaac.lab.scene import InteractiveScene, InteractiveSceneCfg
from omni.isaac.lab.utils import configclass
from omni.isaac.lab.sensors import ContactSensorCfg

# %%
@configclass
class SceneConfig(InteractiveSceneCfg):
    light = AssetBaseCfg(prim_path="/World/Light", 
                        spawn=sim_utils.DomeLightCfg(intensity=3000.0, color=(0.75, 0.75, 0.75)))
    
    ground = AssetBaseCfg(prim_path="/World/defaultGroundPlane", 
                          spawn=sim_utils.GroundPlaneCfg())
    
    cube1 = AssetBaseCfg(prim_path="/World/cube1",
                        init_state=AssetBaseCfg.InitialStateCfg(pos=(0.0, 0.0, 1.5)),
                        # init_state=AssetBaseCfg.InitialStateCfg(pos=(0.0, 0.0, 0.5)),
                        spawn=sim_utils.MeshCuboidCfg(size=(1.0, 1.0, 1.0),
                                                      physics_material=sim_utils.materials.RigidBodyMaterialCfg(),
                                                      activate_contact_sensors=True,
                                                      rigid_props=sim_utils.RigidBodyPropertiesCfg(disable_gravity=False,
                                                                                                   max_depenetration_velocity=5.0),
                                                      collision_props=sim_utils.CollisionPropertiesCfg(),
                                                      visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 0.0, 0.0))))

    cube2 = AssetBaseCfg(prim_path="/World/cube2",
                        init_state=AssetBaseCfg.InitialStateCfg(pos=(0.0, 0.0, 3.0)),
                        # init_state=AssetBaseCfg.InitialStateCfg(pos=(0.0, 0.0, 1.25)),
                        spawn=sim_utils.MeshCuboidCfg(size=(0.8, 0.8, 0.8),
                                                    physics_material=sim_utils.materials.RigidBodyMaterialCfg(),
                                                    activate_contact_sensors=True,
                                                    rigid_props=sim_utils.RigidBodyPropertiesCfg(disable_gravity=False,
                                                                                                 max_depenetration_velocity=5.0),
                                                    collision_props=sim_utils.CollisionPropertiesCfg(),
                                                    visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.75, 0.75, 0.75))))
    
    sensor1 = ContactSensorCfg(prim_path="/World/cube1",
                              update_period=0.0,
                              history_length=6,
                              track_pose=True,
                              debug_vis=True, 
                              filter_prim_paths_expr=["/World/cube2"])
    
    sensor11 = ContactSensorCfg(prim_path="/World/cube1",
                              update_period=0.0,
                              history_length=6,
                              track_pose=True,
                              debug_vis=True, 
                              filter_prim_paths_expr=["/World/defaultGroundPlane/GroundPlane/CollisionPlane"])
    
    sensor2 = ContactSensorCfg(prim_path="/World/cube2",
                              update_period=0.0,
                              history_length=6,
                              track_pose=True,
                              debug_vis=True, 
                              filter_prim_paths_expr=["/World/cube1"])
    

# %%
sim_cfg = sim_utils.SimulationCfg()
sim = sim_utils.SimulationContext(sim_cfg)
sim.set_simulation_dt(1.0/120.0)

scene_cfg = SceneConfig(num_envs=1, env_spacing=2.0)
scene = InteractiveScene(scene_cfg)

sim.reset()

# %%
def output_data(data, name):
    data = [round(f, 3) for f in data.tolist()]
    print(f'{name}: {data}')


# %%
sim_dt = sim.get_physics_dt()

sensor1 = scene["sensor1"]
sensor11 = scene["sensor11"]
sensor2 = scene["sensor2"]

mass1 = sensor1.body_physx_view.get_masses()
mass2 = sensor2.body_physx_view.get_masses()

print(f'mass1: {mass1}')
print(f'mass2: {mass2}')

for i in range(100):
    sim.step()
    scene.update(sim_dt)
    force1 = sensor1.data.net_forces_w[0][0]
    force2 = sensor2.data.net_forces_w[0][0]
    
    filted_force1 = sensor1.data.force_matrix_w[0][0][0]
    filted_force11 = sensor11.data.force_matrix_w[0][0][0]
    
    pos1 = sensor1.data.pos_w[0][0]
    pos2 = sensor2.data.pos_w[0][0]
    quat1 = sensor1.data.quat_w[0][0]
    quat2 = sensor2.data.quat_w[0][0]
    print('-'*64)
    print(f'step {i+1}')
    print(' -- '*16)
    output_data(force1, 'force1')
    output_data(force2, 'force2')
    output_data(pos1, 'pos1')
    output_data(pos2, 'pos2')
    output_data(quat1, 'quat1')
    output_data(quat2, 'quat2')
    output_data(filted_force1, 'cube2->cube1')
    output_data(filted_force11, 'ground->cube1')
    print('-'*64)

Output

When the first cube landed, the force is corrct as:

force1: [0.001, 0.0, 9804.056]
force2: [0.0, 0.0, 0.0]
pos1: [-0.0, -0.0, 0.5]
pos2: [0.0, 0.0, 1.794]
quat1: [1.0, 0.0, 0.0, 0.0]
quat2: [1.0, 0.0, 0.0, 0.0]
cube2->cube1: [0.0, 0.0, 0.0]
ground->cube1: [0.001, 0.0, 9804.056]

When the first cube landed, the force is wrong as:

force1: [-0.004, -0.002, 1227.905]
force2: [0.005, 0.002, 5027.711]
pos1: [-0.0, -0.0, 0.5]
pos2: [-0.0, -0.0, 1.4]
quat1: [1.0, -0.0, 0.0, 0.0]
quat2: [1.0, -0.0, 0.0, -0.0]
cube2->cube1: [-0.005, -0.002, -5027.711]
ground->cube1: [0.001, 0.0, 6255.616]

To cube1, the gravity is 10000*9.8=-9800, force from cube2 is -5027.711, force from ground is 6255.616, the force of cube1 is serious imbalance, right?

System Info

  • Isaac Sim Version: 4.2.9
  • OS: Ubuntu 22.04
  • GPU: RTX 2080Ti
  • CUDA: 12.1
  • GPU Driver: 535.183.01
2

Hello and thanks for your question. Since this is an Isaac Sim related question I’m going to move you over to the Isaac page. However, could you provide the USD file so we can review it?

3

All USD files used are the default assert provided by Isaac Sim, I only used cubes and planes.

4

Well if you want to set up the physics scene and save it as a usd file, we can take a look.

5

I am going to move this to the Isaac Sim forum for better help. Please post all Isaac Sim related questions there instead.