Making a physics prim fixed in the simulation

When a USD prim has physics schemas applied on it, it is affected by physics simulation. This means that the prim can move, rotate, and collide with other prims in the simulation world. However, there are cases where it is desirable to make certain prims static in the simulation world, i.e. the prim should still participate in collisions but its position and orientation should not change.

The following sections describe how to spawn a prim with physics schemas and make it static in the simulation world.

Static colliders

Static colliders are prims that are not affected by physics but can collide with other prims in the simulation world. These don’t have any rigid body properties applied on them. However, this also means that they can’t be accessed using the physics tensor API (i.e., through the assets.RigidObject class).

For instance, to spawn a cone static in the simulation world, the following code can be used:

import omni.isaac.orbit.sim as sim_utils

cone_spawn_cfg = sim_utils.ConeCfg(
    radius=0.15,
    height=0.5,
    collision_props=sim_utils.CollisionPropertiesCfg(),
    visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 1.0, 0.0)),
)
cone_spawn_cfg.func(
    "/World/Cone", cone_spawn_cfg, translation=(0.0, 0.0, 2.0), orientation=(0.5, 0.0, 0.5, 0.0)
)

Rigid object

Rigid objects (i.e. object only has a single body) can be made static by setting the parameter sim.schemas.RigidBodyPropertiesCfg.kinematic_enabled as True. This will make the object kinematic and it will not be affected by physics.

For instance, to spawn a cone static in the simulation world but with rigid body schema on it, the following code can be used:

import omni.isaac.orbit.sim as sim_utils

cone_spawn_cfg = sim_utils.ConeCfg(
    radius=0.15,
    height=0.5,
    rigid_props=sim_utils.RigidBodyPropertiesCfg(kinematic_enabled=True),
    mass_props=sim_utils.MassPropertiesCfg(mass=1.0),
    collision_props=sim_utils.CollisionPropertiesCfg(),
    visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 1.0, 0.0)),
)
cone_spawn_cfg.func(
    "/World/Cone", cone_spawn_cfg, translation=(0.0, 0.0, 2.0), orientation=(0.5, 0.0, 0.5, 0.0)
)

Articulation

Fixing the root of an articulation requires having a fixed joint to the root rigid body link of the articulation. This can be achieved by setting the parameter sim.schemas.ArticulationRootPropertiesCfg.fix_root_link as True. Based on the value of this parameter, the following cases are possible:

• If set to None, the root link is not modified.

• If the articulation already has a fixed root link, this flag will enable or disable the fixed joint.

• If the articulation does not have a fixed root link, this flag will create a fixed joint between the world frame and the root link. The joint is created with the name “FixedJoint” under the root link.

For instance, to spawn an ANYmal robot and make it static in the simulation world, the following code can be used:

import omni.isaac.orbit.sim as sim_utils
from omni.isaac.orbit.utils.assets import ISAAC_ORBIT_NUCLEUS_DIR

anymal_spawn_cfg = sim_utils.UsdFileCfg(
    usd_path=f"{ISAAC_ORBIT_NUCLEUS_DIR}/Robots/ANYbotics/ANYmal-C/anymal_c.usd",
    activate_contact_sensors=True,
    rigid_props=sim_utils.RigidBodyPropertiesCfg(
        disable_gravity=False,
        retain_accelerations=False,
        linear_damping=0.0,
        angular_damping=0.0,
        max_linear_velocity=1000.0,
        max_angular_velocity=1000.0,
        max_depenetration_velocity=1.0,
    ),
    articulation_props=sim_utils.ArticulationRootPropertiesCfg(
        enabled_self_collisions=True,
        solver_position_iteration_count=4,
        solver_velocity_iteration_count=0,
        fix_root_link=True,
    ),
)
anymal_spawn_cfg.func(
    "/World/ANYmal", anymal_spawn_cfg, translation=(0.0, 0.0, 0.8), orientation=(1.0, 0.0, 0.0, 0.0)
)

This will create a fixed joint between the world frame and the root link of the ANYmal robot at the prim path "/World/ANYmal/base/FixedJoint" since the root link is at the path "/World/ANYmal/base".

Further notes

Given the flexibility of USD asset designing the following possible scenarios are usually encountered:

1. Articulation root schema on the rigid body prim without a fixed joint:

   This is the most common and recommended scenario for floating-base articulations. The root prim has both the rigid body and the articulation root properties. In this case, the articulation root is parsed as a floating-base with the root prim of the articulation Link0Xform.

   ArticulationXform
       └── Link0Xform  (RigidBody and ArticulationRoot schema)
   

2. Articulation root schema on the parent prim with a fixed joint:

   This is the expected arrangement for fixed-base articulations. The root prim has only the rigid body properties and the articulation root properties are applied to its parent prim. In this case, the articulation root is parsed as a fixed-base with the root prim of the articulation Link0Xform.

   ArticulationXform (ArticulationRoot schema)
       └── Link0Xform  (RigidBody schema)
       └── FixedJoint (connecting the world frame and Link0Xform)
   

3. Articulation root schema on the parent prim without a fixed joint:

   This is a scenario where the root prim has only the rigid body properties and the articulation root properties are applied to its parent prim. However, the fixed joint is not created between the world frame and the root link. In this case, the articulation is parsed as a floating-base system. However, the PhysX parser uses its own heuristic (such as alphabetical order) to determine the root prim of the articulation. It may select the root prim at Link0Xform or choose another prim as the root prim.

   ArticulationXform (ArticulationRoot schema)
       └── Link0Xform  (RigidBody schema)
   

4. Articulation root schema on the rigid body prim with a fixed joint:

   While this is a valid scenario, it is not recommended as it may lead to unexpected behavior. In this case, the articulation is still parsed as a floating-base system. However, the fixed joint, created between the world frame and the root link, is considered as a part of the maximal coordinate tree. This is different from PhysX considering the articulation as a fixed-base system. Hence, the simulation may not behave as expected.

   ArticulationXform
       └── Link0Xform  (RigidBody and ArticulationRoot schema)
       └── FixedJoint (connecting the world frame and Link0Xform)
   

For floating base articulations, the root prim usually has both the rigid body and the articulation root properties. However, directly connecting this prim to the world frame will cause the simulation to consider the fixed joint as a part of the maximal coordinate tree. This is different from PhysX considering the articulation as a fixed-base system.

Internally, when the parameter sim.schemas.ArticulationRootPropertiesCfg.fix_root_link is set to True and the articulation is detected as a floating-base system, the fixed joint is created between the world frame the root rigid body link of the articulation. However, to make the PhysX parser consider the articulation as a fixed-base system, the articulation root properties are removed from the root rigid body prim and applied to its parent prim instead.

Note

In future release of Isaac Sim, an explicit flag will be added to the articulation root schema from PhysX to toggle between fixed-base and floating-base systems. This will resolve the need of the above workaround.