Decouples rigid object and articulation asset classes (#644)

# Description

Since we override a lot of the functions from RigidObject inside the
Articulation class, we don't need to rely on inheritance anymore.
Duplicacy in the code makes it easier to understand the two classes'
functionalities without severely added overhead from the maintenance
side. Moreover, conceptually, it can be motivated that the two are
independent concepts.

This MR decouples the rigid object and articulation concepts in the
framework.

## Type of change

- Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------
Co-authored-by: David Hoeller <dhoeller@nvidia.com>

source/extensions/omni.isaac.lab/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.20.5"
+version = "0.20.6"
 
 # Description
 title = "Isaac Lab framework for Robot Learning"

source/extensions/omni.isaac.lab/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.20.6 (2024-08-02)
+~~~~~~~~~~~~~~~~~~~
+
+Changed
+^^^^^^^
+
+* Removed the hierarchy from :class:`~omni.isaac.lab.assets.RigidObject` class to
+  :class:`~omni.isaac.lab.assets.Articulation` class. Previously, the articulation class overrode  almost
+  all the functions of the rigid object class making the hierarchy redundant. Now, the articulation class
+  is a standalone class that does not inherit from the rigid object class. This does add some code
+  duplication but the simplicity and clarity of the code is improved.
+
+
 0.20.5 (2024-08-02)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/articulation/articulation_cfg.py

@@ -8,21 +8,25 @@ from dataclasses import MISSING
 from omni.isaac.lab.actuators import ActuatorBaseCfg
 from omni.isaac.lab.utils import configclass
 
-from ..rigid_object import RigidObjectCfg
+from ..asset_base_cfg import AssetBaseCfg
 from .articulation import Articulation
 
 
 @configclass
-class ArticulationCfg(RigidObjectCfg):
+class ArticulationCfg(AssetBaseCfg):
     """Configuration parameters for an articulation."""
 
-    class_type: type = Articulation
-
     @configclass
-    class InitialStateCfg(RigidObjectCfg.InitialStateCfg):
+    class InitialStateCfg(AssetBaseCfg.InitialStateCfg):
         """Initial state of the articulation."""
 
-        # root position
+        # root velocity
+        lin_vel: tuple[float, float, float] = (0.0, 0.0, 0.0)
+        """Linear velocity of the root in simulation world frame. Defaults to (0.0, 0.0, 0.0)."""
+        ang_vel: tuple[float, float, float] = (0.0, 0.0, 0.0)
+        """Angular velocity of the root in simulation world frame. Defaults to (0.0, 0.0, 0.0)."""
+
+        # joint state
         joint_pos: dict[str, float] = {".*": 0.0}
         """Joint positions of the joints. Defaults to 0.0 for all joints."""
         joint_vel: dict[str, float] = {".*": 0.0}
@@ -32,10 +36,17 @@ class ArticulationCfg(RigidObjectCfg):
     # Initialize configurations.
     ##
 
+    class_type: type = Articulation
+
     init_state: InitialStateCfg = InitialStateCfg()
     """Initial state of the articulated object. Defaults to identity pose with zero velocity and zero joint state."""
+
     soft_joint_pos_limit_factor: float = 1.0
-    """Fraction specifying the range of DOF position limits (parsed from the asset) to use.
-    Defaults to 1.0."""
+    """Fraction specifying the range of DOF position limits (parsed from the asset) to use. Defaults to 1.0.
+
+    The joint position limits are scaled by this factor to allow for a limited range of motion.
+    This is accessible in the articulation data through :attr:`ArticulationData.soft_joint_pos_limits` attribute.
+    """
+
     actuators: dict[str, ActuatorBaseCfg] = MISSING
     """Actuators for the robot with corresponding joint names."""

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/rigid_object/rigid_object.py

@@ -120,6 +120,10 @@ class RigidObject(AssetBase):
     def update(self, dt: float):
         self._data.update(dt)
 
+    """
+    Operations - Finders.
+    """
+
     def find_bodies(self, name_keys: str | Sequence[str], preserve_order: bool = False) -> tuple[list[int], list[str]]:
         """Find bodies in the articulation based on the name keys.
 

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/rigid_object/rigid_object_data.py

@@ -33,24 +33,20 @@ class RigidObjectData:
     is older than the current simulation timestamp. The timestamp is updated whenever the data is updated.
     """
 
-    _root_physx_view: physx.RigidBodyView
-    """The root rigid body view of the object.
-
-    Note:
-        Internally, this is stored as a weak reference to avoid circular references between the asset class
-        and the data container. This is important to avoid memory leaks.
-    """
-
     def __init__(self, root_physx_view: physx.RigidBodyView, device: str):
         """Initializes the rigid object data.
 
         Args:
-            root_physx_view: The root rigid body view of the object.
+            root_physx_view: The root rigid body view.
             device: The device used for processing.
         """
         # Set the parameters
         self.device = device
-        self._root_physx_view = weakref.proxy(root_physx_view)  # weak reference to avoid circular references
+        # Set the root rigid body view
+        # note: this is stored as a weak reference to avoid circular references between the asset class
+        #  and the data container. This is important to avoid memory leaks.
+        self._root_physx_view: physx.RigidBodyView = weakref.proxy(root_physx_view)
+
         # Set initial time stamp
         self._sim_timestamp = 0.0
 
@@ -76,6 +72,7 @@ class RigidObjectData:
         Args:
             dt: The time step for the update. This must be a positive value.
         """
+        # update the simulation timestamp
         self._sim_timestamp += dt
 
     ##
@@ -97,7 +94,7 @@ class RigidObjectData:
     """
 
     default_mass: torch.Tensor = None
-    """ Default mass provided by simulation. Shape is (num_instances, num_bodies)."""
+    """Default mass read from the simulation. Shape is (num_instances, num_bodies)."""
 
     ##
     # Properties.
@@ -133,8 +130,7 @@ class RigidObjectData:
     def body_acc_w(self):
         """Acceleration of all bodies. Shape is (num_instances, 1, 6).
 
-        This quantity is the acceleration of the rigid bodies' center of mass frame. The acceleration
-        is computed using finite differencing of the linear and angular velocities of the bodies.
+        This quantity is the acceleration of the rigid bodies' center of mass frame.
         """
         if self._body_acc_w.timestamp < self._sim_timestamp:
             # note: we use finite differencing to compute acceleration
@@ -158,6 +154,10 @@ class RigidObjectData:
         forward_w = math_utils.quat_apply(self.root_quat_w, self.FORWARD_VEC_B)
         return torch.atan2(forward_w[:, 1], forward_w[:, 0])
 
+    ##
+    # Derived properties.
+    ##
+
     @property
     def root_pos_w(self) -> torch.Tensor:
         """Root position in simulation world frame. Shape is (num_instances, 3).
@@ -255,3 +255,19 @@ class RigidObjectData:
         This quantity is the angular velocity of the rigid bodies' center of mass frame.
         """
         return self.body_state_w[..., 10:13]
+
+    @property
+    def body_lin_acc_w(self) -> torch.Tensor:
+        """Linear acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the linear acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 0:3]
+
+    @property
+    def body_ang_acc_w(self) -> torch.Tensor:
+        """Angular acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the angular acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 3:6]