Adds IMU projected_gravity_b and IMU computation speed optimizations (#2512)

# Description

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This PR adds projected_gravity_b data stream as well as speeding up
computation by during batch rotating of all data streams to base frame
in one computation rather than each data stream separately.

This PR is dependant on [PR
2129](https://github.com/isaac-sim/IsaacLab/pull/2129/) and should not
be merged until that PR is merged.

Fixes # (issue)

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## Type of change

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- New feature (non-breaking change which adds functionality)

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## Checklist

- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
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---------
Co-authored-by: Kelly Guo <kellyg@nvidia.com>

source/isaaclab/isaaclab/envs/mdp/observations.py

@@ -270,6 +270,21 @@ def imu_orientation(env: ManagerBasedEnv, asset_cfg: SceneEntityCfg = SceneEntit
     return asset.data.quat_w
 
 
+def imu_projected_gravity(env: ManagerBasedEnv, asset_cfg: SceneEntityCfg = SceneEntityCfg("imu")) -> torch.Tensor:
+    """Imu sensor orientation w.r.t the env.scene.origin.
+
+    Args:
+        env: The environment.
+        asset_cfg: The SceneEntity associated with an Imu sensor.
+
+    Returns:
+        Gravity projected on imu_frame, shape of torch.tensor is (num_env,3).
+    """
+
+    asset: Imu = env.scene[asset_cfg.name]
+    return asset.data.projected_gravity_b
+
+
 def imu_ang_vel(env: ManagerBasedEnv, asset_cfg: SceneEntityCfg = SceneEntityCfg("imu")) -> torch.Tensor:
     """Imu sensor angular velocity w.r.t. environment origin expressed in the sensor frame.
 

source/isaaclab/isaaclab/sensors/imu/imu.py

@@ -96,7 +96,11 @@ class Imu(SensorBase):
         if env_ids is None:
             env_ids = slice(None)
         # reset accumulative data buffers
+        self._data.pos_w[env_ids] = 0.0
         self._data.quat_w[env_ids] = 0.0
+        self._data.quat_w[env_ids, 0] = 1.0
+        self._data.projected_gravity_b[env_ids] = 0.0
+        self._data.projected_gravity_b[env_ids, 2] = -1.0
         self._data.lin_vel_b[env_ids] = 0.0
         self._data.ang_vel_b[env_ids] = 0.0
         self._data.lin_acc_b[env_ids] = 0.0
@@ -135,22 +139,24 @@ class Imu(SensorBase):
         else:
             raise RuntimeError(f"Failed to find a RigidBodyAPI for the prim paths: {self.cfg.prim_path}")
 
+        # Get world gravity
+        gravity = self._physics_sim_view.get_gravity()
+        gravity_dir = torch.tensor((gravity[0], gravity[1], gravity[2]), device=self.device)
+        gravity_dir = math_utils.normalize(gravity_dir.unsqueeze(0)).squeeze(0)
+        self.GRAVITY_VEC_W = gravity_dir.repeat(self.num_instances, 1)
+
         # Create internal buffers
         self._initialize_buffers_impl()
 
     def _update_buffers_impl(self, env_ids: Sequence[int]):
         """Fills the buffers of the sensor data."""
-        # check if self._dt is set (this is set in the update function)
-        if not hasattr(self, "_dt"):
-            raise RuntimeError(
-                "The update function must be called before the data buffers are accessed the first time."
-            )
+
         # default to all sensors
         if len(env_ids) == self._num_envs:
             env_ids = slice(None)
         # obtain the poses of the sensors
         pos_w, quat_w = self._view.get_transforms()[env_ids].split([3, 4], dim=-1)
-        quat_w = math_utils.convert_quat(quat_w, to="wxyz")
+        quat_w = quat_w.roll(1, dims=-1)
 
         # store the poses
         self._data.pos_w[env_ids] = pos_w + math_utils.quat_apply(quat_w, self._offset_pos_b[env_ids])
@@ -170,12 +176,19 @@ class Imu(SensorBase):
         # numerical derivative
         lin_acc_w = (lin_vel_w - self._prev_lin_vel_w[env_ids]) / self._dt + self._gravity_bias_w[env_ids]
         ang_acc_w = (ang_vel_w - self._prev_ang_vel_w[env_ids]) / self._dt
-        # store the velocities
-        self._data.lin_vel_b[env_ids] = math_utils.quat_apply_inverse(self._data.quat_w[env_ids], lin_vel_w)
-        self._data.ang_vel_b[env_ids] = math_utils.quat_apply_inverse(self._data.quat_w[env_ids], ang_vel_w)
+        # stack data in world frame and batch rotate
+        dynamics_data = torch.stack((lin_vel_w, ang_vel_w, lin_acc_w, ang_acc_w, self.GRAVITY_VEC_W[env_ids]), dim=0)
+        dynamics_data_rot = math_utils.quat_apply_inverse(self._data.quat_w[env_ids].repeat(5, 1), dynamics_data).chunk(
+            5, dim=0
+        )
+        # store the velocities.
+        self._data.lin_vel_b[env_ids] = dynamics_data_rot[0]
+        self._data.ang_vel_b[env_ids] = dynamics_data_rot[1]
         # store the accelerations
-        self._data.lin_acc_b[env_ids] = math_utils.quat_apply_inverse(self._data.quat_w[env_ids], lin_acc_w)
-        self._data.ang_acc_b[env_ids] = math_utils.quat_apply_inverse(self._data.quat_w[env_ids], ang_acc_w)
+        self._data.lin_acc_b[env_ids] = dynamics_data_rot[2]
+        self._data.ang_acc_b[env_ids] = dynamics_data_rot[3]
+        # store projected gravity
+        self._data.projected_gravity_b[env_ids] = dynamics_data_rot[4]
 
         self._prev_lin_vel_w[env_ids] = lin_vel_w
         self._prev_ang_vel_w[env_ids] = ang_vel_w
@@ -186,6 +199,7 @@ class Imu(SensorBase):
         self._data.pos_w = torch.zeros(self._view.count, 3, device=self._device)
         self._data.quat_w = torch.zeros(self._view.count, 4, device=self._device)
         self._data.quat_w[:, 0] = 1.0
+        self._data.projected_gravity_b = torch.zeros(self._view.count, 3, device=self._device)
         self._data.lin_vel_b = torch.zeros_like(self._data.pos_w)
         self._data.ang_vel_b = torch.zeros_like(self._data.pos_w)
         self._data.lin_acc_b = torch.zeros_like(self._data.pos_w)

source/isaaclab/isaaclab/sensors/imu/imu_data.py

@@ -25,6 +25,12 @@ class ImuData:
     Shape is (N, 4), where ``N`` is the number of environments.
     """
 
+    projected_gravity_b: torch.Tensor = None
+    """Gravity direction unit vector projected on the imu frame.
+
+    Shape is (N,3), where ``N`` is the number of environments.
+    """
+
     lin_vel_b: torch.Tensor = None
     """IMU frame angular velocity relative to the world expressed in IMU frame.
 

source/isaaclab/test/sensors/test_imu.py

@@ -353,6 +353,14 @@ def test_single_dof_pendulum(setup_sim):
         if idx < 2:
             continue
 
+        # compare imu projected gravity
+        gravity_dir_w = torch.tensor((0.0, 0.0, -1.0), device=scene.device).repeat(2, 1)
+        gravity_dir_b = math_utils.quat_apply_inverse(imu_data.quat_w, gravity_dir_w)
+        torch.testing.assert_close(
+            imu_data.projected_gravity_b,
+            gravity_dir_b,
+        )
+
         # compare imu angular velocity with joint velocity
         torch.testing.assert_close(
             joint_vel,
@@ -495,6 +503,13 @@ def test_offset_calculation(setup_sim):
             rtol=1e-4,
             atol=1e-4,
         )
+        # check the projected gravity
+        torch.testing.assert_close(
+            scene.sensors["imu_robot_base"].data.projected_gravity_b,
+            scene.sensors["imu_robot_imu_link"].data.projected_gravity_b,
+            rtol=1e-4,
+            atol=1e-4,
+        )
 
 
 def test_env_ids_propagation(setup_sim):