Gives sensor_base tracking of dt (#2504)

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This PR give sensors knowledge of dt changing on
scene.update/sensor.update. This does two things:

- This allows for physics update rates to be changed after
initialization so that the contact_sensor correctly fetches the right
data from physics tensors.
- This also removes any issues with IMU calculating the numerical
derivative at time=0 because now it will have an initialized value.

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## Type of change

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- Bug fix (non-breaking change which fixes an issue)

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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
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Co-authored-by: Kelly Guo <kellyg@nvidia.com>

source/isaaclab/isaaclab/sensors/contact_sensor/contact_sensor.py

@@ -326,7 +326,7 @@ class ContactSensor(SensorBase):
         # obtain the contact forces
         # TODO: We are handling the indexing ourself because of the shape; (N, B) vs expected (N * B).
         #   This isn't the most efficient way to do this, but it's the easiest to implement.
-        net_forces_w = self.contact_physx_view.get_net_contact_forces(dt=self._sim_physics_dt)
+        net_forces_w = self.contact_physx_view.get_net_contact_forces(dt=self._dt)
         self._data.net_forces_w[env_ids, :, :] = net_forces_w.view(-1, self._num_bodies, 3)[env_ids]
         # update contact force history
         if self.cfg.history_length > 0:
@@ -338,7 +338,7 @@ class ContactSensor(SensorBase):
             # shape of the filtering matrix: (num_envs, num_bodies, num_filter_shapes, 3)
             num_filters = self.contact_physx_view.filter_count
             # acquire and shape the force matrix
-            force_matrix_w = self.contact_physx_view.get_contact_force_matrix(dt=self._sim_physics_dt)
+            force_matrix_w = self.contact_physx_view.get_contact_force_matrix(dt=self._dt)
             force_matrix_w = force_matrix_w.view(-1, self._num_bodies, num_filters, 3)
             self._data.force_matrix_w[env_ids] = force_matrix_w[env_ids]
 

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

@@ -102,12 +102,6 @@ class Imu(SensorBase):
         self._data.lin_acc_b[env_ids] = 0.0
         self._data.ang_acc_b[env_ids] = 0.0
 
-    def update(self, dt: float, force_recompute: bool = False):
-        # save timestamp
-        self._dt = dt
-        # execute updating
-        super().update(dt, force_recompute)
-
     """
     Implementation.
     """

source/isaaclab/isaaclab/sensors/sensor_base.py

@@ -196,6 +196,7 @@ class SensorBase(ABC):
 
     def update(self, dt: float, force_recompute: bool = False):
         # Update the timestamp for the sensors
+        self._dt = dt
         self._timestamp += dt
         self._is_outdated |= self._timestamp - self._timestamp_last_update + 1e-6 >= self.cfg.update_period
         # Update the buffers
@@ -218,7 +219,7 @@ class SensorBase(ABC):
         # Obtain device and backend
         self._device = sim.device
         self._backend = sim.backend
-        self._sim_physics_dt = sim.get_physics_dt()
+        self._dt = sim.get_physics_dt()
         # Count number of environments
         env_prim_path_expr = self.cfg.prim_path.rsplit("/", 1)[0]
         self._parent_prims = sim_utils.find_matching_prims(env_prim_path_expr)