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Execution order

When a simulation is started with isaacimi sim run path/to/blueprint.yaml, it goes through the following phases: Initialization → Simulation Loop → Finalization

Initialization

  1. Insert a reference to the environment .usd file provided in the blueprint into the USD stage at the specified prim_path. An Xform prim points to the environment asset.
  2. For every robot listed in the blueprint:
    • Insert a reference to the robot .usd file into the USD stage at the specified prim_path. An Xform prim points to the robot asset.
    • Wrap the robot prim into a high-level ImiRobot object, which is an articulation-aware object that exposes APIs for physics, joint manipulation and more,
    • Instantiate each plugin listed in the blueprint and load it onto the robot, on_plugin_load() hook is called for each plugin
    • At this point, the robot object has not been added to the Scene yet
  3. For every ImiRobot robot object:
    • Add the object to the Scene registry, enabling the object to be managed by the simulation. Now, the object will become responsive to simulation events when the simulation begins.
    • set_up_scene() is called for each plugin on the robot
    • At this point, the robot's physics and joint handles have not been initialized yet, i.e. robot APIs like num_dof or get_joint_positions() return None
  4. Initialize physics handles and articulation controllers for all objects in the Scene registry, including the ImiRobot objects. At this point, robot physics and joint handles are initialized, e.g. joint indicies are mapped and robot APIs like get_joint_positions() no longer return None
  5. The initialize() method is called for each plugin on the robot. Here, plugins have access to the robot object with all physics initialized.

Simulaton loop

  1. All ROS nodes across all robots are checked for available callbacks. If a callback is available, execute it in the background by a worker thread. Note, only one callback is started each loop (if there is one available), but each simulation loop is very fast and callbacks can run concurrently.
  2. If it is time for a physics step (configured by the physics_dt field in the blueprint):
    • Call pre_physics_step() for all plugins
    • Complete the physics step (done internally by Isaac Sim)
    • Call on_physics_step() for all plugins
  3. If it is time for a render step (configured by the render_dt field in the blueprint)
    • Render the scene (done internally by Isaac Sim)

Finalization

  • to-do