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Lifecycle of a Hardware Component

Methods return values have type rclcpp_lifecycle::node_interfaces::LifecycleNodeInterface::CallbackReturn with the following meaning:

• CallbackReturn::SUCCESS method execution was successful.

• CallbackReturn::FAILURE method execution has failed and the lifecycle transition is unsuccessful.

• CallbackReturn::ERROR critical error has happened that should be managed in on_error method.

The hardware transitions to the following state after each method:

• UNCONFIGURED (on_init, on_cleanup):

  Hardware is only initialized, but communication is not started and no interfaces are imported into ResourceManager.

• INACTIVE (on_configure, on_deactivate):

  Communication with the hardware is established and hardware component is configured. States can be read, but command interfaces (System and Actuator only) are not available.

  As of now, it is left to the hardware component implementation to continue using the command received from the CommandInterfaces or to skip them completely.

  Note

  We plan to implement safety-critical interfaces, see this PR in the roadmap. But currently, all command interfaces are available and will be written, see this issue describing the situation.

• FINALIZED (on_shutdown):

  Hardware interface is ready for unloading/destruction. Allocated memory is cleaned up.

• ACTIVE (on_activate):

  States can be read.

  System and Actuator only:

  Power circuits of hardware are active and hardware can be moved, e.g., brakes are disengaged. Command interfaces are available and the commands should be sent to the hardware