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Different Clocks used by Controller Manager

The controller manager internally uses the following two different clocks for a non-simulation setup:

• RCL_ROS_TIME: This clock is used mostly in the non-realtime loops.

• RCL_STEADY_TIME: This clock is used mostly in the realtime loops for the read, update, and write loops. However, when the controller manager is used in a simulation environment, the RCL_ROS_TIME clock is used for triggering the read, update, and write loops.

The time argument in the read and write methods of the hardware components is of type RCL_STEADY_TIME, as most of the hardware expects the time to be monotonic and not affected by the system time changes. However, the time argument in the update method of the controller is of type RCL_ROS_TIME as the controller is the one that interacts with other nodes or topics to receive the commands or publish the state. This time argument can be used by the controllers to validate the received commands or to publish the state at the correct timestamp. The period argument in the read, update and write methods is calculated using the trigger clock of type RCL_STEADY_TIME so it is always monotonic.

The reason behind using different clocks is to avoid the issues related to the effect of system time changes in the realtime loops. The ros2_control_node now also detects the overruns caused by the system time changes and longer execution times of the controllers and hardware components. The controller manager will print a warning message if the controller or hardware component misses the update cycle due to the system time changes or longer execution times.