PID controller and diff drive controller chaining

[Juliaj]

Address #568. This is to add a new example_16 based on the use case outlined in the controller manager doc section Motivation, Purpose and Use. It uses example_2 as a base and adds a chained 'diff_drive_controller' and two PID controllers with following flow.

diff_drive_controller     → pid_controllers      → hardware
     (velocities)           (error control)       (motors) 

This requires following changes from ros2_controllers:

• Fix handles for following controllers of diff_drive_controller ros2_controllers#1513
• [pid_controller] Fix logic for feedforward_mode with single reference interface ros2_controllers#1520

New Examples

If you propose adding a new example, make sure that your new example has:

• The correct folder structure (described in the main README.md)
• Example has doc/README.rst with description
• Detailed commands how to run an example
• Output examples of CLI commands
• Screenshots with expected visualizations (if applicable)

[christophfroehlich]

I think you found a bug, I proposed a fix in ros-controls/ros2_controllers#1513
I also changed your configuration a bit, let met know what you think.

[christophfroehlich]

Thanks for your submission! Some thoughts:

• Please cleanup the docs to focus on the controller chain (e.g., remove the MockHardware part)
• Update the test_diffbot_launch.py to check for all controllers
• I suggest that you add some dynamics to the hardware interface, a first-order or second-order low pass, so that the PID controller actually makes sense.

When the following stuff is fixed/merged we should include to the docs (maybe in follow-up PRs)

• view_controller_chains output Fix view_controller_chains ros2_control#2026
• add plots of pal_statistics to show the PID controller action Integrate pal_statistics for introspection of controllers, hardware components and more ros2_control#1918

[Juliaj]

I think you found a bug, I proposed a fix in ros-controls/ros2_controllers#1513 I also changed your configuration a bit, let met know what you think.

Thank you so much for responding so quickly. The configuration changes are really nice, love them. I tested the fix you made, the activation is working now. I'll continue the rest of work and include the suggestions that you made. Left some comments on your PR ros-controls/ros2_controllers#1513.

[christophfroehlich]

see #1528 for a proper fix of the interfaces. could you have a look there and review please?

[christophfroehlich]

Suggested change

# start the base controller in inactive mode

[christophfroehlich]

Suggested change

	# "--inactive",
	"--ros-args",
	"--log-level",
	"debug",