Wow that looks like a great series, thanks for the heads up. I'm just starting to dip my toes into basic PID control and am astounded by how deep (and predictive) the math can get in this domain.
Edit: Also, I learned a lesson about closed-loop systems with the launch (well, EDL) of the Curiosity rover on MARS. It's mentioned in this article [1] but the trigger to start lowering the rover on the crane wasn't some process monitoring distance from the ground...it actually had no direct notion of that. It just watched the throttle setting on the retro-rockets and started unwinding the winch when the throttles reached a reduced steady state setting...something expected once the descent stage had reached its hover altitude.
[1] (search for throttle) - https://www.planetary.org/blogs/emily-lakdawalla/2012/070607...