That is actually done intentionally to allow for changing of the tuning parameters while the PID is running.[1] The author of the PID library has a full writeup

That is actually done intentionally to allow for changing of the tuning parameters while the PID is running.[1] The author of the PID library has a full writeup on the design of the algorithm.[2] [1] http://brettbeauregard.com/blog/2011/04/improving-the-beginner%E2%80%99s-pid-tuning-changes/ http://brettbeauregard.com/blog/2011/04/improving-the-beginn... [2] http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/ http://brettbeauregard.com/blog/2011/04/improving-the-beginn...