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* Arduino PID Library - Version 1.2.1
* by Brett Beauregard <br3ttb@gmail.com> brettbeauregard.com
*
* This Library is licensed under the MIT License
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 - For an ultra-detailed explanation of why the code is the way it is, please visit: 
   http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/

 - For function documentation see:  http://playground.arduino.cc/Code/PIDLibrary

This fork uses back calculation per [Astrom 1989](http://cse.lab.imtlucca.it/~bemporad/teaching/controllodigitale/pdf/Astrom-ACC89.pdf) to manage integral windup.