Quadcopter balance with MPU-6050
There are tho worlds in controll theory: open-loop (ger. Steuerung), closed loop (ger. Regelung). These two worlds can be joint together with addition.


The picture above is a depiction where open and closed loop are binded together. A PI controller only adds some correction to the setpooint derived from LUT (static characteristics) aka feedforward compensation. The stability of closed loop isn't affected, because the feedforward component is open loop - You must take care that you don't make an accidental loop from PV.