Nonlinear PID control of linear plants for improved disturbance rejection

A nonlinear PID (NPID) controller for a class of linear plants with output feedback is studied. The controller gains are modulated by the control error and error rate to yield a closed-loop system for improved disturbance rejection. an optimization problem involving constraints written as linear matrix inequalities (LMIs) is solved to compute the controller parameters. A hard disk drive (HDD) control application shows that the NPID outperforms the conventional PID by 9% in disturbance rejection along with a consequent reduction by 20% in settling time while the stability robustness is maintained.