Add-on controller design based on disturbance observer in H-infinity framework

In the paper, an add-on controller design based on disturbance observer for a class of single-input single-output (SISO) linear systems is proposed in H_∞ framework. Motivated by the previous research of reinterpretation of disturbance observer as an add-on controller, the disturbance observer is designed to minimize reference tracking error which is the ultimate objective of every control system by minimizing disturbance estimation error. Since the disturbance estimation error may amplify reference tracking error in a certain frequency band when the disturbance estimate is implemented to existing closed-loop system, the design methodology for an add-on controller considers a primary feedback controller. Simulation results validate disturbance suppression performance of the proposed method by comparing with Q-filter approach.