On the unified approach to the disturbance observer

The purpose of the disturbance observer (DOB) in the dynamical system is to cancel out external disturbances from environment and inherent uncertainties in the plant through realizing the disturbance. Although a variety of different DOB structures has one common goal of rejecting disturbance, many variations in control structure are present owing to how to estimate the disturbance and how to cure signal distortion from modification. A unified approach to classify DOBs into model-based and non-model based DOBs is present. Several time-delayed DOBs as a non-model based DOB are derived from the ideal time-delayed DOB. Reconfiguration of the simplified time-delayed DOB turns out to be a high gain feedback control structure. The analysis of the time-delayed DOB with acceleration feedback shows that the overall open loop system becomes a nominal inertial system regardless of any second order plants under no time-delay condition. Practical issues are addressed and simulation examples are demonstrated to support the analysis.