ADRC uses an Extended state observer to linearize the Quadrotor's Nonlinear dynamics (similar to Feedback linearization). This makes it capable of eliminating disturbances (robustness).
Disturbance observers are known for their use in the estimation of unmeasurable dynamics in a plant. An ADRC controller as the name implies actively cancels disturbances and unwanted dynamics. It is based on the concept of an extended state observer (ESO). An extended state observer is a type of disturbance observer that estimates combined or total disturbances in a plant by introducing a fictitious state (also known as the extended state) that represents the unknown or unwanted dynamics. After estimation of the unknown dynamics, a feedback linearization-type operation is used to cancel out the total disturbance from the plant dynamics. This implies that the control affine plant transforms to a linear feedback integrator system. As a result, linear control theory can be applied. It is thus intuitive that the performance of the controller is dependent on the accuracy, speed and performance of the ESO.
