“…The concept of 2-DOF control was first proposed and applied to the design of the PI/PID controller by Horowitz in 1963 . With the development of the control technology, 2-DOF-based control systems are generally divided into two types: “feedback and feedforward” and “feedback and disturbance observer (DOB).” The former one mainly consists of the 2-DOF PI/PID controller, − whose tuning rules have been studied by worldwide researchers, such as maximum sensitivity ( M s )-constrained integral gain optimization (MIGO), internal model control (IMC), , relative delay method, desired dynamic equational (DDE) method, multi-objective optimization, and so forth. The latter one mainly focuses on the design of the observer to handle disturbances and uncertainties, including DOB, perturbation observer (POB), equivalent-input-disturbance (EID), uncertainty and disturbance estimator (UDE), generalized PI observer (GPIO), unknown input observer (UIO), extended state observer (ESO), , and so forth.…”