All real control systems are subject to uncertainties and disturbances, so feedback controllers have to be designed such that closed-loop systems possess the desired dynamic and steady-state responses in the presence of any allowable uncertainties and disturbances. It is common practice to address the effects of uncertainties and disturbances via bounding techniques and the almost disturbance decoupling approach, respectively. Linear, affine, and power growth conditions on uncertainties are required for almost disturbance decoupling. However, when these conditions are not satisfied, almost disturbance decoupling is not possible. A new concept called weak disturbance decoupling is introduced to mitigate the effects of disturbances. A weak disturbance decoupling problem is to find a feedback controller so that the close-loop system has the weak disturbance decoupling performance, that is, the closed-loop system is stable and the norm of the output is not greater than the sum of a positive constant and the product of the norm of the disturbance and a positive constant.
Du et al. (Sun,) studied this question.