Non-linear disturbance observer-based robust control for systems with mismatched disturbances/uncertainties

Robust control of nonlinear systems with disturbances and uncertainties is addressed in this paper using disturbance observer based control technique. In this framework, the “disturbance” is a generalized concept, which may include external disturbances, unmodeled dynamics, and system parameter perturbations. The existing disturbance observer based control (DOBC) methods were only applicable for the case where disturbances and uncertainties satisfy so called matching condition, i.e., they enter the system in the same channel as the control inputs. By appropriately designing a disturbance compensation gain vector in the composite control law, a nonlinear disturbance observer based robust control (NDOBRC) method is proposed in this paper to attenuate the mismatched disturbances and the influence of parameter variations from system output channels. The proposed method is applied to a nonlinear missile system in the presence of various uncertainties and external disturbances. Simulation shows that, compared with the widely used nonlinear dynamic inversion control (NDIC) and NDIC plus integral action (NDIC+I) methods, the proposed method provides much better disturbance attenuation ability and stronger robustness against various parameter variations. The proposed method significantly extends the applicability of the DOBC methods.