Design of a Nonlinear Speed Controller for a DC Motor System with Unknown External Torque Based on Backstepping Approach


A nonlinear PI controller for a system with a mismatched and unknown externaldisturbance is proposed in the present paper. A Sigmoidal function is proposed to be usedin the nonlinear integral element to attenuate the disturbance effect. Formally the controllerforces the state to a positively invariant set around the origin. As an application, thenonlinear PI controller is used as a virtual controller for the upper subsystem in the frame ofBackstepping control approach for the DC motor system. In this Backstepping controllerdesign the restriction about the disturbance form in the upper system is relaxed. The resultsshow the effectiveness of the nonlinear PI controller to attenuate the effect of unknown andvarying disturbance torque and force the angular velocity to follow the desired reference.Finally a reduced order observer is designed to estimate the armature current used in thedesigned controller for the DC motor system.