A hybrid adaptive controller for positioning servo systems with saturating actuators and nonlinear fuzzy friction at low velocities

In this thesis, we investigate a hybrid adaptive control algorithm which provides a fast position tracking in a servo system operating at low velocities. The servo system involves saturation of actuators in away from zero velocity regimes while it includes a nonlinear fuzzy friction torque in close to zero velocity regimes. We introduce a continuous friction model by which the behavior of the servo system in all velocity regimes can be analyzed. For away from zero velocity regimes, this model reduces to the classical friction model while for close to zero velocity regimes it allows the realization of stiction and pre-sliding phenomena. The hybrid adaptive control algorithm comprises two parts; one for away from zero, and one for close to zero velocity regimes.;The performance of the servo system which utilizes the hybrid adaptive control algorithm is compared with that of the adaptive linear counterpart with and without Coulomb friction compensator in both velocity regimes. Simulation results show significant performance improvement in terms of tracking time and transient response when using such hybrid adaptive control algorithm.;For away from zero velocity regimes where stiction can be neglected, a time optimal control algorithm is designed in conjunction with a modified adaptive identification scheme which rapidly identifies the nonlinear model parameters including Coulomb friction under poor excitation of bang-bang control. For close to zero velocity regimes where fuzzy stiction is present, an adaptive fuzzy control scheme with new scaling blocks and membership reshaping factors is introduced.