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Observer of Nonlinear Friction Dynamics for Motion Control

Kinetic friction in motion control systems is subject to large uncertainties due to the multiple, and often weakly known, internal and external factors such as roughness, thermal and lubricant state of contacting surfaces, varying normal loads, dwell time, wear, and others. The single modeling of friction behavior, even if comprehensive and accurate enough, appears to be insufficient for accurately compensating the friction disturbances, due to their time- and state-varying nature. In this paper, we propose a novel nonlinear friction observer aimed at the motion control. We analyze the uncertainties of viscous and Coulomb friction and derive an asymptotic observer for two-state friction dynamics without assuming a particular dynamic friction model. The linear observer gains prove to be sufficient for achieving an accurate friction estimate with a controllable eigenbehavior. Furthermore, we analyze the friction observer within the linear feedback loop and describe the required system identification and design of controller. An experimental case study accomplished on a rotary actuator system is provided for evaluating both the friction observer and its use for precise positioning.