Analysis of settling behavior and design of cascaded precise positioning control in presence of nonlinear friction

In several robotics and machinery applications the design of positioning control constitutes a trade-off between the requirements posed on a fast transient response and accuracy in settling. Mostly, neither 'universal' control gains can be found equally suitable for both objectives, so that often gain-scheduling strategies are used, in particularly for the inner velocity loop. However, this can be cumbersome from a systematic and robust control design point of view, and often no analytical solutions are available to ensure the fast and accurate settling. In this paper, the design of precise positioning control is presented which uses an additional feed-forward friction observer (FFFO) in the inner velocity loop. The FFFO approach allows efficiently compensating for nonlinear friction and can be appointed as a plug-in, after designing the surrounding feedback control. Here, the standard cascaded P-PI control is taken as a reference control system. The design of cascaded positioning control is discussed and analyzed in view of the settling behavior. Exposing in details the closed-loop dynamics we discuss the shortcomings of cascaded P-PI and PI-PI feedback regulators in presence of nonlinear presliding friction. The proposed control strategy is evaluated experimentally on a linear stage with drive velocity of 500 mm/s and micrometer positioning accuracy.