2016 Volume 5 Issue 2 Pages 61-68
In various servo applications, the design of a precise positioning control constitutes a trade-off between the fast transient response and the short settling time for the required accuracy. Mostly, neither ‘universal’ control gains can be found to be equally suitable for both of these objectives. In this study, the design of a cascaded precise positioning control is analyzed in the presence of nonlinear friction. The nonlinear friction strongly impacts the reference settling and can deteriorate the positioning control performance. Two common cascaded structures, and that P-PI and PI-PI, are analyzed by exposing the closed-loop dynamics of the control system with friction in details. It is shown that a more ‘traditional’ integral control part is less useful when compensating for the nonlinear friction at the reference position settling. Furthermore, we show how the settling performance can be improved by applying a feed-forward friction observer. This can be included as a plug-in after designing the surrounding cascaded feedback control and without re-tuning. The proposed control strategy is evaluated experimentally on a standard industrial linear positioning axis, with a relatively high reference speed of +/-500mm/s and narrow positioning error band of +/-0.01mm.