A generalized disturbance filter design and its applications to a spinstand servo system with microactuator

Narrow-band position error at mid-frequencies around the open-loop crossover frequency can not be effectively reduced using a conventional peak filter, because the attenuation of sensitivity gains has to compromise with the associated decrease of phase margin. This paper presents a general second-order filter that is applicable to reject narrow-band disturbances at any frequency range. The filter zero is designed to minimally degrade the closed-loop system stability and obtain a smooth sensitivity curve around the disturbance frequency. A nonlinear optimization procedure is developed to select the filter parameters such that the statistical position error is minimized. Experimental results of a PZT-actuated head positioning control system on spinstand demonstrate that the add-on filter can further reduce the mid-frequency PES NRRO by 8% and preserve the stability margin of the original control system.