Resonance identification by commanding a spindle speed change

The present invention relates to positioning actuators of electromechanical systems, and more particular to highly fast and precise servo positioning systems such as those employed in disc drives. Method and apparatus for identifying a resonance by changing a spindle speed, which has the effect of changing a corresponding sampling rate and Nyquist frequency. The invention discriminates (1) between plausible frequencies of true resonance and/or (2) aliased resonance from non-aliased resonances. This is accomplished by monitoring whether and/or how a resonance shifts during this speed change. Methods of the present invention monitor resonances by deriving a resonance indicator such as an apparent frequency (below the Nyquist frequency), a gain or error magnitude (compared to a derived threshold at a calculated frequency of interest), or similar criteria tested by values conventionally illustrated on a Bode plot. The present invention further includes steps or structural features for configuring a controller to attenuate or similarly limit an unwanted frequency component of an actuator control signal. In this way, resonances in the actuator can be reduced, and servo speed and accuracy thereby enhanced.