Optimal Narrow-Band Disturbance Filter for PZT-Actuated Head Positioning Control on a Spinstand

Abstract

Narrow-band position error at midfrequencies around the open-loop crossover frequency cannot be effectively reduced by using a conventional peak filter, because the attenuation of sensitivity gains has to be traded off with the associated decrease of phase margin. This paper presents a general second-order filter that can 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. The paper presents a nonlinear optimization procedure for selecting the filter parameters so that the statistical position error is minimized. Experimental results of a piezoelectrically actuated head positioning control system on a spinstand demonstrate that the add-on filter can further reduce the midfrequency nonrepeatable runout of the position error signal by 8% and preserve the stability margin of the original feedback control system.

Stable Peak Filtering Method to Reject High Frequency Disturbance in Hard Disk Drives

“…From (1), it can be seen that the design of base-line servo loop and the disturbance filter can be somewhat decoupled (18) . The base-line controller C(s) takes care of the plant P(s) to guarantee the basic stability margin, while the disturbance filter F(s) takes care of the pseudo "plant" S h (s) so that the resulting S F (s) has desired attenuation at those disturbance frequencies (16) .…”

“…It is preferable to augment the filters into the existing servo loop in an "add-on" fashion so that the design of each loop can be somewhat decoupled (18) . The phase stable ability of this compensation can be guaranteed by appropriately placing a zero so that the closed-loop is robust against phase uncertainty around the disturbance frequency (16), (19), (20) . In these works such as (15), (16) , the add-on filters were placed at the disturbance frequencies where there were no significant plant dynamics, i.e., lightly damped plant poles.…”

“…The phase stable ability of this compensation can be guaranteed by appropriately placing a zero so that the closed-loop is robust against phase uncertainty around the disturbance frequency (16), (19), (20) . In these works such as (15), (16) , the add-on filters were placed at the disturbance frequencies where there were no significant plant dynamics, i.e., lightly damped plant poles. As mentioned earlier, the disturbances such as air-flow or head-disk interference would excite the structural resonance modes.…”

Adaptive disturbance rejection in the presence of uncertain resonance mode in hard disk drives

“…By doing so, the uncertain resonance modes can be identified, and as soon as the identification is settled down, a peak filter C pf (z) will be designed to replaceN (z) according to the identified results to provide better disturbance rejection at these uncertain modes. Note that bothN (z) and C pf (z) are augmented in an "add-on" fashion so that the performance of the nominal controller is well-preserved [8] and both of them can be easily enabled or disabled to accommodate the fast-changing uncertainties. Fig.…”

“…According to the identification results, the estimated resonance mode is centered at 7574Hz. As soon as the approximation error enters into the dead-zone and stays within it thereafter, at about 30 milli-seconds for this case, the identification loop can be cut off and replaced by a linear peak filter given by (8). Therefore, the natural frequency in (8) can be set as ω 0 =2π7574.…”