I-001

Hard Disk Drive Bandwidth Limitations Due to Sampling Frequency and Computational Delay

Authors: Matthew T. White and Wei-Min Lu
Affiliation: IBM Corporation

Abstract
The improvements in track density and performance expected of future disk drives require increased servo bandwidths. The mechanics are often cited as a bandwidth limiter, but sampling frequency and computational delay also play a part in reducing the achievable bandwidth. Unfortunately, the cost of improving the sampling frequency and computational delay is often high. To isolate the effects of the sampling frequency and the computational delay, the higher frequency resonances often found in disk drives were ignored, and a simple double-integrator plant model was used for the controller designs. The maximum bandwidths (assuming fixed stability margins) were calculated using two controller types, a second-order lead-lag filter and an H-infinity controller, for a range of sampling frequencies and computational delays. The results are summarized in a scaling law that relates the bandwidth to the sampling period, delay time, and an empirically determined constant. The effects of sampling and delay on the sensitivity function, and the implications of the Bode Integral Theorem, are also discussed.

Matthew T. White
IBM Almaden Research Center
650 Harry Road, K62/C3
San Jose, CA 95120-6099
phone:(408) 927-2438
fax:(408) 927-3008
mwhite@almaden.ibm.com