Major HDD TMR sources and projected scaling with TPI

This paper identifies the major sources of ontrack, self-induced, non-synchronous TMR (track mis-registration) in a HDD (hard-disk drive) with a track-density of 4400 TPI (tracks per inch) and a rotational rate of 90 Hz. Experimental measurements of the drive's PES (position-error signal) in ontrack mode are used to determine the spectrum of the perceived TMR, which is then broken down into component parts. The total RMS nonsynchronous perceived TMR of 0.56% of a track is found to be composed of 0.17%, due to torque disturbances, 0.33%, due to disk motions, 0.42% due to PES demodulation noise, with 0.04% unaccounted for (the total being the root-sum-of-squares of the components). With the assumption of a doubling of the PES noise, a factor of two reduction in the disk vibrations, a quadrupling in the servo sample-rate, and a doubling of the loop bandwidth, the authors show that an RMS ontrack, nonsynchronous, self-induced TMR of 1.67% of a track is possible with a track-density of roughly 72000 TPI.