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 non-synchronous perceived TMR of 0.56% of a track is found td 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, non-synchronous, self-induced TMR of 1.67% of a track is possible with a track-density of roughly 72000 TPI.