Hardware Reference
In-Depth Information
writing data. This is a major difference from the typical de
fi
nition of settling
time in control system step responses. In HDD servo mechanism, the error
must be less than 10%(writing) or 15%(reading) of a single track irrespective
of the number of tracks traversed by the seek operation. Let us consider a seek
command asking for movement of the head from track N to track N + 100,
which is equivalent to a step response with y
ref
= 100. According to the de-
fi
nition of 5% settling time in linear control system, the settling time is equal
to the time it takes to bring the position error within ±5tracks. Anditis
equal to ±10 tracks for a seek command of 200 tracks. However, in HDD ser-
vomechanism, the limit of position error is 10% (writing) or 15% (reading) of
one track for all seek lengths.
1.1 History of HDD Technology
The HDD industry has a relatively short but fascinating history. In four
decades it evolved from a monstrosity with
fi
fty 24-inch diameter disks storing
only 5 MB of data to today's drives storing close to 100 GB (100,000,000,000)
of data on one surface of a disk in a 3
2
inch drive. This enormous growth
was made possible by developments in diverse
fi
elds of knowledge including
materials, tribology, mechanics, servo control, signal processing and electron-
ics. Drives of
fi
rst generation were signi
fi
cantly different from the drives we
see now in aspects like size, capacity, and data transfer rate as well as in the
technologies used. For a comprehensive reading on the history of magnetic
recording in general and hard disk drive in speci
fi
c, interested readers may
refer to many published articles such as [5], [74], [186].
1.1.1 The Early Days
The fascinating journey of this marvelous device began with a huge, mon-
strous equipment called RAMAC (Random Access Method of Accounting and
Control),whichusedthevery
fi
rst non-volatile DASD introduced by IBM in
1956 [151]. The disk drive of RAMAC contained
fi
ftydisks,each24inchin
diameter, and could store 5 Megabytes of data at a recording density of 2K
bits/in
2
. The track density and linear density were 20 TPI and 100 BPI, re-
spectively. The disks used to be spun at a speed of 1200 RPM (revolutions
per minute ) and the rate of data transfer was 8.8 kilobytes per second (KB/s)
only [74]. The RAMAC did not use any closed loop control for the head po-
sitioning mechanism. IBM, the only company designing and building hard
disk drives in the early years, was the sole contributor to the growth of this
industry in those days. It designed in 1961 the
fi
rst drive using air bearing
heads, and in 1963 the
fi
rst removable disk pack drive. All these drives used
either motor-clutch mechanism or hydraulic actuators for the head positioning
system operated under open loop control. The
fi
rst HDD with a closed loop
servo control was produced in 1971 (IBM 330 Merlin Drive), and it used the
