Hardware Reference
In-Depth Information
readhead.InmodernHDDs,thereadheadisathin-
fi
lm, metalized structure
that exhibits magneto-resistive effect, that is, its resistivity changes when it
is brought under the in
fl
uence of a magnetic
fi
eld. A current (I
MR
)issent
through the MR head. Voltage across the MR sensor is V
MR
= I
MR
R
MR
,
where the resistance R
MR
of the MR element varies as a function of magnetic
fi
eld in the vicinity of the sensor. The voltage V
MR
is a direct measurement
of the magnetic
fi
eld produced by the binary bits written on the disk surface.
Sensitivity of the MR head is much higher than that of an inductive head.
Moreover, unlike in the case of inductive head, the amplitude of the voltage
signal produced by the MR sensor does not depend on the media velocity. In
hard disk drives, the disks are spun at a constant angular velocity (ω rad/s)
and the media velocity, i.e., relative velocity between head and data track
(v = ωR) depends on the radius (R) at which the track is located. This velocity
(v) is smaller when the head is over an inner track than that for an outer
track. As a result, the amplitude of the read back signal in an inductive head
becomes smaller and smaller as the head moves towards ID. Drives currently
available in the market use the giant magneto-resistance (GMR) heads, dual
spin valve heads, and tunnel junction heads. These advanced heads utilize
new technologies to enhance the magneto-resistive effect and thus the read
head sensitivity.
1.2.2 Electromechanical Components
Two different electromechanical components are used in a hard disk drive - (1)
a spindle motor to spin the disk or disks and (2) an actuator to re-position
the read-write heads on the desired data track and to maintain its position
precisely over the track while data is being read or written.
Spindle Motor
Brushless DC motor is used to spin the stack of disks in an HDD. Both ends of
the spindle are
fi
tted with pre-loaded ball bearing. High end, high performance
drives (spindle spinning at 10,000 RPM or more) use
fl
uid dynamic bearing or
aerodynamic bearing spindles. The spindle speed was 1200 RPM for RAMAC,
and 3600 RPM for the 14-inch drives. Earlier generations of drives with form
factors 5
4
inch and 3
2
inch, designed for desktop applications, had their disks
spinning at 3,600 or 4,200 RPM. Many models of drives for the desktop market
and mobile market still use spindle speeds of 5400 RPM or 6000 RPM. Higher
spinning rates such as 7,200 RPM, 10,000 RPM or even 15,000 RPM are being
used these days for high performance drives.
Speed of the motors must be controlled precisely to ensure conformity of the
rate of read back bits. Variation of speed affects the performance in many ways.
Firstly, the flying height of the slider varies affecting the bit density. Secondly,
if the spindle speed during reading is different from its speed during writing
