HARD DISK DRIVE GUIDE
How a Hard Disk Drive Works
Heads
Last updated: 2/27/2005
HEADS. The heads (or Winchester sliders) are
spring-loaded airfoils and actually fly like an airplane above (or below)
the surface of the platters at a distance measured in micro-inches. The
air stream though which a head "fly" is caused by the motion of
the platters spinning through the air inside the HDA. The platters
drag the air along by friction. The higher pressure air between the
heads and the platters is known as air bearing. The effect is somewhat
like a puck in an air hockey game. The bottom of a head is called an
air bearing surface. This sort of mechanism was introduced in the Winchester
hard disk drive invented by IBM in 1973.
 The
heads are extremely small electromagnets (about 1 mm square) and one is shown
schematically to the right
(for a prettier and more detailed picture with separate read and write elements, click here). Information
is stored on the platters by sending pulses of current from the drive electronics
to the head. The direction of the current and thus the direction of
the diverging magnetic field across the gap in the head determines the direction
the magnetic domains (little bitty, molecular magnets) on a particular spot
on the platter's magnetic coating, and, thus, whether the spot represents
a binary one or zero. The domains essentially retain their directional bent
(whether the computer is on or off) until "told" to do otherwise
by the drive electronics, which take their orders from the rest of the computer
and ultimately from software. The complexity of the mechanisms and
methods associated with doing all of this will be omitted here.
 The
heads are bonded to a metal suspension (or head arm), which is a small arm
that holds the head in position above or beneath a disk. A head and suspension
is called a head-gimbal assembly or HGA. The HGA's are stacked together Into
a head-stack assembly, which is propelled across the disk surface by
the actuator. The actuator on most recent hard disks employs a voice
coil mechanism. It functions much like the voice coil in a loud speaker,
thus its name. It consists of a curved magnet (or magnets--very
strong ones) and a spring-loaded coil of fine wire which is attached to the
read/write heads by head arms. The head arms are attached to, and pivot
about an actuator shaft. When the drive electronics apply an electric
current to the actuator coil, it interacts with the magnet and swings against
the actuator spring. The heads rotate around the actuator shaft in
the opposite direction of the coil movement, inward and outward from the
center to the edges of the platters. If there is a power outage (e.g.,
you turn-off the computer) the spring, which counterbalances the electromagnetic
force between the coil and magnet, takes over and automatically parks (lands
them on skids or nanosliders--like pontoons on a sea plane) and locks the
heads on a part of the platters called a landing zone (like an airport runway
only curved) before they can crash (like an airplane) on, and mar that part
of the surface of the platters where data is stored. When power is
restored, the platters speed-up and the heads take off (like a tethered model
airplane, except the ground moves--and those on the bottoms of the platters
can fly up-side-down) and start flying again--an extraordinary mechanism...
One no longer has to park a hard disk before moving
the computer as was the case in times of old when actuators were moved
by devices known as stepper motors. However, if the power jitters
repeatedly or the drive is subjected whack from a frustrated user, a
crash can occur.
See the BASICS
OF MAGNETIC RECORDING on for a more detailed
treatment of this subject--a good read.
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