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(Anisotropic) Magnetoresistive (MR/AMR) Head
As the disk spins, magnetic fields on its surface
representing data bits pass under the read/write head. These fields generate minute electrical currents in the
head that can be interpreted by the computer as binary data. To write data, the process is reversed, and a current
is sent to the read/write heads. The current generates a magnetic field in the head which polarizes the magnetic
particles on the disk surface in one of two ways to represent either binary zeros or ones.
When the computer wants to read data, the operating system works out where the data is on the disk. To do this
it first reads the FAT (File Allocation Table) at the beginning of the partition. This tells the operating system
in which sector on which track to find the data. With this information, the head can then read the requested data.
The disk controller controls the drive's servo-motors and translates the fluctuating voltages from the head into
digital data for the CPU.
HEAD ARM
The head arms are thin pieces of metal, usually triangular in shape onto which the head sliders (carrying
the read/write heads) are mounted. In a way, the idea here is similar to how the arm of a phonograph is used to
move the stylus from the outside of a record to the inside (although of course the similarity ends there).
There is one arm per read/write head, and all of them are lined up and mounted to the head actuator to form a single
unit. This means that when the actuator moves, all of the heads move together in a synchronized fashion. Heads
cannot be individually sent to different track numbers.
The top head arm of a typical recent-design
hard disk drive. Note that the arm is not solid, but rather has a structural triangular shape. This is done to
reduce weight while maintaining rigidity.
The arms themselves are made of a lightweight, thin material, to allow them to be moved rapidly from the
inner to outer parts of the drive. Newer designs have replaced solid arms with structural shapes in order to reduce
weight and improve performance. This is the same technique used to reduce weight in the construction of airplane
wings, for example. Newer drives achieve faster seek times in part by using faster and smarter actuators and lighter,
more rigid head arms, allowing the time to switch between tracks to be reduced.
FUTURE DRIVERS are designed focusing in the reduction of platters in hard disks towards three or even two
platters. Large number of head arms makes it difficult to make the drive with high enough precision to permit very
fast positioning (on random seeks) due to increased weight in the actuator assembly from the extra arms, and also
problems aligning all the heads.
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