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Hard Disk Head Sliders - Arms - Actuator - Platters
An illustration of a typical PC actuator assembly, showing
the major components. The platters have been removed from the drive to provide a better view of the actuator arms
and heads. There are four sliders but only one of each pair is visible. The spindle motor is visible at the top
right.
Hard Disk Platters
Hard disk platters are accessed for read and write operations using the read/write heads mounted on the top and
bottom surfaces of each platter. The read/write heads must be held in an exact position relative to the surfaces
they are reading and they must be moved from track to track to allow access to the entire surface of the disk.
The heads are mounted onto a structure that facilitates this process. Often called the head assembly or actuator
assembly (or even the head-actuator assembly), it is comprised of several different parts.
Sliders
The heads themselves are mounted on head sliders. The sliders are suspended over the surface of the disk at the
ends of the head arms. The head arms are all mechanically fused into a single structure that is moved around the
surface of the disk by the actuator and they play an important role in the function and performance of the drive.
In particular, advances in slider, arm and actuator design are critical to improving the seek time of a hard disk.
Each hard disk head is mounted to a special device called a head slider or just slider for short. The function
of the slider is to physically support the head and hold it in the correct position relative to the platter as
the head floats over its surface.
Sliders are given a special shape to allow them to ride precisely over the platter. Usually they are shaped somewhat
like a sled; there are two rails or runners on the outside that support the slider at the correct flying height
over the surface of the disk, and in the middle the read/write head itself is mounted, possibly on another rail.
(Anisotropic) Magnetoresistive (MR/AMR) Head
As hard disk read/write heads have been shrinking in
size, so have the sliders that carry them. The main advantage of using small sliders is that it reduces the weight
that must be yanked around the surface of the platters, improving both positioning speed and accuracy. Smaller
sliders also have less surface area to potentially contact the surface of the disk.
Head Actuator
The actuator is the device used to position the head arms to different tracks on the surface of the platter (actually,
to different cylinders, since all head arms are moved as a synchronous unit, so each arm moves to the same track
number of its respective surface). The actuator is a very important part of the hard disk, because changing from
track to track is the only operation on the hard disk that requires active movement: changing heads is an electronic
function, and changing sectors involves waiting for the right sector number to spin around and come under the head
(passive movement). Changing tracks means the heads must be shifted, and so making sure this movement can be done
quickly and accurately is of paramount importance. This is especially so because physical motion is so slow compared
to anything electronic--typically a factor of 1,000 times slower or more.
View a larger image !
The actuator in a modern hard disk uses a device called
a voice coil to move the head arms in and out over the surface of the platters, and a closed-loop feedback system
called a servo system to dynamically position the heads directly over the data tracks. The voice coil works using
electromagnetic attraction and repulsion.
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