How Hard Drives Work - Part 2
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The actuator is responsible for moving
which hold the read/write heads precisely with a specific
track and sector
. The actuator is one of the most important parts of the hard drive that determine the speed of the drive and how many bits it can pack on its platters. The actuator must align the read/write head precisely, if not, data read and write errors can occur.
You can picture the actuator movement like a F-22 fighter jet. Now imagine that this jet is moving at an unbelievable mach 5, one inch off the ground and being able to stop on a blade of grass. Development in actuator technology is a constant affair. To be able to seek information faster, the actuator must be able to move increasingly faster while retaining complete accuracy. However, this is not an easy task. The faster everything moves, the more damage can occur if the read/write heads ever actually come in contact with the platters. The
is attached to the ends of the actuator arms.
It looks like the heads are touching the platters. But in actuality, they are "flying" above the platters using a cushion of air created by the spinning platters. This thin layer of air is what prevents the heads from coming in contact with the platters.
the heads are extremely close to the platters. An excessive shock to the drive during operation can mean disastrous outcomes. Also, remember how we mentioned that these components are sealed in a dust-free chassis? If the heads are flying only about 2 microns above the surface of the platter, can you imagine the consequences if any small particle were to come between the head and the platter?
The heads are responsible for writing to the drive and reading from it. They write data by arranging the magnetic particles on the surface of the platters. When arranged in one direction, the particles will produce a northern polarity and when arranged in the other direction, they will produce a southern polarity.
When reading from the platters, the head will detect the polarity of the particles and translate that into electrical signals and send the signals back to the on-board
hard drive controller
Ferrite Heads: Early hard drives used read/write heads that were constructed of a thin copper wire that was wound around a small ferrite core with a tiny cut out of it. This cut determined the size of the data bits that could be
written to disk and read from disk. An electrical signal would be sent through in one direction or the other causing electrical induction. This creates specific patterns of magnetic fields arranging magnetic particles on the platter to either face north or south. When reading from the disk, the head senses which direction a particle is in, translating that into a 0 or 1 bit.
Thin-film Inducted Heads: This type of head was developed by IBM in 1979. The heads used photolithographic techniques from semiconductor manufacturing processes to create the head structures. The magnetic core and the coil were created the same way. This gave a greater precision than the Ferrite design and allowed for more bit read and write accuracy.
Magneto-resistive Heads: These types of heads use separate read and write components for even greater precision than thin-film inducted heads. The read component is made of a thin-film material that changes its resistance when near a magnetic field (being the bits). This produces a stronger signal that enables greater areal density limits and thus allowing more bits to be crammed into a platter.
Giant Magneto-resistive Heads: This technology is based on magneto-resistive heads with different thin-film materials that produced large amounts of magneto-resistance. That is where the term "giant" comes from - the giant amounts of resistance. In 1997, IBM introduced the GMR heads which replaced the MR heads with different elements and increased areal densities by many times. They are used in today's high-end hard drives.
The hard drive controller
is attached on the exterior of the hard drive chassis. It contains DSP (Digital Signal Processors) chips that control dataflow, etc. It also contains cache modules to keep frequently used data close at hand.
The mini PCB inside the hard drive itself is the controller for the actuator.
You may think that placing a magnet near a hard drive will destroy the data inside. This is true. But what you may not know is that there are two very strong magnets inside of the drive. The magnets are located inside the actuator. One is on top of the actuator arms and the other one is underneath. The reason why these magnets do not affect the platters is that the magnetic field on the magnets lie on a vertical axis. If they were on a horizontal axis, the field would stretch onto the platters,
destroying the data
. Another reason is that the magnetic signals recorded on the platters aren't any regular magnetic fields but they are electro-magnetically recorded. Electro-magnets are many times more stronger than natural magnets.