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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.

Hard disk Read/Write HeadThe 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.

Hard Disk Head AssemblyThe 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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