Editing Hard disk drive (section)

=== Components ===
[[File:Hard disk dismantled.jpg|thumb|left|An HDD with disks and motor hub removed, exposing copper-colored stator coils surrounding a bearing in the center of the spindle motor. The orange stripe along the side of the arm is a thin printed-circuit cable, the spindle bearing is in the center and the actuator is in the upper left.]]
[[File:Circuit board of a Samsung hard disk MP0402H.jpg|thumb|right|Circuit board of a 2.5-inch Samsung hard disk MP0402H]]

A typical HDD has two electric motors: a spindle motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to [[amplifier]] electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 [[g-force|''g'']].

[[File:Kopftraeger WD2500JS-00MHB0.jpg|thumb|Head stack with an actuator coil on the left and read/write heads on the right]]
[[File:HDD read-write head.jpg|thumb|Close-up of a single [[disk read-and-write head|read–write head]], showing the side facing the platter]]

The ''{{visible anchor|actuator|Actuator}}'' is a [[permanent magnet]] and [[moving coil]] motor that swings the heads to the desired position. A metal plate supports a squat [[neodymium magnet|neodymium–iron–boron]] (NIB) high-flux [[magnet]]. Beneath this plate is the moving coil, often referred to as the ''[[voice coil]]'' by analogy to the coil in [[loudspeaker]]s, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives have only one magnet).

The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper [[magnet wire]]. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the center of the actuator bearing) then interact with the [[magnetic field]] of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the [[magnetic field#Force on a charged particle|tangential force]]. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head.

The HDD's electronics controls the movement of the actuator and the rotation of the disk and transfers data to or from a [[disk controller]]. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to [[servomotor|servo]] feedback. These are either complete concentric circles (in the case of dedicated servo technology) or segments interspersed with real data (in the case of embedded servo, otherwise known as sector servo technology). The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil motor to rotate the arm. A more modern servo system also employs milli or micro actuators to more accurately position the read/write heads.<ref>A. Al-Mamun, G. Guo, C. Bi, Hard Disk Drive: Mechatronics and Control, 2006, Taylor & Francis.</ref> The spinning of the disks uses fluid-bearing spindle motors. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.