|Computer hard disk drive|
|Thursday, 23 November 2006|
A hard disk drive employed as a memory device by a computer comprises active components such as magnetic disks, magnetic heads, a disk driving unit, and a controller. In general, a typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCB). The PCB controls the HDA functions and provides an interface between the disc drive and its host. The HDA includes at least one magnetic disk, a spindle motor for rotating the disk, and a head stack assembly (HSA) that includes a slider with at least one transducer or read/write element for reading and writing data. The head stack assembly is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has an actuator assembly that moves in response to the servo control system, a head gimbal assembly (HGA) that extends from the actuator assembly and biases the slider toward the disk, and a flex cable assembly that provides an electrical interconnect with minimal constraint on movement. Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head has an air bearing surface which cooperates with an air flow generated by the rotation of the disks to create an air bearing pressure between the transducers and the disk surfaces, while the disks are rotating. The air bearing pressure allows the head to fly over a rotating disk at a certain nominal fly height to prevent mechanical wear between the disks and the heads. When the hard disk drive accesses data, the location of the data is first read out, and then the magnetic head is moved to the designated location of the magnetic disc to access the data. When the data is stored into the hard disk drive, an available storage location is found first, and then the magnetic head is moved to the designated location to store the data. Each head is attached to a suspension arm to create an subassembly commonly referred to as a head gimbal assembly (HGA). The HGA's are attached to an actuator arm which has a voice coil motor that can move the heads across the surfaces of the disks. A typical HGA includes a load beam, a gimbal attached to an end of the load beam, and a slider attached to the gimbal. The load beam has a spring function that provides a "gram load" biasing force and a hinge function that permits the slider to follow the surface contour of the spinning disk. The load beam has an actuator end that connects to the actuator arm and a gimbal end that connects to the gimbal that supports the slider and transmits the gram load biasing force to the slider to "load" the slider against the disk. Magnetic heads are generally fabricated utilizing photolithographic, electroplating and thin film deposition techniques to create magnetic shields, magnetic poles and other components on an upper surface of a wafer substrate.
The hard disk drive uses a sector as the smallest unit of data recording: a disk surface is divided concentrically into tracks, which are further divided radially into sectors. The nearer to the circumference of the magnetic disk, the higher the track recording density can be made. Each data track is divided into sectors with intersector gaps therebetween, and each sector is subdivided into a data sector and a servo sector. The data sectors are used for the storage of main data or user information. The servo sectors are used for the storage of control data such as automatic gain control (AGC) data, track addresses and tracking servo patterns for use in position a head. In a hard disk drive, data signals are read from and written to a rotating disk by a head which is moved substantially radially with respect to the disk. The head assembly facilitates reading and writing of information on a surface of a rotating magnetic disk. The interconnect assembly includes a plurality of transmission elements, such as wires or traces, for transmitting data to and from the head assembly. The suspension assembly positions the head assembly at a generally constant distance away from the moving surface of the rotating disk. When the electric power is turned off to end the use of a computer, a driving voltage to a spindle motor also stops so that the speed of the hard disk rotated by the spindle motor gradually decreases. At this point, the magnetic head in the state of flying above the hard disk by the flow of air descends onto the surface of the hard disk. The servo control system generally performs two distinct functions: seek control and track following. The seek control function comprises controllably moving the actuator arm such that the transducer head is moved from an initial position to a target track position. The read/write head is typically a part of or affixed to a larger body that flies over the disk and is typically referred to as a slider. The slider has a lower surface referred to as the air bearing surface. The air bearing surface typically comprises one or more rails which generally generate a positive air pressure. The slider body is attached to a suspension via a head gimbal assembly which biases the slider body towards the disk. Typically, a slider is formed with an aerodynamic pattern of protrusions on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive.
The magnetic disk is rotatably loaded on a spindle motor, and the information is accessed by a reading/writing head mounted on an actuator arm rotated by a voice coil motor. The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). Servo position data read from the disk is processed by the processor, enabling the processor to provide servo current command signals to control the VCM for proper positioning of a transducer head relative to a disk. The stator of a voice coil motor is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When the disk is not rotating, the voice coil motor moves the head away from the disk area. When the disk is rotating, and read/write operations are in progress, the head, which is carried on a VCM arm, is positioned above the data storage surface of the disk. When current is applied to the voice coil, torque is generated, and that makes the actuator arm rotate about the bearing assembly. When the actuator arm rotates, the transducer moves across the surface of the magnetic disc. A spindle motor generally consists of a rotating component, a rotor, having an annular permanent magnet, and a stationary component, a stator, having a stator stack wound with coils, wherein the rotor is equipped with an appropriate rotary bearing to enable rotor's rotation with respect to the stator. Spindle motors used in hard disc drives of computers are typically three-phase motors having three terminals. When a computer is first started, the hard drive spindle motor is typically spun up from a stopped condition by a motor controller circuit using six-state commutation, with peak current limiting and a relatively high motor current.