javafx barcode scanner 9: Hard Drive Technologies in Software

Generate QR in Software 9: Hard Drive Technologies

9: Hard Drive Technologies
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The size required by each magnetic flux on a hard drive has reduced considerably over the years, resulting in higher capacities As fluxes become smaller, they begin to interfere with each other in weird ways I have to say weird since to make sense of what s going on at this subatomic level (I told you these fluxes were small!) would require that you take a semester of quantum mechanics! Let s just say that laying fluxes flat against the platter has reached its limit To get around this problem, hard drive makers recently began to make hard drives that store their flux reversals vertically (up and down) rather than longitudinally (forward and backward), enabling them to make hard drives in the 1 terabyte (1024 gigabyte) range Manufacturers call this vertical storage method perpendicular recording (Figure 97) For all this discussion and detail on data encoding, the day-to-day PC technician never deals with encoding Sometimes, however, knowing what you don t need to know helps as much as knowing what you do need to know Fortunately, data encoding is inherent to the hard drive and completely invisible to the system You re never going to have to deal with data encoding, but you ll sure sound smart when talking to other PC techs if you know your RLL from your PRML!
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Moving the Arms
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The read/write heads move across the platter on the ends of actuator arms In the entire history of hard drives, manufacturers have used only two technologies to move the arms: stepper motor and voice coil Hard drives first used stepper motor technology, but today they ve all moved to voice coil Stepper motor technology moved the arm in fixed increments or steps, but the technology had several limitations that doomed it Because the interface between motor and actuator arm required minimal slippage to ensure precise and reproducible movements, the positioning of the arms became less precise over time This physical deterioration caused data transfer errors Additionally, heat deformation wreaked havoc with stepper motor drives Just as valve clearances in automobile engines change with operating temperature, the positioning accuracy changed as the PC operated and various hard drive components got warmer Although very small, these changes caused problems Accessing the data written on a cold hard drive, for example, became difficult after the disk warmed In addition, the read/ write heads could damage the disk surface if not parked (set in a non-data area) when not in use, requiring techs to use special parking programs before transporting a stepper motor drive All hard drives made today employ a linear motor to move the actuator arms The linear motor, more popularly called a voice coil motor, uses a
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Floppy disk drives still use stepper motors
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Figure 97
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Perpendicular versus traditional longitudinal recording
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permanent magnet surrounding a coil on the actuator arm When an electrical current passes, the coil generates a magnetic field that moves the actuator arm The direction of the actuator arm s movement depends on the polarity of the electrical current through the coil Because the voice coil and the actuator arm never touch, no degradation in positional accuracy takes place over time Voice coil drives automatically park the heads when the drive loses power, making the old stepper motor park programs obsolete Lacking the discrete steps of the stepper motor drive, a voice coil drive cannot accurately predict the movement of the heads across the disk To make sure voice coil drives land exactly in the correct area, the drive reserves one side of one platter for navigational purposes This area essentially maps the exact location of the data on the drive The voice coil moves the read/write head to its best guess about the correct position on the hard drive The read/write head then uses this map to fine-tune its true position and make any necessary adjustments Now that you have a basic understanding of how a drive physically stores data, let s turn to how the hard drive organizes that data so we can use that drive
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