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I just saw this demonstration of a hard drive seeking online.

It made me wonder what type of servo motors are used to move the write heads, and how do they move so fast, yet are accurate enough to position themselves precisely over each magnetic track for each seek.

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    \$\begingroup\$ Voice coils are used. Yes, it's magical. \$\endgroup\$
    – DKNguyen
    Oct 30, 2021 at 5:39
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    \$\begingroup\$ they use a feedback loop for positioning \$\endgroup\$
    – jsotola
    Oct 30, 2021 at 5:42
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    \$\begingroup\$ They are fast because they are voice coil motors. They are accurate because they use feedback from the tracks themselves. Note that a voice coil motor itself cannot be driven to a location like a servo, it can only move out, or move in. It's the feedback that controls where it goes to. \$\endgroup\$
    – Neil_UK
    Oct 30, 2021 at 5:45
  • \$\begingroup\$ If you research as fast as the rotary motor can search, imagine how smart you can become. \$\endgroup\$ Oct 30, 2021 at 5:49
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    \$\begingroup\$ A more interesting question is how they make the heads. (Something I worked on at FEI.) The positioning is less interesting. 1988 discovery by Peter Grünberg and Albert Fert got them a the Nobel prize in 2007 for the gagnetoresistive effect (GMR.) It's a quantum effect at the individual electron level being used by modern drives. \$\endgroup\$
    – jonk
    Oct 30, 2021 at 9:06

2 Answers 2

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They are fast because they have a powerful rare earth magnet to propel voice-coil at one end of the moving head arm assembly around a magnetic fluid sealed bearing tower.

The feedback embedded by a servo written pattern that stays for the life of the drive. It is between every track and between each sector in the form of inner and outer bits gated by the index pulse.

The position error signal (PES) depends on the balance of amplitude of these two bits or "di-bits" when on-track or PES=0.

The entire protocol could fill a chapter and all the technology that makes it fast and reliable, could fill a book after decades of improvements.

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  • \$\begingroup\$ Thank you for your response! One more question, on the track of hard drives, do they have super super precise rotary encoders to ensure that the disk is rotating at a constant angular velocity? \$\endgroup\$ Oct 31, 2021 at 22:20
  • \$\begingroup\$ No they use BLDC like a VFD to control RPM, but that speed is variable on some drives for Zone constant density . Thus digital servo clock control with a Xtal reference but On track is read by the data head selected with servo dibit processing . 1st by detecting track crossings with an acceleration velocity profile then velocity then position feedback \$\endgroup\$ Nov 1, 2021 at 0:55
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Piezoelectric elements are used (in two stages) for the fine positioning after the voice coil has got the head in the general position of the track.

Here's a patent describing that: Piezoelectric assembly for micropositioning a disc drive head, and a "tech brief" from Western Digital: Second-generation Micro Actuator for Better Head-positioning Accuracy.

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