3 ms·

The claim was hundreds of picometers, which is an order of magnitude smaller than a few nanometers. Literally nothing can be perfectly flat, nor perfectly align

The claim was hundreds of picometers, which is an order of magnitude smaller than a few nanometers. Literally nothing can be perfectly flat, nor perfectly aligned.

blattimwind 8y ago
It doesn't need to be. The basic idea is that the fly height is self-regulating; if the head goes away from the platter, its "lift" is reduced, so the springiness of the arm forces it back to the platter. If it moves closer to the platter, lift is increased, so it moves away. Similarly the tracks don't have to be perfectly round or concentric, because the lowest-level head control system in the drive actively tracks the head's current disk track on the surface; head movements aren't a "rotate 11.57821° to track 28139123", but rather "track 28139123 is around here somewhere, lets find it".
- Misdicorl 8y ago
  Yes, I understand how feedback control systems work. The question was whether they really have the fidelity to do it at hundred picometer resolution. I am dubious because this is a pretty incredible feat. This is the length scale at which atom-atom interactions become important. That implies that the crystal lattice structure of both the read/write head and the underlying platter will affect the dynamics of the system!
- loeg 8y ago
  > Similarly the tracks don't have to be perfectly round or concentric They have to be pretty dang close at 7200+ rpm.
  - Serow225 8y ago
    Nope. Since a long time, tracks are virtual (array of raw track IDs as a function of sector ID when the arm is held "constant" and averaged for X thousand RPMs as part of the per-drive firmware Calibration at the factory).
    - loeg 8y ago
      How does the second sentence produce the conclusion "nope?" If the arm is held "constant" and the platter is spinning at 7200rpm, it's going to trace something very close to a perfect circle.
loeg 8y ago
The 3nm figure is from 2011. 0.9nm, or 900 picometers, is maybe not an unreasonable progression from 3nm in 7 years. And — it's perfectly flat relative to everything else involved.

The claim was hundreds of picometers, which is an order of magnitude smaller than a few nanometers. Literally nothing can be perfectly flat, nor perfectly aligned.

blattimwind 8y ago

It doesn't need to be. The basic idea is that the fly height is self-regulating; if the head goes away from the platter, its "lift" is reduced, so the springiness of the arm forces it back to the platter. If it moves closer to the platter, lift is increased, so it moves away. Similarly the tracks don't have to be perfectly round or concentric, because the lowest-level head control system in the drive actively tracks the head's current disk track on the surface; head movements aren't a "rotate 11.57821° to track 28139123", but rather "track 28139123 is around here somewhere, lets find it".

Misdicorl 8y ago
Yes, I understand how feedback control systems work. The question was whether they really have the fidelity to do it at hundred picometer resolution. I am dubious because this is a pretty incredible feat. This is the length scale at which atom-atom interactions become important. That implies that the crystal lattice structure of both the read/write head and the underlying platter will affect the dynamics of the system!
loeg 8y ago
> Similarly the tracks don't have to be perfectly round or concentric They have to be pretty dang close at 7200+ rpm.
- Serow225 8y ago
  Nope. Since a long time, tracks are virtual (array of raw track IDs as a function of sector ID when the arm is held "constant" and averaged for X thousand RPMs as part of the per-drive firmware Calibration at the factory).
  - loeg 8y ago
    How does the second sentence produce the conclusion "nope?" If the arm is held "constant" and the platter is spinning at 7200rpm, it's going to trace something very close to a perfect circle.

loeg 8y ago

The 3nm figure is from 2011. 0.9nm, or 900 picometers, is maybe not an unreasonable progression from 3nm in 7 years. And — it's perfectly flat relative to everything else involved.