3.5" floppy failure mode? Hub not attched to media.

[Eric Smith]

On Sat, Feb 28, 2015 at 11:14 AM, Mike Stein <mhs.stein at gmail.com> wrote:
> Are you saying that it actually stepped in and out
> during a rotation to follow the eccentricity?

Yes, it did fully closed-loop servo positioning, with embedded servo
bursts in all inter-sector gaps. That's why they have not one, but TWO
stepper motors for positioning, with a mechanical adder mechanism.
The coarse positioning stepper doesn't have the resolution needed for
track following.

This can also be seen in their US patent 4,630,145:

    "As the track densities are increased, the misalignment error may
approach or become larger than the track spacing. The noise-to-signal
ratio then becomes very large and the desired signal cannot be read."

    "The second stepper motor 46 is stepped by track following control
circuits operating in a closed loop."

    "It is yet another object of this invention to step the head
towards the track centerline by a predetermined discrete fine
increment equal to the maximum amount of track eccentricity per
sector."

They didn't do this stuff because it was cool and they wanted to make
the drive more expensive. They did it because it was necessary to make
it work reliably at all.