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

[Eric Smith]

On Feb 27, 2015 12:58 PM, "Chuck Guzis" <cclist at sydex.com> wrote:
>
> On 02/27/2015 11:40 AM, Eric Smith wrote:
>>
>> On Fri, Feb 27, 2015 at 10:05 AM, Chuck Guzis <cclist at sydex.com> wrote:
>>>
>>> On 5.25" Drivetec floppies, the cylinder density is
>>> double that--192 tpi.
>>>
>>> Now consider the hub-clamp mechanism of a 5.25" drive.  I suspect that
with
>>> a bit of care, one can probably get things right most of the time.
>>
>>
>> And the only way Drivetec was able to get that to work was by using an
>> embedded servo for closed-loop positioning.
>
>
> I'm not sure that the "fine" stepper was used other than to bring the
head initially on-track.  That is, I'm not at all sure that it compensated
for any eccentricity.

They wouldn't have needed fullblown embedded servo for that. They went to a
lot of expense to have embedded servo tracking because it was actually
necessary.

96 and 100 tpi was pushing the limits of open-loop positioning with a
crappy clamping system. Had that not been a problem, higher t r ack pitch
drives would have appeared much sooner. The ability to make heads for
narrow tracks was not the limiting factor.

Even ignoring the hub clamping tolerance, while mylar has very good
dimensional stability (and was chosen largely for that reason), it isn't
good enough for 200 tpi open-loop. Even the tempco is a problem, and isn't
sufficiently uniform in all directions. Since it's not good for much more
than 135 tpi, it's fairly critical just at 135 tpi, and it takes only a
minute centering error for tracks to be only partially readable. If you
modified the positioning electronics for microstepping, you *might* be able
to compensate for increased centering error by attempting to read each
track at a range of offsets, getting some sectors at one offset and others
at another, etc. I've done the math only for the case of normal
positioning, and it looks pretty grim; for large sector sizes even the
microstepping probably wouldn't help with much centering error beyond that
already caused by the tolerance of the hub clamping.

Having looked at the necessay geometey, I'll believe that data recovery by
reattaching the hub of a 3.5" floppy is practical only when someone has
succeeded doing it to multiple disks, and then I'll want to know how many
attempts per disk it took, how much time per attempt, amd what (if any)
special apparatus was used.

The angle of hub attachment is a far less critical problem. If a controller
is used that actually cares about the relative postition of the index pulse
when reading data, a one-shot (or digital equivalent) can trivially be used
to introduce a delay in the index pulse to compensate for the angular
position error.