RK06 alignment pack

[Paul Koning]

> On Jun 17, 2015, at 1:50 PM, Johnny Billquist <bqt at Update.UU.SE> wrote:
> 
> On 2015-06-17 19:40, tony duell wrote:
>> [Writing alignment disks]
>>> As far as I know, in special machines mounted on slabs on stone
>>> weighting tons, standing on dampeners, so that you had absolutely
>>> vibration free environment, and then a very precisely controlled head
>>> control system that could write the tracks at the exact place they
>> 
>> I have an idea that some of these units used an optical interferometer to
>> determine the head position
> 
> Quite possible. But it also requires the movement control being different from a standard drive, in order to drive at the precision, as well as the feedback from the inferometer.

Interferometer would make sense, at least for drives of that era.  I think modern drives have track spacings small enough that a visible light interferometer may not be sufficient any longer.

> ...
>> Incidentally, I once saw a procedure (maybe HP) for rewriting the servo surface of
>> a fixed/removeable drive in the field. It used special electronics, but not any special
>> mechanics. It went like this :
> 
> [...]
> Well, a drive like the RK05 can also be reformatted in the field. So it all depends on the drive…

True, but an RK05 doesn’t have servo data on the platter; positioning is done by reference to an optical widget in the drive.  So it depends on mechanical reproducibility being significantly better than the track spacing.  Higher density drives use on-pack servo to avoid that constraint.  And embedded servo avoids an additional constraint: accurate positioning of one head relative to another.

	paul