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From: rfarris@serene.CTS.COM (Rick Farris)
Newsgroups: comp.arch,alt.next
Subject: Re: The NeXT Problem
Summary: Multiple heads per surface aren't all that new.
Message-ID: <291@serene.CTS.COM>
Date: 16 Oct 88 22:12:48 GMT
References: <26435@ucbvax.BERKELEY.EDU> <7774@gryphon.CTS.COM> <9287@bigtex.cactus.org> <1988Oct16.054306.1884@utzoo.uucp>
Reply-To: rfarris@serene.cts.com (Rick Farris)
Organization: Serenity Systems,  Del Mar, Ca.
Lines: 29

In article <1988Oct16.054306.1884@utzoo.uucp> (Henry Spencer) writes:
>In article <9287@bigtex.cactus.org> (James Van Artsdalen) writes:
>>As an aside, one interesting idea for linear voice coil drives is to
>>place several heads along the arm, dramatically cutting the maximum
>>seek distance...
>
>Uh, Jim, you may not be aware of this, but the Fujitsu Eagle has two
>heads per surface on its linear actuator.  Not a new idea.

(Darn, I wish I had the balls to say "Uh, Henry,"...)

Actually, mulitiple heads per surface is an old idea.  It used to be quite
common to have "head-per-track" drives.  Of course these were generally
single platter machines.  I believe the reason for shifting to a multiple
platter "head-per-surface" scheme was cost.

Actually, there are still a few applications where head-per-track drives
are still used.  I believe Univac (err, Sperry, errr, Unisys) makes one
for military aircraft.  100 MB as I remember.

The problem with placing multiple heads on a *moving* actuator is that each
head adds mass.  It may not have to move as far, but it's slower getting
started (for a given servo system) and slower stopping.  Any physicists
want to talk about the tradeoffs between moving a large mass a short distance,
and moving a small mass a long distance?

Rick Farris            rfarris@serene.cts.com     voice         (619) 259-6793
POB M                          KCBIW              public access       259-7757
Del Mar CA 92014      ...!uunet!serene!rfarris    serene.uucp         259-3704