Date: Mon, 08 Sep 1997 16:28:40 +1000 From: Mike Smith <mike@smith.net.au> To: hcremean@vt.edu Cc: chat@freebsd.org Subject: Re: lousy disk perf. under cpu load (was IDE vs SCSI) Message-ID: <199709080628.QAA01795@word.smith.net.au> In-Reply-To: Your message of "Mon, 08 Sep 1997 02:43:25 -0400." <19970908024325.42427@wakky.dyn.ml.org>
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> > > PS: When did the first voice-coil-seek hard drives come out? This is a > > > question that's been bugging me for some time now... > > > > Linear or rotary actuator? The linear VC actuator probably predates > > the use of stepper motors; certainly some of the more impressive > > magnets in my collection started their lives in disk units. > > Makes sense...I had an old (1985) Seagate ST-4026 that used linear seek, and > the magnets in it were HUGE. There's also a Hitachi DK511-8 here, of about > the same vintage, that uses rotary voice-coil. I was talking pre-1980 stuff; you would have to find a real fossil to tell you what they were using before that. 8) > > High-precision stepper motor controls are more complex than the linear > > displacement sensor approach that you use with a linear VC actuator too; > > it's the mechanicals and the space that made steppers cheaper, AFAIK. > > Yep...most stepper-motor drives I've seen used rack-and-pinion and > sector-and-pinion linkages, joined together with bands or gears--about the > same as a floppy drive. How they got those things to track accurately is > beyond me, though. That's what guardbands are for; you use a bias current to pull the stepper slightly off centre until you find one band, then push it the other way to find the other. Center between the two is the middle of the track. 8) > Also, you should see the hoops Seagate jumped through to > make the ST-251 do 820 cylinders AND auto-park...thing actually used a > 5-phase stepper! It also had a dedicated sequencer chip to run the stepper > off the spindle's back-EMF to park the heads. There's even weirder auto-park > systems around; I've seen one that actually pulled the heads away from the > disks. But I digress... :) Lots of disks use the spindle to power the drive in spindown; listen to any of the 5.25 CDC/Imprimis/Seagate disks winding down; as the power finally drops off the spindle brake comes on. I presume they did this to minimise the wear on the brake pad. Head retraction is pretty common too; naturally on removables it's mandatory 8) mike
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