Diapositiva 9 / 10

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00:00:04:00	Hard drive teardown
00:00:07:00	flying heads, voice coil motors, amazingly smooth surfaces & signal processing
00:00:10:00	series 3 engineerguy videos
00:00:17:00	A home computer is a powerful tool, but it must store data reliably to work well, otherwise it's kind of pointless, isn't it.
00:00:23:00	Let's look inside and see how it stores data.
00:00:30:00	Look at that: It's marvelous.
00:00:32:00	It's an ordinary hard drive, but its details, of course, are extraordinary.
00:00:35:00	Now, I'm sure you know the essence of a hard drive:
00:00:38:00	We store data on it in binary form - ones and zeros.
00:00:41:00	Now, this arm supports a "head"
00:00:43:00	which is an electro-magnet that scans over the disk
00:00:45:00	and either writes data by changing the magnetization of specific sections
00:00:48:00	on the platter or it just reads the data
00:00:50:00	by measuring the magnetic polarization.
00:00:53:00	Now, in principle, pretty simple,
00:00:54:00	but in practice a lot of hard core engineering.
00:00:58:00	The key focus lies in being sure that the head can precisely
00:01:02:00	error free
00:01:03:00	read and write to the disk.
00:01:05:00	The first order of business is to move it with great control.
00:01:08:00	To position the arm engineers use a "voice coil actuator".
00:01:11:00	The base of the arm sits between two powerful magnets.
00:01:14:00	They're so strong they're actually kind of hard to pull apart.
00:01:17:00	There.
00:01:18:00	The arm moves because of a Lorentz force.
00:01:20:00	Pass a current through a wire that's in a magnetic field
00:01:23:00	and the wire experiences a force;
00:01:25:00	reverse the current and the force also reverses.
00:01:28:00	As current flows in one direction in the coil the
00:01:30:00	force created by the permanent magnet makes the arm move this way,
00:01:34:00	reverse the current and it moves back.
00:01:36:00	The force on the arm is directly proportional to the current
00:01:39:00	through the coil which allows the
00:01:40:00	arm's position to be finely tuned.
00:01:43:00	Unlike a mechanical system of linkages there
00:01:45:00	is minimal wear and it isn't sensitive to temperature.
00:01:49:00	At the end of the arm lies the most critical component: The head.
00:01:53:00	At its simplest it's a piece of ferromagnetic material wrapped with wire.
00:01:57:00	As it passes over the magnetized sections of the platter
00:01:59:00	it measures changes in the direction of the magnetic poles.
00:02:02:00	Recall Faraday's Law: A change in magnetization
00:02:06:00	produces a voltage in a nearby coil.
00:02:08:00	So, as the head passes a section where the polarity
00:02:10:00	has changed it records a voltage spike.
00:02:14:00	The spikes - both negative and positive - represent a "one"
00:02:16:00	and where there is no voltage spike corresponds to a "zero.
00:02:19:00	The head gets astonshingly close to the disk surface
00:02:22:00	100 nanometers in older drives, but today under
00:02:25:00	ten nanometers in the newest ones.
00:02:27:00	As the head gets closer to the disk its magnetic field
00:02:30:00	covers less area allowing for more sectors
00:02:32:00	of information to be packed onto the disk's surface.
00:02:35:00	To keep that critical height engineers use an ingenious method:
00:02:38:00	They "float" the head over the disk.
00:02:41:00	You see, as the disk spins it forms a boundary layer of air that
00:02:44:00	gets dragged past the stationary head at 80 miles per hour at the outer edge.
00:02:48:00	The head rides on a "slider" aerodynamically designed to float above the platter.
00:02:52:00	The genius of this air-bearing technology is its self-induced adjustment:
00:02:56:00	If any disturbance causes the slider to rise too high it "floats" back to the where it should be.
00:03:01:00	Now, because the head is so close to the disk surface
00:03:04:00	any stray particles could damage the disk resulting in data loss.
00:03:07:00	So, engineers place this recirculating filter in the air flow;
00:03:11:00	it removes small particles scraped off the platter.
00:03:14:00	To keep the head flying at the right height the platter is made incredibly smooth:
00:03:18:00	Typically this platter is so smooth that it has a surface roughness of about one nanometer.
00:03:23:00	To give you an idea of how smooth that is: let's imagine that this section is enlarged
00:03:26:00	until it's as long as a football field - American or International -
00:03:31:00	the average "bump" on the surface would be about three hundredths of an inch.
00:03:35:00	The key element of the platter is the magnetic layer,
00:03:38:00	which is cobalt - with perhaps platinum and nickel mixed in.
00:03:41:00	Now this mixture of metals has high coercivity,
00:03:43:00	which means that it will maintain that magnetization - and thus data - until it is exposed to another powerful magnetic field.
00:03:50:00	One last thing that I find enormously clever:
00:03:52:00	Using a bit of math to squeeze up to forty percent more information on the disk.
00:03:57:00	Consider this sequence of magnetic poles on the disk's surface - 0-1-0-1-1-1.
00:04:04:00	A scan by the head would reveal these distinct voltage spikes -
00:04:06:00	both positive or negative for the "ones".
00:04:09:00	We would be easily able to distinguish it from, say, this similar sequence.
00:04:13:00	If we compare them they clearly differ.
00:04:16:00	Engineers, though, always work to get more and more data onto a hard drive.
00:04:20:00	One way to do this is to shrink the magnetic domains,
00:04:22:00	but look what happens to the voltage spikes when we do this.
00:04:25:00	For each sequence the spikes of the ones now overlap and
00:04:28:00	superimpose giving "fuzzy" signals.
00:04:30:00	In fact, the two sequences now look very similar.
00:04:33:00	Using a technique called Partial Response Maximum Likelihood engineers have developed
00:04:37:00	sophisticated codes that can take a murky signal like this,
00:04:40:00	generate the possible sequences that could make it up and then choose the most probable.
00:04:45:00	As with any successful technology, these hard drives remain unnoticed in our daily lives,
00:04:49:00	unless something goes wrong.
00:04:51:00	I'm Bill Hammack, the engineer guy.