Com és un disc dur

00:00:04Hard drive teardown
00:00:07flying heads, voice coil motors, amazingly smooth surfaces & signal processing
00:00:10series 3 engineerguy videos
00:00:17A home computer is a powerful tool, but it must store data reliably to work well, otherwise it's kind of pointless, isn't it.
00:00:23Let's look inside and see how it stores data.
00:00:30Look at that: It's marvelous.
00:00:32It's an ordinary hard drive, but its details, of course, are extraordinary.
00:00:35Now, I'm sure you know the essence of a hard drive:
00:00:38We store data on it in binary form - ones and zeros.
00:00:41Now, this arm supports a "head"
00:00:43which is an electro-magnet that scans over the disk
00:00:45and either writes data by changing the magnetization of specific sections
00:00:48on the platter or it just reads the data
00:00:50by measuring the magnetic polarization.
00:00:53Now, in principle, pretty simple,
00:00:54but in practice a lot of hard core engineering.
00:00:58The key focus lies in being sure that the head can precisely
00:01:02error free
00:01:03read and write to the disk.
00:01:05The first order of business is to move it with great control.
00:01:08To position the arm engineers use a "voice coil actuator".
00:01:11The base of the arm sits between two powerful magnets.
00:01:14They're so strong they're actually kind of hard to pull apart.
00:01:18The arm moves because of a Lorentz force.
00:01:20Pass a current through a wire that's in a magnetic field
00:01:23and the wire experiences a force;
00:01:25reverse the current and the force also reverses.
00:01:28As current flows in one direction in the coil the
00:01:30force created by the permanent magnet makes the arm move this way,
00:01:34reverse the current and it moves back.
00:01:36The force on the arm is directly proportional to the current
00:01:39through the coil which allows the
00:01:40arm's position to be finely tuned.
00:01:43Unlike a mechanical system of linkages there
00:01:45is minimal wear and it isn't sensitive to temperature.
00:01:49At the end of the arm lies the most critical component: The head.
00:01:53At its simplest it's a piece of ferromagnetic material wrapped with wire.
00:01:57As it passes over the magnetized sections of the platter
00:01:59it measures changes in the direction of the magnetic poles.
00:02:02Recall Faraday's Law: A change in magnetization
00:02:06produces a voltage in a nearby coil.
00:02:08So, as the head passes a section where the polarity
00:02:10has changed it records a voltage spike.
00:02:14The spikes - both negative and positive - represent a "one"
00:02:16and where there is no voltage spike corresponds to a "zero.
00:02:19The head gets astonshingly close to the disk surface
00:02:22100 nanometers in older drives, but today under
00:02:25ten nanometers in the newest ones.
00:02:27As the head gets closer to the disk its magnetic field
00:02:30covers less area allowing for more sectors
00:02:32of information to be packed onto the disk's surface.
00:02:35To keep that critical height engineers use an ingenious method:
00:02:38They "float" the head over the disk.
00:02:41You see, as the disk spins it forms a boundary layer of air that
00:02:44gets dragged past the stationary head at 80 miles per hour at the outer edge.
00:02:48The head rides on a "slider" aerodynamically designed to float above the platter.
00:02:52The genius of this air-bearing technology is its self-induced adjustment:
00:02:56If any disturbance causes the slider to rise too high it "floats" back to the where it should be.
00:03:01Now, because the head is so close to the disk surface
00:03:04any stray particles could damage the disk resulting in data loss.
00:03:07So, engineers place this recirculating filter in the air flow;
00:03:11it removes small particles scraped off the platter.
00:03:14To keep the head flying at the right height the platter is made incredibly smooth:
00:03:18Typically this platter is so smooth that it has a surface roughness of about one nanometer.
00:03:23To give you an idea of how smooth that is: let's imagine that this section is enlarged
00:03:26until it's as long as a football field - American or International -
00:03:31the average "bump" on the surface would be about three hundredths of an inch.
00:03:35The key element of the platter is the magnetic layer,
00:03:38which is cobalt - with perhaps platinum and nickel mixed in.
00:03:41Now this mixture of metals has high coercivity,
00:03:43which means that it will maintain that magnetization - and thus data - until it is exposed to another powerful magnetic field.
00:03:50One last thing that I find enormously clever:
00:03:52Using a bit of math to squeeze up to forty percent more information on the disk.
00:03:57Consider this sequence of magnetic poles on the disk's surface - 0-1-0-1-1-1.
00:04:04A scan by the head would reveal these distinct voltage spikes -
00:04:06both positive or negative for the "ones".
00:04:09We would be easily able to distinguish it from, say, this similar sequence.
00:04:13If we compare them they clearly differ.
00:04:16Engineers, though, always work to get more and more data onto a hard drive.
00:04:20One way to do this is to shrink the magnetic domains,
00:04:22but look what happens to the voltage spikes when we do this.
00:04:25For each sequence the spikes of the ones now overlap and
00:04:28superimpose giving "fuzzy" signals.
00:04:30In fact, the two sequences now look very similar.
00:04:33Using a technique called Partial Response Maximum Likelihood engineers have developed
00:04:37sophisticated codes that can take a murky signal like this,
00:04:40generate the possible sequences that could make it up and then choose the most probable.
00:04:45As with any successful technology, these hard drives remain unnoticed in our daily lives,
00:04:49unless something goes wrong.
00:04:51I'm Bill Hammack, the engineer guy.