English subtitles for clip: File:Harddrive-engineerguy.ogv
Jump to navigation
Jump to search
1 00:00:04,000 --> 00:00:07,000 Hard drive teardown 2 00:00:07,000 --> 00:00:10,000 flying heads, voice coil motors, amazingly smooth surfaces & signal processing 3 00:00:10,000 --> 00:00:17,000 series 3 engineerguy videos 4 00:00:17,000 --> 00:00:23,000 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. 5 00:00:23,000 --> 00:00:25,000 Let's look inside and see how it stores data. 6 00:00:30,000 --> 00:00:32,000 Look at that: It's marvelous. 7 00:00:32,000 --> 00:00:35,000 It's an ordinary hard drive, but its details, of course, are extraordinary. 8 00:00:35,000 --> 00:00:38,000 Now, I'm sure you know the essence of a hard drive: 9 00:00:38,000 --> 00:00:41,000 We store data on it in binary form - ones and zeros. 10 00:00:41,000 --> 00:00:43,000 Now, this arm supports a "head" 11 00:00:43,000 --> 00:00:45,000 which is an electro-magnet that scans over the disk 12 00:00:45,000 --> 00:00:48,000 and either writes data by changing the magnetization of specific sections 13 00:00:48,000 --> 00:00:50,000 on the platter or it just reads the data 14 00:00:50,000 --> 00:00:53,000 by measuring the magnetic polarization. 15 00:00:53,000 --> 00:00:54,000 Now, in principle, pretty simple, 16 00:00:54,000 --> 00:00:58,000 but in practice a lot of hard core engineering. 17 00:00:58,000 --> 00:01:02,000 The key focus lies in being sure that the head can precisely 18 00:01:02,000 --> 00:01:03,000 error free 19 00:01:03,000 --> 00:01:05,000 read and write to the disk. 20 00:01:05,000 --> 00:01:08,000 The first order of business is to move it with great control. 21 00:01:08,000 --> 00:01:11,000 To position the arm engineers use a "voice coil actuator". 22 00:01:11,000 --> 00:01:14,000 The base of the arm sits between two powerful magnets. 23 00:01:14,000 --> 00:01:17,000 They're so strong they're actually kind of hard to pull apart. 24 00:01:17,000 --> 00:01:18,000 There. 25 00:01:18,000 --> 00:01:20,000 The arm moves because of a Lorentz force. 26 00:01:20,000 --> 00:01:23,000 Pass a current through a wire that's in a magnetic field 27 00:01:23,000 --> 00:01:25,000 and the wire experiences a force; 28 00:01:25,000 --> 00:01:28,000 reverse the current and the force also reverses. 29 00:01:28,000 --> 00:01:30,000 As current flows in one direction in the coil the 30 00:01:30,000 --> 00:01:34,000 force created by the permanent magnet makes the arm move this way, 31 00:01:34,000 --> 00:01:36,000 reverse the current and it moves back. 32 00:01:36,000 --> 00:01:39,000 The force on the arm is directly proportional to the current 33 00:01:39,000 --> 00:01:40,000 through the coil which allows the 34 00:01:40,000 --> 00:01:43,000 arm's position to be finely tuned. 35 00:01:43,000 --> 00:01:45,000 Unlike a mechanical system of linkages there 36 00:01:45,000 --> 00:01:49,000 is minimal wear and it isn't sensitive to temperature. 37 00:01:49,000 --> 00:01:53,000 At the end of the arm lies the most critical component: The head. 38 00:01:53,000 --> 00:01:57,000 At its simplest it's a piece of ferromagnetic material wrapped with wire. 39 00:01:57,000 --> 00:01:59,000 As it passes over the magnetized sections of the platter 40 00:01:59,000 --> 00:02:02,000 it measures changes in the direction of the magnetic poles. 41 00:02:02,000 --> 00:02:06,000 Recall Faraday's Law: A change in magnetization 42 00:02:06,000 --> 00:02:08,000 produces a voltage in a nearby coil. 43 00:02:08,000 --> 00:02:10,000 So, as the head passes a section where the polarity 44 00:02:10,000 --> 00:02:14,000 has changed it records a voltage spike. 45 00:02:14,000 --> 00:02:16,000 The spikes - both negative and positive - represent a "one" 46 00:02:16,000 --> 00:02:19,000 and where there is no voltage spike corresponds to a "zero. 47 00:02:19,000 --> 00:02:22,000 The head gets astonshingly close to the disk surface 48 00:02:22,000 --> 00:02:25,000 100 nanometers in older drives, but today under 49 00:02:25,000 --> 00:02:27,000 ten nanometers in the newest ones. 50 00:02:27,000 --> 00:02:30,000 As the head gets closer to the disk its magnetic field 51 00:02:30,000 --> 00:02:32,000 covers less area allowing for more sectors 52 00:02:32,000 --> 00:02:35,000 of information to be packed onto the disk's surface. 53 00:02:35,000 --> 00:02:38,000 To keep that critical height engineers use an ingenious method: 54 00:02:38,000 --> 00:02:41,000 They "float" the head over the disk. 55 00:02:41,000 --> 00:02:44,000 You see, as the disk spins it forms a boundary layer of air that 56 00:02:44,000 --> 00:02:48,000 gets dragged past the stationary head at 80 miles per hour at the outer edge. 57 00:02:48,000 --> 00:02:52,000 The head rides on a "slider" aerodynamically designed to float above the platter. 58 00:02:52,000 --> 00:02:56,000 The genius of this air-bearing technology is its self-induced adjustment: 59 00:02:56,000 --> 00:03:01,000 If any disturbance causes the slider to rise too high it "floats" back to the where it should be. 60 00:03:01,000 --> 00:03:04,000 Now, because the head is so close to the disk surface 61 00:03:04,000 --> 00:03:07,000 any stray particles could damage the disk resulting in data loss. 62 00:03:07,000 --> 00:03:11,000 So, engineers place this recirculating filter in the air flow; 63 00:03:11,000 --> 00:03:14,000 it removes small particles scraped off the platter. 64 00:03:14,000 --> 00:03:18,000 To keep the head flying at the right height the platter is made incredibly smooth: 65 00:03:18,000 --> 00:03:23,000 Typically this platter is so smooth that it has a surface roughness of about one nanometer. 66 00:03:23,000 --> 00:03:26,000 To give you an idea of how smooth that is: let's imagine that this section is enlarged 67 00:03:26,000 --> 00:03:31,000 until it's as long as a football field - American or International - 68 00:03:31,000 --> 00:03:35,000 the average "bump" on the surface would be about three hundredths of an inch. 69 00:03:35,000 --> 00:03:38,000 The key element of the platter is the magnetic layer, 70 00:03:38,000 --> 00:03:41,000 which is cobalt - with perhaps platinum and nickel mixed in. 71 00:03:41,000 --> 00:03:43,000 Now this mixture of metals has high coercivity, 72 00:03:43,000 --> 00:03:50,000 which means that it will maintain that magnetization - and thus data - until it is exposed to another powerful magnetic field. 73 00:03:50,000 --> 00:03:52,000 One last thing that I find enormously clever: 74 00:03:52,000 --> 00:03:57,000 Using a bit of math to squeeze up to forty percent more information on the disk. 75 00:03:57,000 --> 00:04:04,000 Consider this sequence of magnetic poles on the disk's surface - 0-1-0-1-1-1. 76 00:04:04,000 --> 00:04:06,000 A scan by the head would reveal these distinct voltage spikes - 77 00:04:06,000 --> 00:04:09,000 both positive or negative for the "ones". 78 00:04:09,000 --> 00:04:13,000 We would be easily able to distinguish it from, say, this similar sequence. 79 00:04:13,000 --> 00:04:16,000 If we compare them they clearly differ. 80 00:04:16,000 --> 00:04:20,000 Engineers, though, always work to get more and more data onto a hard drive. 81 00:04:20,000 --> 00:04:22,000 One way to do this is to shrink the magnetic domains, 82 00:04:22,000 --> 00:04:25,000 but look what happens to the voltage spikes when we do this. 83 00:04:25,000 --> 00:04:28,000 For each sequence the spikes of the ones now overlap and 84 00:04:28,000 --> 00:04:30,000 superimpose giving "fuzzy" signals. 85 00:04:30,000 --> 00:04:33,000 In fact, the two sequences now look very similar. 86 00:04:33,000 --> 00:04:37,000 Using a technique called Partial Response Maximum Likelihood engineers have developed 87 00:04:37,000 --> 00:04:40,000 sophisticated codes that can take a murky signal like this, 88 00:04:40,000 --> 00:04:45,000 generate the possible sequences that could make it up and then choose the most probable. 89 00:04:45,000 --> 00:04:49,000 As with any successful technology, these hard drives remain unnoticed in our daily lives, 90 00:04:49,000 --> 00:04:51,000 unless something goes wrong. 91 00:04:51,000 --> 00:04:53,000 I'm Bill Hammack, the engineer guy.