harddrive read-head arm

[Jamie Morken]

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and the
read head has a single coil of wire on it, and there is a NS magnet pair on
each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way it
must work is with an H-bridge driving the coil in locked antiphase, or maybe
the coil in controlled with an opamp? Any ideas on this?

cheers,
Jamie Morken


read head arm:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE006.JPG>

read head arm and two magnets:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE002.JPG>

read head arm between magnets:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE004.JPG>

 

[Ian Stirling]

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and the
read head has a single coil of wire on it, and there is a NS magnet pair on
each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way it
must work is with an H-bridge driving the coil in locked antiphase, or maybe
the coil in controlled with an opamp? Any ideas on this?

The drive reads the signal coming off the head for tracking information.
Back when the largest drives were 60Mb, there were a number of ways.
From one surface in the drive containing track information, to an optical
encoder reading it out, to stepper motors that moved in discrete steps.

 

[Jamie Morken]

The drive reads the signal coming off the head for tracking information.
Back when the largest drives were 60Mb, there were a number of ways.
From one surface in the drive containing track information, to an optical
encoder reading it out, to stepper motors that moved in discrete steps.

I assume for the new larger drives that the current track is read by the
read head magnetically. What I don't understand is what type of signal is
being sent to the coil that moves the read head arm back and forth so
precisely!

cheers,
Jamie Morken

 

[Garrett Mace]

I assume for the new larger drives that the current track is read by the

read head magnetically. What I don't understand is what type of signal is
being sent to the coil that moves the read head arm back and forth so
precisely!

It's just a precisely-controlled current. The force between the coil and
magnet is proportional to the current flowing through the coil. I haven't
taken apart a hard drive any larger than 1 gig, but I believe it used a
multipole magnet with the coil balanced between poles. Supposedly there are
types that balance the magnetic force against a spring, but I don't know if
that would be as fast as the multipole configuration.

Here's a datasheet for an old voice coil driver, it should give you some
more hints on what kind of signal control a hard drive voice coil requires.
http://www.st.com/stonline/books/pdf/docs/4932.pdf

 

[Tim Wescott]

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and the
read head has a single coil of wire on it, and there is a NS magnet pair on
each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way it
must work is with an H-bridge driving the coil in locked antiphase, or maybe
the coil in controlled with an opamp? Any ideas on this?

cheers,
Jamie Morken


read head arm:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE006.JPG>

read head arm and two magnets:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE002.JPG>

read head arm between magnets:
<http://www.rocketresearch.org/new/pics/4gig hardrive read head actu
ator/IMAGE004.JPG>

The position of the head is read off of the "factory formatting" of the
drive; the voice coil is driven in pure current mode so the arm
accelerates proportional to the drive current. Then the whole thing is
wrapped with a PD or a PID controller to bring the arm to the correct
position.

 

[Mike]

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and the
read head has a single coil of wire on it, and there is a NS magnet pair on
each side of the coil of wire.

No, the read head is that little tiny thing at the end of the arm. These
days, the heads are magneto-resistive, and have no coils for reading. The
voice-coil, which I think you're describing, is what moves the heads back
and forth. The arm is part of what used to be the head stack, but these
days most drives are only one or two heads, so I'm not sure if they still
call it a stack.

The servo information used to position the heads is written in between the
data sectors. As the read head moves across the tracks, it can detect the
servo information, which contains information about which track the head is
currently flying over. When it gets to the right track, the servo stops
moving the head across tracks, but at this point the head is still not
positioned precisely. To complete the precise placement, the servo
information is written in a way that part of the information is off-track
in one direction, and part of it is off-track in the other direction. The
servo algorithms have become quite sophisticated, but the general idea is
that if the head is off-track in one direction, one of the off-track servo
bursts will have larger amplitude when it's read than the other one. So, as
the head moves across the servo bursts, the off-track amplitudes are
detected and subtracted from each other to create an error signal. If the
head is positioned precisely in the center of the track, the error signal
will be zero. The error signal is used to drive the voice coil
(electrically, at least) to a high degree of accuracy.

There are mechanical issues as well - probably even larger than the
electrical issues - but I've never been involved with the mechanics, so I
can't say anything knowledgable about that.

-- Mike --

 

[Ian Stirling]

I think if you look closely you'll find that the position control is now a
lot more sophisticated than "mere" PID control. Precision is amazing, and
bandwidth is pretty darn high too (If only I could recall the numbers.....).

Numbers.
My hard drive has 160G, and does 50Mb/s.
Divide by 120 (7200RPM) gives 416K/track.
Assuming that it's got one platter, then that's 832K/cylinder, or about
144000 tracks.

200nm track spacing.
If the track has an average length of 8cm, then that's 100K/cm, or
bit length of 100nm.

Insane.

If you could get a printer that did this resolution, then you could
print perfect holograms.

Speed goes up as the square root of capacity (if the drives are at the same
RPM, and the tracks density increases at the same rate as the bit density)
this leads to being able to read a 60M disk in a couple of minutes to a
160G drive taking the better part of an hour.

 

[Tim Wescott]

Jamie Morken wrote:

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and

the

read head has a single coil of wire on it, and there is a NS magnet pair

on

each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way

it

must work is with an H-bridge driving the coil in locked antiphase, or

maybe

the coil in controlled with an opamp? Any ideas on this?

cheers,
Jamie Morken

[snip]


The position of the head is read off of the "factory formatting" of the
drive; the voice coil is driven in pure current mode so the arm
accelerates proportional to the drive current. Then the whole thing is
wrapped with a PD or a PID controller to bring the arm to the correct
position.

I think if you look closely you'll find that the position control is now a
lot more sophisticated than "mere" PID control. Precision is amazing, and
bandwidth is pretty darn high too (If only I could recall the numbers.....).

Ultimately it is just the "magic" of extremely well designed closed-loop
control (ie negative feedback)

True, but does that help the OP?

It doesn't take much more than a PID loop and good mechanical design to
get the kind of performance that they get with disk drives -- do you
know what additional measures are taken? Do they just add the
obligatory 1st notch filter and a lead-lag, or do they get wild and have
fully adaptive controllers?

 

[Tim Wescott]

No, the read head is that little tiny thing at the end of the arm. These
days, the heads are magneto-resistive, and have no coils for reading. The
voice-coil, which I think you're describing, is what moves the heads back
and forth. The arm is part of what used to be the head stack, but these
days most drives are only one or two heads, so I'm not sure if they still
call it a stack.

The servo information used to position the heads is written in between the
data sectors. As the read head moves across the tracks, it can detect the
servo information, which contains information about which track the head is
currently flying over. When it gets to the right track, the servo stops
moving the head across tracks, but at this point the head is still not
positioned precisely. To complete the precise placement, the servo
information is written in a way that part of the information is off-track
in one direction, and part of it is off-track in the other direction. The
servo algorithms have become quite sophisticated, but the general idea is
that if the head is off-track in one direction, one of the off-track servo
bursts will have larger amplitude when it's read than the other one. So, as
the head moves across the servo bursts, the off-track amplitudes are
detected and subtracted from each other to create an error signal. If the
head is positioned precisely in the center of the track, the error signal
will be zero. The error signal is used to drive the voice coil
(electrically, at least) to a high degree of accuracy.

There are mechanical issues as well - probably even larger than the
electrical issues - but I've never been involved with the mechanics, so I
can't say anything knowledgable about that.

-- Mike --

Hey Mike:

Once they know the position, what sort of algorithms do they use to
locate the arm? I've closed a lot of loops with speaker coils and
torquer motors, I'm curious what one does when one can justify lots of
engineering time with high production numbers.

 

[Mike]

Hey Mike:

Once they know the position, what sort of algorithms do they use to
locate the arm? I've closed a lot of loops with speaker coils and
torquer motors, I'm curious what one does when one can justify lots of
engineering time with high production numbers.

Dunno. Servo is divided into two parts. The first part takes place in the
channel, where the data is read from the disk and the amplitude is
extracted. Simple algorithms were okay until around 1995 or so, when the
track density reached the point that the errors and noise associated with
simple sample and hold peak detectors got to be too large (that's the last
time I worked on a servo detector). The problem with peak detectors is that
noise is biased - one polarity adds to the peak, while the opposite
polarity is ignored. You can filter the noise before it gets to the peak
detector, but sooner or later more sophisticated detectors are required.

The second part is the voice coil driver that moves the heads back and
forth. My recollection is that the actual servo system isn't that
complicated, but I could be mistaken. The complexity comes when the head
has to move rapidly across multiple tracks during a seek (non-linear), then
slow down and finish the acquisition (linear), all in the shortest time.
Additional complexity comes in the head-parking circuits. When the drive
powers down, the back-emf in the motor coils is used to drive the servo
long enough to move the heads onto the landing ramps.

-- Mike --

 

[Mike]

Numbers.
My hard drive has 160G, and does 50Mb/s.
Divide by 120 (7200RPM) gives 416K/track.
Assuming that it's got one platter, then that's 832K/cylinder, or about
144000 tracks.

The data rate on the outer tracks is significantly larger (close to 3X,
IIRC) the data rate on the inner tracks. The limiting factor is transition
spacing - on the outer tracks the speed of the disk is faster, so data can
be written faster for the same lineal transition density.

Here are some specs from a current Seagate Cheetah 15k drive:

Bytes Per Sector 512
Track Density (TPI) 64,000 tracks/inch
Recording Density (BPI, max) 533,000 bits/inch
Bytes/Track (avg) 377,770

They have a press release about a new storage record they set as well:

Areal Density: 101 Gb/in2
Track Density: 149k TPI
Bit Density: 680k BPI
Data Rate: 256 Mb/s
Bit Aspect Ratio: 4.6
On track raw error rate: 5 X 10-5
Raw error rate @ 5% squeeze and 10% OTC: 1 X 10-4

Check out the raw error rate. On average, a modern drive will contain
roughly 1 in 10^4 bits, or about one error per sector. It gets corrected
back to acceptable levels using coding.

-- Mike --

 

[Terry Given]

Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and the
read head has a single coil of wire on it, and there is a NS magnet pair on
each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way it
must work is with an H-bridge driving the coil in locked antiphase, or maybe
the coil in controlled with an opamp? Any ideas on this?

cheers,
Jamie Morken

[snip]


The position of the head is read off of the "factory formatting" of the
drive; the voice coil is driven in pure current mode so the arm
accelerates proportional to the drive current. Then the whole thing is
wrapped with a PD or a PID controller to bring the arm to the correct
position.

I think if you look closely you'll find that the position control is now a
lot more sophisticated than "mere" PID control. Precision is amazing, and
bandwidth is pretty darn high too (If only I could recall the numbers.....).

Ultimately it is just the "magic" of extremely well designed closed-loop
control (ie negative feedback)

 

[Terry Given]

Jamie Morken wrote:


Hi all,

How does a harddrive readhead arm move so precisely to be able to read a
microscopic track of data on a harddrive? I took apart a harddrive and
the

read head has a single coil of wire on it, and there is a NS magnet

pair

on

each side of the coil of wire. The read head arm goes back and forth
between the magnets when the read head coil is energized, the direction
dependant on the polarity of the battery. I was thinking that the way
it

must work is with an H-bridge driving the coil in locked antiphase, or
maybe

the coil in controlled with an opamp? Any ideas on this?

cheers,
Jamie Morken
[snip]


The position of the head is read off of the "factory formatting" of the
drive; the voice coil is driven in pure current mode so the arm
accelerates proportional to the drive current. Then the whole thing is
wrapped with a PD or a PID controller to bring the arm to the correct
position.

I think if you look closely you'll find that the position control is now a
lot more sophisticated than "mere" PID control. Precision is amazing, and
bandwidth is pretty darn high too (If only I could recall the numbers.....).

Ultimately it is just the "magic" of extremely well designed closed-loop
control (ie negative feedback)

True, but does that help the OP?

It doesn't take much more than a PID loop and good mechanical design to
get the kind of performance that they get with disk drives -- do you
know what additional measures are taken? Do they just add the
obligatory 1st notch filter and a lead-lag, or do they get wild and have
fully adaptive controllers?

Alas, I cant remember much about the papers I have read on this subject re.
advanced control. And of course just because people write papers on things
doesnt mean that it gets done that way in practice.

Hopefully someone in the know will elucidate us - Nice work though Ian. The
numbers really are impressive.

Terry