Hard drive head motor question

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
I know that someone who works, or has worked, on hard drives lurks here.

I'm trying to re-purpose a hard drive head motor into a science demo.  It  
appears to have 64 turns of #30 or #31 (or metric equivalent -- 9 turns  
of enameled wire fits into .09", at any rate), with a circumference of  
around 1.4".  That works out to a total length of 90", and a resistance  
of 1 ohm or so.

Yet the measured resistance of the coil is 15 ohms.

Might there be some reason to intentionally add resistance to the coil,  
(buried where I can't see it), or is my math just cracked today?

Also -- some control guys swear by embedding a shorted coil in their  
torquer motors.  Is that common practice in the disk drive world?

--  

Tim Wescott
Wescott Design Services
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
Tim Wescott wrote:

Quoted text here. Click to load it
it is likely aluminum wire, to cut mass for faster seeking.  Seek time is a  
published parameter, and all the drive makers try to make them go faster.

Jon

Re: Hard drive head motor question
On Sun, 08 May 2016 18:01:35 -0500, Jon Elson wrote:

Quoted text here. Click to load it

Hmm.  Doesn't account for all the difference -- but I can see why they  
may do that.

--  

Tim Wescott
Wescott Design Services
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
On Sun, 08 May 2016 18:22:33 -0500, Tim Wescott wrote:

Quoted text here. Click to load it

To expand: Copper has a resistivity of 1.7 * 10^-6 ohm-cm.
Aluminum has a resistivity of 2.7 * 10^-6 ohm-cm.

So you'd expect an aluminum winding to have 50% more resistance, not 10.

--  

Tim Wescott
Wescott Design Services
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
Tim Wescott wrote:


Quoted text here. Click to load it
Well, the wire drawing and winding process may wreck the conductivity.  
Aluminum as cast is really porous and brittle, then they roll it which  
crushes the grains together and makes it more ductile as well as have better  
conductivity.  When pulling it into thin wire, it may go up in resistance.

But, aluminum is not a great conductor when hot.  Possibly they actually  
used something else that has (nearly) constant resistance over temperature,  
like nichrome?  (Sounds really crazy, but you never know what a clever  
engineer will come up with when thinking outside the box.)

Jon

Re: Hard drive head motor question

Quoted text here. Click to load it

Your data more-or-less matches mine (~11 ohms ~4900mm). The random hits
returned by a search also more-or-less matches our data.

My hard drive voice coil actuator contains only 4900mm of wire (no
hidden components). It seems that the resisitivity of the copper in the
voice coil differs from the standard value. FWIW, wikipedia says that a
long thin copper wire has a much larger resistance. [1]

    All copper wires, irrespective of their shape and size, have
    approximately the same resistivity, but a long, thin copper
    wire has a much larger resistance than a thick, short copper
    wire.

Note.

1. https://en.wikipedia.org/wiki/Resistivity

--
Don Kuenz KB7RPU

Re: Hard drive head motor question
On 5/8/2016 10:23 PM, Don Kuenz wrote:
Quoted text here. Click to load it

Don't confuse resistivity with resistance.

Re: Hard drive head motor question
On 5/9/2016 5:42 AM, John S wrote:
Quoted text here. Click to load it

To expand:

R = rho*length/conductor area (R=rho*l/a) where rho is resistivity, l is  
length, and a is the area. So, for a given resistivity, it is patently  
obvious that a long, thin conductor has higher resistance than a short,  
thick one.

The referenced comment is trash.



Re: Hard drive head motor question

Quoted text here. Click to load it

15 Ohms sounds wrong.  Those heads need to move REALLY fast and 12V  
doesn't put much power into 15 Ohms.

15 Ohms might be right for the head.  Maybe it's the wrong wires?

--  
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.

Re: Hard drive head motor question
On Sun, 08 May 2016 22:58:17 -0700, Kevin McMurtrie wrote:

Quoted text here. Click to load it

Those are the wires whose voltage wiggles when I move the coil in the  
magnetic field.

--  
Tim Wescott
Control systems, embedded software and circuit design
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
On 08/05/2016 22:52, Tim Wescott wrote:
Quoted text here. Click to load it

Could the metallic wire be much thinner than you think because the  
enamel thickness is much greater than usual?

piglet

Re: Hard drive head motor question

Quoted text here. Click to load it

Great hypothesis.

Or maybe the wire isn't copper, for some reason.

S.

Re: Hard drive head motor question

Quoted text here. Click to load it

A single strand of voice coil actuator wire from my drive measures 35
AWG. 5X magnification reveals an unknown amount of residual enamel on
the wire. The length of the wire is ~4900 mm with a resistance of ~11
ohm.

The resistance of 35 AWG copper is 1.08 Ohm/m [1]. The theoretical
resistance of 4900 mm of 35 AWG copper is ~5.3 ohms, or about half of
the measured resistance.

A reverse lookup of the measured resistance of ~2.2 Ohm/m yields ~38 AWG
from the table. The possible use of copper-clad aluminum magnet wire
[2] may also explain part the difference.

Note.

1. http://www.daycounter.com/Calculators/AWG.phtml
2. http://preview.tinyurl.com/zujeg4o

--
Don Kuenz KB7RPU

Re: Hard drive head motor question
Is the winding star coupled, so you are in fact measuring two in series?

Cheers  

Klaus  

Re: Hard drive head motor question
Den mandag den 9. maj 2016 kl. 21.50.02 UTC+2 skrev Klaus Kragelund:
Quoted text here. Click to load it

that was my first thought but I think he is talking about the "voice coil" for the head arm, not the three phase spindle motor  

-Lasse

Re: Hard drive head motor question
On Mon, 09 May 2016 12:55:38 -0700, Lasse Langwadt Christensen wrote:

Quoted text here. Click to load it

Indeed I am -- sorry about the unclear title.  I only posted about the  
part that mattered to me, not the part I threw away :).

--  
Tim Wescott
Control systems, embedded software and circuit design
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
On Sun, 08 May 2016 16:52:52 -0500, Tim Wescott

Quoted text here. Click to load it

I don't suppose it's like those transformer winding where
they use a heavier gage for the external leads, with a much
smaller gage just under the paper skin?

Best regards,


Bob Masta
  
              DAQARTA  v9.20
   Data AcQuisition And Real-Time Analysis
              www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
 Frequency Counter, Pitch Track, Pitch-to-MIDI  
FREE 8-channel Signal Generator, DaqMusiq generator    
          Science with your sound card!

Re: Hard drive head motor question
On Mon, 09 May 2016 13:33:20 +0000, Bob Masta wrote:

Quoted text here. Click to load it

Don't think so.  It's an amazing little piece of manufacturing  
engineering, by the by.

--  
Tim Wescott
Control systems, embedded software and circuit design
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
On Sun, 08 May 2016 16:52:52 -0500, Tim Wescott wrote:

Quoted text here. Click to load it

A new coil, using about 20 feet of #34 wire (I calculate about 160 turns,  
but did not count), generates about twice the signal (60mV peak-peak at  
1Hz) with a 4-ohm coil -- and it doesn't fill as much space with wire as  
they used.  So I should be able to improve on their performance.

Whatever was in the drive, they weren't optimizing for conductivity.

--  

Tim Wescott
Wescott Design Services
We've slightly trimmed the long signature. Click to see the full one.
Re: Hard drive head motor question
On 5/9/2016 12:52 PM, Tim Wescott wrote:
Quoted text here. Click to load it

That's not terribly surprising.  The head movements are very fast and  
the currents are in pulses which are likely not resistance limited.

Don Kuenz said his coil wire was larger gauge than estimated by  
resistance and suggested it might be copper clad aluminum.  Given the  
skin effect on fast pulses and the need for low weight, that is not an  
unreasonable suggestion.  Have you tried scraping or cutting the old  
coil wire to see if it was solid copper?

--  

Rick C

Site Timeline