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'[EE] Non-precision Linear Actuator?'
2005\08\26@105717 by Harold Hallikainen

face picon face
Anyone have experience with voicecoil actuators? I have an application
where one might be useful. We're looking at a maximum displacement of
about 50mm and a force of 22 Newtons (about 5 pounds). Using a normal
solenoid (non-magnetized slug), it appears we need about 4,700
ampere-turns to do this (100mm coil length, 30mm coil diameter). It SEEMS
that it would be better to use a permanent magnet instead of a
non-magnetized slug. I'd expect the force to be proportional to the
product of the two fields. But, anyone here have experience with these
types of actuators? Any comments?

THANKS!

Harold



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2005\08\26@113042 by Denny Esterline

picon face
> Anyone have experience with voicecoil actuators? I have an application
> where one might be useful. We're looking at a maximum displacement of
> about 50mm and a force of 22 Newtons (about 5 pounds). Using a normal
> solenoid (non-magnetized slug), it appears we need about 4,700
> ampere-turns to do this (100mm coil length, 30mm coil diameter). It SEEMS
> that it would be better to use a permanent magnet instead of a
> non-magnetized slug. I'd expect the force to be proportional to the
> product of the two fields. But, anyone here have experience with these
> types of actuators? Any comments?
>
> THANKS!
>
> Harold
>

A little more info would be nice. Required actuation speed, duty cycle,
system power limitations, actuation frequency, physical size limitations,
etc.

I don't have any experience using perm magnets vs. steel, but have you
considered geartrain/leadscrew options? Also be aware of automotive starter
solenoids, about 1.5 inches dia x 4 inches long, inch or so of stroke and
about 10 lbs force.

-Denny

2005\08\26@113407 by Bob Blick

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Hi Harold,

I've used DC motors for this, swinging < 90 degrees, but none in that
power range.

Cheers,

Bob


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2005\08\26@120152 by Harold Hallikainen

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Thanks! IF we could get the voice coil thing to work, it would make the
product REAL SIMPLE (only one moving part). Alternatives we're considering
involve a lead screw driven by a stepper or permanent magnet DC motor. We
need to do about one cycle per second.

Harold


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> -

2005\08\26@132028 by Stephen R Phillips

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--- Harold Hallikainen <spam_OUTharoldTakeThisOuTspamhallikainen.com> wrote:

{Quote hidden}

I suggest looking at old bass speaker coils and magnets.

I also suggest looking at speaker manufacturers sites.

In english units it sounds like you are wanting to move something 2
inchs that's 1.2" in diameter.  This is not exactly a simple task since
you need a uniform perpendicular magnetic field the entire stroke
length as well as something to keep the coil aligned properly.  You
mentioned 1 second. I assume this is a repeated cycle therefore?  IE in
and out in 1 second or roughly 4IPS (10cmps?) average speed?

Anyhow you need 2 poles Inner (steel software iron) and outer (BIG
permanent magnet(s)). You also need a linear bearing setup, I assume
the side force on this is negligible? You probably can get away with a
resin tube with the coil embeded into it. Which means you can pop nylon
balls in it so it moves along the steel piece (in the center) smoothly.
It still won't be exactly small.  You need to account for aceleration
etc and mostly importantly you need some sort of elastic return
mechanism (spring) as well as a mechanical stop.

Sounds like you have a lot of work to do! I'm not so sure you can use a
stepper to move that fast without getting into a larger mechanism.

A rack and pinion system might work though with a stepper running the
pinion and the rack being the moving element.

Good fortune :D


Stephen R. Phillips was here
Please be advised what was said may be absolutely wrong, and hereby this disclaimer follows.  I reserve the right to be wrong and admit it in front of the entire world.

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2005\08\26@132216 by Harold Hallikainen

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Thanks for the comments! I've posted a little more info in another post.
This is to do about one cycle per second. It will spend most of the time
resting at one end of the travel or the other (and still have to supply
the force, though the force will decay over the wait time). A motor and
leadscrew is also being considered, but a voicecoil would be simpler, if
it works. The existing system (not voice coil or lead screw) power
consumption is about 60W, but I'd like to reduce that, if possible.

THANKS for the comments!

Harold


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> -

2005\08\26@132914 by James Humes

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Have you ever taken apart a putting machine? Their return mechanism sounds a bit like what you're describing, but I have no idea of the force except that its stronger than one would think.  James

2005\08\26@143609 by Harold Hallikainen

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We are looking at having the magnet inside the coil and having the magnet
move (like a loudspeaker with the coil stationary and the magnet moving).
The magnet will slide within a cylinder with the coil wrapped around the
cylinder. The cylinder could be magnetic or not, whichever works better.
We could use a spring for return, or reverse the current through the coil.
We could end up with a motor of some sort on a lead screw, or a rack and
pinion, or something else. The voice coil is just so simple though, it'd
be nice if it worked.

Thanks for the ideas!

Harold


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> -

2005\08\26@162143 by Peter

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On Fri, 26 Aug 2005, Harold Hallikainen wrote:

> Anyone have experience with voicecoil actuators? I have an application
> where one might be useful. We're looking at a maximum displacement of
> about 50mm and a force of 22 Newtons (about 5 pounds). Using a normal
> solenoid (non-magnetized slug), it appears we need about 4,700
> ampere-turns to do this (100mm coil length, 30mm coil diameter). It SEEMS
> that it would be better to use a permanent magnet instead of a
> non-magnetized slug. I'd expect the force to be proportional to the
> product of the two fields. But, anyone here have experience with these
> types of actuators? Any comments?

I have limited experience with voice coil actuators made from sub-bass
drivers (floor shakers). I think that 5 pounds static is going to be
tough. These devices overheat very quickly if they do not move. In
normal operation they are ac coupled and ventilate themselves. I had 10
mm stroke with ~500 gram load with 5A into 3Ohms. The driver would
overheat in maximum 10 minutes.

Peter

2005\08\26@163828 by Harold Hallikainen

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Thanks for the comment! Maybe it's back to a leadscrew...

Harold

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2005\08\26@170252 by PicDude

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face
Look at automotive door-lock actuators, or automotive linear actuators which
are designed to open doors (not just unlock them).  Then there are
cruise-control actuators both vacuum and electric, but I'm not sure if they
would put out the force you need.  Also, I've seen steppers with no shaft,
but instead had a threaded hole through them.  Seems like that would be
easier.

Cheers,
-Neil.


On Friday 26 August 2005 11:01 am, Harold Hallikainen scribbled:
> Thanks! IF we could get the voice coil thing to work, it would make the
> product REAL SIMPLE (only one moving part). Alternatives we're considering
> involve a lead screw driven by a stepper or permanent magnet DC motor. We
> need to do about one cycle per second.
>
> Harold


2005\08\26@170818 by Russell McMahon

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flavicon
face
The exercise industry use small geared motors with a position pot on
them. Essentially they are a slow fractional turn servo. These are
used to alter the load in eg exercycles, rowers etc. Cost is probably
"low" for what you get due to the volume produced.

These drive a small cable drum of about 20mm diameter. Operation is
very slow (about 30 seconds/revolution) and operating force at the
diameter is 5kg+. I don't know if this meets your specs - speed of
operation, frequency and duration of operation would be key factors.


       RM



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2005\08\26@171808 by Harold Hallikainen

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Thanks!

Harold

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> -

2005\08\26@173427 by Peter

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On Fri, 26 Aug 2005, Harold Hallikainen wrote:

> Thanks! IF we could get the voice coil thing to work, it would make the
> product REAL SIMPLE (only one moving part). Alternatives we're considering
> involve a lead screw driven by a stepper or permanent magnet DC motor. We
> need to do about one cycle per second.

22 lbs at 50 mm would be a ~700 oz-in output gear with a crank 50 mm
long, no ? That's not so hard even without a lead screw (straight gears
and counter spring). Some strong windlass model servos could be used
directly (pricey). A suitable motor+gearbox from China (no name) with 6
mm beveled output axle and 1:100 ratio that I saw in a shop cost about
$18. This was an all-metal servo and gear unit. A magnet that holds
22lbs midway through a 50mm stroke will cost 5 times as much imho.

The power for your throw is only 2.5W (5kg*0.05m/1sec) but you will
likely need 10 times as much for reasonable acceleration and
deceleration within 1 second even without gear losses. The motor I saw
was about 20W I think.

Then there is the question of how long this should last. I made a
thermostat recently that used a relay to switch the heater. I had to fix
the algorythm such that the relay would not cycle more than once every 5
minutes to stay below 100k actuations/year (1 year is 31536000 seconds
and a relay is often rated at 100 k to 1 million cycles). Your gear will
have problems before too long if it moves once a second.

Peter

2005\08\26@183555 by Stephen R Phillips

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--- Harold Hallikainen <haroldspamKILLspamhallikainen.com> wrote:

{Quote hidden}

heat will be your killer problem. Also it requires a uniform magnetic
field to operate correctly.  The type of magnets you need are
DANGEROUS, just telling you now before you actually hurt yourself, they
can break bones etc. they are so strong. They require VERY careful
handling precautions.  You should think very carefully about assembly
before you even touch the magnets in sumamry.  If you just want end to
end movement you can put in stops and sensors for end of travel
information. Then banging it from one end to the other is no big deal.

If you wish the magnet to move, that complicates things a lot.  As I
mentioned before you need a uniform magnetic field. This is a complete
magnetic circuit so you have a core magnet and steal cup shaped pole
piece extending around the coil facing the other side of the coil.
Since you only seem to need 5lbs thrust force and not continuously you
can set it to have a thrust current and holding current.  A current
limited driver in this case will work just fine. Peek current would be
in movement blah blah. I'm sure you understand all that :D

You do need something for the actuator to roll against to keep it
straight unless you already have a linear bearing system in mind.
Since you do not require a real voice coil action you do not need a
spring to return it (as the magnetic field should be adequate to move
it and you don't need seem to need galvonometer type movement). You
might be able to use hall effect sensors to find the end of travel
(that makes it fast and simple). You should have cushioned stops (IE
elastic such as rubber and the like) for end of travel, shock is VERY
bad for any kind of magnet (and rare earth ones tend to break easily).

Here is a link to assorted high power rare earth magnets.
engconcepts.net/List_Of_Disc_Magnets.asp
engconcepts.net/List_Of_Cylinder_Magnets.asp
engconcepts.net/List_Of_Ring_Magnets.asp
http://engconcepts.net/List_Of_Motor_Magnets.asp

I repeat these are very dangerous, as you CAN get seriously hurt if you
aren't careful this is why you see them with those 'handy' plastic
spacers on the web site.

It's preferable if you plan on cycling this thing a lot too use N40
material as it can handle higher temperatures. Cooling should be
considered a very important aspect of such a system. Hehehehe.  Steel
is a lousy thermal conductor, copper is better than that of aluminium.

You will not need 4700 AMP turns, you can compute how much current you
need based on the magnetic field strength of the magnets.  Since the
magnetic field needs to be between the magnetic core and the steel pole
(cup shaped) outside (IE it's field needs to be perpendicular to the
coil's magnetic lines of force) you should read through all the magnet
information on the site and take that into consideration.
The magnets listed for use in motors may be the best suited.

Stephen

Stephen R. Phillips was here
Please be advised what was said may be absolutely wrong, and hereby this disclaimer follows.  I reserve the right to be wrong and admit it in front of the entire world.

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2005\08\26@190831 by Harold Hallikainen

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Thanks for the comments! I've passed them on to the mechanical engineer.

Harold

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2005\08\31@121032 by Alan B. Pearce

face picon face
> Anyone have experience with voicecoil actuators? I have an application
> where one might be useful. We're looking at a maximum displacement of
> about 50mm and a force of 22 Newtons (about 5 pounds). Using a normal
> solenoid (non-magnetized slug), it appears we need about 4,700
> ampere-turns to do this (100mm coil length, 30mm coil diameter). It SEEMS
> that it would be better to use a permanent magnet instead of a
> non-magnetized slug. I'd expect the force to be proportional to the
> product of the two fields. But, anyone here have experience with these
> types of actuators? Any comments?

Sounds like you want a voice coil out of an old 14" pack disk drive. If you
do get one, always keep body parts well clear, as even the old Dynex 3+3 or
6+6MB ones could go with an almighty wack when the drive transistors went.
If you get one out of something like a CDC 300MB pack drive, then you soon
work out that long handled tools are a good idea.

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