# How does a cylinder coil motor work?

I'm trying to study the difference between a simple solenoid coil and one that will change position with voltage. It looks like they work in the same way of a speaker.

I've seen expensive professional cylinder linear actuators, but I don't have one to take apart to see whats inside.

What I want to make is a 6" long rod, that I can control the length/position of - magnetically, instead of using a ballscrew or belt drive type motor (or anything else that makes physical mechanical noise). I'm trying to raise and lower a rod connected to a small trombone style home made musical instrument.

I was hoping I can just wind a long coil and apply a variable voltage or current to it to move the position of the rod. I know hard-drive actuators for the heads, work in this same way - they are very fast and accurate, and make pretty much zero noise. I also seen how mag-lev linear motors work, which are a very complicated series of coil windings placed in a precise array - this is way too complex for my skills I do not want to do that. I just want to know more about a 1-coil winding.

So I was hoping if someone here had some knowledge more then I've done so far, for some advice or links on making my own simple positioning coil linear motor.

Hi,
The type of actuator you are thinking off, is for only very short strokes, like 10 to 15mm, in valve assemblies.

What is your application and how much stroke are you looking for?

Tom…

The force induced by a DC coil is a linear function of the ampere-turns. So if you want a linear motion of a rod you need a means of converting AT (force) to AT (distance). This can be achieved by the compression of a long coil spring as such springs have a linear relationship between force and movement, usually described as N/M or some suitable subdivision (N/mm) (or lbs/inch seeing as how you are using imperial units)

However, this will also result in an undamped oscillation so you will also require some form of damper. Also the AT required to move a 6" rod against a reaction spring over that distance may be surprisingly high.

I feel you really would be far better going for a rack-and-pinion drive system which will give you good speed of response and adequate force for minimal input power.

Or, alternatively, a small hydraulic double acting cylinder which is driven by a remote power-pack.

google linear motor - there are lots of options..

Allan

TomGeorge: To answer your question about the stroke, I haven't thought that part out. I'm curious to know what the relative distance I can make a stroke based on the length of the copper coil, gauge of the wire and number of windings in the lengthwise direction.

I know the distance the core will move is relative to the length of the coil. For example, this solenoid on my desk measures .95" compressed, and 1.13" expanded, with a copper coil length of .75", if I reversed the polarity it gives me .18" x2 = .36" of stroke distance with the .75" coil.

I don't have a collection of various solenoids to measure, so I do not know the relative distance on the common stroke for usually manufactured solenoid coils.

So if this ONE on my desk was 10x the size, lengthwise say 7.5", would the stroke of it be 1.18" (in one direction) ? I do not know (yet). If it were, it would give me a bi-polar stroke of 3.6", roughly about 1/2 the size in length of the coil.

DocStein99: I'm trying to study the difference between a simple solenoid coil and one that will change position with voltage. It looks like they work in the same way of a speaker.

I've seen expensive professional cylinder linear actuators, but I don't have one to take apart to see whats inside.

What I want to make is a 6" long rod, that I can control the length/position of - magnetically, instead of using a ballscrew or belt drive type motor (or anything else that makes physical mechanical noise). I'm trying to raise and lower a rod connected to a small trombone style home made musical instrument.

I was hoping I can just wind a long coil and apply a variable voltage or current to it to move the position of the rod. I know hard-drive actuators for the heads, work in this same way - they are very fast and accurate, and make pretty much zero noise. I also seen how mag-lev linear motors work, which are a very complicated series of coil windings placed in a precise array - this is way too complex for my skills I do not want to do that. I just want to know more about a 1-coil winding.

So I was hoping if someone here had some knowledge more then I've done so far, for some advice or links on making my own simple positioning coil linear motor.

A solenoid actuator will never give you any accuracy (as in playing a particular note based on current input).

While it is true that hard drives use this very method to position the heads, realize that the "voice coil actuator" (which is what it's called in a HDD) operates in a closed loop. The heads read servo information from the platters and use that information to accurately position the heads on the "loudest signal".

To make a solenoid work, you would need some sort of feedback to tell your servo loop the position of the armature and adjust the coil current accordingly. The feedback could be a potentiometer or an LVDT, but in any case it would be much more complex (mechanically and software wise) than a simple motorized lead screw.

The biggest problem you would run into is making the servo loop accurate, fast and properly damped. And since the force applied to the solenoid armature versus coil current varies with the POSITION of the armature, the servo loop would become an absolute nightmare.

Just use a linear actuator... maybe one driven by a stepper motor would be even better.

Hope this helps.

Krupski: Thanks for the reply information. I was going to use a hall effect sensor, reed switches, current sense feedback resistors, scale encoder, or experiment with whatever to gauge the distance of the movement.

I was trying to focus a small device that I could move that had long enough stroke, before I figure out how to position it as the next phase of the project. I am guessing the reason why they don't make 12" long skinny solenoids is because the shape of a magnetic field must somehow be limited to a spherical or torrid geometry where the power of the field extends in a relative direction to it's length or width.

DocStein99: Krupski: Thanks for the reply information. I was going to use a hall effect sensor, reed switches, current sense feedback resistors, scale encoder, or experiment with whatever to gauge the distance of the movement.

I was trying to focus a small device that I could move that had long enough stroke, before I figure out how to position it as the next phase of the project. I am guessing the reason why they don't make 12" long skinny solenoids is because the shape of a magnetic field must somehow be limited to a spherical or torrid geometry where the power of the field extends in a relative direction to it's length or width.

Maybe a small stepper motor with a simple "crankshaft" and "connecting rod" would do what you want.

The total stroke would be approximately the diameter of the "crankshaft".

Only problem would be that motion would not be linear with regard to the position of the crankshaft.

It's action would be sinusoidal... the motion would be greatest at the center of rotation and least at the ends.

A simple lookup table or polynomial could linearize the device....

Good luck.

I found a video which better demonstrates what I am looking to do. I will just need to build 4 coils instead of one.

Linear solenoid motor at most basic level

DocStein99:
I’m trying to study the difference between a simple solenoid coil and one that will change position with voltage. It looks like they work in the same way of a speaker.

Back to the original question. A solenoid is a coil and a soft iron yoke, the current magnetizes the yoke
causing attraction of other magnetically soft materials. The current can be DC in either direction or AC,
the magnetic field is always attractive, whatever the current direction.

A voice-coil motor has a strong permanent magnet (hard magnetic material) causing a permanent
magnetic field that the coil’s turn interact with - the direction of current controls the direction of the
force.

Soft magnetic materials respond to external “magnetomotive force” or MMF, becoming magnetized,
but only temporarily. Hard magnetic materials can be permanently magnetized, or put another
way they have very strong magnetic hysteresis.

MarkT: Thanks for the information, this is actually very helpful to what I am trying to find or build.

I prefer to go the voice-coil motor route - that is my original idea. This device has the least amount of parts and complexity. I have found any research yet to show me how I can make (or someone else who has) a voice coil with a stroke outside of 1". Some info on people hacking hard drive coils, and speaker woofers, and commercial grade devices which also have small stroke.

A linear tube motor so far is the second simplest design.

If I were to make a linear tube motor, I think I need to fill a stainless, aluminum, or iron tube with neo magnets, spaced apart and / or facing opposite -N/S- poles. After I make a magnetic rod, then just 4 copper coils around 4 plastic spindles. But I have to research coil size, spacing before I make an attempt to fabricate something - still need more information. I found some video demonstrations, but their specific designs are not revealed. Commercial manufacturer's do well at demonstrating what can be done with this technology and also the specifics of the design are a mystery to me.

DocStein99: MarkT: Thanks for the information, this is actually very helpful to what I am trying to find or build.

I prefer to go the voice-coil motor route - that is my original idea. This device has the least amount of parts and complexity. I have found any research yet to show me how I can make (or someone else who has) a voice coil with a stroke outside of 1". Some info on people hacking hard drive coils, and speaker woofers, and commercial grade devices which also have small stroke.

Large industrial voice-coil motors will not be cheap, note, which is why home-brew ones are popular - you still need the concentric pole-pieces and the large magnet - speaker drivers are clearly one possible source, but they are often wide and flat, not narrow and long,

I will try to make something on a small scale to for demonstration, and go from there. I'm already designing the fixture to make a coil winder with a couple of steppers, some arbor rods, bearings. I'm lost on the concentric poles.

DocStein99: I'm already designing the fixture to make a coil winder with a couple of steppers, some arbor rods, bearings.

It is worth putting a search up on ebay. Coil winders occasionally appear, often not complete though. Hope you have a lathe.

A bit of information that may or may not be of help. The voice-coil actuator in a hard disk actually operates a lever with the magnetic read head on the longer part of the lever. The length of the arms may be as much as 4 or 5 to one, perhaps more. This is how the read head is able to move quite a distance.

After I posted this, I recall there was a construction article in an old issue of "Home Shop Machinist" magazine for a multi-cylinder electric motor like you are considering. If you have access to those magazine, or go to their web site, you may be able to access an archived issue. I have no clue as to the date.

Paul

Faulhaber make some linear motors with some scary-fast response speeds. They've kind of unrolled a brushless DC motor. The control electronics inside will require some serious mathematics to understand. But we mortals can use them with simple serial commands.

A single coil or single magnet solution are just simply not going to get any useful force on a long stroke like 12". Use a linear actuator - there are a lot available for reasonable prices. (I'm guessing that the Faulhaber ones aren't reasonable for most people.)

Yes, I have access to a lathe, milling, grinding, etc. Is anyone interested in any machining ? They can use some work down there.

Anyway. Thanks for the tip on the article, I will look. It's not easy finding information how to build a linear motor. The simplest one I can find so far, is lining up a bunch of neo's in a metal tube, as the shaft. Create a plastic tube, with 4 separate coils the same size width as magnets. Probably impossible to do if you do not have a lathe.

I can buy a coil winder, someone somewhere sells a kit to assemble the parts. I should be able to bang that out in a day at the shop. It's just 2 steppers, a couple of brackets, shafts and couplers to hold the 2 bobbins. I will CNC the steppers to wind. It will take quite longer for me to learn the basics math on designing a coil, for the strength, size of magnet and what material to use for the tube shaft. There are no basic-beginner's tutorials on how to design a linear motor. I probably need to start searching paperback books.

I have seen the linear rails for motors on ebay cost start at \$75+ with NO carriage. Linear motors are not cheap. The videos I have watched are very impressive, the speed and torque is space age.

I just found a good page for explaining how to build and design one, with the software to help me calculate it. Unfortunately, the company is probably expecting me to take a mortgage out to make payments on the software used to run their calculator script.

Tubular linear motor design

MorganS: I am interested in learning how to build one for myself. I can't justify the cost of a commercial linear motor for what I am trying to do with it (yet).

The only equations you need are the standard motor equations:

F = BLI L = length of wire, B = magnetic flux density, I = current V = BLv v = velocity

L = pi D N (D = diameter of coil, N = number of turns)

The flux, B at the magnet poles is probably best measured, though in theory it can be calculated from the magnetic circuit geometry and magnet size/type.

I'll browse the hardware store tomorrow to get a 12" stack of disc magnets, hopefully I can get neo's. It looks like I am to put iron spacers between each magnet facing opposite poles. I need to design the coils from the size of these magnets I can get.

Do you think the gauge of the wire matters? I guess I'm limited by current to the gauge of wire and length. And I dont think they sell magnet wire at the hardware store, so I may end up scavenging for some microwaves, tv sets or a scrap pile.