[Robotgroup] electromagnets

Sat Oct 14 09:29:06 PDT 2006

I can share a little bit of knowledge about electromagnets. I got it from books mostly, and I reckon the robot group is looking for practical experience.

There are four things I can think of that affect how strong your electromagnet will be. In no particular order these are the shape, the number of coils, the core material, and the amount of current you have running through the thing.

A decent college physics book should tell you more then you ever wanted to know about how the shape of the electromagnet relates to the shape of the resulting electromagnetic field. The most basic rule though is the Right Hand Rule.

For a given electro-magnet, if you wrpa the finger of your right hand in the direction that electric current will flow while sticking your thumb out as if to hitch hike, your thumb will be pointing in the diection of the magnetic field. 

So take a nail, wrap with wire, apply electricity and the magnetic field will come out the ends of the nail. If you wrap your fingers around a nail and stick out your thumb it should point in the direction of the pointy end, or the head of the nail. This is where the magnetic field will be comming out of the nail. This shape is essentially a bar magnet.

The number of coils is pretty easy. The more you put on there, the stronger the magnetic field will be. Since the magnetic field is at right angles to the magnetic field, it doesn't matter if you wrap coils of wire over each other, so long as the current keeps moving in the same direction.

The core material is important. You want something that is a good "conductor" of magnetic fields. (I think the correct term is permeable or permiability.) This is why transformers nearly always have metal innards. It's not just to hold the wire in place, it's to channel the magnetic field. 

Air has poor permeability. Iron has really good permeability. There are some other more exotic materials. Stainless steel is by it's nature non magnetic.

More current means quit simply more electrons. More electrons moving in space in the same direction causes more magnetic field. (Even a straight piece of wire has a magnetic field if current is moving through it.)

Ok, that's my 25 cent tutorial. Tommorrow there will be a quiz.

P.S. As usual Wikipedia has a good article on this topic. It has a picture that explains the the right hand rule. (The right hand rule can also be applied the other way round. If you know the direction of the magnetic field, you can figure out the direction of the electric field with it.) http://en.wikipedia.org/wiki/Electromagnets

----- Original Message ----
From: brooksdesign <brooksdesign at peoplepc.com>
To: Austin Robotgroup Mailing List <robotgroup at puremagic.com>
Sent: Saturday, October 14, 2006 5:18:27 AM
Subject: Re: [Robotgroup] electromagnets

They use 2 solinoids to shift the gears and the complete assembly unit is called "the wig wag" .......For those that need to know .
-brooks

-----Original Message-----
>From: Shane Geiger <sgeiger at ncee.net>
>Sent: Oct 13, 2006 10:50 PM
>To: Austin Robotgroup Mailing List <robotgroup at puremagic.com>
>Subject: Re: [Robotgroup] electromagnets
>
>For all your parts rummagers out there who might have old washing machines:
>
>When I was in sixth grade I did a science project with electromagnets.  
>The strongest magnet I had was a part from an old washing machine.  It 
>was powered by 120 volts--directly coming from the wall.  This thing was 
>about 1x2 inches.  It instantly snapped together with pretty 
>dangerous-to-fingers force. 
>
>
>
>Gray Mack wrote:
>> It sounds like some of us are interested in projects
>> involving electromagnets. It seems we don't really
>> know much about them. 
>>
>> Here are some websites I found:
>>
>> http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html
>>
>> http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/ElectromagneticFields.htm
>>
>> But there is still a lot of unknown things when it
>> comes to creating motion with electromagnets.
>> It will take some experimenting to figure out what can
>> be done with them.
>>
>> Many questions arise about how strong they can be, how
>> much current they draw, how they respond to signals
>> such PWM or alternating current, how opposing coils or
>> magnets can be used to increase the pull capability
>> and provide push capability and how to mechanically
>> amplify the motion created since the field strength
>> dissipates rapidly with distance. 
>>
>> How does the effort and cost of winding our own coils
>> compare with buying commercial solenoids for a project
>> that might contain 10, 100, or 1000 coils?
>> How do the parameters the coil (# of turns, height,
>> radius) affect the distance that it can push/pull a
>> magnet?
>>
>> Some goals could be:
>> *making paper butterflies wings flap
>> *controlling steel balls on a table
>> *creating a tabletop caterpillar robot that moves by
>> pushing small angled feet a tiny distance
>> *changing the height of an array of tiles making up a
>> table
>> *assisting pendulums in an art sculpture to swing so
>> they don't stop due to friction
>> *Cause mirrors to move that reflect laser light in a
>> pattern.
>> *using the feedback from an optical sensor to keep
>> just enough field strength for levitating a metal
>> object 
>> *Interacting with ferrofluid made from printer toner
>> and oil
>> *Play a musical instrument by controlling valves or
>> causing percussion or vibration
>>
>>
>> Cool things about electromagnet coils:
>> *Cheaper than servos
>> *Can last much longer than servos
>> *Can move things quietly
>> *Can be sized to the object they are in or drive push
>> rods or cables
>> *Can be used to generate push,pull, or rotational
>> movement
>>
>> Problems:
>> *Use lots of power
>> *Must remain energized to maintain motion
>> *Need a mechanical (spring,gravity,etc) return due to
>> residual magnetism
>> *Field strength dissipation allows for only small
>> movement of actuator
>>
>>
>> -Gray
>>
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>
>-- 
>Shane Geiger
>IT Director
>National Council on Economic Education
>sgeiger at ncee.net  |  402-438-8958  |  http://www.ncee.net
>
>Leading the Campaign for Economic and Financial Literacy
>
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