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Do you guys think this will work?

  1. May 20, 2009 #1
    I am making a locking mechanism with a very powerful magnet. The electromagnet in the diagram is held in place. Basically when the electromagnet turns on it will push the permanent magnet out. Here is a picture of what I want to do.
    http://www.mypicx.com/05202009/fas/
     
  2. jcsd
  3. May 20, 2009 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. The traditional way to do that function is with a solenoid, which attracts a ferrous bolt into a soltnoid with the current flows through the solenoid (Quiz Question -- why does it attract the bolt?).

    The problem with using a permanent magnet to be repelled by the coil field, is that the coil field will tend to demagnetize the permanent magnet over time. If you could use two opposing electromagnets, you would not have that problem. But the traditional solenoid arrangement is usually the most cost-effective arrangement for generating linear forces with a current input.
     
  4. May 20, 2009 #3
    I see. So what you're saying is that I should build a solenoid of some sort and maybe place an iron bolt inside that is spring loaded? That is very interesting. Do they sell these things or do you think it's better for me to make it myself?

    edit: lol nvm, you don't have to answer that. Just found out what latching solenoids are. heh thx man!

    Another question. Why will the coil field demagnetize the permanent magnet?
     
    Last edited: May 20, 2009
  5. May 21, 2009 #4

    berkeman

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    Just as a magnetic field in one direction is used to magnetize the magnet, a magnetic field in the opposite direction will serve to demagnetize it (or even magnetize it in the opposite direction, if the field is stong enough).

    A permanent magnet is just a magnetic material with magnetization at one end of its hysteresis loop:

    http://en.wikipedia.org/wiki/Magnetic_hysteresis#Magnetic_hysteresis

    .
     
  6. May 21, 2009 #5

    Danger

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    One thing to consider while designing this thing is whether you want it 'fail safe' or 'fail secure'. Ie: if the electromagnet loses power, do you want the door unlocked or locked?
     
  7. May 21, 2009 #6

    russ_watters

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    Ie, many commercial buildings use purely magnetic locks (a magnet literally holds the door closed). They release when power is lost, which can be advantageous for an emergency.
     
  8. May 22, 2009 #7

    berkeman

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    Those are internal doors, obviously. Our elevators are also designed to drop to Floor 1 and open their doors in emergencies. We test those things pretty regularly.
     
  9. May 22, 2009 #8

    russ_watters

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    Yes. And ironically, the opposite also exists (didn't think of it when I wrote that last post). Fire doors can be propped open normally if they are held open by a magnet, closing (but not locking) in the event of a fire to stop it from spreading.
     
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