How can I detect metal contact on u shaped electromagnet and register that contact to an Arduino?

Discussion in 'Embedded Systems and Microcontrollers' started by Michael Campbell, Oct 25, 2015.

  1. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
    36
    1
    Hi Guys,

    I am using an electromagnet to hold a loaded metal plate in place but I'd like to also use it to simply detect the plate's presence and register that detection on an Arduino. For the detection, I don't actually want to hold it in place. Is this possible? Any tips on how I can achieve this? I have a little image below to paint the picture.

    BOrwwGr.png
    http://i.imgur.com/BOrwwGr.png

    Thanks much!
     
  2. #12

    Expert

    Nov 30, 2010
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    More information please.
    How far away do you not want to hold it?
    How much voltage, current, weight, amp-turns, distance, etc.
     
  3. Regular_hobbyist

    New Member

    Mar 9, 2015
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    0
    You could imagine the plate as a switch and use the arduino to detect the continuity of it. If it was high, the circuit formed by the plate would be closed and the plate would be in contact. If it was low, the plate would not be in contact with anything.

    What do you want to use this for and what is the plate doing?
     
  4. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
    36
    1
    Oh yes, of course.

    I want detection to take place at the point of touching, with maybe a couple mm of tolerance. As close as I can get. I do want it to be sensitive enough to detect it in a fraction of a second though as it may only be there for that long.

    10 volts
    1 amp
    Not sure about amp-turns, but it's about 1 lb of copper coil (2 coils), 24 gauge, roughly 400 wraps.
     
  5. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    I have a small spring loaded door that is articulating down. At certain points I want to hold it down, but other times I just want to know if came down. Can an arduino detect continuity through the coils? And is it possible to detect presence of only a few microseconds?
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Are both coils in your image turned the same direction? That us, will both end of the U be North poles of a magnet or will there be one N and one S?
     
  7. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    They're oppositely spun at the moment, but can rewind if necessary.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What is the DC resistance of each coil? What voltage is powering the two coils (connected in series as I see it).
     
  9. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    Not sure about resistance, but's 24 gauge wire and I'm running 10-12 volts through it to actually hold it. I can change that amount for detection only. And yes, this is connected in series.
     
  10. #12

    Expert

    Nov 30, 2010
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    I think you are missing the fact that magnetic circuits change suddenly and strongly at the moment of contact. This requirement to detect the plate when it is NOT contacting is complicating the situation. For that we need to work with, "how much". Is it really important to detect the plate when it is NOT contacting? How far away (2mm). Is that distance merely air? etc.
     
  11. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    I prefer it to register only on contact. I only included some tolerance just in case, as I'm not familiar with this application.
     
  12. GopherT

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    Nov 23, 2012
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    Ok, can we assume 50 feet of wire in each magnet? That is 100ft in total and 2.4 ohms, giving 5 amps of current.

    So, 60 watts (5A x 12V) of power into those coils. Hope you don't mind the heat. The plastic bobbins will likely melt.

    Now, if you have 1000 feet of wire, then you have 24 ohms and 0.5 amps (6 watts). Some heat but the mass of 500 feet of 24 gauge wire should be ok.

    We are getting there. We just need some context here.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    As #12 is say, we can start detecting as the bar approaches but it will be a small effect, then, in the last fractions of a mm, the effect should go up quickly. Telling the difference between a few tens of micrometers will be difficult differentiating from true contact.
     
  14. #12

    Expert

    Nov 30, 2010
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    OK. Sudden change in current will happen. Child's play from here. Glad we got that cleared up.
    I'm going to leave it to Gopher for now.
     
  15. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
    36
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    I didn't count honestly, as I'm just experimenting but it's gotta be on the order of 300 + ft. It certain is getting warm though. Here's a pic:

    pbNzjzK_800x600.jpg
    http://i.imgur.com/pbNzjzK.jpg
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Material lift magnets typically require a bi-polar control, depending on the weight of the lifted item, it may need a reverse pulse to de-mag.
    BTW you do not require laminated core with a DC electromagnet.
    Max.
     
  17. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
    36
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    Let's just go with absolute contact. The only potential challenge would be it potential presence for only a few microseconds (or is that milliseconds).
     
  18. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    Interestingly, when I wrap a fresh electromagnet, it does works great, however after a bunch of abuse, it does seem like it needs a reverse pulse de-mag. I'm not sure what's happening.. is it short circuiting somewhere?

    Are you saying I don't need laminated cores? Or are you asking?
     
  19. Michael Campbell

    Thread Starter Member

    Oct 25, 2015
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    Very interested in this, when is a reverse pulse necessary?
     
  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You are making a permanent magnet.
     
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