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Repulsive electromagnets

  1. Jan 22, 2013 #1
    Hello, I'm Zach. Nice to meet you
    I have an idea I'm trying to work out in my head but don't have much experience with magnetics, especially electromagnetics so I'm helping someone could help me out a bit

    What my end goal is is a essentially a tube full of magnets that acts like a spring. If you put a bunch of round magnets in a tube that is almost the same diameter of the magnets, and turn the magnets to repel each other N S N S N S, you essentially end up with a magnetic spring. Easy enough, now the tricky part:
    I want a spring that is instantly variable. This spring should be able to support say 150 lbs., ideally have a stroke of around 6" and perhaps be 12" in length. These numbers are really only to give you an idea of the scale I'm working with.

    Now I'm trying to figure out what is going to be the best way to go about this.
    Idea 1: The simplest idea is to line up some electromagnets in a tube, perhaps with some sort of teflon plastic encasing and have them repel each other.
    Idea 2: Same idea as number 1, but having neodymium magnets between the electromagnets to perhaps boost the capacity of the "spring". Would this work? Or would I achieve the same capacity as idea 1? Also with the varrying amount of current being supplied to the electromagnets I'm a little worried about the neodymiums losing their magnetism...
    Idea 3: Putting the electromagnets on the outside of the tube and using permanent magnets inside the tube as the "spring", but manipulated by the electromagnets to pull or push the permanents together, in other words weakening the "spring". I don't have high hopes for this idea, it seems limited...

    Please bear with me here, I have a feeling I don't really know what I'm getting into
  2. jcsd
  3. Jan 22, 2013 #2


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    shy_barbarian, Hello Zach. Welcome to Physics Forums. This is a place where real science is discussed and disseminated. You've come to the right place to get authentic answers from real scientists, engineers, and highly experienced folks.

    I believe you could support the weight (say, 150 pounds) with one single large electromagnetic solenoid. You would not need multiple permanent magnets or the other complexities you described. The solenoid would be large (massive) and would need a large variable Direct Current (DC) current source to drive it. But certainly it could all be sized to raise and lower that weight over the distance you want. See:

    "Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug (termed the armature). The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet. The armature is used to provide a mechanical force to some mechanism (such as controlling a pneumatic valve). Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very low reaction times.

    The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil (see Faraday's law of induction). The force applied to the armature will always move the armature in a direction that increases the coil's inductance.

    Electromechanical solenoids are commonly seen in electronic paintball markers, pinball machines, dot matrix printers and fuel injectors."

  4. Jan 22, 2013 #3


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    Looking at your suggested spec, I would say that you would not be likely to achieve it in the way yo suggest. For a structure that is 12" long to produce a variation in length of 6", using electromagnets, sounds a very tall order if you want the layout you suggest in your diagram. The field drops off very rapidly at the end of a solenoid (or bar magnet) and this requires a large spacing between magnets at the maximum extension. This would need very high currents in your coils. I'm not sure that a combination of permanent and electromagnets would actually help that much.

    There is a fundamental problem with trying to simulate a spring, where the force follows Hooke's Law (linear) with a magnetic system which follows something like a 1/x3 law with spacing. Perhaps that wouldnot matter, though. It would depend what you wanted.

    Magnetic 'actuators' can be made to produce high forces over a moderate distance, using a sliding core inside a coil - as with motor car starter motor 'solenoids' but you are after a 2:1 extension ratio.

    You may be in with a better chance if the coils were very large diameter ('pancake' style), when the rate of drop of of field would be less with extension. I also wondered whether a set of coils, in a column, switched progressively, from bottom to top of the column, could give a long throw of the armature.

    An alternative approach could be to simulate a spring characteristic with a servo. This would be a lot more complex but, once you think in terms of a proper control system, there is no limit to what you could do.
  5. Jan 22, 2013 #4
    Are you really interested in having the weight supported by the magnetic field or are you just interested in a continuously variable spring? If it is the latter then I would suggest using a metal spring of the strength you would need most of the time and supplement it with pancake magnets which could attract (to weaken the spring) or repel (to make the spring stronger). That way your electromagnets won't have to be as strong. They will require less current and power to operate, they could be made smaller and less expensive, heat dissipation will be less of an issue, and your load won't come crashing down in the event of a power loss.
  6. Jan 23, 2013 #5


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    If it's open season on solutions, I'd suggest the length of the spring could be varied by progressively clamping more and more of the bottom coils. This would vary the spring constant as you want. It could be done with solenoids, pushing wedges in between the coils. Is that cheating, though?
  7. Jan 23, 2013 #6
    This is an interesting idea. I wonder if multiple electromagnets switched progressively would be strong enough, I imagine they would.
    I was also thinking of using an armature that contains permanent magnets, and the armature is pushed or pulled in a similar fashion that a mag-lev train works, or a coilgun works. I would imagine I could achieve more strength with this configuration than with a regular iron armature.

    Please elaborate a bit as I'm not familiar with servos really.

    Interesting idea, are you imagining the pancake magnet acting directly on the spring to pull it together to effectively make it weaker?
    Also what exactly is a pancake electromagnet?

    Don't think it could act quickly and dynamically enough for my purposes, but keep the ideas coming!!

    Thanks all
  8. Jan 24, 2013 #7


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    A servo is an arrangement with a motor or actuator, of some sort, which uses feedback (in this case Position Feedback) to control the position of a wheel, lever, whatever. It can be used to emulate any force law you like and a Hooke's Law behaviour would be easily achievable. If as you say, you want a quick response to changing the 'spring constant', you are already talking in terms of electronic control.
    For "pancake" see the top of your post and your quote of my first use of the word. What shape is a pancake? Wide and flat. The field of a coil with that shape would extend much further from the plane of the top of the coil. I still think a 2:1 ratio of extension is not practical if you don't use some form of armature - some telescopic core even. Just play with a couple of bar magnets and you'll see what I mean about the rate the force drops off with distance. It's much faster than inverse square law even and a spring law is linear with distance.
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