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I'd like to make some kind of actuator/vibrator but much stronger. I thought of using a static, tubular solenoid, driven with AC current. Inside it would be a mobile permanent magnet cylinder (magnetized along the axis of the cylinder). One end of the permanent magnet would be tied to a membrane and the current would make it vibrate.

-Will the magnet core rock back and forth in a much stronger way than a classic iron core would?

-Will the magnet core require a spring, like an iron core would ?

-Any comments on the conditions such device would work or not are appreciated.

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  • \$\begingroup\$ "Much stronger" just means that you haven't found a speaker with high enough power output. \$\endgroup\$ – Andy aka Sep 1 '16 at 16:48
  • \$\begingroup\$ This is, more or less, how a speaker works: 2.bp.blogspot.com/-oemme74cWEc/U2dwlc9nJ6I/AAAAAAAAADw/… \$\endgroup\$ – user86234 Sep 1 '16 at 16:48
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    \$\begingroup\$ Loudspeakers use fixed magnets and moving coil to minimise the moving mass. With what you're proposing most of the energy will go into accelerating the magnet rather than pushing the diaphragm. \$\endgroup\$ – Transistor Sep 1 '16 at 19:31
  • \$\begingroup\$ that's correct, except the diaphragm will be made of metal, this is not really a speaker, I will edit the question \$\endgroup\$ – Manu de Hanoi Sep 3 '16 at 2:55
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The material that neodymium magnets are made of saturates very fast, faster than soft iron. Therefore, if max strength is desired, soft iron is better than neodymium magnets (and possibly other types of permanent magnet too).

On top of that, a strong electromagnet will pull a permanent magnet no matter what is the polarity, so expecting the permanent magnet to push back the electromagnet wont work if the electromagnet is strong enough.

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    \$\begingroup\$ I replaced soft iron with neodymium magnet and got solid improvement of the force. I suppose that in certain conditions permanent magnets could be better than soft iron. Could you direct me to more information about that topic, so I could understand it better? \$\endgroup\$ – Pygmalion May 28 '18 at 11:18
  • \$\begingroup\$ there @Pygmalion it depends on the magnetic flux en.wikipedia.org/wiki/Saturation_(magnetic) \$\endgroup\$ – Manu de Hanoi May 29 '18 at 15:05
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I was unsatisfied with my solenoid actuator that consist of double solenoid that pulls core in both directions and after replacing original soft iron plunger with the neodymium magnet plunger I got solid improvement in the force.

I do not know the theory behind the magnetic saturation and I don't know which is more efficient in particular cases, but maybe you could give it a try.

There is even an article on using permanent magnets for this application: http://www.mdpi.com/2076-3417/5/3/595/htm

EDIT: After additional research I want to add the following:

Using soft iron or permanent magnet works on different principles:

  1. Soft iron is always attracted to the solenoid. Therefore, you need to have either two solenoids, one pulling up and other pulling down, or you need one solenoid to pull and one spring to push. In the latter case when solenoid is on magnetic force prevails and pulls core into the solenoid and when solenoid is off spring force pushes away.

  2. Permanent magnet will try to go either way depending on the direction of the current within the solenoid. If current goes one way it will push core in one direction, if current goes the other way it will push core in the other direction. You don't need the spring or double solenoid, however you must be able to change the direction of the current.

A more detailed description can be found here: Automatic switcher operation and improvement

Now it depends on you what you really want.

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Solenoids are rather inefficient motors; could you use instead a rotating AC motor with an eccentric weight on the shaft? That would vibrate quite strongly. With a flexible shaft coupling, you could leave the motor stationary while the weight and its bearings vibrate (that's how concrete fill vibrators are constructed, to agitate the mix and prevent voids).

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A linear actuator consisting of an electromagnet and a permanent magnet must be some kind of variation of a voice coil. They are essentially superior by every metric to solenoids, including force-density; except for one aspect, cost.

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  • \$\begingroup\$ i believe solenoids can be much stronger than permanent magnets \$\endgroup\$ – Manu de Hanoi Jan 5 at 2:59

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