# Magnetic pendulum, coil design

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#### ccurtis

##### Well-Known Member
Hello electro-tech fans,

I am interested in improving/optimizing the various magnetic pendulum novelties that exist, mostly in regards to minimizing the use of power (maximizing efficiency). For those who are not familiar with these pendulums, they are configured such that a permanent magnet is attached at the swinging end of the pendulum and a fixed electromagnet (coil) is positioned a small distance from that magnet when the pendulum arm is at its lowest point. An electronic circuit pulses the coil with current at the same rate as the pendulum's natural period, resulting in a force being applied to the pendulum (magnet) to keep it swinging, by way of magnet field interaction. I get the electronic circuit part of the pendulum and all that is necessary to maximize its efficiency. I am not certain how to configure the coil for maximum transfer of energy from the electromagnet pulse current to physical force applied to the magnet. Specifically:

1. What is the most efficient shape (pancake/flat, long, core diameter, outer diameter) for the coil, and why?
2. Should the coil have a solid core (as opposed to air core), and if so what should the core be made of, and why?

Assume a pancake/flat/button shaped magnet, but open to suggestion of a better shape for the magnet.

#### kubeek

##### Well-Known Member
I guess the shape of the coil doesn´t really matter much, what matters is the shape of the core. I think a U-shaped core would work the best, such that one end of the core will push on the magnet, and the other end would pull.

#### alec_t

##### Well-Known Member
Assume a pancake/flat/button shaped magnet
Importantly, how is the magnet polarised? Along a diameter, radially, or between faces?

#### ccurtis

##### Well-Known Member
I guess the shape of the coil doesn´t really matter much, what matters is the shape of the core. I think a U-shaped core would work the best, such that one end of the core will push on the magnet, and the other end would pull.

That is an interesting twist. I'm thinking I would have to switch polarity of the coil depending on what side the pendulum approaches the electromagnet. Not what I have in mind, but worth considering. It makes me wonder, though, if a "horseshoe" magnet would be better than a flat magnet if used with a horseshoe cored electromagnet, as the field would be more intense (focused), not having to travel around the ends of a straight core on the outside of a straight coil in a large volume of space.

Importantly, how is the magnet polarised? Along a diameter, radially, or between faces?

One flat face is north pole, the opposite face is south pole.

#### audioguru

##### Well-Known Member
Buy a solar powered boggle head or other little moving thing at The Dollar Store and see how they use the miniscule solar power to move the thing back and forth.

#### AnalogKid

##### Well-Known Member
The magnet system does not drive the pendulum. It is there only to overcome friction, so it already is a very low power system. And it does not have to goose the pendulum on both halves of the cycle, or even every cycle. An important design point, do you want the coil to attract or repel the armature?

ak

#### ccurtis

##### Well-Known Member
Buy a solar powered boggle head or other little moving thing at The Dollar Store and see how they use the miniscule solar power to move the thing back and forth.

Indeed, I did that and tore it down to take a look and have done some experiments with it. The coil is air core, about 2mm long, 2mm inner core diameter, and about 12mm outer diameter, wound with 45 AWG enameled copper wire. The pendulum magnet is button shaped 10mm diameter, 4mm thick. I replaced the cheap magnet with a rare earth version of same size. That improved the efficiency very noticeably. In bright room light, the swing arc length increased from less than one inch to about three inches. That was with the dancing dog figure replaced with a homemade support for the magnet to swing from an 8in length of thread above the coil. I expect it was designed for low cost, not greatest efficiency or performance.

#### ccurtis

##### Well-Known Member
The magnet system does not drive the pendulum. It is there only to overcome friction, so it already is a very low power system. And it does not have to goose the pendulum on both halves of the cycle, or even every cycle. An important design point, do you want the coil to attract or repel the armature?
ak

That would be my understanding, as well. There must be something more to the picture than only friction of the pivot point, however, because a more powerful magnet results in a greater swing arc. Also, if it swings continuous at all, I would think the system is overcoming that friction, but if I start the pendulum manually at a higher swing arc, it settles down to continuous swing but at a smaller swing arc.

I don't have a preference, repel or attract. What I noticed in the case of the modified toy, discussed previously, is that it works no matter what pole of the magnet faces the coil.

#### MrAl

##### Well-Known Member
Hello there,

Distance is the most important thing to consider in magnetic circuits. The more distance, the less of everything, which includes force.
This mean that if a force F1 has to move the pendulum by a distance of 1 unit, then in most cases LESS than 1/2 of that force would be required to move it 1/2 of the distance. Less than 1/2 of the force means less than 1/2 of the energy required.
This means that probably the best setup is to both push and pull, because a push and a pull should work to exert the same force as just a push or just a pull. So the coil could pull the permanent magnet toward itself, then when it gets close enough, push the permanent magnet away.
Ideally it would pull with a certain forcing function that creates a smooth action, but we'd have to work out the control law and then create a more complicated control system circuit so probably a push and a pull would suffice.
Perhaps a hall effect detector to detect the position of the armature with the permanent magnet, or an opto interrupter.

The efficiency of the coil is highest when the resistance of the coil matches that of the power source, but that may be hard to achieve too. The shape of the coil has a lot to do with it when there is no steel core, but not using a steel core is not a good idea here. The steel core tends to concentrate the field better even though there will be a lot of fringing anyway.

#### dr pepper

##### Well-Known Member
I had a couple of go's at this a while back, I used a Vcr head as the hinge and a relay coil as the electromagnet, another one uses a hard drive read head assembly made into a pendulum that works well as it has neo magnets.
A better hinge can be made, tyou can buy clock pendulum hinge springs from horology centres.

#### atferrari

##### Well-Known Member
The magnet system does not drive the pendulum. It is there only to overcome friction, so it already is a very low power system. And it does not have to goose the pendulum on both halves of the cycle, or even every cycle. An important design point, do you want the coil to attract or repel the armature?

ak

If so, who/what drives it then? Left alone, the pendulum, sooner or later, will stop.

#### dr pepper

##### Well-Known Member
I found an old and naff vid of the hard disk clock thingy.
I used an analogue input on a microcontroller to detect voltage generated by the pendulum swinging, when this was detected the analogue input was switched to a digital output and the coil was driven for a short time, just enough to keep the head swinging, an effective method that doesnt need hall sensors or anything, you had to start it by hand, but I spose you could include that in software.

#### MrAl

##### Well-Known Member
Hi,

I would think a hard drive mechanism or other motor could be driven just like a motor with the motor as the pivot and the energizer too. No extra coil required, and it can start up on it's own without a hand push.

#### dr pepper

##### Well-Known Member
Yes thats how the above works, it just uses the hdd's original servo coil, I've seen one using a dc motor too, but that wont be as efficient as a hdd has ball bearings and no brushes.

#### MrAl

##### Well-Known Member
Hi,

A pseudo pendulum: drive the fan to make it look like the pendulum is swinging back and forth on it's own too

#### ccurtis

##### Well-Known Member
I'm getting some more ideas here. Thanks.

Does anyone know if a pancake shape coil would be better than a long, slender coil for this application (with the coil at the bottom and the magnet swinging over it)? I don't know what gives the greatest magnetic field strength near one end (pole) of those two kinds of coils, if there is any difference. I can't use a ferrous core because then the magnet will gravitate to the core and hang up there. I think I pretty much have to use an air core, or ferrite core. Also, I saw one reference that stated that the core diameter of the coil should be the same as the magnet, but gives no reason why.

#### MrAl

##### Well-Known Member
I'm getting some more ideas here. Thanks.

Does anyone know if a pancake shape coil would be better than a long, slender coil for this application (with the coil at the bottom and the magnet swinging over it)? I don't know what gives the greatest magnetic field strength near one end (pole) of those two kinds of coils, if there is any difference. I can't use a ferrous core because then the magnet will gravitate to the core and hang up there. I think I pretty much have to use an air core, or ferrite core. Also, I saw one reference that stated that the core diameter of the coil should be the same as the magnet, but gives no reason why.

The key here would be the surface area of the object the electromagnet has to attract.
The force decreases as the inverse square of the distance, and increases as the square of the ampere turns. The force also increases as the area increases. These together mean that the closer the turns are to the object the better, but the area of the coil should roughly match the objects area and shape. This means if you have a neo magnet that is round with 1 inch diameter, then the coil should be round and 1 inch diameter or a little larger. The more turns you add the higher the force, but without a core material the force drops off fast with each turn that is farther away from the object.
So the cross section shape here would be round, and with a distance of x away from the object the far end will be a distance x+L away where L is the length of the coil. This means when L gets as far away as the first turn at the front of the coil, the force decreases by a factor of 0.25. That's probably a good place to stop. Going twice as long as the distance to the object we'll only get 1/9th of the force, which is nearly just 0.1, so adding turns that make the coil too much longer than the distance to the object doesnt help much and adds to the resistance as much as the first turn at the front of the coil.
What this could in turn mean is that you should use a core even if it's just a nail or something. Otherwise you might need some heavy current,. Of course you want to get the front of the coil as close to the object as possible when it swings, within reason keeping in mind any bearing wear over time.

#### ccurtis

##### Well-Known Member
Thanks, MrAl. That makes good sense to me. A flatter coil I will try first. I'll see if I can find a ferrite core with at least some permeability greater than air, easy enough to remove and replace and note the difference.

#### Pommie

##### Well-Known Member
I assume that if the pendulum swings freely then a sine wave will be produced in the coil - one half as it approaches and the other as it moves away. This could be detected and a little after it crosses zero the coil could give a small push. I also assume that the push would need to be tiny as it only needs to add the amount of energy lost in a half swing. Could you measure this loss? If you started with a swing with a vertical displacement of (arbitary) 10cm and counted how many swings it takes for the displacement to be 9cm then you could estimate the energy lost per half swing.

As an after thought, if the toy does detect the induced voltage then this would explain why the polarity of the magnet doesn't matter, the electronics would just assume the pendulum was swinging the other way.

Mike.

#### ccurtis

##### Well-Known Member
Hi Pommie,
I started the swing at a 45 degree angle on this one model in front of me that has a very small ball bearing as the pivot and timed it until stop at 2 min 48 sec. Interestingly, just by watching it, I can tell the rate of reduction in swing arc over time is not linear, with the swing arc declining more rapidly at first than at the end, before stopping.

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