Transformer Induction coil (COIL CONSTRUCTION)

  • Thread starter jegues
  • Start date
  • #26
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Thank you for the coil layouts Mike.

Would simply placing the primary coil before the secondary as indicated in the figure work as well, or is it necessary to follow one of the design patterns you've demonstrated?

Initially that is how we were "told" to do it, but it's up to us to make the final decision. Are you certain the other designs will create a stronger signal? Will the plastic PVC tubing interfere at all?

Thanks again.
 
  • #27
702
34
The coils work equally well when placed either way. I'd tune the primary (or excitation winding) with a capacitor. Not knowing any better, I'd try tuning it around 1-2 MHz using ceramic caps. Why? Because you'll get more current in the primary if it's ringing into a cap. That makes for far more signal.

The PVC is a plastic material, and you won't see any signal aberration due to it. In fact, just about any plastic is great.

This configuration won't get more signal as such, BUT you'll get a more discrimination. If you get your coils centered, you won't see much signal until a metal ball comes along.

As the ball rolls into the coils the output will reach a peak, suddenly drop to zero, and then suddenly reach another peak and taper off as the ball rolls away. Very definite. The marble will make no difference at all.


If you do decide to do this, make sure you have a feel for WHY this works. Someone's bound to ask. The idea is to reject the signal from the primary - until the ball disturbs the field.
 
  • #28
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Okay so now I'm confused.

Is it going to make a big difference if I use the orthogonal layout as opposed to simply placing the primary coil followed by the secondary coil?
 
  • #29
702
34
Sorry Jeques,

They're any number of ways to do this. Either one of the two I showed you will work good. A lot of people like the orthogonal coil because they can adjust it easily. Just move it along the other coil until the output disappears, then tape and glue.

The other method would probably be simpler to construct - just keep track of which way the wire is wound, make sure you have the same number of turns on each of the side windings, and that their fairly centered about the primary. If you see a bit of signal squeaking through, you can push the windings about a bit with your thumb nail while watching the signal. When the signal has nulled out, tape and glue your coils

Again, the orthogonal one will adjust better, the other takes less finesse to build. Either one will do a great job of detecting the bearing.
 
  • #30
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So aside from you designs, I can't simply place a primary coil followed by a secondary coil in sequence to generate a signal? (NO EXICITATION WINDING. See figure)
 
  • #31
vk6kro
Science Advisor
4,081
40
So aside from you designs, I can't simply place a primary coil followed by a secondary coil in sequence to generate a signal? (NO EXICITATION WINDING. See figure)

You might get a small signal if the steel ball happened to have some residual magnetism, but generally, no. There would have to be a magnet there somewhere.

This is sounding a little like a metal detector design project.

I saw a metal detector coil in a magazine. It had coils about the size of compact disks (ie about 12 cm) but they were flattened on one side to make a "D" shape. The two coils were then laid over each other with the curved parts facing away from each other.

like this:
http://dl.dropbox.com/u/4222062/metal%20detector%20coils.PNG [Broken]

The idea was that the vertical part of each "D" would get opposing signals from the vertical and curved parts of the other "D" shape and you got a cancellation.

The rectangular coils at right would probably give the same effect. You would feed the signals in where the gaps are in the vertical sections. You would only feed the ends of the windings, of course.

When you had cancellation, bringing any metal into the vicinity would cause the signal radiated from one coil to be reradiated into the other coil.

I have never tried it but it seems like a good idea.
 
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  • #32
4,662
5
Mike's coil-3.jpg in Post 22 is the right way to do it. The two pickup coils, if properly balanced (bucking mode), will give a null signal unless a conducting ball rolls inside the pipe. It is important that the two pickup coils be symmetric about the excited coil. If necessary, the coils could be moved back and forth on the PVC pipe to get a null.

Bob S
 
  • #33
702
34
Hey VK,

Yep, the twin D is common for metal detectors. With the sensing coil overlapping the excitation coil's loop, you can get rejection. It's just a matter of tweaking.

What's really interesting about these detectors is that you can amplify / limit the output of the sensing coil and run that through synchronous detectors. The end result is that you get the phase relationship between the excitation signal and the return signal. This tells you natural time constant (L/R) of the target. Hence you get a good idea what kind of metal is present ;)
 
  • #34
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5
It is useful to look at the circuit design used in the coin pass/ fail detection units in vending machines. They use half a pulsed ferrite pot core on one side of the coin, and a L/R sensing pickup (the other half of the pot core) on the other. It is very fast.

Bob S
 
  • #35
1,097
3
Mike's coil-3.jpg in Post 22 is the right way to do it. The two pickup coils, if properly balanced (bucking mode), will give a null signal unless a conducting ball rolls inside the pipe. It is important that the two pickup coils be symmetric about the excited coil. If necessary, the coils could be moved back and forth on the PVC pipe to get a null.

Bob S

So this is the design I should be aiming for in order to obtain the best consistent signal?
 
  • #36
4,662
5
Mike's coil-3.jpg in Post 22 is the right way to do it. The two pickup coils, if properly balanced (bucking mode), will give a null signal unless a conducting ball rolls inside the pipe. It is important that the two pickup coils be symmetric about the excited coil. If necessary, the coils could be moved back and forth on the PVC pipe to get a null.
So this is the design I should be aiming for in order to obtain the best consistent signal?
Yes, IMHO.

Bob S
 
  • #37
1,097
3
Also, how crucial is it that the two pick up coils are exactly identical? Will it make a huge difference if one pick up coil has more turns than the other?

Remember, the two pick up coils would be consisting of one 15m strand of very thin wire, it might be difficult to have the two coils be symmetric, no?
 
  • #38
702
34
If your coils are off, you can scoot them closer or further with your thumb nail until you get a null. Or, you can use the orthogonal coil. I gaurantees a null simply by scotting it from left to right.

Mike
 
  • #39
4,662
5
If your coils are off, you can scoot them closer or further with your thumb nail until you get a null. Or, you can use the orthogonal coil. I gaurantees a null simply by scooting it from left to right.
The optimum design is probably a ~100-turn primary coil and two symmetric ~50 turn bucking pickup coils. The ideal location is with each pickup coil probably centered over each end of the excited coil, to maximally intercept the dipole field from a conducting (or magnetic) ball. This would require some electrostatic shielding between the excited and pickup coils, however, so probably starting out with Mike's end-to-end coil layout in coil-3.jpg is best. I suspect that the excitation frequency will be in the 20 kHz range. The optimum frequency will be when the skin depth is ~equal to the radius of the balls. If it is too high, you will not be able to discriminate between aluminum and steel balls, for example. Do you know whether the excitation is pulsed or CW?

Bob S.
 
  • #40
1,097
3
The optimum design is probably a ~100-turn primary coil and two symmetric ~50 turn bucking pickup coils. The ideal location is with each pickup coil probably centered over each end of the excited coil, to maximally intercept the dipole field from a conducting (or magnetic) ball. This would require some electrostatic shielding between the excited and pickup coils, however, so probably starting out with Mike's end-to-end coil layout in coil-3.jpg is best. I suspect that the excitation frequency will be in the 20 kHz range. The optimum frequency will be when the skin depth is ~equal to the radius of the balls. If it is too high, you will not be able to discriminate between aluminum and steel balls, for example. Do you know whether the excitation is pulsed or CW?

Bob S.

Thanks again for your input Bob.

I'm not sure whether the excitation is pulsed or CW, I'm sure if I asked he would tell me but as of now we aren't given any detailed infromation about the circuit, just that it resembles a transformer.

We're going to be hopefully testing out coils out today so I'll let you know how our trial and error goes.
 

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