Question about magnets

[Micheal]

I have a few questions for the Knowledgeable ONES ...

The coil on Pi a metal detector creates a magnetic field (which is fairly weak)...which when passed over a metal object eddy current is produced and it is the eddy currents that the reciever picks up...my question is this, if you pass magnet like a strong rare earth magnet over a metal object is eddy currents produced....and it is just an idea in my head that a magnet be used to amplify the eddy currents (which I am thinking will be stronger than what a coil would produce )so that the receiver picks up a stronger signal.

[Qiaozhi]

I have a few questions for the Knowledgeable ONES ...

The coil on Pi a metal detector creates a magnetic field (which is fairly weak)...which when passed over a metal object eddy current is produced and it is the eddy currents that the reciever picks up...my question is this, if you pass magnet like a strong rare earth magnet over a metal object is eddy currents produced....and it is just an idea in my head that a magnet be used to amplify the eddy currents (which I am thinking will be stronger than what a coil would produce )so that the receiver picks up a stronger signal.

When a magnet is passed over a metal object it does indeed generate eddy currents in that object. This is exactly what happens when you drop a magnet down the inside of a copper pipe. The eddy currents generate an opposing magnetic field that significantly slows down the fall of the magnet.

There is a major difference between using a TX signal from a PI, and using a rare earth magnet to create eddy currents in a target. In the case of the PI the pulses are occurring several times each second, but for the magnet there is only one pulse as it crosses the target. Also, the strength of the eddy currents depend heavily on how fast the TX signal is switched off. I would imagine that even passing a rare earth magnet relatively quickly over the target would be much less effective than a PI transmitter.

[Micheal]

Ok then...
must the magnetic field be in pulses to generate eddy currents ...


Imagine this if you disable the TX part of the detector,and stick a 50mm diametre rare earth magnet in the middle of the coil what will happen when you go over a metal object....( I really am keen to do some of these experiments)

shouldnt the reciever pick up some kind of signal.
Or once again with experiments I dont know ..

also my other question is this...If a magnet is over the object and held there for lets say 10seconds does that mean the object will create eddy current for 10 seconds or will the eddy current die out sooner ...
These are the things I am trying to understand

[Qiaozhi]

Ok then...
must the magnetic field be in pulses to generate eddy currents ...

There must relative motion between the magnet and the metal target in order to generate eddy currents.

Imagine this if you disable the TX part of the detector,and stick a 50mm diametre rare earth magnet in the middle of the coil what will happen when you go over a metal object....( I really am keen to do some of these experiments)

shouldnt the reciever pick up some kind of signal.
Or once again with experiments I dont know ..

How are you going to synchronize the receiver sample pulse to the motion of the magnet? One way I could think of is to use an accelerometer, but personally I think this idea is already heading for the trash. Good luck with the experiment though.

also my other question is this...If a magnet is over the object and held there for lets say 10seconds does that mean the object will create eddy current for 10 seconds or will the eddy current die out sooner ...
These are the things I am trying to understand

No!! ... and sorry for laughing!

As I said before, there needs to be relative motion between the magnet and the metal target in order to create eddy currents. If the magnet is stationary then no current is generated. If this wasn't the case then generators would not need to rotate. A PI transmitter creates eddy currents by producing a rapidly varying magnetic field, and is synchronized to the receiver, thus allowing the decaying magnetic filed to be sampled.

[Aziz]

Hi Micheal,

congratulations. You have invented the first reliable discriminating metal detector (provided that the magnet is very very strong). So you will pull out small ferro magnetic targets directly from the ground without digging them out.

To induce a voltage in the receive coil, you have to move the coil rapidly. The faster, the higher the induced voltage.
Only a changing magnetic field (dB/dt = first derivation of magnetic field) will induce a voltage in a coil (Faraday's Law) or generate an eddy current in targets.

A rough comparison between your idea and a PI detector:
Your coil movement: 1 meter/second
PI detector: 1000 Pulses/second (for 1 meter coil sweep)

So a PI detector is generating at least 1000 times higher dB/dt and hence 1000 times higher eddy currents (rough calculation of course).

Read more at wiki:
http://en.wikipedia.org/wiki/Faraday...w_of_induction
http://en.wikipedia.org/wiki/Electromagnetic_induction
http://en.wikipedia.org/wiki/Magnetic_flux

But you also will magnetise magnetic minerals in the ground like the black sand (magnetite). If once magnetised, it also will give a signal. And of course, they will stick on your coil.

One plus: Your detector will be a very low power detector (no TX current necessary).
One plus more: You have a motion detector.

Aziz

[Aziz]

Hi Michael,

I want to encourage you with the following rough calculations again. Some other guys should be surprised about it though.

Again a comparision between a PI coil and magnet coil:
PI coil:
Number of turns: 22
Coil diameter: 10 inch
Coil current: 10 A (we use a strong PI machine)
Inductivity: approx. 300 µH
Target is nearby the coil wires, magnetic field is changing in the time domain

The coil will induce a magnetic field density of approx. 30 mT (see below graphics) at the coil wires (we need 5 mm minimum distance to the coil wires and let saturate the coil current to 10 A). We let damp the magnetic field in about 5µs and get a dB/dt = 30 mT/5µs = 6000 T/s (dB/dt = magnetic field change, that is really huge).

Now let us make the calculation for a magnet coil if we can get a 6000 T/s slope for equivalent eddy current stimulation.

Magnet coil:
Magnet field density: 1 T (neodym type, easily achieveable)
Magnetic field is stationary (not changing in the time domain), but the target is moving

Lets assume, we have still 1 m/s coil sweep and the target is small (1 cm) and is passed over the magnet coil. As we know, the magnetic field is rapidly diminishing, if the distance to the magnet gets larger. Lets assume, that the target is sensing almost the whole magnetic field within 5 cm distance. The target takes 5cm/100cm/s = 1/20 s or 50 ms during this time and we have a magnetic field change of approx. 1T/50ms = 20 T/s.

Bad comparison?
The PI coil is 6000 T/s / 20 T/s = 300 times better.

Now the encouragement for you:
- We can take more magnets, filling a big flux area of the receive coil.
- We can take much more windings for the receive coil.
- We have a much higher relative permeability due to the magnets in the flux area and this will increase the inductivity of the receive coil much.

As the receive coil has now much higher inductivity, it could compensate the drawback and be maybe compareable to a PI machine as the receive coil now amplifies the eddy current responses much much more.

Unfortunately, the target frequency response is lying in the low frequency range (0 .. 100 Hz), in which your measurements will be disturbed due to EMI noise. Anyway.

So just try it.

Aziz

[Qiaozhi]

As the receive coil has now much higher inductivity, it could compensate the drawback and be maybe compareable to a PI machine as the receive coil now amplifies the eddy current responses much much more.

Unfortunately, the target frequency response is lying in the low frequency range (0 .. 100 Hz), in which your measurements will be disturbed due to EMI noise. Anyway.

So just try it.

Aziz

How would you synchronize the receiver sampling to the magnet?

[Aziz]

How would you synchronize the receiver sampling to the magnet?

I wouldn't synchronize. Just detecting signal changes. There should be some measureable eddy currents or magnetic field changes caused by ferro magnetic materials.

But this principle would have its own disadvantages of course (many).

Aziz

[Micheal]

thank you aziz and freinds even the one that says trash the idea...

ok the magnetic feild and the edyy current are emitted in different frequencies so it shouldnt be too hard to make a reciever to detect the eddy current emitted and eliminate the magnets field.....

I am thinking along the lines of aziz ,that the magnets be spinning which can be made to spin also using magnets thus eliminating the need for power to spin the magnets....I will put a diagram of an idea ...
the other magnets I am think are the circle magnets used in large P.A speakers 30cm diametre.

Yes I the whole idea is whacky but I am just trying to come up with something new rather than follow the traditional methods to make detectors ...and I think its worth discussion...

[Micheal]

here some very basic ideas of what Im thinking

[Micheal]

this another idea

[Qiaozhi]

I am thinking along the lines of aziz ,that the magnets be spinning which can be made to spin also using magnets thus eliminating the need for power to spin the magnets.

So you're planning to make a permanent magnet motor?
You do realize that such a device is known as a perpetual motion machine?

[bama]

magnets on LRL receiving rods and dowsing?

I don't mean to butt in, but since magnets is the topic, I have a question.
Is there any advange in using magnets on LRL receiving rods and on
dowsing rods?
regards

[Micheal]

Qiaozhi.....perpetual motion machine?


Is that how you interpret it
You like the idea dont you,just why you didnt think of it till now...
The perpetual motion -magnet coil (PPMC)
Whats wrong with innovation and different ideas?