Eddy current strength?

[wyndham]

I am trying to learn a bit more about the way an eddy current works. I believe that I read here or somewhere that if a tx coil is passed over a target faster(as oposed to very slow) that it can induce a greater eddy current in the target. I may have this all wrong but the idea made me wonder about how induction works.
In a farady flashlight, doesn't the speed of the magnet+ the gauss strength thru the coil indicate the amt of current generated for the flashlight?
how Does the gauss of the magnet and the winding relate as to the current it produces and the force(speed) that the mag reaches as it goes thru the winding. Is there a simple equation that I could understand that someone might post?
Long way around to say If a Tx coil on a PI were spinning as it transmitted, would it increase the field strength into the target and then get a greater responce?
Any thoughts Wyndham

 

[Prospector Al]

Alas, there is no simple equation...

I will show the equations involved, but they are several and not simple. Nevertheless, it's possible to draw conclusions from them that answer your questions...

The voltage generated in a target is defined by E = -d Flux/dt
which says that the magnitude of the voltage is proportional to the flux change per unit time.

The amount of flux that penetrates a target is:

Flux(Maxwells) = B (Gauss) x A (cm squared)

The current that flows in response to the voltage is:

I = E/Z where Z is the impedance of the eddy current path,

Z = R + iX where R is the resistance and X the inductive reactance.

The current does not attain its maximum value immediately. It builds up according to the formula:

I = I(0) x (1-e.sup.-t/T) where t is the running time and T is the time constant of the target.

T = L/R where L is the inductance and R is the resistance of the eddy current path.

In a PI detector, the voltage is generated in the target while the coil current, and thus the flux is changing. After the coil pulse, the eddy current decays according to the formula:

I = I(0) x e.sup.(-t/T)

The practical consequences of the above:

A silver coin has a long time constant ( T = L/R R is small)
It takes a relatively long time to "charge" it and the current decays slowly.

An aluminum foil disk of the same size is charged fast and the current in it decays fast ( T = L/R R is large--T is short.)

The current in the transmitter coil, and thus the flux, changes in
microseconds. Any mechanical movement of the coil is too slow in comparison to change the eddy currents significantly. A slight increase would result if the coil is moved linearly toward the target--spinning would not help at all...

Finally, the flux generated by a coil is directly proportional to the current and the number of turns. The field strength depends on the size of the coil. If you spread the flux (Maxwell) over a greater area you get a lower field strength (Gauss).

The above is a simple representation of what's going on in a PI detector--there are second-order effects which I haven't mentioned, for fear of going beyond of what is called for in the post...

Prospector Al

 

[Reg]

Hi Wynham,

I can't speak as to what happens if the tx head is spinning, but my head is spinning after reading your question and Al's answer (LOL).

Prospector Al has presented some of the equations involved and when looking at them you can see there is a level of complexity to the answer to one of your question(s).

Now, I say questions since you really did make a statement and ask multiple questions and they are sort of like mixing apples and prunes. However, they (your questions and not the apples and prunes) do point out something important.So, I will try to add a little more information to the confusion.

You mentioned if you move the search coil faster as opposed to slower you can generate a greater eddy current. This is true, but this is not the main technique used in a PI. However, this action, the coil movement will also generate a response because of the earth field effect. This means that moving the coil faster within the earth's magnetic field will generate a signal in a PI and it may be the stronger response. Normally, this particular signal is compensated for in the design of the PI and this earth field effect is cancelled. However, some of the most basic PI designs do not have any compensation, so there is response that may be heard.

Now, for the discrimination buffs, this coil movement technique can be taken a step farther and used. Remember, iron objects concentrate the earth's magnetic field, so moving the coil above an iron target should create different signal strengths depending upon the sweep speed. So, one should be able to distinguish iron from non-ferrous objects. BTW, I consider this idea in the public domain now.

Ok, lets get back to PI's and how your statements relate. Without going into great detail, the pulse current is turned on and shut off quickly, much faster than any sweep speed. The quicker it is shut off the better. If you sort through the various formulas involved, you will see the flux change will be faster also if the current is turned off faster, and this will ultimately result in a greater signal in the target.

So, it is the rate of change of the pulse current and not the speed of the sweep coil that is the main factor in primary target response in this case.

Hope this helps.

Reg

 

[wyndham]

Prospector Al
That was a very clear and concise explanation. It also gives insight to the problem of very small targets that can not hold a charge so becomes invisible to the detector.
I can see this in my mind as the old Ed Sullivan shows act that spun plates on sticks and having to go back often to spin them up again.

"In a PI detector, the voltage is generated in the target while the coil current, and thus the flux is changing. After the coil pulse, the eddy current decays according to the formula:"

Is the flux change a polarity change flopping poles every pulse?

Thanks for your time and explanation, wyndham

 

[wyndham]

Thanks Reg. It takes me about 6 months to digest all the info from these posts, but it is sinking in a bit at a time.
"iron objects concentrate the earth's magnetic field"
Does blk sand, because it is so small, even in so quanity become invisible or does the concentration of the field help the induction or hinder.
Example a blk sand layer over a target, better or not or nothing?
Thanks Wyndham

 

[Reg]

Hi Wyndham,

Black sand also concentrates the magnetic field and, as such, has a very pronounced effect on VLF's.

Fortunately, the signal from the black sand is gone before the sample is taken on most PI's. However, there is some thought that black sand can influence other target signals what are in close proximity.

Reg

 

[Prospector Al]

The field just changes between zero and a max. value.

(There are some detectors that reverse the polarity of every other pulse, but even then, for a particular pulse, the change is between zero and a max. value...)

P. Al

 

[Rudy]

Assuming you could somehow move the coil so fast that you could actually materially influence the size of the induced eddy current, you need to consider that the receiver is inactive until sometime after the TX current is hut off. When the receive electronics is finally enabled, you better still have the coil somewhere over the target or you won't sense it.

Secondly, the answers so far have dealt with inducing an eddy current, but that is only half the job. Perhaps the easy part. The more challenging electronics is the detection part. The signal generated by the target is as mentioned by Prospector Al is proportional to the mass of the target, its impedance and the cross section that it presents to the TX field. For a small target, the signal strength generated by the target decreases by the square of the distance to the coil (I am assuming the target geometrry is small compared to the distance to the coil and thesize of the coil). In other words, a "point source". A large target at similar distances will not have a signal strength that decreases as the square of the distance to the coil. It will be a more linear drop off. This is the underlying effect that is used by experienced detectorists to tell a coin sized target from a buried soda can, or a small piece of foil near the surface.

 

[Largesarge]

Wyndham:
If I read your last question correctly you're talking about having the coil spinning like say a CT scanner but the vert. plane.

Me thinks you'd be opening a big can'o worms. First the spinning coil would be a bare to swing, going one way the spin would aid you and the other way you'd be fighting it. Second you'd have to have slip rings to transfer the signals to and from the coil. These rings would a source of noise as would the drive motor. You could I suppose put a pre-amp on the coil to boost the return signal enough to overcome the noise but then you'd have put a counter balance on the coil to keep it spinning true.

It occurs to me that the coil would end up costing a good bit more than the detector itself.

 

[wyndham]

Thanks for the info. I realize the limits of a portable detector but the idea of the inductance has intrigued me so my thought is to a stationary platform and how gold and blk sand react to a induced field as they pass thru it. The relationship between size and field strength and time give me so ideas to reinvent the wheel. Thanks again for the help, the principals remain the same as to how a body is seen in a fluxed field. Either to detect it or act on it and how long the field will be active in that window.
A trammel does not get all the gold in the screen but the riffles get more, but how much is lost in the tailings that remain.
If you induce a field into the stream of materials and sample downstream with a rx coil, how small could a target be in practical terms to "set of" signal. gram, 1/2,1.4....?
any thoughts?

 

[Frank-S]

Wyndham,
Reg is right about black sand on VLF,I learning to use a Dowser
here in north FL.I located a target on the beach,after diging down
to dept of 41".my Minelab SOV.gave me a week response under black
sand after removeing it,it turned out to be a quarter (25 cents)The
black sand here is not magnet-et,it had a influence on the targets
dept under black sand.
Frank-s

 

[Carl-NC]

As P.Al said, eddy current strength is proportional to dB/dt. So for a DC magnetic field (say, an ordinary magnet), the faster you sweep a piece of metal through the field the higher the eddy current.

But metal detectors use AC magnetic fields. The dB/dt is already built in, either as the slew rate of a sinusoid (VLF detectors) or as the turn-off slew (PI detector). The speed of the coil sweep can add to these slews as a Doppler shift, but you would have to have a *wicked* fast sweep rate to make even the slightest difference.

So, the answer is that the sweep rate of a normal metal detector won't affect eddy current strength.

- Carl

 

[gatorguy]

I'm not sure I quite understand, first a trommel is a classifier, then the Sluice box with riffles does the catching. There is sufficient knowledge to build any type of sluicing system with multiple underboxes to trap gold of almost any fineness. I don't see how inducing a stream of material? is going to help. The master principle behind classification is to classify as to size then gold will drop out of the mixture because of it's heavier weight compared to other paricles. You don't need to detect gold in the system if that is what you are trying to do, the system itself takes care of that. If you are trying to re-invent the wheel with PI, there would seem to be no need, unless I don't understand your idea. Don

 

[wyndham]

Don, the reason for this rather convoluted idea, comes from a fellow who was losing gold in his system and I thought in theory this might be a way of finding out how much loss there is , if it could detect small enough gold.
The earlier post gives me the math.I thought this could show where the point of diminishing returns shows up.
I know that I've always looked at things from a different POV and thanks to every one for the patients with this thread.
If you feller are scratchen your heads, just think what my wife goes thru with the other ideas I don't post. Thanks again Wyndham

 

[gatorguy]

I understand your ideas, but I think your friend needs to do some research on new sluice designs and catching fines. Flour gold is another subject and I also doubt seriously, you will have any type PI system to read it for losses. With the ability to retain fine gold, if you have that much, you have a system which is running at the high 90%. At that point you are near optimal and I doubt anything will go higher, you just need to process material. If you go to Mike's 49er forum(old Tom Ashwoth forum), there is a discussion back a little ways with design I am talking about and research the man did. Good Luck...Don

 

[Buz]

n/t

 

[bbsailor]

Reg,

What motivated you to make the following statement?

"BTW, I consider this idea in the public domain now."

If this technique is a general PI principle, it gives new value to having a signal strength meter to watch when sweeping a target slow and then fast while observing a different signal strength, espicially for deep targets if they are close and the difference is not clearly detectable by the ear. This is one more technique for extracting additional intelligence from an unknown target, attempting to make it known.

Thanks Reg.

bbsailor

 

[Reg]

Hi bbsailor,

Since there has been a discussion of the technique, I consider the idea in the public domain, so no one can patent it. It seems some companies and some people are patent crazy.



Reg

 

[Reg]

Hi Buzz,

If you have a PI that does not have compensation to eliminate the earth field effects, then, if the detector is sensitive enough, just swinging the coil will generate an audio response. Usually, this is something like a groan or a whine and can be direction dependent.

On the same detector, if you make a sudden stop, you will or can get a distinct audio response.

Now, realizing that iron objects concentrate the earth's field, then sweeping over an iron object should generate a distinct response, again, providing the sensitivity is high enough. This will not happen if the target is a non-ferrous object.

Now, on a PI that has compensation built in, such compensation would have to be inhibited during testing. Other changes may have to be made also, depending upon the PI itself.

Now, all is not perfect, since the signals would be direction sensitive, meaning things will change depending upon the way you are facing and the coil is swinging.

This same thing happens on a simple magnetometer also. If you use something simple such as a linear hall effect generator, amplify the signal sufficiently, you will be able to distinguish iron objects reasonably well. I know, I have done it. Now, in this case, it is best to build in a form of compensation (a second hall chip) so the motion of the coil is not what generates the response, but rather the distinct difference in field strength caused by an iron object. This makes a separate unit better in many respects. Also, in this case, one could build an absolute value detector so any pulse signal becomes a positive one, which would minimize problems of being direction sensitive.

So, it is almost as easy to build a complete mag unit mentioned rather than try to modify an existing PI. This way, they are separate but could be used together. Since the hall generator is so small, they could be mounted on the coil and used at the same time as the PI. Some trickery may be needed to minimize the problems caused by the pulse signals.

Reg

 

[bbsailor]

Reg,

What you said, about obtaing a different signal from iron by changing the sweep speed, suggests a Hammerhead PI modification which temporally disables the secondary sample signal before the integrator stage by using a simple push-button switch. If so, where would you place the N.O. pushbutton in the circuit for the best results?

bbsailor