Targets frequency response

[Tinkerer]

Latest test results:

1. Original coin
2. 5mm long slot from circumference towards center.
3. Slot extends from circumference all the way to the center.

There was not detectable difference in the response to these targets.

Regarding the possibility of using magnetic field paper to view the actual field ... of course, that was a stupid idea. As Carl said, the field is a.c. not d.c.
In addition, the magnetic field paper acts as a target to the metal detector.

If you imagine the huge number of small eddy currents being generated in the target, and the manner in which they superimpose each other, then it is clear why the slotted coins have the same response as the original coin.

Thanks for doing the experiment.
In other words, it confirms Carl's drawing of the eddy currents in a coin.

Tinkerer

[ivconic]

"...Two years ago my goal was to design a metal detector for meteorites location..."

This is interesting. Few years ago i 've been talking with one friend from EU on this subject. He is meteorite prospector. He used Pulse Star II Pro with more or less succes. Main remark was PS stability at trashy areas. So we considered use of Delta Pulse, adapted for such conditions.

What you didn't mentioned are usual depths you are expecting and types of soil you've planing to search on. So...what depths and what type of soil?
Third; what is the usuall/common size of meteorites that you can find there?
It is simply impossble to design metal detector with which you could cover all the possible cases , no matter the technology and type - still impossible. Pitty. Provide here more detailed informations.
Regards!

[Qiaozhi]

Thanks for doing the experiment.
In other words, it confirms Carl's drawing of the eddy currents in a coin.

Tinkerer

Yes, I think that is true.

If the eddy currents were really flowing as a set of concentric rings, then cutting a 5mm long hole from the circumference to the center would effectively break the outer rings and reduce the target response. Also, the coin with the full cut would have a very poor response. The experiment showed that this was not the case.

What appears to be happening is that the eddy currents more or less cancel each other throughout the whole target, except at the circumference. This effectively leaves one current path around the edge of the coin. The interior of the coin tends to inhibit the target response by absorbing energy. By removing the center of the coin, and only leaving a thin outer ring, the TX energy is concentrated in the remaining material, and the target response is improved. When you break the ring, you also break the current flow at the circumference.

This also explains why the partially cut coin gives the same response as the original coin. The current path around the circumference is still there, but it is slightly diverted around the cut. Likewise for the full cut that goes all the way to the center. In fact, I would expect to get the same result if I extend the cut through the coin to within 5mm of the opposite side. Yet another experiment for tomorrow.

[Aziz]

Hi guys,

the gap in the coin does not interrupt the inner circulating eddy currents. You can cut it almost to the other side and it should still give response without any significant response loss.
The response losses are due to flux area loss of the cutting line and the increased current path resistance (PI: time constant change, IB-VLF: phase change).

Here is something to think of (see below):
Aziz

[WM6]

Hi Aziz, if your statement is correct then we have in case of spring washer or open ring such current distribution:



Clearly opposite direction. So spring washer or open ring have to be invisible, but it is not invisible?

[ivconic]

All the statements and suggestions here on this topic are still on assumption level. Hypothesis level.
That's why is so tough to give proper answer.
We should be able to see eddy currents on coin surface so to know exact behavior and be sure in right answer.
Is there any method, instrument or apparatus to visualy display and observe eddy currents on coin surface?
Aziz recently presented software model that can be adapted and used in those matters. But that model as any other software model is based on algorithym with mathematical functions and calculations - not neccessary 100% true and illustrative in real life. Yet good as something to start with, for sure.
So...again:
Is there any method, instrument or apparatus to visualy display and observe eddy currents on coin surface?

[Aziz]

Hi WM6,

Hi Aziz, if your statement is correct then we have in case of spring washer or open ring such current distribution:



Clearly opposite direction. So spring washer or open ring have to be invisible, but it is not invisible?

on your open ring, there is still some flux area for induced eddy currents. As long as the flux area is not zero (inducing voltage) and the current path is closed (currents can flow), the object will be visible.

There are other effects, which change the detecting parameters:
- relative permeability (material dependent)
- coupling of coils (magnetic field path changing effects, can change the balance of coils on IB-VLF systems)
- ohmic losses (when eddy currents flow)
- electro magnetic induction effects (currents will even flow, if the current path is obviously not seen as a closed loop -> "antenna" ).

Consider the open ring: it is an inductor + capacitor (LC-circuit).
We could bring the "LC circuit" into resonance.

Aziz

[WM6]

Hi WM6,

on your open ring, there is still some flux area for induced eddy currents. As long as the flux area is not zero (inducing voltage) and the current path is closed (currents can flow), the object will be visible.

There are other effects, which change the detecting parameters:
- relative permeability (material dependent)
- coupling of coils (magnetic field path changing effects, can change the balance of coils on IB-VLF systems)
- ohmic losses (when eddy currents flow)
- electro magnetic induction effects (currents will even flow, if the current path is obviously not seen as a closed loop -> "antenna" ).

Consider the open ring: it is an inductor + capacitor (LC-circuit).
We could bring the "LC circuit" into resonance.

Aziz

Hi Aziz

we are discuss frequencies of about 7kHz, not 700 MHz. So most of your indications here are practically not important for eddy current events inside small open ring (e.g. LC-circuit etc.).

Here You agree with Carls primary driving:

"currents will even flow, if the current path is obviously not seen as a closed loop"

Yes, current will even flow inside many small eddy current areas and not as closed loop follow periphery shape.

[ivconic]

http://www.mac-ndt.com/NDT/EddyCurre...FZ1h4wodzDTMJA

[Qiaozhi]

This also explains why the partially cut coin gives the same response as the original coin. The current path around the circumference is still there, but it is slightly diverted around the cut. Likewise for the full cut that goes all the way to the center. In fact, I would expect to get the same result if I extend the cut through the coin to within 5mm of the opposite side. Yet another experiment for tomorrow.

We're in the middle of a snow storm here, so this simple experiment took me a little longer than expected.

After extending the cut all the way through the coin, to within 2mm of the other side, the target response was identical to the original uncut coin. Exactly as I predicted. Conclusion - the concentric ring theory is busted.

[mikebg]

Quiaozhi,
The frequency of conducting your experiment falls in low frequency region of the target.
As smaller pieces you split the target, the more bass is the TX frequency for them.
In order to avoid this effect, do the same experiment with large equal coins placed close to each other, as shown in the sketch.
However, you must know the cutoff frequency of a coin, to see if the experiment is in the LF region or in HF region.

[Qiaozhi]

Quiaozhi,
The frequency of conducting your experiment falls in low frequency region of the target.
As smaller pieces you split the target, the more bass is the TX frequency for them.
In order to avoid this effect, do the same experiment with large equal coins placed close to each other, as shown in the sketch.
However, you must know the cutoff frequency of a coin, to see if the experiment is in the LF region or in HF region.

I don't really understand what you're saying. This might be because you are using a translator, and English is not your first language. Please can you explain again?

Also, there is no "u" in Qiaozhi.

[Esteban]

I don't really understand what you're saying. This might be because you are using a translator, and English is not your first language. Please can you explain again?

Also, there is no "u" in Qiaozhi.

Make the test with a lot of coins each other.

[ivconic]

"... the cutoff frequency of a coin..."

???
I think Qiaozhi is confused mostly with this statement. Me too.

[Qiaozhi]

Make the test with a lot of coins each other.

That's the only part I could understand. But what is the point of the test?