Targets frequency response

[Qiaozhi]

Hi Qiaozhi,

it doesn't matter. I just want you to discover your wrong assumptions.

Particularly in the coin #2, #3, #4 and #8, the green opposite current flow is wrong. As long as the ring is not open, the current direction must be the same as on the outer side. The current density is lower in the inner area (could be visualised in light red).

I am sorry, but the circular eddy currents theory is not busted yet.

Aziz

If the concentric ring theory was correct the original coin (#1) would give the best response, as all the "rings" would contribute to the RX signal. Likewise coins #4 and #7 would have the worst response, as a large number of the "rings" are missing. In reality coins #1 and #7 give the same response, and coin #4 gives the best.

[Qiaozhi]

Hi Qiaozhi,

thanks for posting this excellent work.

Now, how would it look for a sphere instead of a coil? My guess is, probably not much different, since the VLF runs at low frequency.

It does look different for a PI though.

The di/dt at switch off of the PI can be a 100 times higher, so we are definitely in the skin effect region.

Tinkerer

The formation of eddy currents in the target is slightly more complicated than my simple diagrams suggest, as they assume a 2-dimensional shape. However, any currents that circulate across the top and bottom of the coin will be perpendicular to the the plane of the coil and will not contribute to the target response.

Therefore I agree that a sphere, with the same cross-section as the coin, will most likely give a similar response.

[mikebg]

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

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

Ivica,
the cutoff frequency "fc" of a conductive target is explained in many postings of this thread. This is the point M shown in the lesson of posting #8, Exercise 1 in the posting #18, the figure in posting #42 etc. The Bode diagram is shown in several postings and was explained why it is called "cutoff frequency". If needed, I can explain it again with this draft, which is more clear than in the present posting #42. Is there a need?

[Aziz]

Hi Qiaozhi,

it doesn't matter. I just want you to discover your wrong assumptions.

Particularly in the coin #2, #3, #4 and #8, the green opposite current flow is wrong. As long as the ring is not open, the current direction must be the same as on the outer side. The current density is lower in the inner area (could be visualised in light red).

I am sorry, but the circular eddy currents theory is not busted yet.

Aziz

[Qiaozhi]

Hi Qiaozhi,

thanks for your excellent work.
Could you make a tenth coin?
Take coin #3 and make a cut version of this (open the ring).
Than compare #3 and #10 (new one).

Aziz

Unfortunately I do not have coin #3 anymore. During the first set of experiments with the steel-based coins I managed to blunt several drill bits. Therefore I filed out coin #3 to make coin #4. However, I still have coin #2, which I have cut down to the inner circle. The response of this new coin (coin #10) is identical to the original coin. This of course now means that coin #2 no longer exists.

[Tinkerer]

Hi Qiaozhi,

thanks for posting this excellent work.

Now, how would it look for a sphere instead of a coil? My guess is, probably not much different, since the VLF runs at low frequency.

It does look different for a PI though.

The di/dt at switch off of the PI can be a 100 times higher, so we are definitely in the skin effect region.

Tinkerer

[Aziz]

Hi Qiaozhi,

thanks for your excellent work.
Could you make a tenth coin?
Take coin #3 and make a cut version of this (open the ring).
Than compare #3 and #10 (new one).

Aziz

[Qiaozhi]

To remove any confusion, here is a summary of the tests I have performed so far, and my conclusions following these experiments. Please refer to the diagram below.

Different colors have been used to denote different parts of the experiment, as follows:

• The coin is shown in "brown".
• The individual eddy currents (gross representation) are shown in "blue".
• The areas of maximum current density are shown in "red" and "green".

First - some explanation. When the coin is close to the search head, eddy currents are generated throughout the body of the coin. These eddy currents tend to overlap each other, and due to the principle of superposition they more or less cancel each other, except around the circumference of the coin. This is where the current density is highest, resulting in a current flow around the circumference. So there are two things happening here:

1. The current (shown in "red") generates a magnetic field that is detected by the RX coil.
2. The body of the coin absorbs some of the TX energy, but fails to produce any significant magnetic field. i.e. the body of the coin actually weakens the target response.

It should be noted that the restriction of the current to the coin circumference has nothing to do with skin effect. This phenomenon is only significant at frequencies above 100KHz, and not at the VLF frequencies we are talking about here. Anyway, the skin effect only limits the depth of penetration of the TX signal, and hence the creation of eddy currents. Therefore in these experiments the whole volume of the coin has an effect on detection, not just the surface.

Coin #2 is identical to coin #1 except for a small hole drilled in the center. By the same reasoning you can determine that there will be a current flowing around the inner circumference of the hole, but in the opposite direction. This current is too far from the outer current to cause any cancellation. The result is that coin #1 and coin #2 produce no measurable difference in detection distance.

Coin #3 has a larger hole in the center than coin #2. In this case there is still insufficient material removed from the coin to provide a detectable difference.

It is not until enough material is removed (coin #4) that the target response is improved. What seems to be happening here is that more of the TX electromagnetic field is concentrated in the ring, and less energy is being lost in the coin body, which has essentially been removed. There is still one more experiment to perform, and that is shown by coin #8. This represents a situation where the ring is now very narrow. The expectation here is that the inner and outer currents will start to interact and the target response will again be reduced.

The experiments performed with coins #5, #6 and #7 all confirm the assumption that a current path is established around the circumference. It is unlikely that the air gap is being bridged by displacement current as the gap is too wide. These three coins all give a target response that is indistinguishable from coin #1. The only conclusion is that the current follows the edge of the cut. If this was not the case then the cut coins would have a reduced target response, whereas they have neither an improved or a reduced response.

All of this, of course, assumes that the coin is lying flat in the same orientation as the search coil.

If the concentric ring theory was correct, the three cut coins would show a massive reduction in target response. Which reminds me that there is one experiment I forgot to show in the diagram. That is a ninth coin, identical to coin #4, with a cut through the ring. This particular coin has a very poor target response, but the concentric ring theory would predict that coin #7 and coin #9 (not shown) would both show the same reduction, which they do not.

It is this last experiment that is probably the most interesting. As we have seen, you can drill out material from the center of the coin and the response improves. But if you remove material by cutting in from the edge, the response will initially be the same as coin #1. However, removing more material by this method causes the response to be dramatically reduced. The cut ring is an extreme example.

[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?

[ivconic]

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

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

[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.

[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.

[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]

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.

[ivconic]

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