Targets frequency response

[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.

[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

[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?

[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?

[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

[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.

[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!

[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

[Qiaozhi]

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.

[Carl-NC]

Mike,

I agree, the discussion has shifted from the original topic, although a good understanding of eddy response is not a bad thing to know.

What needs to be done is a thorough investigation of target phase response vs frequency. This is easier said than done. You either need a excellent phase-linear wideband detector, or you need to characterize its phase response and compensate target results. The latter would require a target with a predetermined phase response.

I would suggest a DDS-based TX circuit for easy programming and a good clean sinusoid. Then a traditional I/Q receiver driving an ADC+micro, where the micro computes a phase angle and drives a simple display. The hard part is designing the coil, and ensuring the whole thing has a good inherent phase response. But it can be done.

The analogy with light doesn't hold for detectors. Changing frequency just shifts target phase responses. This would be like something appearing to be blue under a red light, but orange under a green light.

- Carl

[mikebg]

What to experiment?

What information is needed on one amateur designer, to design metal detector possibly surpassing existing models?
Can we acquire this information through strange experimens as drilling of coins?
Two years ago my goal was to design a metal detector for meteorites location. Some of these "heavenly stone" are purchased at prices higher than gold. I began to look for relevant information and because I could not find it in the WEB, I turned to this forum with a question of interest to me "frequency response". Note that the question is in frequency domain. He could be asked in another way in frequency domain: "Does published somewhere cutoff frequencies of different targets?"
If I had to formulate both questions in time domain, they would be respectively:
1. "Does published somewhere impulse characteristics of different targets?" And
2. "Does published somewhere time constants of different targets?"
This information is necessary to know how targets differ in "color". My "meteorite locator" would receive signal not only from stones stuck in the dust, but dust and if there are any other targets. To identify targets by "color" the TX must illuminate them with white light. Using wide band TX and RX has disadvantaged for two reasons. The first reason is the huge power consumption. Wide band of TX can be achieved only by dumping process which means waste of energy in resistances. This implies a large and heavy batteries.
The second reason is that the wide band RX provides an increased level of thermal noise and interference (note the dimension "sqrtHz").
What information about targets can be obtained at SI (Sine Induction)? This is like lighting objects with colored light? You can experiment with lighting by green light to a green and a white object. They will look identical green in color because their spectral response is the same in the green region of the spectrum. Then illuminate both items with a red light. The white object will appear red, but the green - black.
This means that you can discriminate targets with SI (narrow band) metal detector, if appropriately chosen the operating point in frequency spectrum. The narrow band machine has three advantages:
1. Economical consumption of battery.
2. Low noises and low interferences.
3. At SI can be increased modulation index of the useful signal by automatically compensating signal at the input of RX preamp. If so, we can increase the gain of RF amplifier in RX, which will provide greater depth of detection.
When I asked my colleagues on the hobby (the REMI group) how to make a compromise between two conflicting requirements for wide band and narrow band, they simply answered: "Using a key SI / PI (Sine Induction - Pulse Induction)". Then they gave me 3 old circuit diagrams (over 100 years!). These are ancient AF metal detectors made of telephone parts.
I think we need to measure spectral characteristics of different targets and soils. Computer sound card is adequate to do the job. Wolfgang Buescher created such a program for amateur radio. Search WEB for DL4YHF's Spectrum Lab (Audio Signal Analyser). Maybe Aziz will offer a program designed specifically for measuring the spectral characteristics of targets and soils.

[Carl-NC]

These methods work well for DC magnetic fields, probably not for AC fields.

[Qiaozhi]

Hi Ivconic,

I have a large sheet of magnetic field paper that I could use.
Here's a website that shows something similar -> http://www.coolest-gadgets.com/20071...c-field-paper/

[ivconic]

Or this!!!!

http://www.youtube.com/watch?v=H9b3lePwN4I

[ivconic]

http://www.youtube.com/watch?v=wuA-dkKvrd0&feature=fvst

http://www.youtube.com/watch?v=uj0DFDfQajw

http://www.youtube.com/watch?v=Dn6VVZ3mrs4