Targets frequency response

[Tinkerer]

Different target response

Bruce, hope the move went well.
If I understand the different screens you posted that for a given induction most of the decay, regardless of density or surface area(except the ring) of the material, decayed almost the same. Does that mean that the targets producing the eddy currents did this or the RX only saw this amt of decay?
Maybe I'm reading the charts wrong, but shouldn't diferent target show more of a difference in the decay(except for the ring)?
I am very inexperienced(none) in the nuts and volts of circuits and the math that goes with it, so I'm limited in many areas here.
It would seem that non magnetic metal objects might have a min decay time(that doesn't seem right) and though you didn't post it yet, is the decay time in linear or exponential relationship to the induction. I would guess exponential since that's what I seem to see on other screens and post that indicate the exponential amt of tx energy to add a modest amt of depth.
Also would it be possible to use a 24 bit sound card in a pc for the input of a induced signal and sound software to see the transient signal? That's about all I have that might be useful.
Thanks for your time when you have some for a wannabe
Wyndham

The target response is composed of several different components.

The resistive component
The reactive component
The skin effect eddy currents
The core eddy currents
And maybe a few more.

To visualize or read a specific component, the processing of the signal has to be specifically designed for that specific component.

For example: to read the response of the skin effect, the sample has to be taken very early, meaning after 5 to 10 us after the transient.
The skin effect relates more to the surface area presented to the field.

The effect of the core eddy currents last much longer, they are related to the TX time and represent mostly the TC of the target.

If You want to see the reactive response of FE targets, you need to know where and how to look for them.

A tip from Eric Foster: If you present a thin, FE disk the flat way to the coil, you see mostly the resistive component of the target.
If you turn the same disk on edge, you see mostly the reactive component of the target.
Try this and compare by doing the same test with a thin disk of similar dimensions of non magnetic material. The results will prove without doubt.

Starting with this good piece of advise, I developed a fully discriminating PI platform. Thanks Eric.

Tinkerer

[wyndham]

induction test

Bruce, hope the move went well.
If I understand the different screens you posted that for a given induction most of the decay, regardless of density or surface area(except the ring) of the material, decayed almost the same. Does that mean that the targets producing the eddy currents did this or the RX only saw this amt of decay?
Maybe I'm reading the charts wrong, but shouldn't diferent target show more of a difference in the decay(except for the ring)?
I am very inexperienced(none) in the nuts and volts of circuits and the math that goes with it, so I'm limited in many areas here.
It would seem that non magnetic metal objects might have a min decay time(that doesn't seem right) and though you didn't post it yet, is the decay time in linear or exponential relationship to the induction. I would guess exponential since that's what I seem to see on other screens and post that indicate the exponential amt of tx energy to add a modest amt of depth.
Also would it be possible to use a 24 bit sound card in a pc for the input of a induced signal and sound software to see the transient signal? That's about all I have that might be useful.
Thanks for your time when you have some for a wannabe
Wyndham

[Aziz]

I have found a method, to compute the target frequency response.
The way is to setup two differential equations of the second order for the primary coil and for the target (coil). As the system is inductively coupled, these two differential equations are also coupled together.

For the analytical solution, it is not trivial and often it can not be done. Using numerical methods (like finite elements, numerical derivations) gives a good approximation.
The mutual inductance between the primary coil and the target (coil) must be taken into account. Also any capacitance of the primary and target.

I will search for more infos..
Aziz

[B^C]

Gday Aziz,

I will set things up & get some data for you, just leaving now.
I'll be in touch.

[Aziz]

Hi B^C,

I would appreciate the following data (if possible):
- noise characteristics on the signal decay curve (e.g. after the pre-amp)
- ground signal (+noise) on the decay curve (without target) on most critical mineralized grounds (I live in Germany, so I have no idea about Oz ground).

This would help, to analyse and eliminate the noise problem. As the lock-in amp can not be used easy and conventional on the PI's, I have to look for other solutions. So using lock-in amp sucks a little bit here.

Regards,
Aziz

[B^C]

Hi All,

I have intruments that i can collect some good data on.
Is there any particular data that will be useful to others?
If so make a list & in the coming weeks i will set about doing some tests & give the results. I am moving to a new location tomorrow & will start to set some things up for testing.
I have a lot of things to test out for myself but if i can help out just let me know.

[amtech2005]

Targets frequency response

There is quite a bit posted on the web "www" about coin characteristics and also archaeomagnetic coins. You just have to be carefull how you type your search!! If not You will get 50,000 "metal detector for sale" sites!!! . I started reseaching today on Magnetic fields of coins and I found this on a public website!! Eugene

[Aziz]

Hi B^C,

have a nice trip. I am looking forward to your progress. You are enviable.

Aziz

[TheWizard]

......Green--Red is COLOUR.......

What signals, how its make, why red is before green.... for me, yours pictures is blah blah....

[B^C]

Gday Aziz,

Yes, the instrument is ok, i have spent considerable time building things up & to eliminate all noise problems, your link to noise problems has helped very nicely. Now i have a very nice signal---Thanks!

I have been testing many things with surprising results & also how different coils respond to very small recieved signals--very interesting to see this now on computer. I need to setup things more accurately to show extended test results.
I have a good idea now on various diameter & types of coils vs there sensitivity from small target signals, i will setup up things better & do multiple tests to give accurate results & post them when i am finished--very interesting, this will take some time as i am now moving to a new location--Gold Area .
I will be setting up tests in actual Ground conditions to see target sensitivity at depth from various signals & frequencies & how this affects the recieved signals in differing ground conditions.

I should be able to give some informative information if i can test these things accurately.

Mrs B^C is now going, BLAH,BLAH,BLAH,BLAH so i will have to go pack up some things ready to move.


TheWizard,

Green--Red is COLOUR.

[Aziz]

Hi B^C,

the fourth picture showing the difference between ring and surface targets best.
As on the ring, the induced eddy currents have to go through the ring path. Defining a relative constant radius of the "coil".
However on the flat targets, there are more (concentric) paths induced in the flux area of the target. These different current path cancels some of the outer eddy current induced magnetic fields. Also the small radius current path have small magnetic fields result. Therefore the total response is smaller compared to ring targets.
On the other side, one would expect, that flat targets would consume more magnetic field energy due to eddy currents producing heat on the target. Probably, the magnetic field is collapsing to fast, to see the difference. It would be interesting to see a response of super conducting target (more decay time)!

To see the differencies on the decay curve better, its is probably practical to make the first derivation of the normalized signal curves and comparing this.
Otherweise, the differencies are to small to see.

Regards,
Aziz

PS: Nice instrument. I would dream of such an instrument.

[TheWizard]

I see good device - 20 Mega Samples - LOL

What is GREEN, RED... explain pls

[B^C]

Hi All,

I have setup an experiment of inducing various metal objects with the same magnetic flux intensity etc. I wanted to read the induction & rise time values & decay times of various objects to get a better understanding of things.
From what i have done the induction values, rise time etc change & is very responsive to area of contact & material but the Decay times are all near identical.

For some test Standards i used a 25mm square piece of Aluminium foil & also a single turn loop of AWG18 Magnet winding wire--1 Turn formed around an AA Battery giving an outside diameter of 17mm. This is the first Attachment, Green reading is the Aluminium Foil & the Red reading is the single turn of AWG18 wire.

I then tested a 1grm reasonably FLAT Gold Nugget vs the Single turn AWG18 loop, the results i found interesting & is the second attachment.
Flat Gold Nugget is Red, Single turn loop is Green.

The Third Attachment is the 1grm Flat nugget vs another 1grm Nugget that is 1mm narrower, Flat Nugget is Red, Narrower Nugget is Green.
I did this test to see how responsive the readings were compared to surface area of the the same type of target.
The Nuggets are the same length but the width is 1mm different. The test shows as things should be with the wider Nugget a little stronger, so the tests seem to be responsive to change.

Fourth Attachment is the 1grm Flat Nugget vs a very Fine Gold Ring, Nugget is Green & Gold Ring is Red.

I did a lot more testing on ferrous objects etc with good results, i won't attach them unless somebody will find them usefull.

You will see there is variation in the produced signal & the rise time of the different objects but the decay is almost identical, what's your thought's, besides i wasted a ****load of time?

I thought with the Decay being almost the same on all tests done that the Driving Test Coil was interfering with the Decay times. I found this not to be the case by moving the Test Coil Across the test pieces just like a Metal Detector & the same Decay time results applied.

Have i done this correctly, have a look at the attachments & you will see what i mean?

[TheWizard]

AZIZ
This HI freq. Noise in Pulse Induction is NOT problem
The big problem is 50/60 Hz and low freq. noise.....

[mikebg]

Re: mneteorites

TNX B^C for your posting about meteorites.
It contains information, which I can't find in the WEB. I have questions about results shown in the figures, concerning method used for measurements. However, before to ask you, I should browse WEB for answers, because it is for shame one to ask for something, which is written, explained and even animated in the WEB.
Unfortunately, more beauteful and more expensive meteorites have very little permeability. Their price however is larger than price of gold. See in the WEB that one gram meteorite can be sold at price greater than one gram gold. The most expensive meteorites can't produce signal in air at all. That forced me to use discontinuity in matrix for meteorite location and to use as sensor coplanar gradiometer (differential configuration of RX coils).
Fortunately, experts on magnetic tape recording, showed me recently that my idea to use air gap as demagnetizing factor and to change hysteresis cycle, is very stupid (to put it mildly). Air gap is ineffective passive demagnetizator. There is active method used for linearisation in magnetic recording of continous audio signals. Now engineers on audio recording study (instead me) patents of B. H. Candy, to explain me how he with short bidirectional pulses demagnetizes matrix.
I start studying magnetic tape recording to understand how HF bias erases magnetisation and how linearises magnetic properties of tape. May be the future PI detectors will have an additional sinusoidal HF generator at 150 kHz, in order to make magnetic soil transparent for EMI sensor (instead comprehensive TX pulses used in B.H.Candy's designs). This becomes ever more and more interesting.
Regards. Mike.