Targets frequency response

[gwzd]

...
...For "homework" you can SPICE simulate the attached circuit diagram...

No thanks, the only thing I "spice" is my "pasta del diablo".

[gwzd]

To gwzd:
Your opinion that “EMI methods are absolutely useless for finding plastic mines” is quite wrong.

As I said before, make one and get rich beyond imagination.

A suitable designed "metal" detector can locate buried in the soil nonmetalic items and targets like: voids, nonmagnetic stones, nonmetalic pipelines, terracotta and even the place where recently a hole is filled in with soil.

Do not miss the line "There must be enough mineralization in the ground so the cave or void represents a marked reduction in this amount of mineralization". Now, what did I tell you?

The attached file contains pages from the previous edition of Instruction Manual for TM808 (in the latest edition these pages are omitted). Once upon a time, the "metal" detector TM808 was designed to detect voids, despite its not suitable sensor and despite its not suitable TX frequency. The suitable differential sensor was invented in 19-th century, but it is used effectively for soil clearing since 1917. The differential EMI sensor is used and now for demining - read patent US 6,853,194 issued to Nelson et al. in 02/2005. This sensor is most suitable to locate filled in holes and signal is stronger if there is a nondetectable in air object (plastic mine) buried inside the hole.

Commercial BS nothing else, if this would work there wouldn't be so many mutilated people in southern Africa and Asia. You have no idea what you are talking about.

If you measure resistance with wire probes and an ohmmeter using AC source, the measured value will depend on the frequency because of skin effect. You can use an EMI sensor as wireless ohmmeter, but a conductive (eddy current) target convert the EMI sensing system in twice differentiating network (see equivalent circuit diagram in my previous posting). Because of that, unlike an ohmmeter, the measured resistive component is zero if the TX frequency is zero. There is a frequency for maximum resistive signal.

Then again your miss conception of the technique, the measured value is "apparent conductivity". You have a paper but I have the equipment. check this:

http://www.geophysical.com/Profiler.htm

I use this equipment only when the conductivity of the ground is so heavy that our GPR units become useless.

Another example: The lossy ferrite is roughly nonconductive (infinite resistivity measured with ohmmeter), but imports transient resistance in EMI sensor, which increases with frequency.

Wrong again, it doesn't measure transient anything.

In one of my next "homeworks", we will exercise how to increase ground signal in order a differential type sensor to locate a filled in hole (where may be a plastic mine is buried).
Please keep to read my future postings and "homeworks", because I hope experts to find in them errors.

I don't know if it is worth to do that. I have told you many times that it doesn't work based on my practical experience and theoretical background but you keep ignoring the main issue: you cannot induce electromagnetic field in non metallic objects.

I'm only an amateur (ham radio) designer.

Now, this was a surprise! I always understood Ham's were very technical people, at least the ones I know are. Please read an elementary book on RF so you get a better understanding on the nature of RF circuits:
RF Circuit Design, by Chris Bowick
That should give you a good start.

My ambition to design meteorite locator and the lack of WEB information for target frequency responses

A bit too late for that:

http://www.meteorlar.com/detector.html

In one of the future my postings I will send a "homework" written especially for you,

Don't waste your time and read some books and build something instead. That's going to give you more profit than trying to convince me that you can bend the laws of Physics.

how to find errors in a publication from 1999 for demining.
Yours Mike.

Books and articles do have errors, no one argues about it. I just hope the errors you find are not the same kind as the ones you have exposed here so far.
Regards

[mikebg]

"Metal" detector locates nonmetalic targets

To gwzd:
Your opinion that “EMI methods are absolutely useless for finding plastic mines” is quite wrong. A suitable designed "metal" detector can locate buried in the soil nonmetalic items and targets like: voids, nonmagnetic stones, nonmetalic pipelines, terracotta and even the place where recently a hole is filled in with soil. The attached file contains pages from the previous edition of Instruction Manual for TM808 (in the latest edition these pages are omitted). Once upon a time, the "metal" detector TM808 was designed to detect voids, despite its not suitable sensor and despite its not suitable TX frequency. The suitable differential sensor was invented in 19-th century, but it is used effectively for soil clearing since 1917. The differential EMI sensor is used and now for demining - read patent US 6,853,194 issued to Nelson et al. in 02/2005. This sensor is most suitable to locate filled in holes and signal is stronger if there is a nondetectable in air object (plastic mine) buried inside the hole.
If you measure resistance with wire probes and an ohmmeter using AC source, the measured value will depend on the frequency because of skin effect. You can use an EMI sensor as wireless ohmmeter, but a conductive (eddy current) target convert the EMI sensing system in twice differentiating network (see equivalent circuit diagram in my previous posting). Because of that, unlike an ohmmeter, the measured resistive component is zero if the TX frequency is zero. There is a frequency for maximum resistive signal.
Another example: The lossy ferrite is roughly nonconductive (infinite resistivity measured with ohmmeter), but imports transient resistance in EMI sensor, which increases with frequency. In one of my next "homeworks", we will exercise how to increase ground signal in order a differential type sensor to locate a filled in hole (where may be a plastic mine is buried).
Please keep to read my future postings and "homeworks", because I hope experts to find in them errors. I'm only an amateur (ham radio) designer. My ambition to design meteorite locator and the lack of WEB information for target frequency responses, forced me to reinwentthe wheel. Users and amateur designers of metal detecting equipment need this information and need exercises for self-education. Believe me, after a brief self-education I can find errors in other publications and designs of metal detecting equipment and I can teach others how find them for "homework". In one of the future my postings I will send a "homework" written especially for you, how to find errors in a publication from 1999 for demining.
Yours Mike.

[mikebg]

AC analysis

Addition to Exercise No 1.
An "Absorption metal detector" is sensitive only to resistive component of received signal. For each conductive target, there exists a frequency, at which the absorption (resistive component) of the received signal is maximal. For nonferrous conductive target this is the cut-off frequency, sometimes named also: 3dB frequency, corner frequency, break frequency, roll-off frequency, or characteristic frequency. It is represented by pont M in figures 1 and 3.
CONCLUSION: Despite absorption MD is narrow band (single frequency) machine, for best results it should have a control to select suitable TX frequency. Highest frequency is needed to detect wires in the wall.
For "homework" you can SPICE simulate the attached circuit diagram. This is simplifyed equivalent circuit of sensing network with target having a single eddy current. SPICE will show how EMI sensor transforms a low pass filter in high pass filter and how the Re part of eddy current transfer admittance, transforms in Imaginery part of transfer impedance (and vice versa).

[gwzd]

Here is the first exercise with principles explained in the "Lesson". The title is

"Normalized Transfer Impedance of an Eddy Current"

You can post me your answers and solutions as private message if you wish my corrections. If you wish others to discuss and correct your answers, post it direct in the thread.

You should do your home work yourself, then perhaps you might learn that a non ferrous metal is still a metal. Ah..., if you meant that impedance is dependent on frequency then, of course it is. But the nonsense you argued about was that the "resistance" was dependent on frequency. Impedance and resistance are related but not the same thing.
One last thing, mathematics are nothing without a good understanding of the physical processes taking place.
I wish you luck with your school assignment and when you can detect plastics with a metal detector then file a patent and get rich. Why do they call them metal detectors anyway?!

[mikebg]

Exercise No 1

Here is the first exercise with principles explained in the "Lesson". The title is

"Normalized Transfer Impedance of an Eddy Current"

You can post me your answers and solutions as private message if you wish my corrections. If you wish others to discuss and correct your answers, post it direct in the thread.

[gwzd]

Hi gwzd,
The thread is Geotech>Forums>Coils>DD-coil Analysis (Sensitivity Profile).
The patents are US 3,002,262 and US 3,823,365.

Hi mikebg,
As I suspected it, plastic still remains non-magnetic.
The thread doesn't say anything about UXO, or perhaps I missed again.

As for your patents, well you probably misunderstood but they refer to *metal* detectors with the emphasis in metal.

Pat.No 3,002,262
...so that they are balanced to a null output when no target is present, but so that they produce a resultant signal in the presence of targets having MAGNETICALLY PERMEABLE or ELECTRICALLY CONDUCTING masses.

Pat. No. 3,823,365
Patent title: "Metal detecting apparatus having improved ground-effect immunity"

summary of the invention: "In accordance with the present invention, an apparatus for detecting METAL OBJECTS includes...."

As said before, if you want to detect non-metallic objects magnetic permeability is not your friend, dielectric constant on the other hand is your definite allied. Think of GPR and you'll be safer in a mine field, and pay attention I typed "safer" not "safe".

Have a very nice day mikebg

[mikebg]

Hi gwzd,
The thread is Geotech>Forums>Coils>DD-coil Analysis (Sensitivity Profile).
The patents are US 3,002,262 and US 3,823,365.

[gwzd]

To gwzd,
Boy, I have posted in an other thread for Aziz the description of a sensor used to detect metallic and nonmetallic UXO. ...

Hi mikebg,
I'm really bad at searching stuff in forums. I couldn't find your post, could you please post the link again.

Essentially, what you are saying is that you can induce an electromagnetic field in a piece of plastic, am I wrong? or this is something in the same league as the real part of the impedance being dependent on frequency? or to put it bluntly, next time I'll use a resistor should I check first the operating frequency of the circuit? am I wrong again?

The metal content in plastic mines is so minimal that you would have really hard time discriminating it from the minerals in the earth. The closest we have got to the moment is using GPR combined with metal detectors and I can tell you is far from being perfect or 100% reliable.

There people here who really wants to learn, please stop typing your nonsense and confusing people, specially in a subject so serious like UXO detection.

Waiting for your answer

RA

[mikebg]

To gwzd,
Boy, I have posted in an other thread for Aziz the description of a sensor used to detect metallic and nonmetallic UXO. It can detect by EMI and plastic mines if they are buried. The sensor creates powerful ground signal, which changes in the place where is the nonmetalic object. Read patents citted in that posting.

[Aziz]

Mikebg,

Look at the web link below. It is a very detailed (178 page) document that covers metal detectors (UXO) in technical terms. This is a good read for all. http://www.serdp.org/Research/upload/UX-1122-FR-01.pdf

Pay attention to the three stages of eddy current decay. This relates to surface eddy currents, deep eddy currents and total target TC.

bbsailor

Hi bbsailor,

thanks very much for your hint. There is much usefull information therein, that takes me some time for dealing with.
Aziz

[gwzd]

Mikebg,
... covers metal detectors (UXO) in technical terms.

UXO stands for unexploded ordnance, buried bombs and mines. EMI methods are absolutely useless for finding plastic mines. So, before going into the field with one of those I'd think twice
Regards

[bbsailor]

Technical Link

Mikebg,

Look at the web link below. It is a very detailed (178 page) document that covers metal detectors (UXO) in technical terms. This is a good read for all. http://www.serdp.org/Research/upload/UX-1122-FR-01.pdf

Pay attention to the three stages of eddy current decay. This relates to surface eddy currents, deep eddy currents and total target TC.

bbsailor

[gwzd]

...having complex resistance Z = R+jXL.

Z is called impedance and is the sum of the resistance R and the reactance X

Also the skin-effect would affect the resistance Z.

skin effect at 20 to 40 kHz and tiny currents? you should be more worry about proximity effect, skin effect is more like for isolated wires but you are talking about coils.

Maybe looking more in detail to transformator equations

good idea, I thought you said you are a software engineer.

would bring more success. Target and TX coils are more or less complex coupled transformators.

Care to elaborate that?

Any hints will be appreciated.
Aziz

See above,
regards

[Aziz]

This is also very interesting to me and would bring more accurate modelling results in sensitivity map creation of a coil arrangement. As I noted, modelling a target isn't easy.
But it can be modelled by simplification: short circuited small coils, having complex resistance Z = R+jXL. Also the skin-effect would affect the resistance Z. Maybe looking more in detail to transformator equations would bring more success. Target and TX coils are more or less complex coupled transformators.

Any hints will be appreciated.
Aziz