Targets frequency response

[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]

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]

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]

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]

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).

[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.

[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

[gwzd]

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

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

[Aziz]

I think, we have to consider also the capacitance of the coils and the target.
Hence it follows that, particularly high-frequency domain responses could be taken more better into account due to dominating capacitive effects on high frequencies.

This applies especially to PI MD's. We have more harmonic parts of high freq. domains both on the coils and the target.

Also the ground has increased capacitance effects.

This capacitance is seen as a parallel connected C to coil L in series with resistance R.
.--R--L--.
|^^^^^|
*---C---*
|^^^^^|
°^^^^^°
^=space (can't use real spaces here)

This would explain the real physical background more better.

[B^C]

Are there published anywhere frequency responses of metal detected targets: different coins, rings, hot rocks, junk etc.? I can,t find in WEB even one frequency characterstic of target in complex (impedance) plane! Mike ***

Gday mikebg,

Not much written on the subject due to the fact that the Eddy Currents which are confined to the Conductive zones, set up there own secondary magnetic fields having the SAME Frequency but not necessarily the same phase as the primary inducing field.

By inducing a ground Matrix or Target does not change the frequency of the target response signal. There are not to many things in Nature that can be induced with one Frequency & we have another Frequency come out of it.
I suppose that's why we Discriminate via Phase shift rather than Target Frequency.

I may be wrong like always & i am all ears waiting for comments to the contrary.

[mikebg]

Frequency Responses

Hi B^C,
The only wrong thing in your posting is your term "Target frequency". Target may be identifyed by its characteristic frequency, and even by its resonance frequency (if metal is ferrous). It has no own frequency.
Your expression:
"I suppose that's why we Discriminate via Phase shift rather than Target Frequency", means that my "Lesson" and "Exercise" are written too briefly.
Because the rest of your posting is correct, I will explain it in great detail.
Fig. 2 in the "Lesson" is valid for both linear and nonlinear systems. If a network is linear, it responds with the same Frequency as excitation. Then the magnitude and phase of received signal can be represented by a point M in the impedance line shown in fig. 3. If we change TX frequency from zero to infinity, the point M moves on arc in fig. 3 from origin to point 100% and the phase of response changes from 0 to -90 deg. The arc represents in complex plane response of an eddy current for all frequencies.

What represents the straight lines OA and OB in fig. 1? The answer is - response for fundamental frequency. If the system is nonlinear, it also responds with the same frequency as TX, but adds harmonics. For example, if we excitate an ferrous object with transmit TX frequency 2kHz, we receive also this fundamental frequency 2kHz, but there is in output also remarcable magnitude of second harmonic 4kHz, and especially of third harmonic 6kHz because the third harmonic is attributable to magnetizm. The complex plane can,t represent correct response of nonlinear properties as magnetizm. The straight lines OA and OB in fig. 1 are only an approximation, which shows that phase of fundamental frequency not changes with change of TX frequency. We will use them in future exercises to analyse frequency response of ferrous conductivity.

For a linear system, we can plot a normalized transfer impedance with relative magnitude 100% as shown in fig. 3. For a nonlinear system, transfer impedance in complex plane may represent only the response for fundamental frequency. For example, if we increase TX ampereturns twice, this no means that the signal will increase twice moving point M in twice distance from origin on straight lines OA or OB. Hysteresis cycle shows, that magnetism has saturation and this limits the maximal possible magnetic signal. The point M shifts less than twice if the magnitude of TX field increases twice, but the oppsite happens when the magnitude of TX field decreases. Very important thing for a nonlinear system is BIAS. We will discuss this in future exercises.
* * *

[B^C]

Gday mikebg:

I am as dumb as a horses *** at the best of times being a full time Prospector but i still don't see any frequency response change.
Some Formulas may help.
I may of course be reading & summarising things differently--or totally wrong!
Maybe there is confusion & a lot of different technologies are being combined instead of concentrating on one technology at a time?.

For my sake & maybe others i will attach a few things so we all can comment without confusion & maybe you can run us through a few things & formulas to start off with & progress from there.

Adding a few numbers to the equations to the attached formulas & showing the results will certainly help a lot of people out.
I'm sure there are quite a few who would like to learn & many need to start from the begining.
The following information may be helpful to start things off, i have just blown the cobwebs of my metal detector bible, i'll attach some things..

If we are going to get lessons we may as well start at the beginning to benefit many other readers who just can't catch on to the stage things were started at--me included, then again i may be the only dumb prick that can't get it!.

I'm sure there's many on this forum who could make this interesting with there comments & Formulas to prove the facts please.

Back to school, boys!

I'll be interested & an eager participant, maybe start off with the formulas for:

Magnitude of Magnetic Field B=

The Magnitude of Magnetic Field at an axial point P a distance X from the centre of the loop is given by B=

Magnitude of flux through a loop is:

Magnitude of induce current in the coil while the electric current is changing:

Then keep progressing, i'm sure you'll get a ****load of questions, it will be very informative.

I have attached Frequency Domain Data in this post, i will post another with Time Domain Data next.

[B^C]

Time Domain Data Attached:

[Unregistered]

MIKEBG
You're not going to emit one frequency at a "passive target" and get a different frequency back except for harmonics, Doppler shift, or stimulated emissions (none of which would apply in this case senario).

PHS

[Aziz]

Hello B^C,

your attached information is quite interesting to me. Many thanks!
I will check this on the coming week out.

Aziz