Targets frequency response

[mikebg]

Answer for B^C

Bruce C,
It is useful for you to be student in the forums because:
1. You will grow younger, despite in this thread we study knowledge from 19-th century.
2. You will grow able to find errors in Bruce Candy's articles, patents and designs. Please read carefully old postings in
the technology forums since last 9 years (avoid postings from Sean Goddart).
3. You will be able to use LTSPICE and similar methods for computer aided analysis and design. They will help you to understand why comrehensive TX pulses in Minelab mashines are delusion.
4. You will find that the articles in your old Bible are useless for design purposes.
5. You will know which metal detector to buy for prospecting and how to use it effectivelly.
6. You will understand that Chinese engineers are not important danger for Minelab. Dangerous are Dave Emery and Carl's
"Commercial Design Groups". China and Bulgaria can be favourable producers.
To start with selfeducation, please download LTspice (or SWCAD III) and read [2] and [7]. Afther you post in this thread your answers to exercise, I will send you via email an interesting publication. You will ascertain the fact that you already can find in it errors.
Yours Mike.

[mikebg]

Answer for unregistered PHS
You are welcome PHS,
To understand what I'm going to emit, you should read [2] and [7]. Your posting is very important for me. It forced me to plan exercise for analysis of modulation, caused by sensor movement above target. Please register in this forum to become able to receive private messages. In next my posting I will shed light on term "Frequency Response". Mike.

[mikebg]

Frequency Response = Spectral Characteristic

Hi to all,
Let's we determine what is FREQUENCY RESPONSE before to continue with exercises.
1. [Wikipedia] FREQUENCY RESPONSE is the measure of any system's spectrum response at its output, to a signal of varying frequency
(but constant amplitude) at its input.
2. [Steve Smith] FREQUENCY RESPONSE is the magnitude and phase changes that sinusoids experience when passing through a linear system. Usually expressed as a function of frequency. Often found by taking the Fourier transform of the IMPULSE RESPONSE. (This is in [2] page 636. Frequency response is described in [2] page 177).
3. [My reinvention] FREQUENCY RESPONSE is complex transfer function of a linear system in frequency domain. It can be expressed by two frequency functions:
a) Re(f) and Im(f) in rectangular coordinates, or
b) Magnitude vs. Frequency and Phase vs. Frequency in polar coordinates.
Trasfer functions in time domain are STEP RESPONSE and IMPULSE RESPONSE.
4. [My explanation] For a linear system, frequency response is its own characteristic because it is independent on amplitude of excitating signal. For example, the gain of an amplifier is its own characteristic, supposing it is linear (if gain is independent on input amplitude).

Fellows, I know that the terms "frequency response" and "impulse response" are misleading, but they are in English and
this forum is in English. In the nopnenglish rest of Europe is used correct term "Spectral characteristic" or "Spectrum response" or "Frequency characteristic" etc. In SPICE is used the term "AC analysis".
To avoid misleading, in my next postings I will use the term "spectral characteristic" instead "frequency response" because for analysis we assume that the sensing network is linear. The only advantage of term FREQUENCY RESPONSE is that it is valid also and for nonlinear system.

[B^C]

Gday Mike,

I will read your posts & reply but i thought i would would leave a couple of quotes by the man himself first.

Any intelligent fool can make things bigger, more complex,
and more violent. It takes a touch of genius, and a lot of courage,
to move in the opposite direction.

Albert Einstein

Technological progress is like an axe in the hands
of a pathological criminal.

Albert Einstein

If you can speak of technical things only in technical terms,
you do not understand them.

Albert Einstein

[B^C]

Mike,

PHS was me, i was in a hurry & didn't log on, had to go test something out, i meant to put PHYS on the bottom being short for PHYSICS. I will log on in the future to avoid confusion.

[B^C]

1

[B^C]

Bruce C,
It is useful for you to be student in the forums because:
1. You will grow younger, despite in this thread we study knowledge from 19-th century.
2. You will grow able to find errors in Bruce Candy's articles, patents and designs. Please read carefully old postings in
the technology forums since last 9 years (avoid postings from Sean Goddart).
3. You will be able to use LTSPICE and similar methods for computer aided analysis and design. They will help you to understand why comrehensive TX pulses in Minelab mashines are delusion.
4. You will find that the articles in your old Bible are useless for design purposes.
5. You will know which metal detector to buy for prospecting and how to use it effectivelly.
6. You will understand that Chinese engineers are not important danger for Minelab. Dangerous are Dave Emery and Carl's
"Commercial Design Groups". China and Bulgaria can be favourable producers.
To start with selfeducation, please download LTspice (or SWCAD III) and read [2] and [7]. Afther you post in this thread your answers to exercise, I will send you via email an interesting publication. You will ascertain the fact that you already can find in it errors.
Yours Mike.

Mike,

Answers to:
1)
Hahaha, have to love that, you promise much my friend, you will need to start your lessons from the beginning of technology to be able to make quantified steps to bring us up to date.
Leaving old technology out of the picture gives us nothing to take example from. We must be able to compare technologies side by side to make judgement to which is better suited to the application at hand.

2)
What was written in Bruce Candy's articles & what Bruce Candy does in reality may be two different things, it's no good telling everybody exactly what your doing or they'll all do it.

3)
Delusion or not Minelab machines are the top dog, "actual" field conditions prove this time & time again. If you can place something in my hands that is better then i will use it, until then it is all hypothetical jargon written on forums.

4)
Not sure about that one, knowledge is the key to understanding, can you supply me with actual factual Formulas to verify your statement? That is the only way to prove something wrong before design.

5)
That's easy, Minelab only, nothing competes at this stage, i have tried plenty.

6)
Not concerned about Dangers to Minelab, good luck to all who can out do them, just wish they would hurry along before Minelab introduce there new Subsurface Visualisation Technology that will stand this market on it's head.
Some previews have aleady circulated about this, it is a quantum leap ahead & will bring us all plenty to think about.
The only good thing that China makes is Fireworks--end of story, give them time & they will improve though.

If you can prove "mathematically" what your saying is RELEVANT to the detection of PASSIVE targets then i will sit down & learn something.
At the moment all we have is a lot of mumbo jumbo written on the forum with no actual proof any of this is relevant to the application.
I have posted what is relevant & can post another 500 pages on the subject with formulas to prove FACTS.

Your turn .

[B^C]

Hi to all,
Let's we determine what is FREQUENCY RESPONSE before to continue with exercises.
1. [Wikipedia] FREQUENCY RESPONSE is the measure of any system's spectrum response at its output, to a signal of varying frequency
(but constant amplitude) at its input.
2. [Steve Smith] FREQUENCY RESPONSE is the magnitude and phase changes that sinusoids experience when passing through a linear system. Usually expressed as a function of frequency. Often found by taking the Fourier transform of the IMPULSE RESPONSE. (This is in [2] page 636. Frequency response is described in [2] page 177).
3. [My reinvention] FREQUENCY RESPONSE is complex transfer function of a linear system in frequency domain. It can be expressed by two frequency functions:
a) Re(f) and Im(f) in rectangular coordinates, or
b) Magnitude vs. Frequency and Phase vs. Frequency in polar coordinates.
Trasfer functions in time domain are STEP RESPONSE and IMPULSE RESPONSE.
4. [My explanation] For a linear system, frequency response is its own characteristic because it is independent on amplitude of excitating signal. For example, the gain of an amplifier is its own characteristic, supposing it is linear (if gain is independent on input amplitude).

Fellows, I know that the terms "frequency response" and "impulse response" are misleading, but they are in English and
this forum is in English. In the nopnenglish rest of Europe is used correct term "Spectral characteristic" or "Spectrum response" or "Frequency characteristic" etc. In SPICE is used the term "AC analysis".
To avoid misleading, in my next postings I will use the term "spectral characteristic" instead "frequency response" because for analysis we assume that the sensing network is linear. The only advantage of term FREQUENCY RESPONSE is that it is valid also and for nonlinear system.

Mike,

I still can't see how this relates to a Passive target imbeded tightly in a conductive matrix that has been there for millions of years. When the only things that will change the frequency are harmonics, Doppler shift, or stimulated emissions (none of which would apply in this case senario) as stated previously.
There are a number of conductors including:

1. <LI type=a>amount of negative charge on the clay particle; <LI type=a>clay content; <LI type=a>concentrations of salts in the soil solution; and, <LI type=a>soil moisture content <LI type=a>temperature, and
2. bulk density.

This is not to mention ironstone etc in mineralised soils, in free air conditions i can understand we could setup a harmonic response but for gold that has been buried for millions of years or locked between fault lines or in quartz viens to setup harmonics of a particular target in this enviroment doesn't seem applicable.
To induce gold to the magnitude to setup a harmonic response, what sort of signal is going to be required to ignore or punch through the matrix to do it.

Like they say seeing is believing, you will need to produce more than words to get people interested though.
Put some formulas up & then everybody will want to go further, anybody can write words & i am am example of this. If you put up some mathematics then people can knock it on the head or take it further.
Otherwise there is nothing to learn, if someone walks up to you on the street & says he can create a hole & fall through it, we don't all sit around & take lessons. On the other hand if this is a proven FACT then everybody takes interest & wants to learn.

[mikebg]

Education

Thanks Bruce,
Here a saw of Bulgarian amateur designers:
IF YOU CAN'T DESIGN IT SIMPLE, YOU SIMPLY NEED EDUCATION.

[Aziz]

I will investigate more on the eddy current effects on targets in the future. Since I haven't a measuring instrument for this yet, I couldn't check the formulas and my simulation results.

Now I am able to measure small AC voltages and currents using lock-in amps.
I also have to look for some physics background.

I found the typical eddy current measurements by the industry are based on the change of impedance Z=R+jXL of the primary air cored coil influenced by targets. According to the following web-site, both R and XL are changing.

http://www.uni-magdeburg.de/iwfzfp/E...EddyCation.htm

Sorry, it's in german. Look particularly to the impedance graph (right bottom).
Regards,
Aziz

[mikebg]

Re: German site

Thanks Aziz,
This site has english version. To select, goto home page.

[mikebg]

Exercise No 2

Exercise 2 "Normalized Transfer Impedance of conductive nonferrous object "
Attached FIG. 6 shows transfer impedance of a conductive nonmagnetic object in which flow whole set of eddy currents. You can see, in above mentioned WEB site, an animation - how moves point M when we increase frequency and how - if there is different cracks in the structure. The shown impedanceform is valid for most nonferrous targets and for halfspace (salty water and conductive soils without ferrous mineralisation). Only metal foil has slight different impedanceform because skin effect is inessential at low frequencies (the whole thickness of foil is used by eddy currents).
Because this impedanceform is normalized, it is independent on:
- diameter of target,
- diameter of TX loop,
- conductivity of metal or of halfspace,
- distance between sensor and object,
- ampereturns of TX current,
- turns of RX winding.
NOTE the following regularities:
- The impedanceform at low frequencies seems like those of an object having single eddy current.
- The cutoff frequency fc seems as if no difference with such one object.
- The magnitude of signal at high frequencies is remarkable greater (approximatelly 30%).
IMPORTANT: It follows that if we design a wide band metal detector displaying impedanceform in complex plane, we can discern the shape of nonferrous targets. If the impedanceform is half-circumference, the target is like bracelet or ring. If the impedanceform is stretched at high frequencies, the object has no large opening.
Using this normalized impedanceform, you can solve following problems and answer to following questions:
1. Plot in normalized form frequency functions in Cartesian coordinates Re(f) andIm(f).
2. Plot in normalized form frequency functions in polar coordinates: Magnitude vs. frequency and Phase vs. frequency:
a) in linear scale
b) in log scale,
c) approximate log scale as Bode plot.
3. Explain what is the difference in comparison with some frequency functions of single eddy current.
4. Are there errors in the figure on the WEB site?
HINT: Pay attention to inscriptions on axes; they are different from our exercises. Note that at nonferrous conductivity, imported inductance (coordinate Im or X) is always negative.

[mikebg]

Z=R+jX

Hi Aziz,
You have used two times the incorrect notation
Z=R+jXL
The correct notation is
Z=R+jX.
Please read [2] Chapter 30 to understand why.
In our case L is an irreal imported inductance, because is negative and transient (depends on time or on frequency). As you knows, a real coil has positive inductance which is almost independent on frequency. This will become visible when we start exercises with SPICE simulator.

[Aziz]

Hi Aziz,
You have used two times the incorrect notation
Z=R+jXL
The correct notation is
Z=R+jX.
Please read [2] Chapter 30 to understand why.
In our case L is an irreal imported inductance, because is negative and transient (depends on time or on frequency). As you knows, a real coil has positive inductance which is almost independent on frequency. This will become visible when we start exercises with SPICE simulator.

Mike,

thanks, I meant with XL (w*L) the reactance of the coil of course.
I will write only X in the future.
Aziz

[bbsailor]

Mikebg and Aziz

Look at a Burce Candy patent number 5506506 at this link http://geotech.thunting.com/cgi-bin/...le=patents.dat.

Bruce Candy does a methematical analysis on page 9 to describe how he can separate the time constants of various targets to attempt to identify them even in the presence of highly mineralized ground. See his figure 3 where he graphs the responses of various long and short TC targets. I believe the vertical axis of the graph has an error where the upper -1 should be a +1.

He states on the bottom of page 14 that his analysis works for PI as well as continuous wave detectors. This is related to your recent discussion.

Look at the figure 4 where he labels the timed switches as "synchronous demodulators", which is esentially the function of the lock-in amplifier.

He uses three sets of synchronous demodulators to separate and analyze the long and short TCs of targets and filters out the relatively slowly changing ground to recognize and indentify target signatures with a microprocessor constantly adjusting and cancelling out the ground and only responding to the faster target TCs.

You will notice that all targets do not produce an absolute response as their orientation in the ground may alter their response. That is why it is important to get a good understanding of the range of various target responses in all common orientations. Unfortunately, sometimes the desired and undesired target responses may overlap under certain orientation conditions or ground conditions.

Here is an idea that may help your analysis. Most targets are initially located while the coil is in motion over the ground. What if the coil were stopped over a target and placed firmly on the ground where a push button is activated to enter another target ID mode where the last ground signal, held in memory, is subtracted from the target signal? Then, the ID mode analyzes the target response during the TX period using the target to couple the RX coil to the TX coil, as in a loosley coupled transformer, and analyze the target with another set (along with the target parameters stored while in motion) of target ID parameters to minimize any target ambiguity due to target orientation in the ground.

The big tradeoff of lock-in amplifiers is the time needed to extract the signal from the noise. Once the coil is stopped over the target, the coil motion speed is no longer an issue, and the synchronus demodulators can collect many thousand samples to better identify the target.

You should think about optimization in two modes. (1) The motion mode where the design is optimized for target acquisition. (2) The still mode where the design is optimized for target identification.

Here is where your target analysis and coil analysis could lead to some interesting new design opportunities.

bbsailor