NEW !!! Full differential PI front end.

[moodz]

What about a DSP?

Could make things easy.

Aziz

I already tried FFT .... because the pulses are broadband ... unlike VLF detectors the energy and phase dispersal is too spread out ... especially on small targets and very weak so poor resolution. Worse the power and phase vectors vary greatly with orientation to coil. Just coil moving across target causes great changes in phase and power with no definite target signature except on quite large targets close to coil.
So presently the design is fully synchronous demodulation which removes more than 60 dB of noise and oversampling extends effective sample bits to 19 ( 14 bit native ADC ) The resulting wave form is passed through 2nd order derivative function to obtain descrimination information and power integration for target volume / range. Alternating polarity Tx waveforms and late sample differencing provide Earth field compensation and reference differential analysis compensates for ground effect. Single and multi pulse waveforms may be changed on the fly ... eg alternating 100us pulse to triplet 60 us pulse every 50 ms ( 50 ms or 20 hz is the low pass cutoff ). This avoids patent trouble as multi-pulse Tx waveform is transmitted and analysed in separate time periods.

If you know of way to apply FFT with sufficient sensitivity then I am interested to learn .. however my experience is that FFT is very poor on weak signals ( small or deep targets ).

moodz

[Aziz]

What about a DSP?

Could make things easy.

Aziz

[moodz]

FPGA update

Changed the pulse generator back to 100 Mhz clock ... step size is 10 ns .. 0 to 65535 ( 16 bit control ) or 0 to 655.35 us in .01 us steps.

The sample clock is 1.3 Mhz and the pulse repetition is 196 us or 5.098 Khz .... pretty high for a PI but lots of samples = lots of sensitivity.

Now the question is how many sample points ????

1 sample during transmit.
1 early sample just after fly back
1 mid sample 50 to 100 us
1 late sample at 196 us

How to process ???

moodz

[Tinkerer]

That one is expired ??? ..... amazing how all these patents re-patent parts of earlier patents. How long do US patents last anyway ...

This whole patent scheme is really collapsing. With the Internet it is possible to find so much more "prior art" that it is really difficult to invent something new. However, this does not seem to prevent some companies of misusing the patenting system to eliminate competition.

Most of these patents would not stand up to litigation, but deep pockets to pay good lawyers seems more important than honesty in patent applications.

I was of the opinion that discussing ideas on the open forum would classify an idea as "prior art". However, not even older patents seem to be respected as "prior art".

So, should we worry about infringing patents? We don't want to expose ourselves to bullying tactics by the "patent grabbers".

Lets look at a new idea, a new and innovative way to skin the cat.

We do not sell a metal detector. We sell an Electromagnetic Pulse Testing Platform. EPTP

The platform consists of an FPGA brain and many peripherals, designed to test and analyse mineral ground response, ground conductivity and homogeneity as well as ground contamination by metals etc.

The platform can transmit a great variety of electromagnetic pulse sequences and shapes through a great variety of transducers and sensors, coils, probes and arrays.

DSP software samples and analyses the response captured by the transducers, sensors, coils, probes and arrays and produces a variety of outputs, audio, digitally stored, visual, oscilloscope etc.

The EPTP is sold in modular form. The FPGA brain with a basic firmware and
a basic transducer/sensor.
The EPTP can then be upgraded and expanded with additional firmware and more peripherals.

There is virtually no limit to expansion modules that could include GPS, Laptop, temperature sampling, soils acidity/alkalinity, etc., etc.. even cold beer dispenser. Ooops, change the last one to cold milk dispenser, we don't want to mess with alcohol laws.

Anyway, I hope they don't patent this idea. Maybe we should discuss the pros and cons a bit so it becomes public knowledge and "prior art".

Tinkerer

[moodz]

Is this patent not basically doing the same thing?

http://www.geotech1.com/forums/showpost.php?p=113804&postcount=3

Tinkerer

That one is expired ??? ..... amazing how all these patents re-patent parts of earlier patents. How long do US patents last anyway ...

[Tinkerer]

Is this patent not basically doing the same thing?

http://www.geotech1.com/forums/showpost.php?p=113804&postcount=3

Tinkerer

[moodz]

Hi moodz,

have a look into this patent:

http://www.google.com/patents/about?id=EQUVAAAAEBAJ&dq=6927577

Aziz

ouch ouch ouch ouch and ouch ..... this is almost exactly the design of the FPGA PI processor . I had to read this 4 times ....however they made one crucial mistake ... the english language is wonderful how the meaning of things can be totally different by the use ( misuse ) of a single word. The primary claim ( and dependant claims ) uses the word "mode" to imply that the detector has different states ( ie fine calibration mode, course calibration mode and detect mode ) My detector is always in detect mode so I am safe ... according to my personal patent attorney ..
The FPGA determines the calibration data by removing the target data from the signal. Not the other way round .

close shave ... but thanks for alerting me to this Aziz !!!

Regards,

Moodz.

[Aziz]

and the following things are ( because I accidently pressed submit too early ... he he )

1. One or more pulse generators that can be numerically controlled ( duty cycle and frequency ). Obviously need this to control the MOSFET switching. However my initial design will be very similiar to an older minelab.

2. One or more ADC drivers to get analogue data into the system.

3. One or more DAC drivers to generate analogue signals either for feed back to amplifier front end, coil balancing, meters .... whatever.

3. One or more rotary knob controller so you have user controls.

4. A sound synthesiser for user alerts and maybe music during lunch breaks.

5. one or more LCD driver so you can display stuff . ( 2 x 16 char currently )

6. Multichannel VGA digital oscilloscope so you can see signals not only what the ADC captures but also results of DSP computations. Also very useful for checking coil performance etc. This single feature alone is the most useful to have as you can see if your design works or not ... in the field.

7. VGA also displays text console so you dump lots of info about things that wont fit on LCD.

8. serial port so you can connect to computer. On the larger Xilinx Board there is ethernet and more RAM / Flash also.

9. A 32 bit MIPS compatible RISC processor on the FPGA for user code. ( the fast DSP is done in FPGA though ... more efficient) This does the menus, reads the rotary knob values, controls the displays and maybe does the high level descrimination.

10. DSP code blocks that can switched into the signal chain ...

11. All of the above is coded inside the FPGA, tested and works and I have only used up 59% of the resources of a cheap ( $20 ) FPGA.

moodz.

Hi moodz,

have a look into this patent:

http://www.google.com/patents/about?id=EQUVAAAAEBAJ&dq=6927577

Aziz

[moodz]

FPGA update.

Someone went and bought up all the AVNET spartan 3a boards. Cant buy them online any more.

I will have to revert to Spartan 3e 500 starter board by Digilent for now. No problem .... it will still work.

The pulse generator code has been upgraded to run off a 200 Mhz clock ... this provides 5 ns resolution on pulse timing. I was originally running a 1 us resolution however when you get up close and personal with the flyback pulse the 5 ns gives you alot more dance space ( actually 200x more ). It can also be modulated on a pulse to pulse basis .... this may turn out to be important down the track.

The front end drive circuitry has been designed, simulated and awaits installation with other bits in a field box. The front end will use bipolar pulses ... I thought for a moment I would be in trouble with a Bruce Candy patent US6653838 where he seems to be citing bipolar pulses as one of his claims .... however the good old Corbyn patent has bipolar pulses so I am back in business again. ... Corbyn specifies a "dual field" TX coil also ... I will chase that one up with Carl ...hmmm.

moodz

[moodz]

I guess that if I can get the FPGA project completed ... we will be able to compare some real world results.

moodz

This paper sums up the waveform analysis process used in the DSP code .... no requirement for FFT.
See page 4. ( excellent explanation )

mags.pdf

[moodz]

Hi moodz:

I'm assuming the flux must always be consistent with the currents in the coils (since that is how we calculate the flux, from the currents and coil turns). Both coils are producing flux, and initially I believe the sum of the fluxes is not changing anywhere nearly as fast as that step we see when looking at a single coil. Since the sum of the currents does not have that big step, the target doesn't see any extraordinary dphi / dt, just the typical exponential (my second graph shows the sum of the currents, I(L1) + I(L2), and there is no sudden step).

(I don't know how to compare it to moving capacitor plates apart. You definitely put in physical energy to pull the plates apart, so I'd expect the voltages would rise according.)

However, I think I'm getting the clear advantage of "differential" (bifilar/center-tapped) design from playing more with the sims.

It seems for the same "flyback" voltage across the MOSFET (switch in my sims), you can get more dphi/dt with the center-tapped design, maybe double?

This seems to be due to the fact you get double the voltage across the damping resistor (I guess that shouldn't be an engineering problem) without increasing the MOSFET flyback voltage.

Combined with better noise canceling, it seems like a winner combination.

Regards,

-SB

I guess that if I can get the FPGA project completed ... we will be able to compare some real world results.

moodz

[simonbaker]

Hi Simon,
yes the graph does produce a curve that is consistant with conservation of energy. This is a standard principle of physics however you are only considering the current and the turns in the coil. How was the energy lost from one coil and transferred to the other so suddenly ?.... the flux transfers the energy .. and what is also in the flux ? ... the target....does the target also experience a rapid current change ?... anyway food for thought.

This problem is very similiar to charging a capacitor then moving the plates apart ... what happens to the voltage across the plates ... even though the charge on the plates has not changed?

moodz.

Hi moodz:

I'm assuming the flux must always be consistent with the currents in the coils (since that is how we calculate the flux, from the currents and coil turns). Both coils are producing flux, and initially I believe the sum of the fluxes is not changing anywhere nearly as fast as that step we see when looking at a single coil. Since the sum of the currents does not have that big step, the target doesn't see any extraordinary dphi / dt, just the typical exponential (my second graph shows the sum of the currents, I(L1) + I(L2), and there is no sudden step).

(I don't know how to compare it to moving capacitor plates apart. You definitely put in physical energy to pull the plates apart, so I'd expect the voltages would rise according.)

However, I think I'm getting the clear advantage of "differential" (bifilar/center-tapped) design from playing more with the sims.

It seems for the same "flyback" voltage across the MOSFET (switch in my sims), you can get more dphi/dt with the center-tapped design, maybe double?

This seems to be due to the fact you get double the voltage across the damping resistor (I guess that shouldn't be an engineering problem) without increasing the MOSFET flyback voltage.

Combined with better noise canceling, it seems like a winner combination.

Regards,

-SB

[moodz]

Hi Moodz:

Try graphing I(L1) + I(L2) -- I believe that accounts for the total magnetic field seen by the target, the sum of the ampere-turns from both coils. Even though one falls very fast, the other grows very fast, and the sum does not have that sharp step -- but I'm not sure if that initial step is just a Spice artifact or physical.

-SB

Hi Simon,
yes the graph does produce a curve that is consistant with conservation of energy. This is a standard principle of physics however you are only considering the current and the turns in the coil. How was the energy lost from one coil and transferred to the other so suddenly ?.... the flux transfers the energy .. and what is also in the flux ? ... the target....does the target also experience a rapid current change ?... anyway food for thought.

This problem is very similiar to charging a capacitor then moving the plates apart ... what happens to the voltage across the plates ... even though the charge on the plates has not changed?

moodz.

[simonbaker]

Simon ... in order to see if there is an actual benefit in diff coils consider the case of the single coil vs the diff coil.

Below I have even reduced the inductance of the single coil greatly yet it cannot match the step response of the differential coil. At the decay / switch off point the current falls much faster than the single mono coil. It does not matter that the current does not actually fall to zero. The flux in the coil decays much faster in a step response.This is ONE of the TWO key features of the diff coil that I have been trying to make you guys see. Critical to remember that only ONE of the coils is carrying substantial current during the TX pulse in a diff coil. Trying to analyse the diff coil on its own with no comparative reference is like trying to find who came second in a one horse race

...and this is backed by physical experimental results not just math ... which is good but needs to be in the framework of experimental data.

moodz.

[ATTACH]12608[/ATTACH]

Hi Moodz:

Try graphing I(L1) + I(L2) -- I believe that accounts for the total magnetic field seen by the target, the sum of the ampere-turns from both coils. Even though one falls very fast, the other grows very fast, and the sum does not have that sharp step -- but I'm not sure if that initial step is just a Spice artifact or physical.

-SB

[moodz]

Thanks Aziz, I will study your comments.

First, I was not sure if my LTSpice simulation is physically accurate - I gather you feel it does represent reality, correct?

Second, my purpose with the sim is to identify what we really are gaining with the bifilar / center-tapped coil. Are we gaining any magnetic field advantage, or is it basically the same as a single coil with same amount of wire? I understand (I think) the benefits of the differential design.

Third, some previous discussion suggested that the current in the two coils became instantaneously identical at MOSFET shutoff. However, my LTSpice sim indicates that the current briefly grows in one coil while decreasing in the other, and the difference is made up for by current coming through the diode from V+. When the two coil currents become equal, the sim shows the diode current stops, and both coil currents (really the same current) decaying at a slower exponential rate. I'm not clear whether that initial transition period is physically realistic, or a Spice artifact.

I'll try to look over your equations, probably some answers in there.

Regards,

-SB

Simon ... in order to see if there is an actual benefit in diff coils consider the case of the single coil vs the diff coil.

Below I have even reduced the inductance of the single coil greatly yet it cannot match the step response of the differential coil. At the decay / switch off point the current falls much faster than the single mono coil. It does not matter that the current does not actually fall to zero. The flux in the coil decays much faster in a step response.This is ONE of the TWO key features of the diff coil that I have been trying to make you guys see. Critical to remember that only ONE of the coils is carrying substantial current during the TX pulse in a diff coil. Trying to analyse the diff coil on its own with no comparative reference is like trying to find who came second in a one horse race

...and this is backed by physical experimental results not just math ... which is good but needs to be in the framework of experimental data.

moodz.