NEW !!! Full differential PI front end.

[moodz]

Hi Moodz:

(still gnawing on it...)

You mentioned a spice simulation helped show the currents in the two bifilar coils during MOSFET shutoff.

I'm still befuddled by it, but I made this little LTSpice circuit to try to help. It seems to show current in the energized coil decreasing while the current in the other coil increases, then they meet and both decay together. I have no idea if this is realistic or even represents your circuit.

Can you comment?

Regards,

-SB

P.S. Nice stuff Moodz, Aziz, Tinkerer... I think

Thanks Simon ...your simulation matches experimental results. The coils are coupled quite tightly together .... any target within range is also coupled. So energy is transferred to target during flux build up and released on field collapse. The point of collapse is a discontinuity in the signal ( this is why I feel spectral analysis is inappropriate ) however you have pretty much nailed it in your next post regarding differential analysis ..( which is also the key to descrimination IMHO).

Regards,

Paul.

[simonbaker]

gnawing

Continued from prev post... More interesting graph showing sum of currents in coils too. Derivitive of sum gives induced current in target???? (of course this is without MOSFET breakdown).

-SB

[simonbaker]

Thanks for the detailed explaination. It seems reasonable, the difference between motivating the current with an applied voltage versus a magnetic field. I'll have to believe Spice and reality as I'll probably never work the equations to my satisfaction.

I think the disconnect I still have trouble with is: if the second coil suddenly has the same current as the first coil, you would expect twice the magnetic field compared to the first coil alone -- yet that I don't believe happens. I guess I'll just gnaw on it all for a while.

Cheers!

-SB

Hi Moodz:

(still gnawing on it...)

You mentioned a spice simulation helped show the currents in the two bifilar coils during MOSFET shutoff.

I'm still befuddled by it, but I made this little LTSpice circuit to try to help. It seems to show current in the energized coil decreasing while the current in the other coil increases, then they meet and both decay together. I have no idea if this is realistic or even represents your circuit.

Can you comment?

Regards,

-SB

P.S. Nice stuff Moodz, Aziz, Tinkerer... I think

[Tinkerer]

Hi Moodz

I will preformulate a question to be much explicite.
Is this a multi period sampling PI or not ?

For quite complete understanding of pulse waveforms in PI we need viewing the pulse in terms of its frequency component. Coz, we have to use Fourier analysis.

Mathematical apparatus is well known in the prior art of DSP.
If you have developed quite a new mathematical expressions let me know.

And another question is:
Do you considering bipolar or unipolar transmit system and multi period rectangular waveforms ?.

Given link is good for students but for engineers there is excellent book
published by CRC Press LLC in cooperation with IEEE press.

Google it.
Key words "Digital Signal Processing Handbook","Vijay K.Madisetti and Douglas B.Williams"

Reg. Don

Don,

I think the posts below might be relevant to your questions:

http://www.geotech1.com/forums/showthread.php?t=16793

http://www.geotech1.com/forums/showpost.php?p=113760&postcount=17

http://www.geotech1.com/forums/showpost.php?p=113765&postcount=20

Tinkerer

[moodz]

Hi Moodz

I will preformulate a question to be much explicite.
Is this a multi period sampling PI or not ?
Yes this code version is.

For quite complete understanding of pulse waveforms in PI we need viewing the pulse in terms of its frequency component. Coz, we have to use Fourier analysis.
I do not agree .... the frequency domain can describe a signal in phase frequency and amplitude but not time location ...this was one of the reasons wavelet transforms were developed. Also the TD and FD are interchangeable ...proven by operator transforms equivalence.... eg a multiplication in the time domain is the same as addition in the frequency domain and vice versa .. similiar for division / add / subtract. When you understand this you can process stuff in time domain or frequency domain ..just depends what is easiest and most appropriate.

Mathematical apparatus is well known in the prior art of DSP.
If you have developed quite a new mathematical expressions let me know.
I have not developed new techniques ...just using old analogue techniques but in digital domain. You have heard of analogue computer ... same thing .. old music synthesisers use same techniques.

And another question is:
Do you considering bipolar or unipolar transmit system and multi period rectangular waveforms ?.

All of the above the pulse generator will generate what you program it and connect appropriate cct.

Given link is good for students but for engineers there is excellent book
published by CRC Press LLC in cooperation with IEEE press.

Not all the readers are up to speed on this so I publish appropriate links.

Google it.
Key words "Digital Signal Processing Handbook","Vijay K.Madisetti and Douglas B.Williams"

Reg. Don

answers above ...regards. M.

[doncaffe]

Hi Moodz

I will preformulate a question to be much explicite.
Is this a multi period sampling PI or not ?

For quite complete understanding of pulse waveforms in PI we need viewing the pulse in terms of its frequency component. Coz, we have to use Fourier analysis.

Mathematical apparatus is well known in the prior art of DSP.
If you have developed quite a new mathematical expressions let me know.

And another question is:
Do you considering bipolar or unipolar transmit system and multi period rectangular waveforms ?.

Given link is good for students but for engineers there is excellent book
published by CRC Press LLC in cooperation with IEEE press.

Google it.
Key words "Digital Signal Processing Handbook","Vijay K.Madisetti and Douglas B.Williams"

Reg. Don

[moodz]

Hi moodz !

I am very interested with what kind of waveform processing you plan to do ?
This prototype is multi frequency or single frequency system ?
Spectral analysis was done in multi periods of waveforms or not ?
If you plan extreme accuracy of measurements you need some scrupulous selection of data tapering windows and appropriate averaging.
Tell me more ok!

Reg. Don

Hi Don .... I did consider doing a complex domain analysis and I certainly have the code blocks to do it however the conventional DSP techniques are overly obsessed with obtaining results in utilising these techniques IMHO. Mainly because I am focussing on a PI detector all the waveform processing is done in the time domain using the techniques utilised in the conventional analogue processing circuitry. So I am just implementing the same devices like sample and hold, averaging, gain, adders and subtractors etc in the digital domain ... the technique avoids noise and drift.
Some of the old analogue ways have been forgotten and AFAIK are no longer taught in engineering.

This is a definition of the technique ... ( this site is excellent resource BTW )

http://cnyack.homestead.com/files/aconv/convfil1.htm

I will be making a VLF / IB version probably in the same chip and that will be using the Frequency domain processing. ( multi & single ).

Also in one of my past lives I worked with wavelets so I will be coding one of those just for the hell of it ... if I have time.

Regards,

Moodz.

[doncaffe]

Hi moodz !

I am very interested with what kind of waveform processing you plan to do ?
This prototype is multi frequency or single frequency system ?
Spectral analysis was done in multi periods of waveforms or not ?
If you plan extreme accuracy of measurements you need some scrupulous selection of data tapering windows and appropriate averaging.
Tell me more ok!

Reg. Don

[moodz]

FPGA update

As a sidebar I have posted a surface mount process in

http://www.geotech1.com/forums/showthread.php?p=113218#post113218

Works for me anyway ..

On the FPGA side of things I have now taken delivery of the rotary encoders, 2 x 16 LCD and MOSFET drivers so watch this space !

One thing I did not have in the design is the audio channel. Of course I could have written a routine to generate audio ( tone etc ) using the CPU however there is still heaps of space in the FPGA so I have added a 3 channel synthesiser and noise source module which will be modulated by the CPU but will not involve any messy waveform synthesis by the CPU. Even better the volume control is also fully digital and the audio output is PWM ( so it uses only one output pin on the FPGA ) A simple resistor and cap form a low pass filter and the audio resolution is 16 bit. In this way very complex sounds can be generated to indicate all sorts of things at the same time.




Some of you are probably wondering when I am actually going to get around to demonstrating an actual metal detector .... please be patient ... there are about 30 'chips' ( modules ) in this design and each one has to be designed or modified then added to the total design then the whole thing retested to make sure that I did not break anything. However it is getting there. So now I have added the sound module I have basically got everything I need functionwise. I now need to code the user menus and fiddle the basic ground balance. Then I have to build a basic front end .. put the whole thing in a box and rod and get out in dirt. ( My bench testing days are over ... that is why I put the VGA scope interface in this design )

Hope this explains more ....

Moodz.

[moodz]

how simple can it be ?

Here is the vga circuit ...you can assign any pins as video interface.

Spartan 3e 3a ... whatever ... it will still work !

moodz.

[moodz]

This board is ideal for this type of FPGA development.

http://www.sparkfun.com/commerce/product_info.php?products_id=8458

Check out the schematic ...very simple.

A bit pricey at $99 but OK. The MooDz detector code will easily fit into this.

[nick_f]

Nicolae,

Google "avnet spartan 3a evaluation kit". It looks like avnet is out of stock right now.

Hi All,
Thanks for help. Now I understand why I could not find it. It was refered to as "Spartan 3e" instead of "Spartan 3a". I thought the difference is in lowercase e, compared to Spartan 3E...

Best regards,
Nicolae

[moodz]

Alternative to commercial FPGA board.

If you cant get a board consider making your own board

ALL the functionality I have made is INSIDE the fpga. The VGA interface is a few resistors. The rest is just digital I/O pins.

The other bits on the commercial board are mainly there to demonstrate interfacing FPGAs to different peripheral devices like temp sensors and flash mems etc.



XC3S400A-5FTG256C

digikey part no is 122-1580-ND

price for chip is $US 19 each.
you would also need an Xtal oscillator pack and a serial prom and 3.3 volt regulator.
( bout another $5 )

It is a BGA chip however these are easier than you think to solder.
( I can provide detail on the process I use but have not time to write instruction / photo at moment. )

moodz.

[moodz]

Hi Paul,
I am very impressed with what you achieved with these Spartan boards. I checked on the internet where these boards can be purchased from and I could not find them, either on xilinx web site or from avnet. Could you please post a link where you bought them from?

Regards,
Nicolae

Thanks Nick

avnet part no is .. AES-SP3A-EVAL400-G

moodz

[hobbes_lives]

Nicolae,

Google "avnet spartan 3a evaluation kit". It looks like avnet is out of stock right now.