Let's made a PC-base metal detector with usb interface !!!

[TheWizard]

ABOUT http://digital.ni.com/worldwide/sing...25703D00446948

in last:
....Using sound card with computer for the Lock-In Amplifier, we have received some satisfactory results. As this is our first time experiencing with this application, we believe there can be further improvements to the system....

I think - this method(sound-card) is have not precision @ 7-10kHz signal and is litle "blah-blah"
To best precision you need to write software driver for sound-card to make fast DMA read/write to card and you need make best RX/TX synchron or
you need make external hardware Lock-In Amplifier...

[Aziz]

Hi Vistac2000,

I am using the TDA2040 Hi-Fi power-amp IC. Even this IC provides up to 20 W, I am not using the full range. But the electrical characteristics are well enough. The output power depends on the supply voltage and impedance of the output (speaker/coil). Also you have some reserve of output power for experiments.
See TDA2040 data sheet and circuit:

http://www.ortodoxism.ro/datasheets/stmicroelectronics/1460.pdf

May be there is a better IC now. Just look on the web.

For the input signal, just use a low noise op-amp circuit. This is quite easier to develop.

Aziz

[vistac2000]

Hi AZIZ,
I found this schematics for audio amplifiers for Tx and RX,
did these good for this job?
Please help me about coil that good for this purpose...
Thanks... with best regards.

[Aziz]

Designing a Digital Lock-In Amplifier with Sound Card

http://digital.ni.com/worldwide/singapore.nsf/web/all/474BA4D976DEFC548625703D00446948

Hi JC1,

this seams very interesting related to laptop MD.

BTW, I was using similar method as the lock-in technique combined with balancing the search coil (ultra-balancing) on the second channel of stereo-output. Then the SNR goes up and I had a ultra-sensitive MD. But also ultra-sensitive to grounds. In experimental state, I neglected the ground effects.

As I mentioned, that it is possible to detect the signal on the edge of noise level, the lock-in method goes even through this barrier. There is much more potential, as I thought.
I am looking forward to develop my laptop detector soon.

Aziz

links

Designing a Digital Lock-In Amplifier with Sound Card

http://digital.ni.com/worldwide/singapore.nsf/web/all/474BA4D976DEFC548625703D00446948

A low-cost, high-performance, digital signal processor-based lock-in amplifier capable of measuring multiple frequency sweeps simultaneously

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=RSINAK000076000002024703000001&idtype=cvips&gifs=yes

A first version of a low supply voltage (2 V) fully integrated lock-in amplifier, designed in a standard CMOS technology (AMS 0.6 μ), is presented.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6THG-43HJVDR-1D&_user=1496926&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000053094&_version=1&_urlVersion=0&_userid=1496926&md5=2d6707887f3620a3272f83222488d033

A Microcontroller Based Digital Lock-In Milliohmmeter

http://www.cappels.org/dproj/dlmom/dlmom.html

Take you pick

[bbsailor]

Implementing Digital Lock-In Amplifiers Using the dsPIC DSC
http://www.microchip.com/stellent/id...pnote=en532447

Carl and Wizard.

On 8-12-2005 I posted the following on this forum in response to Ferric Toes (Eric Foster).

"Ground Noise...one more thing
Ferric,

Great answer.

There is one more thing that needs to be considered when looking at ground noise...coil sweep speed. This is related to the speed of the main pulse clock and the amount of samples collected during the integration period. If you have a longer integration time to allow more samples during the integration period, you must sweep the coil slower. That is why higher frequency PI machines in approximately the 10 Kpps to 15Kpps range are used with low power pulses so you can have more samples during a shorter integration period and still sweep the coil at a good speed for fast ground coverage.

The integration circuit and the sampling RX window timing circuit actually work like a lock-in amplifier. Go to the following web site to see a good explaination of what lock-in amplifiers are and how they work.

http://www.cpm.uncc.edu/lock_in_1.htm

This will give you and other forum readers a good understanding about how to extract desired signals from noise using sampling windows. Simply, by adjusting the period of the sample window to synchronize with the desired signal, you can extract the desired signal even though burried in high noise.

You can pick up a lock-in amplifier, very inexpensively, on e-bay. Try to replace the differential input to the PI integrator IC chip of the Hammerhead with the lock-in amplifier to see how much more sensitivily you can squeeze out of adjusting integration time with coil sweep speeds. This is only for bench-top experimentaion unless you can find some lock-in amplifier IC chips.

The above web site will provide a very good tutorial on various lock-in amplifier applications and techniques, all of which are very related to PI circuits, timing and signal extraction.

bbsailor"

It looks like lock-in amplifier chips that function as a Digital Signal Processor (DSP) may now be available and be useful to use in a digital PI MD. Use the forum search feature and search on the words "lock-in amplifier" and see my other posts where I compared the function of a lock-in amplifier to extract weak signals in the presence of noise, to the similar processing done in a PI circuit.

The lock-in amplifier reference signal should be synchronized off the TX pulse. The tradeoff that needs to be calculated before any PI applications of a lock-in amplifier are tried is the sweep speed, coil size and TX PPS rate where the number of integrations that can occur with the target within the coil's field is scaled to the sweep speed. As the coil is swept slower the potential sensitivity is increased.

I hope this stimulates some renewed interest in this topic for the next generation of digital PI circuits.

bbsailor

[Carl-NC]

Implementing Digital Lock-In Amplifiers Using the dsPIC DSC
http://www.microchip.com/stellent/id...pnote=en532447

Now THAT looks very, very interesting.

[TheWizard]

Implementing Digital Lock-In Amplifiers Using the dsPIC DSC

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en532447

[99thpercentile]

I'm not sure if this is what you want, but there are systems currently available that already do this. I use them all the time. Do you want a time domain or frequency domain EM instrument. Geonics makes time domain instruments and Geophex makes frequency domain. Geomar makes the PDA software for the Geonics instruments. If you want to log instruments to a laptop, you can use MAGLOG NT from Geometrics. All of these systems integrate the system output with GPS.

https://www.geophex.com

https://www.geonics.com

https://www.geomar.com

https://www.geometrics.com

[Aziz]

Hi Vistac2000,

regarding your email:
Yes, I am using simple hi-fi (high fidelity) audio amplifier for TX.
Hi-Fi means: low noise, low distortion, low klirrfactor, accurate power amplifier.

For RX: either low-noise microphone pre-amp or simple op-amp circuit. I used the last one (AC coupled op-amp).

The coils should be used in induction balanced mode (concentric co-planar, omega-coil, DD=double-D or OO=double O).

Signals can be fed by the laptop: any frequency range or time-domain signal.
You will have definitely some latency+jitter: Transmitted signal, acquired signal. Jitter is on accurate sound cards very low.

Driver part (sound card related functions) must be in real-time priority. See all my posts in the past for some details.
Regards,
Aziz

[Aziz]

Hi AZIZ,

Please help me about laptop detector,
what kind of amplifier need for this job?
How much power need for these amplifiers?
what is the best coil for this job?

I waiting for your answer...
with best regards...

Hi vistac2000,

for signal input, any kind of high-fidelity, high impedance and low noise pre-amp is convenient. This should be an AC coupled pre-amplifier with the high-pass frequency cut-off above 500-1000 Hz (minimizing 50/60 Hz + multiple harmonics distortions from power lines). Of course, a low-pass frequency cut-off should lie on half of the sampling frequency (48/2 or 96/2 kHz depending on your sound card, called typical nyquist-shannon-criteria). Most of the sound cards has built-in such a low-pass filter. You can neglect the second frequency cut-off in most cases. A typical microphone pre-amp is convenient (very low noise). The rest of the signal can be adjusted on the sound card by software (amplification).

For signal output, any kind of high-fidelity, low distortion, NF-AC-power amplifier is convenient (AC coupled). The output power depends on what SNR-rate you want to search. The higher the output power, the higher the induced signals will be but also the higher the power consumption will be. I tested directly the stereo output lines from the PC/laptop without a power amplifier between the search coil and output lines. So just using ultra-low-power (few milli-Watts) and will also work but signal detection will be very small. Very important is the impedance matching of the coil and power-amp. So you will likely match the impedance either to 4 or 8 Ohms (very typical to power amplifier). So the total output power from the power-amp can be dissipated (consumed) from the coil and will cause then more effects. Typical output power should lie between few 100 mW to 1 W (big enough).

I tested a very bad, low Q, high impedance coil in VLF mode (induction balance mode). Even with this coil, I got amazing results. The most common coils will not work for this purpose. A new coil needs to be developed and I am working just on this now (see my efforts on coil analysis). I tested the coil not on resonant circuit (no capacitor connected). If you decide to develop the MD on a particular operating frequency, all the induction effects will be increased very much due to resonant effects (on every radiant, effects are summed). So there is much more potential for this yet.

All the parts you need are common and well-known on the web. There are hundreds of circuits for this purpose (NF-amplifier, microphone-amp, power-amp, etc.).

But all the physics/math must be done in software and in a tricky way. This is the big challenge on such a project. You will have very low part hardware (5 %), but all the rest in software (95 %).

Regards,
Aziz

[vistac2000]

Hi AZIZ,

Please help me about laptop detector,
what kind of amplifier need for this job?
How much power need for these amplifiers?
what is the best coil for this job?

I waiting for your answer...
with best regards...

[Aziz]

It would be great, if there is a PDA with 24-Bit 96 kHz sound card built-in. So metal detecting on the edge of natural noise (SNR up to -110 dB!) would bring you hyper-sensitive MD's. It will also work with 16-Bit / 48 kHz as well.
My experimental results with the laptop detector is amazing. You can even detect the type of ground below the search coil!

On the other side, the science of detecting can be implemented on a simple laptop and later be adapted to any convenient PDA. This would be the best case. Of course, I am missing such a PDA today. So I wouldn't use the laptop, if I could have such a PDA and all the specifications and tools (for free of course). May be we have to wait some time for such a PDA. But today, almost everyone has a laptop at home (besides me - my one is broken).

Despites the operating system (OS) of the laptop isn't a real-time OS (Windows XP/Vista), the way of software programming isn't too much difficult to handle this. I have to deal with some latency and jitter. But I can decode and correct the signal in real-time (like TDMA-frames on GSM technolgy). Far more, I can also use multi-core CPU's to increase my number of measurements per second. So laptops todays gives you much more computation power to make the science behind metal detecting better. Also the implementation and experimenting is quite easy and fast with laptops.

I will continue my laptop detector soon.
Regards,
Aziz

[gwzd]

Hi Aziz,
I think you meant well. I did misunderstand you and I'm afraid many others did too. I'm just fed up of people taking the shortest road just because they can't handle the heat. You must probably wanted to make it easier for the OP to get the job done.

The company where I work (I'm not posting the link because it might seem spam or self promotion and I understood it is highly discouraged to do so, or?) is not a PDA company. We do work with PDA's occasionally and with PC most of the time. We have all the specifications and information because we pay for it. But as you pointed out it might be limiting or actually prohibiting for a private person to do so.

I'm not a software engineer as said before, but Shark DSP and Altium TSK's are used to be on my table. I just don't consider that kind work to be programming, it's just an extension of the EE, nothing else. As a matter of fact these courses are taught in the EE part in most Universities. So, actually is not a big deal. Many people make a great deal of embedding applications but the reality is that you have everything already fixed ahead. You know all the ins and outs forehand.

A software engineer like yourself working in a Windows environment (awful!) never knows exactly what to expect. We deliver applications for countries in Europe, Asia and US all with very different ways of doing things. That's pretty much programming, putting on the whiteboard (blackboards are obsolete by the way) all the possible scenarios but still there's always something one misses.

Don't get me wrong I wish you well, but please when saying something so categoric like "Forget PDA's" try to explain that it might be difficult but also try not to disregard the advantages of it. Something like, "PDA's are light weighted and that's an advantage in the field although keep in mind that the information, hardware and commodity compilers might be expensive". I try not to discourage people but to encourage. Try it out! It might get tough but when done you will get the satisfaction of doing something others failed to do.

I'm not German but Swedish, in many ways we are very much alike so I'll take you wouldn't like me neither. But then again I'm not your boss so it should not be a problem to communicate.

Finally, I'm not trying to convince you to do anything just to not discourage other people who might want to do so. You never know, perhaps they can get the specs from a person in this forum, a compiler from the GNU community, a hardware done by some smart fellows here and put together a nice project they could share with all the other here.

As for myself I think I made it clear from the start, "Who would pay for it?"

My best regards to you Aziz, I wish you well
RA

[Aziz]

Hi gwzd,

Your personal experience is limited, as usual, by your personal skills. I doesn't appear to me you have programmed many PDA's successfully otherwise you shouldn't be that terrify of them. Because it is hard for you it doesn't necessarily mean it's hard for other people too. Don't flatter yourself that much, it is not healthy. I on the contrary will always encourage people to try themselves in new stuff and new challenges. The OP wanted to do something apparently he doesn't know much about it, why not use the opportunity for him and learn something he might actually use?

I think, you misunsderstood me. The problems regarding PDA's is commonly to the lack of specifications and informations. If you not work directly for a such PDA company, it is hard to get some technical informations and tools.

It's always easy to say take two more terabytes of memory for making a calculator, "anyway the customer has no other choice!" This kind of attitude is what bugs me at the first place.

I would like to have such a calculator. Particularly in my Laptop!
Calculation is proportional to the memory capacity! Particularly true, when you have much more to calculate. Why this is so: just ask a software engineer.

About your stories, I do believe you are truly sure about the nonsense you are talking about. But it doesn't make it right anyway. I will let the blackboard alone and you should go back to school. The first step into getting hired with us is to prove you have an open mind and can see the problem from different angles. Now, that's an important step!

Yes indeed, I was gone to the blackboard. And last night again:

http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoLectures/index.htm

It is amazing! I like the professor.

If you are your own boss take my advice and learn a bit about solving a problem with the smallest possible amount of resources.

Yes I did it! Only the coil and laptop. No electronics!

BTW, I do not like german bosses. Also I do not like krauts anymore and therefore I am the boss of myself. I am doing this stuff fulltime now. And I like it.

So see this as a big challenge to convince me your projects with PDA's!

Aziz