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

[simonbaker]

nulling question

Aziz -- this question about nulling interests me.

We null coil to get rid of TX signal. Let's pretend "perfect" case, perfect null, no TX signal, only signal from coin. Then phase of RX signal is constant no matter how deep coin is -- signal gets weaker, but phase same. Our "lock-in" amplifier will get same phase no matter how deep.

Now let's think of real case. RX signal = residual TX signal + coin signal.

So we are adding Asin(wt + phi1) + Bsin(wt + phi2), in other words, two signals with different amplitudes and phases. Result has a phase between the two I believe, depends on amplitudes.

In this case phase shift depends on how deep coin is and how well we null signal. If not nulled well, our discriminator will kill our weak signals because the residual signal will dominate.

Some people say don't completely null coil, but I don't know, it seems like the closer we come, the better. It seems really important to get good null when using discrimination if we want deep coins. For all-metal mode, not so important since lock-in can detect tiny shift.

Now, maybe we can use amplitude information of RX signal to change our discrimination -- if amplitude very small, then we should allow smaller phase shifts in our discrimination logic. However, amplitude is usually very noisy I think. But are you planning to do something about residual TX signal from imperfect nulling? What is your nulling strategy?

Regards,

-SB

[bbsailor]

I made big steps towards a portable VLF laptop MD realisation.
The MD is now running on my Asus Eee PC 901. The laptop speaker sound is loud enough. Without external electronics, I can detect 1 Euro coin at 50 cm distance (air-test). The external USB sound-card will decrease the operation duration time. I think, there should be 3-4 hours operation time possible with the internal battery. I will extend this by optimizing the code.

Next steps:
1. Portable Coil
2. Optimizing the software for faster response. I have too much debugging utilities and informations displayed on the screen. Also full of unused code is running there, which they consumes too much milli amps and processing power.

Aziz

Aziz,

You have done well.

If you can find a simple way to build a phantom powered preamp stage (gain 10X) in the RX coil circuit (in the housing) you can improve the signal to noise ratio and eliminate the effect of cable capacitance from the RX circuit performance of PI detectors.

I see a new class of PI coils being developed that use an active RX circuit with small surface mounted components in a module built right in the coil housing in a location that is least affected by the TX radiation pattern.

It is nice to see you picking up on the lock-in amplifier posts I have made in the past. Next, look into how to use the boxcar integration instrument functions so that you can simulate the back end processing of a PI circuit.

You may be able to simply build the coil interface to connect the coil TX and RX circuits to a PDA (Personal Digital Assistant) for portable use. You will need an external MOSFET coil driver and first amplifier stage with a gain of a hundred or two with clamping diodes in the RX input. You could even use the PDA screen to observe the RX return signal to help identify the nature of the target.

Keep on pushing the envelope!!!

bbsailor

[Aziz]

I made big steps towards a portable VLF laptop MD realisation.
The MD is now running on my Asus Eee PC 901. The laptop speaker sound is loud enough. Without external electronics, I can detect 1 Euro coin at 50 cm distance (air-test). The external USB sound-card will decrease the operation duration time. I think, there should be 3-4 hours operation time possible with the internal battery. I will extend this by optimizing the code.

Next steps:
1. Portable Coil
2. Optimizing the software for faster response. I have too much debugging utilities and informations displayed on the screen. Also full of unused code is running there, which they consumes too much milli amps and processing power.

Aziz

[simonbaker]

I have good news!


The Creative X-Fi Surround 5.1 (USB) runs and meets the specifications.

Timings: No timing problems. Lock-in amp is stable. No phase drifting or different clockings between output and input channels.
Features: Full-duplex, 24 Bit, 96 kHz (USB 2.0 only).
SNR: Better than 100 dB!
LineOut/Earphone Output: 1.1 V rms
Line Input: 1 V rms (low for this application and the signal needs to be amplified)
Mic Input: with microphone booster, the receive coil can be directly connected to mic input (but noisy). Voltage range not measured yet.

Operating frequency can be up to 23 kHz. Above 20 kHz, there is more total harmonic distortions. But this does not matter much.

I have same detection depth results despite of the lower output voltage (1.1 instead of 1.4 V). The signal quality is almost same or a little bit better.
Now, it is time to develop an external amplifier module. The external booster module will allow to work with more sound-cards and also more detection depth.

Without an external booster module, it has enough detection depth to knock out other VLF detectors.

Aziz

That is good news. What a neat gadget, so easy to use with any PC. Wouldn't it be nice to stuff the processor inside too, there's your detector in a little box! (plus batteries). Hope you take it outside soon, let us know what you find!

Cheers,

-SB

[Aziz]

I have good news!


The Creative X-Fi Surround 5.1 (USB) runs and meets the specifications.

Timings: No timing problems. Lock-in amp is stable. No phase drifting or different clockings between output and input channels.
Features: Full-duplex, 24 Bit, 96 kHz (USB 2.0 only).
SNR: Better than 100 dB!
LineOut/Earphone Output: 1.1 V rms
Line Input: 1 V rms (low for this application and the signal needs to be amplified)
Mic Input: with microphone booster, the receive coil can be directly connected to mic input (but noisy). Voltage range not measured yet.

Operating frequency can be up to 23 kHz. Above 20 kHz, there is more total harmonic distortions. But this does not matter much.

I have same detection depth results despite of the lower output voltage (1.1 instead of 1.4 V). The signal quality is almost same or a little bit better.
Now, it is time to develop an external amplifier module. The external booster module will allow to work with more sound-cards and also more detection depth.

Without an external booster module, it has enough detection depth to knock out other VLF detectors.

Aziz

[Aziz]

I have received my new external USB sound-card (Creative X-Fi Surround 5.1 USB) . It has crystal clear sound on the head-phones so far. The installed XP driver doesn't work properly. So I have to use the standard windows mixer software. The sound-card will be used purely in stereo mode (no advanced features - switching them all off).

I will test the sound-card more in detail for application.

Aziz

[Aziz]

Hi miki73,

Can you share with us some software to work with dis detector?Or you use simply scope prog?

Regards?

I am sorry to say, that this project is not an open-source project. Software development is my daily bread and I can not give any source-codes. I also don't want some people stealing my good ideas and huge effort built-in in the software. But I am giving many hints and results to make this project possible to others.
Just start reading Win32 coding. It is well documented. You only need the wave input/output programming interface on the Win32 SDK.

Aziz

[Aziz]

Hi all,

I have ordered a new external USB sound-card for some tests. I will check the sound-card whether it is suitable for this application.
SNR is -100 dB and is a bit better than my built-in HD sound-card on the mainboard. I hope, there will no timing problems.

Ordered sound-card:
Creative Sound Blaster X-Fi Surround 5.1

http://us.creative.com/products/product.asp?category=209&subcategory=669&product=17751

Aziz

[miki73]

Can you share with us some software to work with dis detector?Or you use simply scope prog?

Regards?

[Aziz]

Hi Simon,

So essentially you are saying you first "null" the coils mechanically as much as possible, then add a very small bucking coil aligned with the RX coil and electrically independent from the TX coil, driven by its own signal (using the other stereo output channel not used by the TX coil). The bucking coil voltage (and phase) can then be programatically controlled by software to achieve any kind of nulling you want.

It sounds like a very good idea, especially for a double-D coil where most of the nulling is done physically and only a small residue needs to be corrected. I think that technique could be added to any metal detector. It does mean an extra wire running down the cable for the independent nulling coil. For conventional detectors, a couple of extra trim pots could be added for tweaking the balance to compensate for aging coils, or different add-on coils.

Actually it would work for concentric coils as well -- two bucking coils: one big one in series with the TX coil, and one small independent one to tweak the residue.

I like it.

Exactly described. I have used this method already former. One can balance the coil exactly and correct phase shifts with software. Building the search coil gets then much easier.

I just thought of a problem with the independent bucking coil -- as the TX coil moves over the ground, it's current and magnetic field change a lot in response to ground mineralization and targets (because of its high Q resonance).

Because the independent bucking coil is not in series with the TX coil, it's current and magnetic field would not change proportionately, and the nulling would be lost.

Wouldn't that be a problem?

-SB

I suppose, the laptop detector will be very sensitive to ground anomalies. I have no idea until I saw the field testing results. It depends then on the results which way or method I will develop further.

I would like to build a portable search coil this month. I also have to get an external HD USB sound-card. I hope, I can go some field testing this month.

Aziz

[Tinkerer]

coil interaction

I just thought of a problem with the independent bucking coil -- as the TX coil moves over the ground, it's current and magnetic field change a lot in response to ground mineralization and targets (because of its high Q resonance).

Because the independent bucking coil is not in series with the TX coil, it's current and magnetic field would not change proportionately, and the nulling would be lost.

Wouldn't that be a problem?

-SB

All the coils within a coil assembly are interconnected by transformer coupling to some degree.
Any change in the field or current of any one coil, will affect all other coils by the amount determined by the k or coupling factor.
Tinkerer

[Mustafa]

Merhaba Aziz,

USB high definition (HD) sound-card wanted!

If some of you know a good USB sound card, then please feel free to post the product type here.
The sound-card should support:
- full-duplex
- 24 Bit
- 96 kHz or more sample rate (both for input/output)
- USB-2.0 interface (small, without external DC supply demand)
- Line-in/Mic-in + Earphone Out
- Input frequency range >20 kHz

While searching for GNURadio, I hit these links.

Analog to Digital Converter with 16 bits and 448000 Samples per second based in the Bt878A

http://www.domenech.org/bt878a-adc/index-e.htm

btaudio.c module modification to get 896000 Samples per second with the Bt878A ADC

http://www.domenech.org/bt878a-adc/index-decimator-e.htm

My First Software Defined Radio (SDR) Receiver - 1st Section

http://www.domenech.org/homebrew-sdr/receiver-1.htm

Perhaps we can use usb tv card instead of HD sound-car.

[simonbaker]

I just thought of a problem with the independent bucking coil -- as the TX coil moves over the ground, it's current and magnetic field change a lot in response to ground mineralization and targets (because of its high Q resonance).

Because the independent bucking coil is not in series with the TX coil, it's current and magnetic field would not change proportionately, and the nulling would be lost.

Wouldn't that be a problem?

-SB

[simonbaker]

I have recognized the most difficult part of this project: the search coil

Initial balancing and phase shift is very important. To get the right properties, it is very difficult to meet these requirements. Additional critical point is the mechanical stability with inherent temperature coefficient (mechanical shock, tension, temperature drift). Further are the coil cables. Shielding is also critical but can be realized easy (double layer graphite shielding: coils itself and the coil housing).

Due to varying sound-card techical spec's regarding the output impedance and voltage, input voltage sensitivity, I will use an external low noise amplifier for the TX and RX part. TX will be driven on one channel (amplified). RX coil will have additional independent winding for right balance and phase shift adjustments fed from the other output channel (not amplified). I will discard the balance adjusment circuit part. Balancing is made purely with magnetic field coupling.

Using an external amplifier gives more flexibility and depth performance.
Now focusing on how to make a portable and tight search coil.

Aziz

So essentially you are saying you first "null" the coils mechanically as much as possible, then add a very small bucking coil aligned with the RX coil and electrically independent from the TX coil, driven by its own signal (using the other stereo output channel not used by the TX coil). The bucking coil voltage (and phase) can then be programatically controlled by software to achieve any kind of nulling you want.

It sounds like a very good idea, especially for a double-D coil where most of the nulling is done physically and only a small residue needs to be corrected. I think that technique could be added to any metal detector. It does mean an extra wire running down the cable for the independent nulling coil. For conventional detectors, a couple of extra trim pots could be added for tweaking the balance to compensate for aging coils, or different add-on coils.

Actually it would work for concentric coils as well -- two bucking coils: one big one in series with the TX coil, and one small independent one to tweak the residue.

I like it.

[Aziz]

I have recognized the most difficult part of this project: the search coil

Initial balancing and phase shift is very important. To get the right properties, it is very difficult to meet these requirements. Additional critical point is the mechanical stability with inherent temperature coefficient (mechanical shock, tension, temperature drift). Further are the coil cables. Shielding is also critical but can be realized easy (double layer graphite shielding: coils itself and the coil housing).

Due to varying sound-card techical spec's regarding the output impedance and voltage, input voltage sensitivity, I will use an external low noise amplifier for the TX and RX part. TX will be driven on one channel (amplified). RX coil will have additional independent winding for right balance and phase shift adjustments fed from the other output channel (not amplified). I will discard the balance adjusment circuit part. Balancing is made purely with magnetic field coupling.

Using an external amplifier gives more flexibility and depth performance.
Now focusing on how to make a portable and tight search coil.

Aziz