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

[gwzd]

Just investigating, where the very small drift is coming from (few µV/sec).

Temperature?
Regards,

[Aziz]

Temperature?
Regards,

Hi gwzd,

yes could be a matter. But should be constant over some time. I want to change the coils in the coming days. Making tighter and better temperature conductive. The fibre glass fleece with the right resin is quite good for making very tight and stable coils. Just testing some resin types at the moment.

It could also caused by the sound-card output stage. I will test this with a simple resistor divider. Don't know yet where this small drift is coming from. I will fix this, before I start to implement the software drift compensation.

Also a matter is the coil place position. When the coils are balanced (lying on each other), it detects even very tiny seismic shock (going in the room, wind blow, etc.). I want to build a stable desk for this in the coming days. It can also be used as a seismic/geophon detector with proper sensors.

Once fixed this problem, I can break new detection records. I also started to remove the experimental code in the software. Making faster and more power efficient (saving battery).


Aziz

[Tinkerer]

Laptop noise

I have finally traced one source of sporadic noise.

When using the laptop sound card input and the Spectrum Lab oscilloscope, I kept getting sporadic noise interference.

I have now traced this noise to the Logitech cordless mouse.
When I move the mouse, I get the noise. Not easy to find out because of the time delay between the action and the output of the oscilloscope.

Tinkerer

[Aziz]

I have finally traced one source of sporadic noise.

When using the laptop sound card input and the Spectrum Lab oscilloscope, I kept getting sporadic noise interference.

I have now traced this noise to the Logitech cordless mouse.
When I move the mouse, I get the noise. Not easy to find out because of the time delay between the action and the output of the oscilloscope.

Tinkerer

Hi Tinkerer,

with the built-in spectrum analyzer and scope in the MD, it is easy to detect some noise. The laptop/PC generated and correlated noise does not matter to the MD. In the frequency spectrum up to 24 kHz, there are more regions for quite noiseless operating frequencies. I can see lots of noise sources: PC, 50/100 Hz line EMI, DC/DC converter of the PC, CRT monitor, external sound-card amplifier (for my TV application), etc..
These noise sources does not disturb the real operation. Lock-in amplifiers have a very small bandwith inherent filtering (~50-100 Hz).

On the VLF frequency spectrum, there are some military radio stations, which transmits sporadically. Submarine is using VLF frequency spectrum!!! But this can be shielded out with faraday shielding.

Now my stable wood desk is finished. No more seismic shocks (I hope) and big metal frames nearby the search coil.

I have tested with two component epoxy glue to fix the coil with fibre glass fleece. It is really quite easy to fix the coil. And it is getting very very tight. I will try to get some other resing for the coil today.

The IB coil should not to be balanced perfect. The opposite occurs: it has to be left a little bit unbalanced. So it will work on 5-90% of dynamic range of the sound card input voltage (Umax = 100mV rms). The lock-in amplifier does not work on perfectly balanced coils due to bigger phase errors on very small signals on receive coil. It is enough, to avoid the overloading of the input voltage range.

Aziz

[gwzd]

Hi gwzd,

yes could be a matter. But should be constant over some time.

More or less yes, depending on how close to the window or the ventilation you have your circuit. Then again, you said it was somewhat constant, but perhaps is something else.
Regards,

[Alz]

sonar for Pocket PC devices.
http://nerdipedia.com/tiki-index.php?page_ref_id=27

I can't wait for the metal detector for pc or ce. How long do you think its going to take before we get to play with your pc md?

[Aziz]

Drift problem partial localized:

I have now analyzed the temperature influence by a simple resistor divider connected to the sound card output. Yes, it has some influence due to high sensitivy of the lock-in amplifier.

Unfortunately, I made use of a thermal isolated coil. The high current through the coil produced some heat, which it has conserved this for a long time. It will therefore produce some drift for some period of time. I will use other coils with a better temperature conductivity (fully resin filled and fibre glass fixed coils). Temperature drift can not be totally compensated. It changes the resistance of the coil and the geometry. But the software can compensate this very elegant.

But the very big influence and drift comes from mechanical unstable coils:
- top overlapping coil (DD configuration) is bending due to its own weight
- any tension caused by balancing takes some time to get out thus changing the geometrics of the coil.

I got some experience in making fibre glass +resin fixed coils. The result is expecting more stability. And it is quite easy to build such coils. I will make two layers of fixing: 1. fixing the coil windings, then graphite shielding after the first layer is dryed and 2. finaly the last layer over the graphite shielding. So coils can also be water resistant. This mechanical drift can be totaly compensated with better coil construction.

More results probably next week. Coils take some day to dry.

Aziz

[gwzd]

Hi Aziz,
As long as you know what the problem is, then half of your problems are fixed!
Wish you luck with that.
Regards

[Aziz]

Found another drifting cause: (the mysterious one!)
Coil Cables.

If they are not fixed tight, they also have inductive coupling to the receive coil (more or less one winding). They were lying or hanging free in air and the gravity force + wind blow changed their position and thus the coupling. This inherent effect was caused by experimental assembly of the MD (on the fly assembly).

I am glad of this discover.

Aziz

[Aziz]

Hi Aziz,
As long as you know what the problem is, then half of your problems are fixed!
Wish you luck with that.
Regards

Hi gwzd,

that's really true. I would be glad to achieve only the half.

Aziz

[Aziz]

sonar for Pocket PC devices.
http://nerdipedia.com/tiki-index.php?page_ref_id=27

I can't wait for the metal detector for pc or ce. How long do you think its going to take before we get to play with your pc md?

Hi Alz,

the pocket sonar is really an interesting application. The metal detector can be ported to the pocket PC one day. The speed of the pocket PC would be enough and the operation time and portability makes it very attractive. The sampling rate of 44.1 kHz at 16 Bit is enough but not the best. Only laptops with high definition sound cards yield the best performance at 96 kHz and 24 Bits at the moment.

I have to solve some problems until I can develop the real MD software. At the moment, the software is only a:
- PC scope
- PC spectrum analyzer
- PC lock-in amplifier
So much to do yet to get this MD working perfect.

Aziz

[FrankMD]

Hello Aziz

I've just caught up on all you've posted about your Laptop Metal Detector. Congradualtions on your achievments so far. I can appreciate your attacking the challenges head on and using your imagination too!

In a post way back you mentioned some drift maybe related to the coil.

Maybe you could design (maybe sotware) into the system a way to automatically keep the receive coil nulled by sampling the transmit coil voltage?

Just a thought as I have seen this done before.


Frank in Maryland, USA

[Aziz]

I've just caught up on all you've posted about your Laptop Metal Detector. Congradualtions on your achievments so far. I can appreciate your attacking the challenges head on and using your imagination too!

In a post way back you mentioned some drift maybe related to the coil.

Maybe you could design (maybe sotware) into the system a way to automatically keep the receive coil nulled by sampling the transmit coil voltage?

Just a thought as I have seen this done before.


Frank in Maryland, USA

Hi Frank,

I have localized the most of drifting problems so far. It was mainly mechanical related. Before implementing the software drift compensation, I was interested on the source of drifting. So drifting can be minimized and the software can do the rest. I can do this on the receive signal. The transmit signal will not be observed. Finally only the temperature change of the coil would cause some drifting and this will be compensated elegantly by the software.

The total cost for this MD will be:
- Laptop (if you don't have one)
- Appr. 10-20$ for coil (coil wire, cables, connectors and 3 capacitors) <-- This is the only passive electronics part of the MD.
- Free MD software (provide by me one day when finished).

And it will compete with the professional ones! I hope, some of you would also start to implement such a MD. Nobody wants to compete with me.

Aziz

[FrankMD]

Hi Again Aziz

I'd love to give it a try! I have a nice fast laptop already and an electronics/metal detector/EMI/EMC background. I have a lot of other interests taking my time too! But I love treasure hunting.

When I mentioned the transmit signal I should have said using the transmit signal as a reference to electronically and or via software to null any residual signal present on the receive coil.


Talk to ya later,

Frank

[FrankMD]

Aziz, coil

I could purchase an already manufactured coil, correct? As long as I knew the design frequency? A fisher, teknetics, whites, etc.


Frank