Laptop Metal Detector utilising Digital Lock-in Amplifier

Is a powerful metal detector possible by using only laptop and sensor (search-head) without any additional electronics?

See the generation and progress of this experimental project on:

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The metal detector uses only the sound-card (output/input) for interfacing the search-head. ;-) Aziz

Reply to
oeguet
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Powerful in what sense? As compared to a real metal detector, no.

John

Reply to
John Larkin

I haven't looked into metal detectors, but my Daqarta system has a signal generator that can probably create any sort of driver signal you want, plus real-time spectral (or waveform) analysis of the input signal. It might be useful for development purposes, to try out concepts before you devote a lot of time to writing your own code.

If you can explain the basic principles involved (or point me to a Website), I'll be able to give you a better idea of whether Daqarta can handle the task. (And if it can't do it now, it might be something to add to the next version!)

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

That page was too much to follow, too little detail and too many links.

Many metal detectors work by having an oscillator with a coil that is open to the world. When metal comes close to the coil, that shifts the oscillator's frequency and that shift is the giveaway that there is metal.

That won't work by merely feeding the coil with a signal, the coil has to be part of the signal generator.

I seem to recall there are things where the metal acts as coupling between two coils, one excited by an oscillator and another a pickup, and then the amplitude in the second coil varies depending on the unknown metal. I suspect that is less workable than the first method, though the excitation coil does not have to be a part of the oscillator.

From a memory of a Carl & Jerry story, I think the magnetometer uses a standalone excitation, and not that much more than a milk bottle filled with water and wound with wire. I can't remember what they used as a pickup.

The problem is the question is based on some discussion somewhere, rather than based on a foundation of what a metal detector is. If the original poster had done some basic reading before asking here, he'd have a better idea of what was required, and maybe an idea of whether it's possible.

Michael

Reply to
Michael Black

Most common today are the induction balance types. The various patents have a lot of thory.

Reply to
Rick

Hey, I remember that story! I think the gadget was called a "proton spin magnetometer" or something like that. The idea was the same as Nuclear Magnetic Resonance. They hit the coil with an RF pulse which put the protons of the hydrogen atoms in hte water into an altered state, then they monitored the decay back to baseline. (Or some such.) I seem to recall that they were trying to do some sort of archaeological search, but it turned out that their version of the gadget had some problem... I think it was too sensitive to background objects, or the Earth's field, or something.

Boy, *that* takes me back!

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

Hi Bob,

my experimental measurement system has already a scope, spectrum analyzer, digital lock-in amplifier, signal generator and much more.

So this experiment is a typical proof of concept which is saying: it is working!

Basic operation of the laptop metal detector: Harmonic sine wave is sent through the earphone output. The impedance of the earphone output is round about 20 Ohms which delivers enough current for the transmitter coil. The transmitter coil has a capacitor connected defining a LC resonant tank. The transmitted frequency is the resonant frequency of the LC tank. The receive coil is in inductively balanced position (less coupled to transmitter coil) and has also a capacitor which also defines a LC resonant tank (same of transmitt frequency). The receive coil is inducing a small signal. The signal amplitude and phase will change upon a metal target nearby the search coil appears. The sound-card is used in full-duplex mode (while transmitting a signal, the receive signal is acquired). The digital lock-in amplifier (a two channel I & Q lock-in amplifier) detects the signal magnitude and phase of the receive coil. While the laptop knows the reference frequency (internally generated), it detects the changes by the receive coil.

This is the most simple and sentive metal detector ever designed. You don't need any active electronics between the laptop and search coil. Search coil has only some capacitors and the inductors (transmit & receive coil). All the rest is done by the software using a high definition sound-card operated at 96 kHz sampling rate and 24 bit resolution.

The experiment is show> > >Is a powerful metal detector possible by using only laptop and sensor

Reply to
oeguet

Aziz:

Thanks for the explanation. This sounds like a very intriguing project!

There are a couple of details I am still not sure about. You mention using a digital lock-in amp, and it sounds like you mean an external hardware device when you say "two channel I and Q lock-in amplifier". But then you mention running the sound card in full-duplex mode, which sounds like you are doing the operations in software... which would certainly be the way to do it. Is that the case?

Does the sound card really need 24 bit, 96 kHz? What frequency do you send out to the transmitter coil?

Finally, how difficult is it to tune the two coils? As I see it, it would be simple enough to automatically tune the sound card frequency to match the transmit coil, using any capacitor that was "close enough", by watching the amplitude of the received signal. But then you'd have to manually tune the receive capacitor. True?

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

Hi Bob,

indeed, this is really an interesting and fascinating project. The digital lock-in amplifier is a pure software implementation which is applied on the input signal (A/D converted receive signal). The lock- in amplifier is a very sensitive phase detector even the signal is buried in high noise. Any small changes can be detected with it (=B5V measuring). Using a 24 bit sound-card at 96 kHz sample rate increases the signal-to-noise ratio and dynamic range. It will also work on 16 bit and 44.1/48 kHz with reduced SNR and dynamic range.

It is quite time-critical application. You must not loose the synchronisation of the transmitted to received signal. The continious wave form is buffered to the sound-card to avoid signal gaps due to operating system task switches. But this is easy to handle and only the DMA is busy and relieves the CPU. Laptop has enough CPU power for doing this and much more in real-time (FFT, digital filter, lock-in amp, detection, signal generation, synch generation, graphical output ..).

The operating frequency for the sensor is between 5 to 24 kHz (VLF range). It depends only on the resonant frequency of the search head (L,C resonant defining elements) and sampling rate (fmax=3DSR/4). The higher the operating frequency, the better the sensitivity of the sensor (Faraday's law). So it is mostly defined by the sensor specification.

The sensors are typically D shaped coils with same inductivity L for transmit and receive coil. This will allow a simple matching of the capacitors (same for transmitter and receiver). The coils are in overlapped co-planar position and forming a circle (two D's). The receive coil should have a minimum of signal level (10-50 mV rms). This position must be found by moving one of the coils.

What about digital lock-in amplifier for your application? This would be a quite useful feature.

Regards, Aziz

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Reply to
oeguet

Aziz:

Thanks for the explanation. Yes, I am quite familiar with real-time issues. Daqarta needs perfect sync to do synchronous waveform averaging for noise reduction, so I've been there and done that!

A digital lock-in would be a definite possibility for Daqarta. I'll put that on my "Wish List" for future enhancements. I probably won't offer the "lock-in" (PLL) part that hardware lock-ins have, since I've always thought that was pretty silly unless you really do need to sync to an external signal.

For those who are following this thread and aren't familiar with lock-in amplifiers, they are essentially a single-frequency Fourier Transform. You separately multiply the incoming signal by the sine and cosine of the reference signal, and low-pass filter the results. From the old high-school formula for the product of sinusoids, you get only terms at sum and difference frequencies. It's the difference term we want here. The low-pass removes the sum and produces an output only if the input is exactly the same frequency as the reference (difference = 0), or very near.

(See for an FFT explanation that goes into more detail about this.)

The only real difference between a lock-in and the output of an FFT is that the FFT has a very crude low-pass filter (one for each spectral line) and the lock-in usually has a better filter (longer time constant in lock-in terms). That statement assumes that the FFT has a spectral line just where you wanted the lock-in reference frequency. This is no big deal if you are generating the output frequency yourself... just make sure it lands squarely on a spectral line.

Daqarta can already do this. And it can get the noise reduction by synchronous waveform averaging before the FFT. The only thing is that it doesn't display the data in lock-in format, with separate sine and cosine or magnitude and phase readouts. That would be a good addition!

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

Reply to
Bob Masta

Great discussion, excellent commercial. Thanks guys.

Reply to
Don Bowey

Late at night, by candle light, snipped-for-privacy@gmx.de penned this immortal opus:

Why lug along a lap-top if the needed circuitry fits in a box the size of a cigarette pack? All you need is a LED display and possibly a beeper.

- YD.

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Reply to
YD

you make the lock-in amplifier sound very much like a synchronous detector. in-fact if you're not implementing the PLL it seems more like a synchronous detector than it is a lock-in amplifier.

Bye. Jasen

Reply to
Jasen Betts

well. the size of a mobile phone anyway :)

you'd probably want to be able to tune it to discriminate between scrap iron and lost coins.

Bye. Jasen

Reply to
Jasen Betts

Hi YD,

the development platform is a PC/laptop. But this project can be ported to a handheld Pocket PC. The reason, why it can't be done to a size of cigarette box pack is simple: the demand for high number crunching computing performance. That's the reason, why this project becames very simple. All the detector is done by pure software programming.

Aziz

Reply to
oeguet

But which comes first? Are you actually getting a significant improvement by using a computer, or is it so you don't have to wire up much circuitry?

In other words, unless the "high number crunching" is adding soemthing to make this better than the average metal detector, it's only purpose is to compensate for the lack of circuitry.

A software modem needs a lot of number crunching, but that does not make it better than a hardware modem. Just cheaper, though of course I can use my hardware modem with any computer hardware, including something from 30 years ago.

Michael

Reply to
Michael Black

Exactly so! Hardware lock-in amplifiers are just sine and cosine synchronous detectors with PLLs to sync to an external reference. I don't know what the current situation is, but for years the lock-in designers were off in the ozone somewhere. The typical design had a built-in reference oscillator that appeared to be an afterthought... it was completely separate, and even in models that allowed you to change the reference frequency remotely (via control voltage or serial bus), it still had to go through the PLL. Which meant you had to wait for the PLL to settle ("acquire lock") instead of instant response like they would have gotten had they just allowed the oscillator to

*be* the reference.

It seems there are not that many applications that really need to sync to an independent external reference these days. The original lock-ins were used with optical choppers, where the reference depended on the rotation of a wheel. (And they used simple switches instead of true multipliers, so they were effectively multiplying by square waves and thus were sensitive to all the odd harmonics.)

Ahh, the good old days...

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

Reply to
Bob Masta

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