Laptop Metal Detector utilising Digital Lock-in Amplifier
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Posted by Bob Masta on August 15, 2008, 8:06 am
Please log in for more thread options wrote: <snip>
>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. 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 www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card! | |||||||||||||
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Posted by on August 16, 2008, 12:10 pm
Please log in for more thread options 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 showing amazing sensitivity results. It can compete with professional VLF detectors. Aziz On 14 Aug., 14:15, NoS...@daqarta.com (Bob Masta) wrote: > On Wed, 13 Aug 2008 17:29:33 -0700 (PDT), oeg...@gmx.de wrote:
> >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:
> >http://www.thunting.com/geotech/forums/showthread.php?t=3D14102 >
> >The metal detector uses only the sound-card (output/input) for
> >interfacing the search-head. > >;-) > >Aziz >
> 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. =A0It 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. =A0(And if it can't do it now, it might be > something to add to the next version!) > > Best regards, > > Bob Masta > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 DAQARTA =A0v4.00 > =A0 =A0Data AcQuisition And Real-Time Analysis > =A0 =A0 =A0 =A0 =A0 =A0 =A0www.daqarta.com > Scope, Spectrum, Spectrogram, Sound Level Meter > =A0 =A0 =A0 =A0 =A0 =A0FREE Signal Generator > =A0 =A0 =A0 =A0 Science with your sound card! | |||||||||||||
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Posted by Bob Masta on August 16, 2008, 1:30 pm
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On Sat, 16 Aug 2008 09:10:50 -0700 (PDT), oeguet@gmx.de wrote: >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 showing amazing sensitivity results. It can compete >with professional VLF detectors. >Aziz 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 www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card! | |||||||||||||
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Posted by on August 16, 2008, 2:06 pm
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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 > Aziz:
-
> > Thanks for the explanation. =A0This sounds like a very intriguing > project! > > There are a couple of details I am still not sure about. =A0You 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. =A0Is that the case? > > Does the sound card really need 24 bit, 96 kHz? =A0What frequency do you > send out to the transmitter coil? =A0 > > Finally, how difficult is it to tune the two coils? =A0As 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", =A0by watching the amplitude of the received signal. =A0But then > you'd have to manually tune the receive capacitor. =A0True? > > Best regards, > > Bob Masta > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 DAQARTA =A0v4.00 > =A0 =A0Data AcQuisition And Real-Time Analysis > =A0 =A0 =A0 =A0 =A0 =A0 =A0www.daqarta.com > Scope, Spectrum, Spectrogram, Sound Level Meter > =A0 =A0 =A0 =A0 =A0 =A0FREE Signal Generator > =A0 =A0 =A0 =A0 Science with your sound card!- Zitierten Text ausblenden = >
> - Zitierten Text anzeigen - | |||||||||||||
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Posted by Bob Masta on August 17, 2008, 8:47 am
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On Sat, 16 Aug 2008 11:06:52 -0700 (PDT), oeguet@gmx.de wrote: >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 > 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 <www.daqarta.com/eex01.htm> 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 www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card! | |||||||||||||
