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

[Aziz]

Promissed Full Schematics and Simulation Model.

Hi all,

below is a ZIP-file, which contains all relevant files for a LTspice simulation. I recommend you to play with it to understand the marvellous transmitter in action.
I have put my LTspice in "Alternate mode". To speed up the simulation, I have replaced the system voltages, as these take large time to get built up. It otherwise could cause convergation troubles.

It is the full schematics of the Laptop IB-PI of the first cut. It takes some time to get stabilized operation (>15 ms time). The coil transmitter is a self regulating circuit. Somewhere later, the clock signals will be switched off (testing the absent clock signals).

Have fun with spice simulations.

Attached below:
Picture of a running spice simulation.
Spice simulation ZIP-file.

Aziz

[Tinkerer]

Tinkerer's circuit reminds me very much of a class-e amplifier driving a tank circuit. The timing is a little different, but perhaps that's the inspiration of his design?

I used a class-e amplifier to drive a tank circuit for the transmit coil in a VLF detector when I first started trying to design metal detector circuits. The timing was conventional, and it was very efficient. I didn't know about coil shielding back then, so there were some stability and drift issues. I may have to revisit the circuit again in the future.

The major difference between the VLF sine wave and the RE-PI TX, is the asymmetry of the wave.
To charge a 300uH coil to 5A current takes some time. But then the whole charge is dumped into the capacitor in a much shorter time. This produces a very high di/dt, that excites eddy currents not only on the surface of the target, but also deeper eddy currents.

At present I am trying to resolve the signal into it's parts. The part of the skin eddy currents and the part of the deep eddy currents.

I have not tried to extract the signal the VLF way.
Multiple sampling gives me a lot of information about the target.

Tinkerer

[Aziz]

Hi hobbes_lives,

Tinkerer's circuit reminds me very much of a class-e amplifier driving a tank circuit. The timing is a little different, but perhaps that's the inspiration of his design?

I used a class-e amplifier to drive a tank circuit for the transmit coil in a VLF detector when I first started trying to design metal detector circuits. The timing was conventional, and it was very efficient. I didn't know about coil shielding back then, so there were some stability and drift issues. I may have to revisit the circuit again in the future.

indeed, something looks like similar. It could be well worth to look at it deeper. I am sure, the published schematics can not be made more simpler. Class-E amplifiers need to know the zero crossing current point. This is not the case in this design and the duty-cycle can be much higher than 50%. The recycle phase and charge phase are put together to get optimum efficiency in the continious mode (=continiously current flowing in the coil).

The idea wasn't my own. Tinkerer is fully responsible for this (thanks for this). The transmitter is also known as the Tuned TEM Transmitter and is a well known technique in the geophysics explorations. Well, they often use SCR's to push hundreds of Ampere trough the coil.

Well, if we achieve an efficiency of 90%, we can push 10A at only 1A power consumption.

Aziz

[WM6]

Aziz thank you for all your efforts and kind & clear explanations.

Two question:

1.) Why you do not exchange L2 by proper power diode?

2.) Mean "Avoid Mosfet Avalanching" the same as "Avoid Mosfet Leak", what is adequate German term?

[WM6]

the INA163 is causing some problems. The low pass filter with the feedback capacitors Cfb2 are causing some trouble and do not work in this case.

Hi Aziz,

if you wish to try with INA121 I would be glad to send you 2 or 3 pieces to experiment. Please check its data sheet for first view if you do not know this op-amp yet.

[Aziz]

Hi WM6,

Aziz thank you for all your efforts and kind & clear explanations.

Two question:

1.) Why you do not exchange L2 by proper power diode?

2.) Mean "Avoid Mosfet Avalanching" the same as "Avoid Mosfet Leak", what is adequate German term?

1) Of course, you could place a forward biased schottky diode. But every diode has a forward voltage drop. Multiplied with the flowing current is the power loss. An inductor is much more efficient (=less power loss).

Let's make a comparison at 1 A current drain:
Diode: (Schottky: 350 mV voltage drop), P loss = U*I = 0.35V*1A = 0.35 W
Inductor: (Series resistor: 50 milliOhm), P loss = I²*R = 1A*1A*0.05Ohm = 0.05 W

The inductor is much more efficient.

That's also the reason for, why in the enhanced Phase 4, the mosfet is switched on again to omit the voltage drop of the internal body diode.

2) Avalanching means, if the Drain - Source voltage is exceeding the specified max. voltage limit. It will break down then. The mosfet will be conducting during the higher voltage period. If a mosfet is only specified for 200V, the flyback voltage may not exceed this level. But the breakdown voltage is temperature dependend and you need some spare not to exceed its maximum voltage level.

Well, its not a problem, if the mosfet avalanches. Just increase the coil energy buffer capacitor. It would have a positive effect: eddy current frequency spectrum will be lowered and the eddy currents can penetrate the target deeper. So the target can store effectively more energy, which will last longer for detection.

But if the mosfet breaks (avalanches), a lot of energy will be lost and heated up in the mosfet.

Avalanching in german? I don't know. Maybe: "Durchbruch"

INA121:
No, thank you. I do not need the INA121. My NE5532 is much better (half the noise density). And the INA163 is superior, if I drop the frequency compensation caps.

Aziz

[Tinkerer]

Dear Tinkerer, thank you very much!

Honestly, without Aziz I will never know that idea started under Tinkerer project although I am trying to follow all his posts.

The TEM-TX is an evolution of the TINKERERS_V1 project, where I used the capacitor to avoid the Mosfet avalanching.

Then I started to look for a way to prolong the Flyback to get deeper eddy current penetration in the target. Using more power I looked for ways to recycle the power.

I found this way that recycled most of the power and gave me a very good target response. This is when I asked Aziz to help me developing the idea further.

It is Aziz that you have to thank, for developing my idea and making it understandable.

Tinkerer

[Aziz]

Improvement of the TEM transmitter to work with MONO coils

Hi all,

the tuned TEM transmitter can be modified to work with pure MONO coils.


After phase 2:

When the flyback voltage arrives its maximum (at 1/4 sine wave flyback voltage), i.e. the transmitter energy is transferred into the buffer capacitor, the buffer capacitor will be switched out of the tuned LC circuit. It will hold the coil energy.

The coil then can be cricitally damped (damping resistor switched in) and after some time (decay), the signals can be usually amplified and sampled.
It is a delayed phase 3. The delay can be as much as needed.

After the sampling of the signals, the phase 3 can be started (damping resistor switched out, coil energy buffer capacitor switched in). The rest of the cycle is as usual (phase 4).

You know, I like MONO coils..


Aziz

[WM6]

Hi all,

the tuned TEM transmitter can be modified to work with pure MONO coils.


You know, I like MONO coils..


Aziz

Hi Aziz

I do not understand you? There is pure mono coil already on yours transmitter part schematics. What modification do you speaking about?

U like mono coil too.

[Aziz]

Hi WM6,

Hi Aziz

I do not understand you? There is pure mono coil already on yours transmitter part schematics. What modification do you speaking about?

U like mono coil too.

The published solution uses a transmitter coil (LTX) and a center-tapped receive coil (LRXA, LRXB). The TX/RX coil is a induction balance configuration (inductive and galvanic decoupled).
As there is always current flowing in the TX coil, one can not use the TX coil as receive coil.

I mean, it is possible to modify the transmitter, that the receive signal can be directly get from the transmitter coil. Only one mono coil should be enough. This would make the standard PI detectors more powerful (more depth) with the benefit of high power efficiency (less battery drain).

But the modification can not be used with the published schematics. This would be something for the PI controller project (I have searched a good solution till now - and I should have it now).

Aziz

[WM6]

Hi WM6,



The published solution uses a transmitter coil (LTX) and a center-tapped receive coil (LRXA, LRXB). The TX/RX coil is a induction balance configuration (inductive and galvanic decoupled).
As there is always current flowing in the TX coil, one can not use the TX coil as receive coil.

I mean, it is possible to modify the transmitter, that the receive signal can be directly get from the transmitter coil. Only one mono coil should be enough. This would make the standard PI detectors more powerful (more depth) with the benefit of high power efficiency (less battery drain).

But the modification can not be used with the published schematics. This would be something for the PI controller project (I have searched a good solution till now - and I should have it now).

Aziz

Yes Aziz, thank you, great idea again.

I am not happy to saw such Rx / Tx combination of coil. Maybe interesting for experiment but for practical work at least too heavy. I always hate heavy detectors.

What you say is fantastic, an real "Durchbruch".

Can we then still speaking about IB/PI detector or it would be only PI?

[Aziz]

Hi WM6,

Yes Aziz, thank you, great idea again.

I am not happy to saw such Rx / Tx combination of coil. Maybe interesting for experiment but for practical work at least too heavy. I always hate heavy detectors.

What you say is fantastic, an real "Durchbruch".

Can we then still speaking about IB/PI detector or it would be only PI?

Well, the IB/PI has it's own benefits. Even the coil is difficult to make, it gives quite good discrimination possibilities. Particularly ferromagnetic materials nearby the coil are changing the coil coupling of TX/RX. The receive coil will detect this easily and the response is more distinctable to eddy current responses. I can see this clearly in my frequency spectrum response. To achieve a good depth performance, a stable coil is required. Nevertheless, I will continue the IB-PI project as it is quite simple to implement with the benefit of good performance.


The new transmitter for mono coils should only be used in a standard PI of course (transmitter coil is also a receive coil). I have tested this with spice simulation and it works. The power efficiency is really phantastic and we can increase our coil current as lots of it will be recovered back. This will give us more depth performance. But I will focus to this project later (after finishing the IB-PI). I just let you know, that the principle in general works.

Aziz

[WM6]

To achieve a good depth performance, a stable coil is required. Nevertheless, I will continue the IB-PI project as it is quite simple to implement with the benefit of good performance.

I am sure, we all support you Aziz in this project.
Your technical knowledge are excellent and your filigree work is worthy of admiration.

Especially praiseworthy is your willingness to share this knowledge with others in accordance with good tradition of exceptional people such as Carl and others.

What are possibilities to process IB-PI signal data under Windows Mobile?

[Aziz]

What are possibilities to process IB-PI signal data under Windows Mobile?

I generally use two sound card devices:
One for beeping the target signal and a good one as an ADC/Clock device. The one for beeping could be the internal sound card of the processing front-end (laptop/netbook). The good one is usually an external USB sound card (24 Bit @96 kHz samping rate).

The question are:
Are there any device drivers for these external USB sound cards available?
How is the support for target platform development?
Ok, this isn't an issue with Microsoft platforms. They are well documented and all development tools are available.

But closed/proprietor platforms are not convenient for such a development. The Win32 is widely used and the support for device drivers is very good. Due to mass product market, such devices are quite cheap too.

Windows Mobile should be almost as good as Windows XP/Windows 7 platform. The whole application is not consuming much processing power. It can be optimized for less power consumption.

The schematics in the spice file above can be used to make a very powerful IB-PI detector.

Aziz

[Aziz]

High Efficiency Mono Coil Transmitter Signals.

Hi all,

below is the delayed phase 3 for mono coil configuration PI with efficient energy recovering described above. The delay is called phase S1. This phase is critically damped (damping resistor switched in, coil energy buffer capacitor switched out). After the damping, the samples can be taken directly from the TX coil. The delay period can be as long as necessary. The coil energy will be held in the capacitor during this time.

After the samples has taken, the phase 3 will be continued to recover the energy back. If the Phase 4 is finished, either another sampling is possible (phase S2) or it can be continiued with Phase 1 for continious operation.

Ignore the voltages and currents after phase 4. The control signals are not set and the coil has some minimum energy that begins to oscillate. The minimum energy left back in the coil or capacitor can be damped with the damping resistor if there is another sampling phase (phase S2).

The principle is quite high efficient and makes pure mono TX coils possible. If you don't make the damping resistor switchable, some energy will be lost. Best efficiency will be given, if the damping resistor is switched in at its usage only.

Aziz