Moodz' Awesome Gold Pulse Induction Version 3 - MAGPI V3 Project

[moodz]

Target saturation ? You could try reducing the pulsewidth until the sensitivity noticably drops then crank back to where the sensitivity is 'normal' then alter the freq for that PW. Maybe there's a sweetspot for PW and/or freq for that target size/material in air/ground.

Of course it's going to be different for every coil !?

BTW whats the lower freq breakpoint for your RX amp, higher than the 1kHz TX freq ?

I tried your suggestion and the highest stable TX frequency I got was 25 Khz ... but I backed it off to 20.3 Khz and the range on the 10cm x 10cm foil was 1.1 meters ( more than 2.5 coil diameters ). The sensitivity to the 10 x 8 mm foil remained the same ( approx 20 cm )

The coil is a Minelab commander mono 300 uH 0.5 ohm.

[Gunghouk]

I guess in air the frequency change has no discernible effect with the small target but in ground who knows. My understanding is that lower frequencies propagate through ground better anyway though I suspect the optimum frequency will vary depending on multiple variables the major one being soil type/composition.

Was the 1.1m range for the 10x10 target unchanged aswell?

My bare bones 26 cm dia. homemade coil is 320uH and 1 Ohm so it should perform reasonably well.

[moodz]

I guess in air the frequency change has no discernible effect with the small target but in ground who knows. Was the 1.1m range for the 10x10 target unchanged aswell?

The previous range was just on 1 meter ... the range increased by 10 cm to 1.1 meters at 20 Khz. ( up from 16 Khz )

I have also been considering the reason for this and its related to the maths behind PI target detection which is proportional to SAMPLERATE x Integral ( K X 10-6 )

Where K is a value dependant on coil inductance / area / current / target size / target decay time etc etc.

The main thing is the 10-6 ... the target response is a 6th power down.

Now if we log that or voltage we get 20log(10-6) = -120db. WIth the latest change to the preamp the preamp gain the overall system gain in the MAGPI is now +135db.
Because this exceeds the target path loss from TX to RX we now have above break even target detection. ( break even means the target signal = the noise floor in the amplified demodulated target signal)

Generally speaking unless the ground is highly reactive the ground depth wil exceed the air tests due to halo effect.

[moodz]

Added target modulated volume control TMVC. The MAGPI uses a software VCO that responds to target voltage. The only problem with that is you have to listen to the tone when there is no target. When there is a target the tone frequency rises proportionaly to the target signal strenth the volume stays the same.

The TMVC now drops the no target volume to around 5% .... when a target is near the coil the volume will increase proportional to the target signal strength in addition to the VCO tone changing.

Much nicer ... like changing from a B&W TV to color.

Heres the video

https://youtu.be/X7eVNNC9yq4?si=BFzb0bdBHND8VilW

[Gunghouk]

Nice. How does the hardware compare with the mods we've been discussing eg. the steeper low pass front end hardware, new op-amps or TX fundamentals?

[moodz]

Nice. How does the hardware compare with the mods we've been discussing eg. the steeper low pass front end hardware, new op-amps or TX fundamentals?

Its the same hardware ... I am waiting for bits from element14 but they are coming from the UK. In the meantime I am amusing myself by working on the software.
This latest change was just software. It works fine on the original hardware.

[Altra]

Added target modulated volume control TMVC. The MAGPI uses a software VCO that responds to target voltage. The only problem with that is you have to listen to the tone when there is no target. When there is a target the tone frequency rises proportionaly to the target signal strenth the volume stays the same.

The TMVC now drops the no target volume to around 5% .... when a target is near the coil the volume will increase proportional to the target signal strength in addition to the VCO tone changing.

Much nicer ... like changing from a B&W TV to color.

Heres the video

https://youtu.be/X7eVNNC9yq4?si=BFzb0bdBHND8VilW

Nice audio moodz!

[moodz]

Nice audio moodz!

Thanks ! It worked better than I thought it would.

[moodz]

There are various sources on the web that show that the detected target response from the transmit signal is proportional to a 6th power path loss.

If this is true then as the target approaches the coil the response should be very non linear. To test this we move a target to where the detector just barely detects the target then measure how much closer the target must be for maximum signal / overload.

So I took a coke can which just registers at around 50 cm off the side of the coil. With the MAGPI just moving the coke can in by 1 cm caused a full scale target deflection.
This demonstrates the target signal to noise ratio is highly non linear as the target approaches the coil.

Is it a 6th power ? Dunno .... But it must be quite significant to go from a sniff to a sneeze in just 1 cm.

Anyway heres the video
https://youtu.be/TsX0H9jwNN8?si=fAGdo4BgWHX5gKLd

[Gunghouk]

Thanks Moodz, lovin' these videos and envious of your workshop. Please keep 'em coming

From the freq response sim it would appear that there's only about.. 3.5dB of gain in the front end op-amp. Is this incredible sensitivity from the IN125 instrumentation amp?

[pito]

Is it a 6th power ? = It is safe to assume that the signal strength decreases with the cube of distance (1/8 the signal at twice the depth), and that location errors build up with the square of depth.

[moodz]

Thanks Moodz, lovin' these videos and envious of your workshop. Please keep 'em coming

From the freq response sim it would appear that there's only about.. 3.5dB of gain in the front end op-amp. Is this incredible sensitivity from the IN125 instrumentation amp?

The preamp gain is 1000 given by 100k ohm / 100 ohms.

That simulation above is from JLKing demonstrating the benefit of a 2nd order feedback filter element.
If you plot the gain of the preamp you do get 60 db gain.

So the overall gain is preamp gain x diff amp gain = 1000 x 6000 = 6 000 000 ( 6 million ) . This is a voltage gain so we have 135 db.

[moodz]

Is it a 6th power ? = It is safe to assume that the signal strength decreases with the cube of distance (1/8 the signal at twice the depth), and that location errors build up with the square of depth.

Hi Pito .. I dont know for sure it is a 6th power. I can reference the work of Corbyn and this formula he derived for voltage in a recieve coil of a pulse induction metal detector.



In this case the determining variable is the h^^6 term. This is the target depth to a 6th power.

I wont repeat the calculation here but for a coke can you do expect about microvolt range signal at 1 meter from the RX coil and so I would expect a few volts of signal from the MAGPI with gain = 135db - approx 20 db of losses.
The MAGPI gets around 1 volt of deflection at 1 meter .. indicating I am in the ballpark of corbyns formula.

[moodz]

Progress update .... found a bug in the software while field testing. Had to pack up and back to base. However fixed now.

The proposed final hardware configuration for MAGPI V3 is in place.

Hopefully more field testing this week.

In the mean time here is video of the MAGPI detecting a 0.1 gram nugget..... well under 1 us tau

[Gunghouk]

Hi Moodz, my breadboard Pic test cct transmits at 10kHz with a 50uS TX pulse width, did a timer change since v1.4?