Minelab 8000

[Aziz]

Hi all,

I have made the target response for a 20 mm diameter wire loop target (my standard target). See the zipped Excel file in the attachment.

[Aziz]

Hi all,

it seems that I have to impement the coil model generation of vertical spider/basket weave coil type yet. I have only implemented the horizontal type.
Time to change this. This is required for designing a better coil. Much better than Z18.
A lot of work for reverse engineering my software code...
Cheers

[Aziz]

Hi all,

ok, I have added a very basic vertical basket weave coil model to the coil software now (red coil). There is a reference solonoid coil above (yellow) to check the code.
I have zoomed in, out, rotated, moved and checked its correctness. It seems to be correct.

This gives us more flexibility to design better new and easy coils. Let's weave some baskets soon.

[Aziz]

Interesting reading:
https://www.facebook.com/61564997111...5561712499903/

No user manual? -> Wrong usage
Faulty detector? -> First releases might have some unknown issues.

No, I think the 8000 is a good machine.

[Aziz]

I see a good potential and chance to outperform the Z18 coil with a new coil design.
By just looking at the magnetic field strenght lines shown below (transmit coils not shown):

[moodz]

Interesting reading:
https://www.facebook.com/61564997111...5561712499903/

No user manual? -> Wrong usage
Faulty detector? -> First releases might have some unknown issues.

No, I think the 8000 is a good machine.

Some of the those guys are like sailors ... they think that if they buy a carbon fibre boat ... it will win races for them.

[moodz]

... another thing to consider. Do you reckon if you cut the cable to one of these latest ML boxes that you would find a transmit , ground and rx .... and maybe some data cables to the secret chip ??

No way ... they use an TX levellling system and RX balancing system and probably a DSP controlled compensation system. The electronics is what makes this type of complex coil manageable.
Trying to build a coil that will be anywhere as good using pure mechanical arrangements of coils will never work.

If you have an active system you can "move" any target holes around in real time many times a second ... so removing the issues that occur when you use fixed coils.

For instance balancing .... you might be able to balance a coil manually when you construct it to around 80 db if you are wearing the right color underpants on the day
.... but a DAC feedback system ( eg 12 bit DAC ) can crack 140 db balance.

No smarts = no free lunch.

BTW I cracked the VLF monocoil ...

[Aziz]

Hi all,

I have already found a coil configuration, which outperforms the Z18 coil.
But I am searching for further optimal solutions.

How about a 2-in-1 coil?
A two sided coil for:
- one side for mid to large deep range targets on high mineral ground and
- the other side for small to mid range targets on low mineral ground?
Just flip the sides of the coil. It requires a different coil mounting system. A dual-side coil mounting system.

You pay once, but you get two coils.

Oh man!, it was too easy to beat ML.
Cheers,
Aziz

PS: I won't have time for one week. The coil analysis must be interrupted...

[Aziz]

A flip-over coil mounting system..
This is high speed coil changing rocket science technology.

[Aziz]

Look, this is beating Z18 coil:


You can make it basket-weave too.

[Carl-NC]

For instance balancing .... you might be able to balance a coil manually when you construct it to around 80 db if you are wearing the right color underpants on the day
.... but a DAC feedback system ( eg 12 bit DAC ) can crack 140 db balance.

It's rare that you can ever get a null below 60dB. Electric field coupling and parasitic C in the coils, cable, and connector inject just enough signal that the resistive null limits how deep you can get the total null, even if you could get a perfect reactive null. Also, there is little reason to get a better null because as soon as you put the coil to the ground, you lose almost all of it. It almost makes sense to shoot for a 30dB null @ 180° so that when you lower the coil to the ground, the null always gets better.

[Aziz]

I did do almost perfectly induction balancing with a DAC feedback system down to the noise floor level (in my case -120 dB).
But this isn't really required. 10 - 20 mV residual RX voltage is good enough. A stable phase angle calculation at low level RX is more important.

BTW, the coil above is much easier to build and balance. Fine IB is done with slight distance variation of big RX coil to the other coil arrangement. Just add more or less distance spacers.
I like solenoids now!

[moodz]

It's rare that you can ever get a null below 60dB. Electric field coupling and parasitic C in the coils, cable, and connector inject just enough signal that the resistive null limits how deep you can get the total null, even if you could get a perfect reactive null. Also, there is little reason to get a better null because as soon as you put the coil to the ground, you lose almost all of it. It almost makes sense to shoot for a 30dB null @ 180° so that when you lower the coil to the ground, the null always gets better.

hmmm... below is my DD coil operating with 148 volts PP on the transmit and 64 mv on the RX .... its connected to the "breadboard" ... you saw one of my boards ... this is a new one though and has the modified E class driver but no preamp and the same software you saw running no shielding no filters just a blocking cap connected to the 1.5 volt bias before the RX hits the ADC.

The coil is a DD minelab ... stock .... the one you saw also. 64 mv / 148 volts is -67 db of nulling. It would possibly be a little better if I had a layed out board with better sheilding.

This gives us an interesting insight into the coils used for PI and VLF .... its probably true what you say regarding 60 db for VLF type coils ... because the inductances are much larger ... more wire, more capacitances, harder to balance. However this DD coil is a PI coil but I am using it for VLF. Less wire, less capacitances and it would have been balanced in the factory for a wideband signal much higher than the maximum frequencies I use ( 100 khz tops ).

The TX coil in this DD is 410 uH ... this has a reactance of 82 ohms at 32 Khz .... its plenty of impedance for this type of work. You can put big TX power through it without rediculous voltages like you get on the 1 mH plus coils.

I will tweak the software to see if I can extract a db reading from my software null.

Aziz I dont agree that you need a baseline phase reading .... whether its phase or signal amplitude .. there is an integration going on inside the software to extract weak targets ( phase and amplitude ) ... at the noise floor the baseline phase is all over the place so you dont want it .... to get to the noise floor you need to remove the leakthrough... otherwise your measuring the leakthrough which is an imbalance signal ... not a target signal.
There is a difference between coil balance and a change in mutual inductance between a target and the RX coil ... if the balance changed ... my ground balance method would not work and this can be proved by moving the coils mechanically with respect to each other vs approaching ground.

[moodz]

Just looking at the noise floor in the FCMD with no target no ground the we get a figure of -65 db which is the residual signal after we have software cancelled the residual input from the unbalance of the coil.
So our effective balance from the coil is -67db from the mechanical balance of the coil and -65db from the software cancellation so we end up with -132db.

This is pretty accurate given the unsheilded board .. the noisy lab environment etc.

I bring a ground sample in till it touches the coil ( when ground balanced ) and this figure will not change .... so that means the "balance" of the coil is not affected by ground.

This is shown in the second picture ... we are present a ground signal to the coil with ground balance on. The target detection ( red and yellow ) does not respond .. but the magnitude and angle does does ( this is calculated without the ground balalance applied ).
The signal level has increased from around 4.8 mv to 206.8 mv ... an increase of 32 db. The rejection says -31.8 db .. and thats the point .... the input has increased by 32 db so the difference is still around -65 db.
This means the overall coil balance is still around -132 db.

I am using a noisy 16 bit ADC and no amplifier from the RX coil to the ADC. I am pretty confident this will be a very good detector when I move to 24 bit and preamp.


No need for fancy coils. ( yet )

[Carl-NC]

The coil is a DD minelab ... stock .... the one you saw also. 64 mv / 148 volts is -67 db of nulling.

True, DD coils are usually easier to null to a deeper level, especially if you manage the overlap points well.