Field testing modified MPP

[CAS]

_{Out at Bendigo testing my modified version of the MPP with mixed results.
Several versions of timings have been quite successful but I think the circuit isn't quite up to speed. I think if I am going to continue to use this circuit I may have to make changes to the integrator. I have to swing very slow to get reliable results.
I have been using a small coil today so depth wasn't an option.}

[6666]

Very interesting, your timings work, thats great news.

[6666]

BTW what coil were you useing in the gold field ?

[6666]

Several versions of timings have been quite successful

Hi Cas would it be possible for you to describe any of the timings ?
thanks.

[CAS]

BTW what coil were you useing in the gold field ?

I was using a small 4 1/2 coil I wound using Litz wire and graphite shielding. I did have a basket coil there as well but really didnt want to have too many variables for my tests. One thing i do want to do is get a commercial fast mono coil and run tests with that. I am concerned that maybe my coil isnt up to a good standard even though workshop tests indicate its ok.

[CAS]

Hi Cas would it be possible for you to describe any of the timings ?
thanks.

I found 2 sets of timings worked the best. All sampled between 7 and 12us after coil turn off.
1st set was taken from the SD2000 timing chart in one of the threads in this forum.
2nd was an adaptation of what Davor proposed in http://www.geotech1.com/forums/showt...Ground-Balance
I did have 5 other methods to use as well. All worked to some degree and maybe I will refine them in the future.
If anyone wants the ChipKit code (basically Arduino) I can post that as well. Its had quite a few changes so its not all that neat but I do tend to comment most of it.

What I have taken out of all this is the MPP is a great PI but has its limitations. I think as a relic machine, it is great. To use on small gold or in highly mineralised soil, further mods will be required.
It has certainly helped me understand how a PI detector works and what is needed for difficult targets/soil types. I already have a few different ideas in my notebook that I will try over the next couple of months.

[Qiaozhi]

What I have taken out of all this is the MPP is a great PI but has its limitations. I think as a relic machine, it is great. To use on small gold or in highly mineralised soil, further mods will be required.
It has certainly helped me understand how a PI detector works and what is needed for difficult targets/soil types. I already have a few different ideas in my notebook that I will try over the next couple of months.

The sort of development work that CAS is doing is exactly why I spent some of my valuable time back-engineering the original Minipulse (MP), and then enhancing the design to provide the Minipulse Plus (MPP). The original MP was intended for beach work, and therefore it wasn't able to sample below 25us.

With the improved MPP design it is now possible to sample down to about 7us, which brings with it other issues such as ground response. Of course, for beach work or relic hunting you can adjust the sample delay appropriately, but for the gold fields it does really need some form of ground balance. It will be interesting see how this develops.

[6666]

Hi Cas if you could post the code please that would be very helpfull, and maybe a mudmap of the connections
is this the chipkit uno that you used ?
http://chipkit.net/wpcproduct/chipkit-uno32/

[Chet]

Hi Cas

In your field test was the small coin very deep? With the 5" coil I wouldn't expect great results. The 5" is good for small gold nuggets using early timings or coins at shallow depth. A coil of approximately 10" would be better for coins and larger nuggets. The large junk would be expected with most coils.

Thank you for sharing your results,
Chet

[CAS]

Hi Cas if you could post the code please that would be very helpfull, and maybe a mudmap of the connections
is this the chipkit uno that you used ?
http://chipkit.net/wpcproduct/chipkit-uno32/

Yep thats the board I used. I got mine from Element 14 and was pretty cheap. I will draw up the ChipKit/MPP connections later and post them with the code.

[CAS]

Hi Cas

In your field test was the small coin very deep? With the 5" coil I wouldn't expect great results. The 5" is good for small gold nuggets using early timings or coins at shallow depth. A coil of approximately 10" would be better for coins and larger nuggets. The large junk would be expected with most coils.

Thank you for sharing your results,
Chet

Where I was out in the field there is mainly only small gold left around the mullock heaps. Thats why I stuck with the small coil and early timings. The 5c piece was at about 2". I did detect some larger bits of iron at around 6".
I had made a current limiting mod to the MPP which limited coil current to 1A. In the workshop it didnt seem to make any difference allowing more coil current but in hindsight I think that maybe I should have made it variable with a pot for field testing.
Just one other thing that may be of interest is I used a 15.8V 5000mA LIPO battery (with some diodes in series to drop the voltage so I didnt kill MPP) and probably used it for around 8 - 10 hrs in total. Probably a little overkill but yesterday I measured its voltage and its down to 15.2. It could probably be used for a week without charging.

[6666]

Yep thats the board I used. I got mine from Element 14 and was pretty cheap. I will draw up the ChipKit/MPP connections later and post them with the code.

Thanks Cas.

[Chet]

Where I was out in the field there is mainly only small gold left around the mullock heaps. Thats why I stuck with the small coil and early timings. The 5c piece was at about 2". I did detect some larger bits of iron at around 6".
I had made a current limiting mod to the MPP which limited coil current to 1A. In the workshop it didnt seem to make any difference allowing more coil current but in hindsight I think that maybe I should have made it variable with a pot for field testing.
Just one other thing that may be of interest is I used a 15.8V 5000mA LIPO battery (with some diodes in series to drop the voltage so I didnt kill MPP) and probably used it for around 8 - 10 hrs in total. Probably a little overkill but yesterday I measured its voltage and its down to 15.2. It could probably be used for a week without charging.

Hi CAS

The current limit should be set along with the pulse width to produce a flyback pulse of just under the breakdown voltage of 400v with the IRF740 MOSFET. Any current or charge time that exceeds this will not gain anything worth the gamble that it may shorten the live of the transistor. If you change to a IRF840 you can increase the current or/and pulse width up to a 500v flyback spike. I look forward to seeing your software code. I hope you find one of those 4.4 kilo nuggets in the Bendigo area.

Have another great day,
Chet

[CAS]

Hi CAS

The current limit should be set along with the pulse width to produce a flyback pulse of just under the breakdown voltage of 400v with the IRF740 MOSFET. Any current or charge time that exceeds this will not gain anything worth the gamble that it may shorten the live of the transistor. If you change to a IRF840 you can increase the current or/and pulse width up to a 500v flyback spike. I look forward to seeing your software code. I hope you find one of those 4.4 kilo nuggets in the Bendigo area.

Have another great day,
Chet

I did change the FET to a FS4KM12A as the other was getting a little warm

[CAS]

MPP Mod Schematic and ChipKit Code

I have attached a drawing of the highspeed opto and isolator I used to interface the ChipKit to the MPP as well as the ChipKit code. I had to re-draw it all as most of it I just wired without any formal drawing so hopefully its right.

The code also contains stuff for the display and rotary encoder but if you ignore that you should be able to understand it all. Again, I tend to do most of my work on the fly and have loads of different code versions in my experimental folder so hopefully this is the correct version. I think it is though as it has the latest date stamp.

The Schematic mods show the 2 ics I removed (U6 and U7) as well as the main attachment points.

In my opto drawing, I may have gone a little overboard by adding another switching FET but I wasnt sure how the Opto would handle switching the TX FET.

MPP_CHIPKIT MODS.pdf & Document1.pdf & MPP Rev-D - Schematic mods.pdf