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Hello to everybody! Hope this is the correct place to post this. About a month ago decided to design a sensitive metal detector. I do not know how well these new machines (minelab heard is the best) are doing so I would like to get your opinion and knowledge... Can someone tell me what is considered a very good metal detector; Typical performance measurements at the lab and at the field so I'll know what the heck I am doing...have started the design already and getting close to do first tests. Half analog (front-end and filters) rest is all digital with a nice 320X240 color LCD.
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Originally posted by straton View Post
Hmmm nobody is interested!!! How about If I tell you that last night I did my first tests using aluminum copper and iron. Machine is almost not responding to iron! A piece of 1 Kgr was detected only at 15cm! when a piece of 2 X 2 X 0.05 inch copper is at 97 cm. so again how do these results compare to the very good detectors out there?? anyone? -
Hi Straton,
One of the problems is that there are no standards for making performance measurements. The best thing you can do is get another detector and make direct comparisons. What are you designing for? Gold hunting? Then, yes, Minelab is highly regarded. White's TDI is also competitive.
A 2x2 copper at almost a meter is impressive, but I assume that's in the air. Sensitivity is more than getting a large copper target deep, it's also getting sub-gram low-conductive nuggets at a couple of inches in highly mineralized soil. Besides getting a comparison detector, get a variety of test nuggets. Also samples of hot rocks and a good mineralized test bed. Soil testing is where most designs completely fall apart, so don't put this off for long.
- CarlComment
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Thanks Carl, for pointing in the right direction... Where I live there are no nuggets anywhere near that I know off,( have to ask someone to buy ...) and unfortunatelly do not have experience yet with the different kind of soils. The machine was designed mostly for treasure hunting but I would like to do nugget "hunting" too!There is no ground balance yet but it is going completely to be done in software. From what I see I have to be careful not to use any patented methods... All these tests are done here at my lab with a 25inch coil in air. There are no sample current/integrators. Signal is digitized from 2 fast ADCs. Talking about noise! Spend 3 weeks designing a homemade PCB for very low noise. Computations are done using a low power DSP. Metal discrimination for above targets (2inchX2inch plate) is at 50-60cm. After that I can't distinguish metal quality due to sample noise...If this design turns to be average I'll post schematics and code for anyone to build. There wil be no point for further development. One test that I did is to compare with a very old minelab! I found in my father's garage! It a blue box and trying to find its name. guess have to open it. It seems to be VLF. There is no comparison though. After all you told me, I think this will turn out to be pretty goodComment
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Testing garden
Hello Stranton, You can use split shot fishing weights to approximate gold nuggets. To create the minerals you can go to the garden shop and buy a sack of ironite (iron pellets to add iron to the soil) keeps plants green. I've used it in my test garden and it seems to do the job. Add as much as you need.
Ambitious project,
K.
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I'm interested in this topic as I'm doing something similar. I have a few questions if you don't mind:
1) How much gain are you using before sampling with the ADC?
2) What resolution is your ADC (8bits, 10,12,16,etc.)?
3) If you use a US quarter what kind of air test depth are you getting?
4) What OP amps are you using for the analog portion of the circuit?
5) Can you provide some tips on how you reduced noise in your circuit?
I'm getting between 7" and 9" on a US quarter (air test) using brute force methods. This seems low to me with a 10" coil so I'm looking for ways to increase depth. One factor that has really hurt performance is noise. This thing is currently put together on a breadboard in my basement so the environment is certainly less than ideal.Comment
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I've completed my design
What do you think of these results? Do they compare?
My design is all analog. It's big and heavy. In my lab (bedroom) I am getting 39" on an aluminum coke can. With a 18" coil, I get 10 feet on a jeep. With my large PVC coil, I am getting 11feet on a 1 gallon paint can. I'm thinking about offering them for sale some day.
Thanks,
Owl FaceComment
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Owl Face, I think those are pretty good results for large items. How does it do for small items? How much depth on a US quarter?Comment
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Hello guys. I am pretty close at completing the project. I am writing the final code for the dsp/fpga. That includes ground balancing( I loose maybe 7% of depth under most cifcumstances, max 15%, tests done in my country),detection algorithm modes, battery management, iron reject mode, 50HZ or 60HZ rejection interferece etc. My current setting is a 10inch coil. Here are some latest typical measurments in air but under heavy metalic disturbance: (Heavy metalic objects in close proximity, floor armored cement - my chair was being detect and I had no idea why the readings were chaging when
I would get up and move it a bit to test the coil. It didn't register in the beggining... Machine detects a less that a gram copper sheet that is 3mmX3mmX0.4mm at over 4 inches. (you do weight calculation) An 8 gram gold coin at over 20 inches.(was found !!!!!!!!!! in testing field!!!) An iron coin of 10 gr is detected at 2.7 inches.(iron reject mode) Can of coke is above 85cm. Concering the instrument, I being having some drifting/stability problems due to very high amplification of close to X11K. There is much that can be done in analog electronics. I am getting to the limit of commercial available technology. Only digital is left now. I am going to disclose some information but not details on parts. Front-end uses three stages to get close to X11200 amplification which includes 4 9th order low pass filters and two high pass ones. Next two 18 bit ADCs convert signal to data. That means a hugh amount of data to process. An Fpga does the heavy number crunching. A dspic does rest , the LCD update and other stuff. The machine is going to be able to detect size and depth. It became apparent after collection of a lot of data and tests that size and depth can be approximated. This is wil take another month though and not in priority!- Hopefully soon I'll post pictures of completed machine. Thank you all for moral support. If it wasn't for all the people I see talking about and being involved in metal detection I would not have the interest and courage to do it...Comment
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Originally posted by Owl Face View Post
Hi ! They are pretty good results indeed! but I lately realized that it is not detection depth that is so important. Iron reject and ground balancing are extremely important too!!! Drifting problems are the worse. Think about you try you detector after a month (let's say summer time) and you lost 30% of depth rarge... and you don't even know it!Comment
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stability? Drifting?
Actually, I have no problems and I have been using, not testing, my detector for 6 years now almost every weekend. It is very stable, once set I don't have to touch it. I was actually surprised to hear you are having difficulty because 11K of gain is not that much. My magnetometer has 800K of gain and that was a little tricky to get right. My metal detector has less than 11K of gain. If you don't lay out the PCB correctly, you will have problems with a PI detector. I'd probably look at that first if you are having any kind of drifting or warbling. Keep digital grounds and analog grounds seperate. Tie them together at one point, usually at the reservoir capacitor.
As for depth on small objects, I really don't know how it does because I built it to hunt larger objects. And funny you should say it has to reject iron because I primarily hunt for KGC treasure so I mostly look for iron objects left by them. Since I originally built this for my own use, I don't want any rejection or discrimination
Good luck with your detector.
Owl FaceComment
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Originally posted by Owl Face View PostActually, I have no problems and I have been using, not testing, my detector for 6 years now almost every weekend. It is very stable, once set I don't have to touch it. I was actually surprised to hear you are having difficulty because 11K of gain is not that much. My magnetometer has 800K of gain and that was a little tricky to get right. My metal detector has less than 11K of gain. If you don't lay out the PCB correctly, you will have problems with a PI detector. I'd probably look at that first if you are having any kind of drifting or warbling. Keep digital grounds and analog grounds seperate. Tie them together at one point, usually at the reservoir capacitor.
As for depth on small objects, I really don't know how it does because I built it to hunt larger objects. And funny you should say it has to reject iron because I primarily hunt for KGC treasure so I mostly look for iron objects left by them. Since I originally built this for my own use, I don't want any rejection or discrimination
Good luck with your detector.
Owl Face
Your detector is for very specific needs. Mine is of general use. Where I live the major concern is that of iron discrimination...I have asked a lot of people and they all complain that nails, bottle caps,even grenades... make it very difficult for them! - not to mention dangerous!!! I myself went to a busy beach three years ago to try my father's old garret and I had so many false alarms from bottle caps that never tried it again.
Concering the electronics, you know very well that an all(almost) analog detector has very low noise. Mine is a hybrid and all the digital electronics make very specific noise patterns but it is still noise. I benefit (I believe) overall because I have now digital data that I process through my computer. I do all my testing there and whatever works I try to implement it in the detector. About drifting...you set the detector everytime! because of drift and sensitivity concerns but in any case, try to design a wide bandwidth DC amplifier with 11K amplification and low noise and let's see if it drifts or not!!! A 1 millivolt offset (let's say that temperature increased 20Deg Celsious and a resistor with a typical TC of 200PPM changed 4000ppm thus gave an offset at input of 1 millivolt) becomes 11V. - but If you know how to do it please let me/us know!!! I'll appreciate your help... Coil size makes a big difference in depth detection.There are few ways to increase the SNR of the system but the most effective one is to increase coil diameter.
I think enough said...Comment
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