Alternating pulse width

[Kev]

Hi,
I think (?) I noticed something interesting with my GQ in the weekend, I need to check it out a bit more, but there could be some theory behind it?

I've been altering the program on my riser card so that I can move the second earth field-eliminating sample closer to the first sample. I've been hoping to be able to eliminate some of the ground noise. I appreciate that it would also cancel certain nuggets that fall onto Reg's seesaw balance point, but there

 

[Prospector Al]

Hi Kev,

As I understand it, the signal from the second sample is subtracted from the first one. Therefore, one must wait at least four time constants, before sampling for ground.

Are the pulse lengths generated in code? Does the operating frequency increase when the second sample is moved closer to the first one? If that's the case, that would explain the increase in
sensitivity, when the second sample is moved closer to the first one. A higher pulse rate means that more energy from the target is intercepted, per unit time.

( Of course, the sensitivity to big nuggets will decrease if the second sample is too close. A 3 oz. nugget I measured has a T of ca. 30 uS. Thus, optimally, the second sample should be taken no sooner than 120 uS after the first one...)

Prospector Al

 

[Eric Foster]

Hi Kev,

Bringing the second sample closer in, won't cancel the signal from certain nuggets, but it will reduce it. If the second sample picks up some of the object decay, it will subtract it from the first sample, but as you are sampling on an exponential decay, there will always be some signal difference, with the first always being the greatest, so no see-saw point.

You only get a see-saw if you have extra amplification on the second sample, to bring it up to the amplitude level of the first. The problem then is that movement of the coil in the earth's field will no longer cancel. That is why true ground balancing PI's have two or more receiver channels.

Your TX pulsewidths are interesting, although I am not quite sure on its effect, at the moment.

Eric.

 

[Kev]

Hi Eric,
Thanks for the reply. I also have the facility to widen the second sample width, and thereby effecting some applification. I had hoped that the earth field signal included in the second sample would be enough. The first tests with stubs code were promising but, when I refined the code it didn't work that well.

It's the middle of winter here, and I'm not getting out to the goldfields, so to break the boredom I've been messing around a bit. I will though add a second channel as you recommend and incorporate a variable main sample, keeping both channel earth field samples relative.

Cheers
Kev.

 

[Kev]

Hi Prospector Al,
Thanks for your comments. The variable sample doesn't affect the pulse repetition rate. I subtract the second sample delay from 50 and add the remainder onto the wait before the next pulse, so that the time from the end of one pulse to the start of the next is always the same, even if I change the wait before first sample or the width of the second sample, the only change between each sequence is the pulse width.

I'm certain that it's not the alteration of the second sample that changes the sensitivity to the small bits, I'm sure its the different pulse widths. It doesn't make any noticeable change with larger pieces. The vial also has some bits smaller than a grain, but the accumulated response is what I'd expect from a solid slug.

A tc of 30us for a 3oz nug seems very short. Thanks for that observation. What pulse width was used with that thanks?

Cheers
Kev.

 

[Prospector Al]

Hi Kev.

The length of the xmtr pulse was 200 uS--long enough to allow the eddy currents generated at the leading edge of the pulse to die out before the pulse current returned to zero.

The time constant depends not only on the weight of the nugget but also its shape. An irregular
shape accounts for the short time constant. ( That was, by the way, an Australian nugget.)


I have also experimented with smaller size gold in a bottle. When the pieces are in electrical contact, they provide an eddy current path, but owing to the resistance of the path, the time constant is very short.

I wonder, though, if that kind of a condition would ever exist in nature...

P. Al

 

[Kev]

Hi Prospector Al,
Thanks for filling me in. I'd always been under the impression that a collection of small bits, even in electrical contact, wont respond with a signal representative of it's true weight. Take chains for instance.

As for actual useful value, well gold chains, ( I must check this later ) surficial aeolian deposits, and to a lesser degree mid-stream crevices.

Cheers Kev.

 

[Kev]

Hi Eric,
I wonder if the semi-asynchronous pulse train is what gives the Minelab PIs their sensitivity to smaller bits, as opposed to shorter reject sample times?

Cheers
Kev.

 

[Reg]

Hi Kev,

I would be inclined to think that the wider xmit pulses are causing a change in just when the sample is taken on the receive decay curve. In other words, the receive signal is getting wider with a wider xmit pulse, causing the sample to be taken a little too soon.

What I have found is if the sample is taken so it is partly up on the decay curve, the sensitivity increases noticeably. It is something I have even built in at times. The down side is, the coil becomes very sensitive to the slightest vibration. As such, bumping it against something will generally cause a false signal.

Reg

 

[Kev]

Hi Reg,
That maybe the answer. I've been testing on the bench so I don't know how touchy the coil is? There's nearly always a downside, but if the balance point is found one should be able to get the best of both worlds.

I wonder why coil vibration comes into the picture? Surely complete rigidity would rule out interwinding and shield movement, I know you make good coils Reg and will have payed close attention to this. Could the shield be causing a microphonic effect, altering the capacitance of the front-end?

Getting too deep for me.

HH
Kev

 

[Prospector Al]

Hi Reg and Kev,

I have been reading your posts with interest. In my experience, if the signal gating pulse occurs before the oscillations in the coils have been completely damped out, the system becomes sensitive to the slightest mechanical vibrations. Thus, the nature of the damping network design is important. There is a "critical damping" that provides for maximal energy absorption so that the return to zero occurs in the shortest possible time. The receiver and transmitter coils have to be damped independently.

On a slightly different subject, you mentioned the Minelab PI detector. Is that the "gold standard", as far as sensitivity is concerned?

There are at least two factors that affect the sensitivity to small nuggets:

1) The transmitter power and 2) the gating pulse delay. These parameters are not totally independent, since a higher transmitter coil current inevitably requires a longer time for the coil to "settle down".

Unfortunately, an increase in power does not translate linearly to detection depth. Owing to the exponential laws involved, a doubling of power may yield only a 20% improvement in depth.

I have been designing industrial detectors for many years. Now that I'm retired from that field, I plan to put my experience to use in a hand-held detector.

The main question that pops up in my mind is: "Is worth lugging a heavy gel-cell battery around to gain a couple of inches of detection depth?"

I'd value your input...

P. Al

 

[Wirechief]

Hi Al, Well to be truthful for me it's not worth it. If I were a proffessional gold prospector then it would. I'm not able to give an honest opinion on any of the PI detectors at this time. I have never owned one but Eric's products appeal to me much more than Minelab because of the extra stuff you lug around with Minelab PI's. I have much to learn about the PI detectors but I know I'm in the right place and among people in the know. Just my opinion Al. Thanks everyone. Wirechief.

 

[Kev]

Hi P. Al,
I find it funny that physical vibration frequency, can interfere with electrical vibration frequency, given the micro g forces involved. Still the coil energy that is being dissipated is not miniscule. At work we stress our products in 5grms increments up to 30grms and a particular vibration frequency will cause software, or hardware to crash just as easily as thermal factors.

Anyway, the Minelab is not a standard as such, it's just that Eric on a number of occasions has commented on how he finds it hard to believe that a ML PI with it's relatively long sample delay can locate gold of the minimal size often quoted by operators. Theory dictates that for a small nugget with a short tc, a correspondingly short delay must occur before sampling otherwise the target field will have diminished below detection level. I wondered if a pulse train such as that utilised by MLs could enhance sensitivity to smaller bits.

I use one of Eric's lightweight Goldquests. I often find myself clinging to the sides of gorges, climbing straight up to avert waterfalls and the like. There's no way I could lug a ML PI into such places. My wife has trouble getting herself alone into some of the places we end up, let alone lugging a cannon of a detector as well.

I find the high pulse rate detector ideal. I'm modifying it all the time to suit my needs. It may not have the full depth range of a ML PI but we don't have large numbers of big nuggets like you have in Oz, nor do we have the Fe oxides, so I'm making it more in tune with what we have here. I'm certainly no Rambo, and get horrible backache from my Explorer these days.

Cheers
Kev.

 

[Prospector Al]

Hi Kev and Wirechief. Thanks to you both for you input about heavy battery-packs. I agree.

Regarding the Ml PI pulse train: By having different pulse characteristics one can optimize one channel for smaller nuggets and a second one for larger targets--I believe that's what is happening in the GP-3000. (Of course, the output of the two channels can be outputted at the same time, using different audio characteristics. )

The brochure refers to a patented "Dual Voltage Technology", but I can't find any patents that describe anything like that.

I don't believe it's possible to get any kind of cumulative effect with a "magical" pulse sequence. The only obvious fact is that if the pulse duration is too short, the eddy currents generated at the leading edge of the pulse don't have time to decay in large targets, before the currents are generated at the trailing edge of the pulse. This "carry-over" effect causes some cancellation of the signal, because the lingering eddy current in the target has the opposite polarity from the current used for detection.

One doesn't have to worry about this cancellation with short TCs, thus, a pulse train with more closely spaced pulses will retrieve more signal from small targets...

P. Al

 

[Wirechief]

Hello again Al, thank you for answering a question that I had on my mind about the polarity of the eddy current in the target compared to the current used for detection. CU LATER kEV AND Al and have a good night. Wirechief.

 

[Wirechief]

Hi Kev and I have the same outlook as you do. It would be nice to have a Minelab but I just can't justify one. Is the Goldquest a software defined instrument? The Goldquest is the one I'm interested in and would be more inline price wise. I like the way Eric packages his detectors too. Have you found many gold nuggets with your Goldquest Kev? Also I understand his instruments use thru hole PCB's so it would be easier to work with even though I work with SMT boards on my regular job. CU later Kev and thanks and God Bless. Wirechief.

 

[desertdon]

The GQ is hardware. Don

 

[Wirechief]

Hello Don and thank you for that info. I didn't know since I have never had any of Eric's instruments and was just curious. I hope all is well with you Don and have a good one. Wirechief.

 

[Kev]

Hi Wirechief,
No I've not scored as many nuggets as I would like, but I'm improving in my ability to identify likely areas. Working fulltime and living a couple of hundred miles from the auriferous country also inhibits a swelling poke! I'm also having to find ways to overcome the rich fe3+ that results from the rotting schist basement. Eric's GQ with a DD coil will find them, no doubt about that, success, or lack of it really boils down to the operator most times.

I've built an add on card for my GQ so that I can manipulate the settings a bit more. (see here for details) http://www.geocities.com/kejamnz/Riser.htm
Yep I too work with SMT, often modifying prototypes, 12 layers, up to 6000 components many being 0402 packages, I used to think 0805s were small, replacing BGAs with over 1000. After all this, through hole is great to modify.

Did you say earlier that you've scored a Beachscan? Reg knows how to tune them up a bit, and if you build a DD you'll be crackling, a nice setup to get you started.

Must do some chores.
Cheers
Kev.

 

[Wirechief]

Hello Kev,
Thanks for the response and your right after a day of SMT's thru hole looks good! I don't work with anything as multilayered as you but it still gets awfully hard on the eyes even with help.
I'm sure you will have your day and hit the motherload and I have no doubt that your Goldquest is a fine instrument. Yes your right when we are still working full time jobs it is hard to find time to look around. My big problem is I'm even further from gold country because when you live in West Texas surrounded by farm and ranch land and of course this land is privately owned and no gold to boot, well you get the idea Kev.
Thank you for the link and I will check it out for sure. That would be a lot of fun to add a controller board to change things a bit.With that heavy FE3+ in the soil you must be further West of me. At least your closer to the gold.
Yes Kev I pickedup an older Beachscan on Ebay over in the U.K. from a metal detector dealer there. I'm waiting for it to get here right now. It will be my first PI instrument and I want to use it as a learning platform. The price was right and I couldn't buy a PI any cheaper. I thought Reg is known for his expertise on the units. Eric's stuff simply is more appealing than something like the Minelab's.
Ok Kev I enjoyed the reply and I am going to go check out that link you gave me so CU later and we will visit again. God Bless. Wirechief. John's Detectors