Coil sensitivity in a horizontal plane is what?

[GT Blocker]

...I've searched the forum and haven't been able to find a reference to these questions.
From the flux plots Aziz has provided on several posts, it seems to me that coils of most (all?) types are fairly insensitive in a horizontal plane, i.e.- the coil really doesn't care about what is beside it as long as the object isn't below or above the coil, given a reasonably small mass and conductivity/eddy currents.
Question 1- Is this assumption correct?
Question 2- If the above assumption is correct, would it be a reasonable experiment for me to move part of my HH circuit (Q1 to the input of R23 or Q1 to TP3, Rev D1, DDcoil) to a small faraday shielded cage just off the side of the coil? (After succesful no-mod testing, of course).
Question 3- Would such a move in circuitry help to minimize the Q1 capacitance delays from the cable while (if possible) also minimizing the noise picked up on the return cable to the first stage amp, IC6?
I realize that there are several signals/pwr connections to route and that switching coils might require some re-adjustment, but I thought that I might pick up enough of an advantage in capacitance delay and noise to be worth it on the very short sample times needed for small gold nuggies.
Any thoughts on this or am I way off base? GTB

[Tinkerer]

Moving circuit near the coil

...I've searched the forum and haven't been able to find a reference to these questions.
From the flux plots Aziz has provided on several posts, it seems to me that coils of most (all?) types are fairly insensitive in a horizontal plane, i.e.- the coil really doesn't care about what is beside it as long as the object isn't below or above the coil, given a reasonably small mass and conductivity/eddy currents.
Question 1- Is this assumption correct?
Question 2- If the above assumption is correct, would it be a reasonable experiment for me to move part of my HH circuit (Q1 to the input of R23 or Q1 to TP3, Rev D1, DDcoil) to a small faraday shielded cage just off the side of the coil? (After succesful no-mod testing, of course).
Question 3- Would such a move in circuitry help to minimize the Q1 capacitance delays from the cable while (if possible) also minimizing the noise picked up on the return cable to the first stage amp, IC6?
I realize that there are several signals/pwr connections to route and that switching coils might require some re-adjustment, but I thought that I might pick up enough of an advantage in capacitance delay and noise to be worth it on the very short sample times needed for small gold nuggies.
Any thoughts on this or am I way off base? GTB

Moving the damping resistor, diodes and preamp near the coil would definitely help to shorten the sample delay.

This subject has come up quite frequently, but I have not heard of any results.

A small box for the circuit about 10" up the shaft might be a good place.

Tinkerer

[Unregistered]

Hi Tinkerer,
Thanks for the reply. I had seen a few references to moving the op-amp, and, as you pointed out, no follow-up on the results. Since I've seen none regarding moving the MOSFET section, if I'm following you, there would be no reason to move the IRF740 down closer as well?
I was under the impression that the cable capacitance added to the coss of the FET.
Of course I'm as likley wrong as not, since I'm a rank newbie to PI theory and practice. I chose the HammerHead, over a PIC micro design, since I felt that I would be more in my element with a PIC chip and learn far less. Better to get out of my comfort zone and raise the bar a bit (though wtf this has to do with my question, I don't know...I digress).
Anyway, thanks again for the input, Tinkerer

[GT Blocker]

sorry

Oops forgot to log in on the above post...duh

[Tinkerer]

Changes on the HH

Hi Tinkerer,
Thanks for the reply. I had seen a few references to moving the op-amp, and, as you pointed out, no follow-up on the results. Since I've seen none regarding moving the MOSFET section, if I'm following you, there would be no reason to move the IRF740 down closer as well?
I was under the impression that the cable capacitance added to the coss of the FET.
Of course I'm as likley wrong as not, since I'm a rank newbie to PI theory and practice. I chose the HammerHead, over a PIC micro design, since I felt that I would be more in my element with a PIC chip and learn far less. Better to get out of my comfort zone and raise the bar a bit (though wtf this has to do with my question, I don't know...I digress).
Anyway, thanks again for the input, Tinkerer

The HH is an excellent design to use as an experimenter platform. However, if you are familiar with PIC's I suggest that you use a PIC for the timing.

I would place the TX part and the RX part including the preamp near the coil. Already placing only the damping resistor at the coil makes a difference and if you want to sample really early, every fraction of a uS counts.

I am sampling now at 4uS using a gain of 1000 on the preamp, but I believe that 2uS is possible.

Tinkerer

[GT Blocker]

I think 4uS might be a bit much for me ATM lol

Tinkerer,

"The HH is an excellent design to use as an experimenter platform."

I agree whole heartedly (like I know what I'm talking about....). I spent a lot of time 'lurking' around the board and reviewing the options for learning more than just how to assemble someone else's "no options - so there ya go" project. The HH seems to meet my learnig curve needs perfectly.

"...if you are familiar with PIC's I suggest that you use a PIC for the timing."

I am fairly well aquainted with PIC micros and see no reason why the 12C508 series couldn't replace the 555's. And on and on my list of thoughts for PIC's goes. However, I really think I should get this puppy going stock before I venture on. I feel I need to understand exactly what I am trying to achieve before moving to uP timings, etc..

"I would place the TX part and the RX part including the preamp near the coil. Already placing only the damping resistor at the coil makes a difference and if you want to sample really early, every fraction of a uS counts."

That was my thinking in starting this thread. Although my original question about lateral coil sensitivity is as yet unanswered, I am still considering the poss. of moving the 'extended circuitry' to just behind the coil in horizontal plane. Just a thought.

"I am sampling now at 4uS using a gain of 1000 on the preamp, but I believe that 2uS is possible."

That just blows my mind...not sure what to say, except...any tips?
Thanks for the help, Tinkerer

[Tinkerer]

One step at the time

Tinkerer,

"The HH is an excellent design to use as an experimenter platform."

I agree whole heartedly (like I know what I'm talking about....). I spent a lot of time 'lurking' around the board and reviewing the options for learning more than just how to assemble someone else's "no options - so there ya go" project. The HH seems to meet my learnig curve needs perfectly.

"...if you are familiar with PIC's I suggest that you use a PIC for the timing."

I am fairly well aquainted with PIC micros and see no reason why the 12C508 series couldn't replace the 555's. And on and on my list of thoughts for PIC's goes. However, I really think I should get this puppy going stock before I venture on. I feel I need to understand exactly what I am trying to achieve before moving to uP timings, etc..

"I would place the TX part and the RX part including the preamp near the coil. Already placing only the damping resistor at the coil makes a difference and if you want to sample really early, every fraction of a uS counts."

That was my thinking in starting this thread. Although my original question about lateral coil sensitivity is as yet unanswered, I am still considering the poss. of moving the 'extended circuitry' to just behind the coil in horizontal plane. Just a thought.

"I am sampling now at 4uS using a gain of 1000 on the preamp, but I believe that 2uS is possible."

That just blows my mind...not sure what to say, except...any tips?
Thanks for the help, Tinkerer

Just take it one step at the time.

Start with separating the HH circuit into 2 parts.

If the part near the coil is about 10 " up the shaft, it will be also besides the coil in the vertical axis. It should work.

However do not expect everything to work on the first try.

Perseverance will get you there.

Tinkerer

[GT Blocker]

Guess I should provide some background

Thanks again, Tinkerer, I appreciate the help.

Thought I'ld provide some background, if that's ok.
On another post of mine , Carl's reply mentioned a sweep function generator ( I lost my last Ebay attempt...oops). Looks like I'll need to keep looking. Within his reply, he also mentioned that I might be able to use my pulse gen to excite the coil. I've studied the references on the forum, but I have no idea how to proceed in that direction (guess I really should ask Carl. eh?). I'll get a freq gen sooner or later tho, so no biggie. I'm winding a DD with 30awg/7-strand Teflon receive and 26 awg 7- strand Teflon for trans. We'll see how I do.
At any rate, I guess that I should explain my 'goals', since it makes no sense to ask for help without any parameters.... so here goes (please bear with me...).
A few years ago I was mining the N. Fork of the Yuba river in N. Ca. I was running a Honcoop highbanker and getting pretty good gold for the work. A couple guys showed up with some snorkle gear and told me that they were 'sniping', IE- breaking open and fanning out underwater cracks in the bedrock.
As a long time SCUBA diver (1973 NAUI) I was fascinated. I had brought most of my dive gear to N. Ca., since I have friends up that way that dredge and I figured I might be able to play too. So I hopped into the river (short version) and checked out this 'new' way to get gold. As I dove with these 'snipers', and worked the U/W cliff faces and shallow bedrock, I found that a conbsiderable amount of gold was lodged in places too deep for the folks with 'sniper scope' viewers, and that these crevices/cracks held not nearly enough material for the dredgers to bother with. And, lo and behold, the gold was there all right.
Unfortunatly, I found that I had about a 1-5% success rate in gold bearing cracks to 'dead' cracks. While I floated down the river, busting crack after crack open, all I thought about was how sweet a MD would be. When I returned to AZ, I set about finding a MD to do what I needed done. At a GPAA show in Mesa, AZ, I asked a Minelab rep about U/W dectectors. He replied (in what I think was an attempt to placate me and sell a detector) that they had one ....well you know the rest... (but, I didn't buy it).
Instead I purchased a Pulse 8 U/W detector and found the it couldn't find very small gold. I called the manufacturer and was told that the 'pulse' simply couldn't go short enough for what I wanted. My dealer was kind enough to refund my greenbacks.
Confused and befuddled, I decided that PI MD's suck for small gold. So......
I decided to fall back on my special effects and U/W film experience and modify a VLF detector to fit my needs.
I purchased a ML Eureka Gold to place into an U/W housing. I then purchased the U/W housing and control components (after all, wtf, I've built far more complex housings for film work). Then I discovered the Hammerhead files and this forum on the web. This has changed my opinion about PI detectors and small gold. I have a unique need and this appears to be well worth investigating.
The ML Eureka Gold will remain on dry land. The Hammerhead U/W V.1 is the current project. So.....

For diving in a fast river, I will have VERY little shaft to deal with (for saftey among other reasons). This means that any circuits that need to be moved are moved as close as possible to the rear of the coil (not the shaft {if any still exists}) if they are to be moved at all. I need to punch through about 6" OB total depth in most cracks and in fresh water only. Most of the PI discussions assume that U/W use will mean salt water envirionments. That is not the situation in my case. Since I already have all that I need for housing the circuitry, I think that the HH is the perfect platform for this type of experimentation.
I welcome any thoughts or input on the project. GTB

[bbsailor]

GT Blocker,
Tinkerer,

If you want to explore underwater cracks for gold, make a pulse induction probe coil using a soft ferrite. I have used these ferrites and they do work for a PI probe. http://www.surplussales.com/Inductor...s/FerRods.html
Use the (ICH) ROD7.5/50-3C80. Order a few of them so you can experiment with different configurations to match the sensitivity for the type and size nuggets you are seeking.
1. Single rod
2. Two rods in series, super glued together
3. Three rods in a triangular bundle
4. Three sets of two series rods in a trangular bundle (six rods total)
5. Seven rods in a circular bundle

You should be able to slip one rod in a 5/16" ID to 3/8" ID plastic pipe with one or two layers of wire and make a waterproof probe to fit into small cracks. Just make the probe inductance the same as it would be for an air core PI coil, 300 uH.

Caution, do not drive a PI coil directly with a pulse generator (PG). The flyback pulse will fry the some pulse generator's output transistors. To drive a coil with a PG first build the Hammerhead MOSFET TX coil driver circuit and power that with a battery or a ground isolated power supply. You should be able to drive the gate of the MOSFET with the Pulse Generator, but you need to keep the pulse width or duty cycle at about 10 percent.

You can use a PG to find the resonance of a coil by connecting a 1meg ohm resistor is series between the PG and the coil and then observe the voltage across the coil as you sweep the PG frequency in the range of 500 kHz to 1.5Mhz, looking for the sharp rise in the observed signal on the scope at resonance.

What you want is a coil or probe and coax cable length with the highest resonance so that the damping resistor can be the highest value to make for a fast current turn-off to better stimulate small gold targets. It would seem that Tinkerer is right about putting the TX and RX circuits as close to the coil as posible. This eliminates the coax capacitance that seems to slow down the coil somewhat. A design like this would make a very sensitive probe because the value of the damping resistor would only need to dampen the few 10s of pfs of coil capacitance, not the 100 or so pfs of the coax capacitance. You could get away with a using a small MOSFET with a low COSS to drive the probe in the .5A range with a series resistor to limit the current and optimize the Turn-on pulse TC to that of the desired targets. You should install this circuit right inside the probe housing for the best results.

bbsailor

[GT Blocker]

Ferrite probe sensitivity

bbsailor -

First off, I want to say howdy and thanks for your input. It's folks like you, Tinkerer, Carl, et al, that have allowed me to have a clue as to the questions to even begin to ask. I have read through as many of your posts as I could find on coil design (and hope to remember even half of it), and I really appreciate the time that you and the others spend helping folks and sharing ideas.
As for a probe...I don't know. The cracks and crevices are packed with material. I carry a 2lb. sledge and chisels/screw drivers to get into the cracks. The whole point of this detector is to avoid all that nonsense if there isn't a target to start with. Once the crack is open, finding the gold within is actually fairly easy as the dirt is fanned away. The trick is...which crack to spend the time opening. I think I need a small coil rather than a probe. Remember, there is no crack to put a probe into until you dig/bust it open. I need to narrow down my likely targets and save time (especially if I go for bottled air).
Thanks for the heads up on the PG. I probably would have killed it without your warning.
On another note, I have noticed that a vial of fine gold produces very little response, possibly due to individual eddy currents never adding up to the whole. I figure that smaller SMD components should be far harder to detect than DIPs for the same reason, and so should have less influence on the coil as I place them nearby. We'll see.....
On a non- coil topic, I have a few LMC7101 op amps. Have you (or anyone else) tried them for the first stage?
And lastly, I really like the low COSS idea at about .5A, but what would be about my max on a small coil dia.?
"You could get away with a using a small MOSFET with a low COSS to drive the probe in the .5A range with a series resistor to limit the current and optimize the Turn-on pulse TC to that of the desired targets. You should install this circuit right inside the probe housing for the best results."
Adjusting the above discussion for clogged and packed cracks, would a small coil be more efficient at quick crack scans or a probe?

Thanks

[bbsailor]

GT Blocker,

See my comments in bold below.

bbsailor

bbsailor -

First off, I want to say howdy and thanks for your input. It's folks like you, Tinkerer, Carl, et al, that have allowed me to have a clue as to the questions to even begin to ask. I have read through as many of your posts as I could find on coil design (and hope to remember even half of it), and I really appreciate the time that you and the others spend helping folks and sharing ideas.
As for a probe...I don't know. The cracks and crevices are packed with material. I carry a 2lb. sledge and chisels/screw drivers to get into the cracks. The whole point of this detector is to avoid all that nonsense if there isn't a target to start with. Once the crack is open, finding the gold within is actually fairly easy as the dirt is fanned away. The trick is...which crack to spend the time opening. I think I need a small coil rather than a probe. Remember, there is no crack to put a probe into until you dig/bust it open. I need to narrow down my likely targets and save time (especially if I go for bottled air).

How deep are the target gold nuggets that you seek?

What size are these nuggets typically?

A 4" diameter coil with 35 turns will make about a 300uH coil. If you make the total resistance of the Coil (3.6 ohms), MOSFET on-resistance (2.4 ohms) and the series resistor (18 ohms) equal about 24 ohms, then the coil TC will be 300/24 or 12.5us. A 50us pulse width (4 Time Constants, 12.5 X 4) will allow the current to rise to about 97 percent of .5A with a 12V power source. Even a 3TC pulse width of 37.5us will put you at about 94 percent of .5A

Thanks for the heads up on the PG. I probably would have killed it without your warning.

A PG, using a sine wave, is good for finding the resonant frequency of the coil. With knowledge of the coil inductance and the resonant frequency you can calculate the coil capacitance that causes that particular resonance. When you are in the 300uH range with about 1.2MHz resonance you are in the 50 to 60 pf coil capacitance range. When the coil is shielded you will add a about 20pf for a 4" coil to the total capacitance and drop the resonance a little closer to 1MHz. Adding one foot of coax adds 22 to 32 pf depending on the coax type and drops the resonant frequency below 1MHz in the 800 to 900KHz range.


On another note, I have noticed that a vial of fine gold produces very little response, possibly due to individual eddy currents never adding up to the whole.

I went gold panning in Alaska and retrieved a small amount of gold extracted from my gold pan with an eye dropper that is impossible to detect even putting the vial in the center of a 1" coil at 7.5us.

You need to try to detect some of the typical nuggets that you seek. Sweep those targets past the coil using the same speed that you would use underwater. The size of the Hammerhead capacitors C17 and C18 along with the resistors R26 and R27 determine the integration time which is closly related to coil sweep speed and the number of pulses that are integrated which helps determine the sensitivity to some targets. Those tiny grain of sand nuggets may not be able to be detected as their TC may be too short for the coil and RX circuit to sense. A 4" coil will be more sensitive to small, match-head-size nuggets than a larger coil. A probe concentrates the signal and allows detection of smaller nuggets than even a 4" coil can detect. You need to run some tests to confirm this observation on your particular target samples.

I figure that smaller SMD components should be far harder to detect than DIPs for the same reason, and so should have less influence on the coil as I place them nearby. We'll see.....
On a non- coil topic, I have a few LMC7101 op amps. Have you (or anyone else) tried them for the first stage?
No

And lastly, I really like the low COSS idea at about .5A, but what would be about my max on a small coil dia.?

Smaller coils have more turns for the same inductance compared to larger coils. This means the TX pulse is more concentrated and the RX detection is a little more sensitive to the near smaller targets but the far field detection will be less. Optimum coil size is highly dependent on the target size.

There are 4" coil housings available for Hays Electronics. Try one of these with about 35 turns of AWG 30 Teflon coated wire. Put a spiralwrap spacer over the wire and then use a Scotch 24 mesh shield. Put another layer of spiralwrap or electrical tape over the Scotch 24 mesh shield to secure it.


"You could get away with a using a small MOSFET with a low COSS to drive the probe in the .5A range with a series resistor to limit the current and optimize the Turn-on pulse TC to that of the desired targets. You should install this circuit right inside the probe housing for the best results."
Adjusting the above discussion for clogged and packed cracks, would a small coil be more efficient at quick crack scans or a probe?

Build the Hammerhead and get it working first with any coil. Try building a 4" coil with 35 turns of AWG30 Teflon wire. Shield it with Scotch24 mesh shielding for lower capacitance than a solid shield material. Test this coil on a range sample nugget sizes. Keep the coax short to improve the coil speed and use the highest damping resistance adjustment value that produces a clean signal after IC6 (TP4). You will want to reduce R42a (delay limit resistor) to about 820 or ohms or less for a shorter main delay.

Then, make a probe using the soft ferrite rods that I recommended. Simply switch out the coil and install the probe in it's place and test again on a range of sample nuggets. Small coils have less detecting range than larger coils so keeping the circuit about 12" to 14" away might be the way to go. You could probably get a little closer (8 to 10") to the soft ferrite probe without too much of a problem. Keep good notes and post your results so others can jump in and offer suggestions.
Thanks

[bbsailor]

Fast Sampling Analysis

The HH is an excellent design to use as an experimenter platform. However, if you are familiar with PIC's I suggest that you use a PIC for the timing.

I would place the TX part and the RX part including the preamp near the coil. Already placing only the damping resistor at the coil makes a difference and if you want to sample really early, every fraction of a uS counts.

I am sampling now at 4uS using a gain of 1000 on the preamp, but I believe that 2uS is possible.

Tinkerer

Tinkerer,

If you are sampling at 4uS with an RX gain of 1000, that is pretty good!!
Here is mahematical analysis that should shed some light on the challenges you will face when trying to get down to 2uS.

I used the MiscEl program to do this analysis.

I assumed that the input resistor (Rin) to the RX amp is 1000 ohms. I also assumed that the damping resistor (Rd) is 1000 ohms with the TX circuit and RX amp being close to the coil.

The bulk of the flyback discharge is governed by the peak current of the TX pulse and the most effective Rd value to sample the fastest when the signal is flat. While the flyback voltage is above 0.6V the clamping diodes are conducting thus placing Rd and Rin in parallel. The discharge curve is thus defined by an effective Rd value of 500 ohms to 0.6V. The time it takes for a 300 uH coil at .5A to drop to 0.6V is 3.6uS at 1.19412 mA of current at this instant. Now enter 1.19412 at the new starting current and the new load resistance of 1000 ohms because Rd is no longer in parallel with Rin. It takes an additional 2.1 uS (total of 5.7 uS) to reach 1mV and an additional 4.18 uS (total 7.78 uS) to reach 1 uV. The current discharge must be done in two stages to accurately predict the discharge curve slope.

If you increase the value of Rin to 2K than you will have more room to play with the value of Rd to minimize the coil capacitance to make a really fast coil. You can also try the following. Reducing coil capacitance has a point of diminishing returns. The first 100pfs are the easiest to get rid of. Successive capacitance (pf) reductions get very hard as you approach the design limits of your chosen components, coax type and length, and coil design.

1. Use tin-plated, Teflon insulated stranded wire.

2. Minimize coil to shield capacitance by using Scotch24 or a similar shield.

3. Use lower COSS MOSFETS.

4 Make up for the reduced gain by boosting the gain in the IC8a and IC8b stages.

5. Try a low gain buffer, gain 10, at the coil or near the coil. Then feed the buffer into IC6 with a gain of 100 with the input clamping diodes removed.

6. Isolating the RX and TX stages can help you gain a uS or two in speed as the slope of the RX coil discharge curve of is steeper, only governed by the value of the RX coil Rd.

7. Reduce the flyback voltage by using a series resistor to limit the current.

8. Increase the TX PPS speed (3KPPS to 10KPPS) to integrate more samples.

9. Optimize the coil diameter for the size target you seek.

10. Increase the first op amp supply voltage to improve the headroom and improve recovery time.

11. Keep good notes so you can isolate the variables and see how all these things interact.

bbsailor

[Tinkerer]

bbsailor,

thanks for the many good tips. I have implemented some already and am sure that every one of them will give some improvement.

I should have explained also before, that I use only 0.22 A coil current for the 4uS sampling. I do use 10k PPS at present.
Tinkerer

[Tinkerer]

Horizontal detecting range

...I've searched the forum and haven't been able to find a reference to these questions.
From the flux plots Aziz has provided on several posts, it seems to me that coils of most (all?) types are fairly insensitive in a horizontal plane, i.e.- the coil really doesn't care about what is beside it as long as the object isn't below or above the coil, given a reasonably small mass and conductivity/eddy currents.
Question 1- Is this assumption correct?
Question 2- If the above assumption is correct, would it be a reasonable experiment for me to move part of my HH circuit (Q1 to the input of R23 or Q1 to TP3, Rev D1, DDcoil) to a small faraday shielded cage just off the side of the coil? (After succesful no-mod testing, of course).
Question 3- Would such a move in circuitry help to minimize the Q1 capacitance delays from the cable while (if possible) also minimizing the noise picked up on the return cable to the first stage amp, IC6?
I realize that there are several signals/pwr connections to route and that switching coils might require some re-adjustment, but I thought that I might pick up enough of an advantage in capacitance delay and noise to be worth it on the very short sample times needed for small gold nuggies.
Any thoughts on this or am I way off base? GTB

I moved a few things around in my lab today while I had the coil and amplifier running and connected to the scope.
When moving the cannon ball test target I noticed movement on the scope screen, that is when I remembered your post about horizontal detection range.

Well, large objects like a 3.5 inch cast iron cannon ball is detected about 60 cm horizontal distance from the edge of the coil.
An aluminum beer can 45cm.
The coil is 28cm diameter.

Does this help?

Tinkerer

[GT Blocker]

Hey Tinkerer

Howdy,
Since I've been busy learning my new capture and routing software (see my HH Project post), I forgot to explore this question! Dumb me.
I have my HH working nicely, so I'll try a few small circuit boards in either plane (not cannon balls, lol) and see if it detects them.
I'll also try a 'barley-pop' can in both planes, since they're easy to detect.
I'll post some results in a while (if I don't empty too many 'barley-pops' and forget!).
Thanks for the reply. Good huntin! GTB