NEW TECH PHOTOS

[Tinkerer]

My quick reply wasn't ample, huh?



My suggestion would be to initially design a light-weight unit for coin/relic hunting utilizing an 8" coil. (Hays has a couple of 8" housings that would work well.)

The TX power should be determined as the design progresses, due to all of the trade-offs between TX power, battery consumption, battery size, unit weight, etc., which are all dependent on the pulse width, pulse frequency, coil parameters and driver circuitry.

A primary goal should be to achieve some form of ferrous/non-ferrous discrimination. Your IB-PI experiments should obviously be the stepping stone for this goal.

Very good observations. Could you post a drawing of the Hays 8" coil housing, so that we can design a coil that fits?

Yes, everything is interdependent. This is why I want to take the decisions now, taking into consideration the wishes of the team members.
As a starting point, lets use a middle ground. Medium everything.
But lets define the numbers.
Say: TX 100uS, max 2A coil current, 12V, 600 PPS?

Tinkerer

[Tinkerer]

Tinkerer, I was looking at the graphs and I am puzzled why there is such a big difference between the cannon ball and the steel lugs and steel nails. Isn't the cannon ball made of iron too? I would suggest to take another photo just with a very rusty nail (because we have much more (mis)chance to find rusty iron than steel lugs). And if it happens that you have some hot rocks in your collection, maybe you should take a shot of those ones too. By the way, I discovered that my hot rock attracts magnets, in fact I can hold the rock suspended by a neodymium magnet.

Regards,
Nicolae

Yes, at first sight it is puzzling.
However, remember that the cannon ball and Alu plate are 30cm from the coil and the other targets are 3mm from the coil.

What I tried to do with the cannon ball, is to simulate the worst Australian ground. As a simulation, I think it worked.
How close it imitates the bad ground, I don't know.

If we keep trying we may succeed. If we do nothing we will never.

Tinkerer

[hobbes_lives]

Here's a link to the Hays coil housings...

http://www.hayselectronics.com/parts.htm

Take a look at B and N, and perhaps W. I don't have technical drawings of the housings, but perhaps exact dimensions can be obtained from Mr. Hays once we've settled on a size. He can be contacted at the bottom of his page.

Say: TX 100uS, max 2A coil current, 12V, 600 PPS?

These values all sound like a reasonable starting point. I realize there's some discrepancy about the optimum pulse width for a given target, but I think 100uS is a good starting point. For a coin-shooter, the pulse width should perhaps be longer, but let's start with 100uS and work from there.

What about sampling timing? Should we set some goals for that from the onset? Perhaps just some basic timings that we can work from for prototyping?

I assume we are planning on using a microcontroller. I have some experience with the dsPIC30f and PIC18f devices.

What about the RX coil and preamp? Are we going with a differential design? I believe that would be optimal.

Also I suggest VCO audio, as human hearing is more sensitive to frequency shifts than to volume changes.

We should throw together a block diagram.

[nick_f]

My quick reply wasn't ample, huh?
A primary goal should be to achieve some form of ferrous/non-ferrous discrimination. Your IB-PI experiments should obviously be the stepping stone for this goal.

How can we analyze the waveforms to obtain the discrimination? Using Sample and Hold circuits followed by a A/D converter? How many bits would be required? How many samples during the Rx response?
In this case, do we need to use a microcontroller?
Or maybe we could use something simpler, like a comparator (or a few comparators and a multiplexer).

Regards,
Nicolae

[Tinkerer]

Tinkerer, I was looking at the graphs and I am puzzled why there is such a big difference between the cannon ball and the steel lugs and steel nails. Isn't the cannon ball made of iron too? I would suggest to take another photo just with a very rusty nail (because we have much more (mis)chance to find rusty iron than steel lugs). And if it happens that you have some hot rocks in your collection, maybe you should take a shot of those ones too. By the way, I discovered that my hot rock attracts magnets, in fact I can hold the rock suspended by a neodymium magnet.

Regards,
Nicolae

Hi Nicolae,

I did a quick repeat on the cannon ball photo. The experiment shows several features.
The TX voltage is not exactly the same. screwing around with the pot of the voltage regulator. It shows the reason for using a regulated TX voltage.
The damping is not exactly the same, dito with the pot.

Anyway the sampling window looks close enough to get the same idea.

Picture # 1 NO TARGET
#2 CANNON BALL
#3 CANNON BALL at 30cm and the silver ring at 3mm. this is to show, that with the simulated hot ground present, the 0.55gram silver ring still gives a similar response.

Tinkerer

[Tinkerer]

Here's a link to the Hays coil housings...

http://www.hayselectronics.com/parts.htm

Take a look at B and N, and perhaps W. I don't have technical drawings of the housings, but perhaps exact dimensions can be obtained from Mr. Hays once we've settled on a size. He can be contacted at the bottom of his page.

These values all sound like a reasonable starting point. I realize there's some discrepancy about the optimum pulse width for a given target, but I think 100uS is a good starting point. For a coin-shooter, the pulse width should perhaps be longer, but let's start with 100uS and work from there.

What about sampling timing? Should we set some goals for that from the onset? Perhaps just some basic timings that we can work from for prototyping?

I assume we are planning on using a microcontroller. I have some experience with the dsPIC30f and PIC18f devices.

What about the RX coil and preamp? Are we going with a differential design? I believe that would be optimal.

Also I suggest VCO audio, as human hearing is more sensitive to frequency shifts than to volume changes.

We should throw together a block diagram.

I have to work with the parts at hand, to get parts takes me months. this is why I suggest we use a PIC16F690 for the first version.
For V2 I would like to use a PIC18F4550 and make the firmware USB programmable, so that we can really experiment with various ways to extract a maximum of information from the signal.
Once we know where we are going, a dsPIC30f will probably open new horizons.
Agreed on the variable pitch audio. However, we need at least 2 tones. One for all targets, one for the presence of FE. It could also be one pitch and modulation thereof for FE. I would like to try to generate this with PWM.

Initial sample timing schedule: 5uS after switch ON, 15uS after switch OFF, 5 uS before TX pulse switch ON. Duration of the samples 5uS.

There is nothing that prevents us using a differential preamp directly from the RX coil. On the breadboard I have 2 limiting diodes only for safety sake for when I goof up.

Tinkerer

[Tinkerer]

How can we analyze the waveforms to obtain the discrimination? Using Sample and Hold circuits followed by a A/D converter? How many bits would be required? How many samples during the Rx response?
In this case, do we need to use a microcontroller?
Or maybe we could use something simpler, like a comparator (or a few comparators and a multiplexer).

Regards,
Nicolae

The PIC16F690, has a 10 bit A/D. That is good enough for the beginning.
We need to build the front end to see exactly where to put the sample points. If they are far enough from each other, there is enough time to sample directly.

Tinkerer

[hobbes_lives]

The PIC16F690, has a 10 bit A/D. That is good enough for the beginning.
We need to build the front end to see exactly where to put the sample points. If they are far enough from each other, there is enough time to sample directly.

It's not the sampling itself that's an issue, it's what's done with the samples after the fact. I suggest we go with a dsPIC30f device right off the bat. The instruction set is designed for DSP, there are way more accumulators to work with, the instruction size is 16-bit, I can go on and on, really. It'd suck to write code for the PIC16, and then have to rewrite it for the dsPIC30 when we realize that our project has outgrown the PIC16.

[Tinkerer]

It's not the sampling itself that's an issue, it's what's done with the samples after the fact. I suggest we go with a dsPIC30f device right off the bat. The instruction set is designed for DSP, there are way more accumulators to work with, the instruction size is 16-bit, I can go on and on, really. It'd suck to write code for the PIC16, and then have to rewrite it for the dsPIC30 when we realize that our project has outgrown the PIC16.

There are the basic parts that will be the same for V1, V2, and V3.
I appoint you as master of the V3 version then, while I concentrate on V1.
Maybe somebody turns up to take over V2.

My problem is as mentioned above. I can not get parts. What I have will do, marginally, for V1 and V2.

Ok, now lets see what we could do with a dsPIC30f. Could we take enough samples to reproduce the critical part of the sample window? It would be nice to see some curves.
Probably we can do some mathematical functions. Once we know how to read these curves and convert the information into mathematical formulas.

It is going to be interesting fun.

Tinkerer

[B^C]

Gday All,

I have been following this along --as usual & everytime i look at the images supplied it says that maybe the observations are due to the Eddy Current Impedance Plane of various targets.
Then again i may have no idea, see what you think?
The cannon ball is an odd one though, in saying that, the biggest steel sphere i have is a steel ball about 25mm diameter & it also gives a strange reading compared to non spherical targets.


I read this a long time ago & copied it & also the image which i will attach.

The impedance plane diagram is a very useful way of displaying eddy current data.
As shown in the figure below, the strength of the eddy currents and the
magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways.

If the coil is balanced in air and then placed near a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil, which shows up as resistance) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). Changes in conductivity will cause the eddy current signal to change in a
different way.
When a coil is placed near a magnetic material such as steel, something different happens. Just like with aluminum (conductive but not magnetic), eddy currents form, taking energy away from the coil, which shows up
as an increase in the coils resistance. And, just like with the aluminum, the eddy currents generate their own magnetic field that opposes the coils magnetic field.
However, you will note for the diagram that the reactance increases. This is because the magnetic permeability of the steel concentrates the coil's
magnetic field. This increase in the magnetic field strength completely overshadows the magnetic field of the eddy currents.

[Tinkerer]

Gday All,

I have been following this along --as usual & everytime i look at the images supplied it says that maybe the observations are due to the Eddy Current Impedance Plane of various targets.
Then again i may have no idea, see what you think?
The cannon ball is an odd one though, in saying that, the biggest steel sphere i have is a steel ball about 25mm diameter & it also gives a strange reading compared to non spherical targets.


I read this a long time ago & copied it & also the image which i will attach.

The impedance plane diagram is a very useful way of displaying eddy current data.
As shown in the figure below, the strength of the eddy currents and the
magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways.

If the coil is balanced in air and then placed near a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil, which shows up as resistance) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). Changes in conductivity will cause the eddy current signal to change in a
different way.
When a coil is placed near a magnetic material such as steel, something different happens. Just like with aluminum (conductive but not magnetic), eddy currents form, taking energy away from the coil, which shows up
as an increase in the coils resistance. And, just like with the aluminum, the eddy currents generate their own magnetic field that opposes the coils magnetic field.
However, you will note for the diagram that the reactance increases. This is because the magnetic permeability of the steel concentrates the coil's
magnetic field. This increase in the magnetic field strength completely overshadows the magnetic field of the eddy currents.

Hi B^C,

I think you are on the right track there.
My cannon ball as shown above is probably not comparable with a steel sphere, although, I do not know it's exact composition. Most of the cannon balls that I found on shipwrecks were of very porous, very high carbon cast iron. They had usually been penetrated with salt water to the core and where thus very difficult to preserve.
This one cannon ball preserved much better than the others, so there are differences.
I chose the cannon ball as "practice target" because I thought that it might serve as an example of very rusty old iron or maybe even simulate the extreme iron content of some of the Australian soil.

MAXWELL SV http://www.ansoft.com/downloads.cfm

This software, free to download, lets you visualize the magnetic field lines.
I think, if we play a bit with that, we will get a much better idea of why we read certain responses from the soil and from different targets.

Basically we build a magnetic field around the coil. The shape of the field is influenced by the magnetic field of the earth that surrounds it as well as any conductors within the field.
Magnetic conductors concentrate the field lines. This generates large distortions in the coil's field.
The changing coil field generates eddy currents in all conductors within it.
The eddy currents generate their own fields and since these fields are within the coils field, they cause distortions.

The best way to read these distortions is at the time they are generated.

Tinkerer

[B^C]

Gday Tinkerer,

Yes your right, the canon ball would be cast, for some reason i assumed it to be steel but at the time they made these of course they would be cast & is the reason for the different readings compared to steel.

I am working on a highly magnetically susceptible soil sample at the moment trying to replicate the test readings i get with this.

I'm trying to find an easy way for people to replicate this type of soil when testing & i think it may be able to be done with things that are available to everyone. This will take a lot of guess work out of things if i can get it right, i will post the results as soon as i have it correct.
Aziz also wanted some noise readings from the same type of soil, because everybodies detectors seem to differ a lot a suitable accurate Aussie soil sample replica should help.
I have it close to the mark but it's not quite right, i'll get it & post the results & we can all test, like field testing in bad ground.

[Monolith]

UPDATE?

How is the project going?
Are we going to get and update soon?

Monolith

[6666]

Tinkerer
where in the world are you located ?

[Tinkerer]

Tinkerer
where in the world are you located ?

Venezuela