Degaussing Magnetic Material

[B^C]

Gday Guys,

Something Aziz mentioned a few times has been bugging me so it would be good to get some thought's on it.

Aziz mentioned about Degaussing the ground matrix, ok if it is possible it's worth a closer look.

To degauss-demagnetise hand tools or drill bits say, we pass them through a coil induced with a sine wave slowly back & forward & then move the tool slowly away from the coil. This certainly works but how can this proceedure be done with a detector with a single pass of the coil.
Does this work without a sine wave?

Simon has mentioned the Bell Ringing analogy & tinkerer has mentioned that you hit a bell once & it rings, hit it twice it stops ringing, hit it three times ?????

Does or will this degauss the ground matrix????

Or can this actually be done?

[Tinkerer]

Gday Guys,

Something Aziz mentioned a few times has been bugging me so it would be good to get some thought's on it.

Aziz mentioned about Degaussing the ground matrix, ok if it is possible it's worth a closer look.

To degauss-demagnetise hand tools or drill bits say, we pass them through a coil induced with a sine wave slowly back & forward & then move the tool slowly away from the coil. This certainly works but how can this proceedure be done with a detector with a single pass of the coil.
Does this work without a sine wave?

Simon has mentioned the Bell Ringing analogy & tinkerer has mentioned that you hit a bell once & it rings, hit it twice it stops ringing, hit it three times ?????

Does or will this degauss the ground matrix????

Or can this actually be done?

Hi B^C,

the degaussing of the soil is often mentioned in connection with PI detectors.
It could be looked at as the long TX pulse magnetizing the magnetic susceptible parts in the soil, and the short but powerful Flyback somehow not be capable to degauss this magnetism.
Your reference to magnetized tools could probably also be used for the magnetizing effect.
Offhand I seem to remember that long weak magnetic fields do not have much magnetizing effect. For magnetizing magnets they use very powerful short pulses.
PI pulses? hit the bell 3 times, in which direction?

Tinkerer

[B^C]

Gday Tinkerer,

Ok, lets have a look at this, Hmm, i will quickly put together a circuit to have a varying pre set number of pulses & lets see if the target response increases with a certain number of pulses due to degaussing the ground.

How to do this without a microcontroller, hmmmmmmmmmmmm HHHHmmmmmmmmmmm ???????

Ok, i can do this easily & i should be able to set the pulse number i want before a delayed dead time between each pulse cycle.
1-2-3-4-5-6-7 etc etc or a constant varying number say 3 then 5 then 3 etc or 5 then 7 any number we want or a random number of pulses or just cycles of 3 etc etc.

I'll put it together & have a look.

[Tinkerer]

Gday Tinkerer,

Ok, lets have a look at this, Hmm, i will quickly put together a circuit to have a varying pre set number of pulses & lets see if the target response increases with a certain number of pulses due to degaussing the ground.

How to do this without a microcontroller, hmmmmmmmmmmmm HHHHmmmmmmmmmmm ???????

Ok, i can do this easily & i should be able to set the pulse number i want before a delayed dead time between each pulse cycle.
1-2-3-4-5-6-7 etc etc or a constant varying number say 3 then 5 then 3 etc or 5 then 7 any number we want or a random number of pulses or just cycles of 3 etc etc.

I'll put it together & have a look.

B^C,

sorry I explained badly. It is not the number of pulses in itself.
A steel bar can be magnetized by hitting it with a hammer, while it is exposed to a steady magnetic field. More strokes more magnetism. However, the strokes need to be always from the same direction and the steel bar needs to be always aligned in the same direction in the magnetic field.
When I spoke of 3 magnetic pulses of a PI, I mean one complete cycle.
1) TX start, coil is charging, current increases and with it the field, it grows larger and larger around the coil. Lets consider 6 TC, so the field has reached a steady state and stays a while in that steady state.
2) Cut off the current. The magnetic field collapses. It shrinks back into the coil. The Flyback grows to a high voltage because the magnetic field, we visualize it as field lines, the field lines cut across the coil at very high speed and therefore generate a high voltage. This voltage is of the opposite polarity as the first applied TX voltage. That means that the polarity of the magnetic field has inversed also.
So the Flyback and its magnetic field, grows to a point, then it collapses again. That is, again it changed direction. This is the 3) third stroke.
It is common to use a negative TX pulse, so that means the voltage goes down, up and down again.
Slow down, slow moving field builds weak eddy currents.
Fast up, high di/dt, strong eddy currents
Down again, slower and with little energy left, this kills a good part of the eddy currents generated on the "stroke" before.

Two strokes in the one direction and only one stroke in the other direction, who is the winner? Ah, but the two strokes are weaker that the single stroke, this is why there are eddy currents left over from the single stroke.
What about the magnetizing? Who wins? Ah, one more problem, the eddy currents generate magnetic fields of the opposite polarity. But they are weak. Enough, enough I go to sleep.

Tinkerer

[B^C]

Gday Tinkerer,

Yeah, i understand about the magnetism etc, i think it's like everything in that things can & often do be viewed differently, i guess that what makes us all individuals. I see the way your looking at it.
There is a site with an animation, below, we can use it for discussion purposes.
Scroll down to how the PI works & click the animation on.

http://www.thomasathomas.com/How_Met...ctors_Work.htm

In this Animation it would seem that there first pulse-stroke energizes the target the 2nd stroke the target would not see? & the third stroke is very minimul--shows more intensity in the animation than real life.

Ok lets talk Polarity of the first TX Pulse so were on the same page so to speak.

Using a Zero Datum line in the same plane as the coil windings so above the Coil is Sth going Polarity & below the Coil is Nth going Polarity, The coil Windings being Zero Datum reference.

For example just say we have the first stroke as you say, the main switch on TX pulse of Nth Polarity pulse into the ground.
On the second stroke, the flyback amplitude increases & is of Sth Polarity so the target would not be affected by this?, infact this may be of a help to us..

This would seem to be the end of the polarity changes as the third stroke as you say only returns to zero datum of the coil & doesn't seem to go back to Nth Polarity like the animation shows until the next stroke number 1, the TX switch on. There may be some very minimul magnetic field humming in Nth Polarity but no intensity. Or we could possibly dampen the coil so we get the third stroke to go below 0v & maybe then we would get a Nth going field?


The magnetic field strength is diminished at the point where the flyback falls again to Zero Datum--no intensity left for another polarity change-small maybe.

I agree the third stroke heads back to the zero datum in a Nth direction on a signal reading but it is only fading away to Zero--falling with no intensity, decreasing in magnitude all the way down.

So it would appear that the only pulse the targets get is the first TX switch on pulse of Nth Polarity?

I have added a picture of the polarity changes from some TX pulses taken from a Hall device. The picture has some bad noise from the Hall device, something affects the readings at night around here pretty bad.
More than likely the house is on a big mineral deposit.

In the Picture the bottom trace is from the hall device, Nth Polarity is the Bottom of the Picture Sth is Top.
You will see the slight Nth going magnetic field of the switch on then a good south going pulse but then it settles back to the Zero Datum line & doesn't really go of Nth Polarity again.

See what you think.

[Max]

Gday Guys,

Something Aziz mentioned a few times has been bugging me so it would be good to get some thought's on it.

Aziz mentioned about Degaussing the ground matrix, ok if it is possible it's worth a closer look.

To degauss-demagnetise hand tools or drill bits say, we pass them through a coil induced with a sine wave slowly back & forward & then move the tool slowly away from the coil. This certainly works but how can this proceedure be done with a detector with a single pass of the coil.
Does this work without a sine wave?

Simon has mentioned the Bell Ringing analogy & tinkerer has mentioned that you hit a bell once & it rings, hit it twice it stops ringing, hit it three times ?????

Does or will this degauss the ground matrix????

Or can this actually be done?

Hi,
yes... it's an interesting topic. Don't know if can really do that but if so... this will solve some annoying issues about e.g. PI detectors (probably not only).

The problem is that soil magnetic behaviour is due to e.g. iron oxides...crystals... magnetite etc.

Now... in e.g. CRT degauss the principle is like this: you have a rapid rise in the current at degaussing coil inside the monitor... or tv... then a PTC component (thermistor) open the circuit cause heat generated internally make resistance rise.

During PTC operation on/off there are initial hi-current that charge inductor's magnetic field (stored energy in that field) ...and increasing resistance due to PTC values till current stops flowing abruptly. When this happens the magnetic field is kinda sinusoidal decreasing (degauss coil self-oscillate at fundamental frequency of its resonant tank... inductance+parasitic capacitance+internal resistance dumping) making rapid inversions of strong magnetic field decreasing in amplitude with a rapid slope.

That makes things like shadow mask... to demagnitize to a very low residual level (bias magnetic level) that's compatible with clear, color matching image formation on the screen.

In some PIs... the double pulse ones... there's something similar (though not cleary visible as in tv/monitors by effects)... that is:
there are 2 strong pulses at very close distance in time (say 10-20us between) in wich two fast decaying sinus slopes of opposite magnitude are delivered to soil/target.
The net effect is that a residual small bias is left to magnetized bodies under coil.

That's really different from mono-pulse PI... where there's always a strongest positive or negative part of slope that , instead, tend to magnetize stuff.

That's this way in double pulse PI for mostly two reasons:
1. (most important) are mine detectors... so double pulse minimize the risk of an e.g. magnetic flow fuse ignite the explosive in the mine... triggering an explosion (that's e.g. in AN/PSS12 mine detecting set by Shiebel/NATO)
2. S/N increase a few... cause there's less influence to magnetic related "noise" due to "heavy" magnetic targets... e.g. iron/steel nails etc but also from soil components due to magnetic viscosity effects

Now, I have to say I made some tests with double pulses designs... and they was not so good about results, less than expected.

But if a truly demagnetising effect is perfomed I think there could be substantial advantages, cause detector could then focus only on eddy currents due to conductive targets than magnetic effects.

Till now I never heard of that really happened ever in both commercial or homemade desigsn but the idea is not bad at all.

If you look at (now very old, 1980) Corbyn's article on PI (wireless world issue) you'll see he talks about that... and uses double opposite pulses to increase whole S/N. So the idea is not new... just need great refining I think.

Kind regards,
Max

[Tinkerer]

B^C,

I look at your scope screen and only see the voltage on the top trace and a bad repeat on the bottom trace.

Look at the current trace, not the voltage trace.
As you know, it is the current that produces the magnetic field.

In which direction does the current move in the AC mains?
You can visualize the PI cycle as a distorted sine wave, it goes down, then up, then down again.

Tinkerer

[B^C]

Gday Guys,

Max,
Thanks for the reply.
Yes, it may be worth looking into a bit further, you said:

"But if a truly demagnetising effect is perfomed I think there could be substantial advantages, cause detector could then focus only on eddy currents due to conductive targets than magnetic effects:.

Now this i find interesting as it is the way my new circuits work & they work very well. I have not been focusing on magnetic affects but more the effects of conductive targets.

There is an issue i have though that i still can't work out & have no idea actually.

I can detect very low conductive targets such as 316L stainless steel as good as i can detect targets with very high conductivity??????
This i don't understand?

Tinkerer,

Sorry about the picture, not sure what happens there?

I have re attached it & just the bottom half --the Hall device signal incase it doesn't attach correctly again, i hope it comes through, it clearly shows the magnetic field.

[B^C]

I was just having a brain strain about this demagnetising, isn't this exactly what we are doing anyway.

Eg:

If our first TX Pulse is say of Nth Polarity & then the switch off creates a Sth Polarity, this Sth Polarity pulse would seem to be the Demagnetizing pulse?

The strength & duration of this Sth going pulse would seem to be an important issue.
If it is to weak or to strong or to short or to long in duration it would have not enough or to much of an effect, but if the field strength & duration of this Sth polarity pulse is perfected we could infact make it so it just demagnetizes the soil matrix leaving only the target signal to read.

Maybe!

[Max]

Gday Guys,

Max,
Thanks for the reply.
Yes, it may be worth looking into a bit further, you said:

"But if a truly demagnetising effect is perfomed I think there could be substantial advantages, cause detector could then focus only on eddy currents due to conductive targets than magnetic effects:.

Now this i find interesting as it is the way my new circuits work & they work very well. I have not been focusing on magnetic affects but more the effects of conductive targets.

There is an issue i have though that i still can't work out & have no idea actually.

I can detect very low conductive targets such as 316L stainless steel as good as i can detect targets with very high conductivity??????
This i don't understand?

Tinkerer,

Sorry about the picture, not sure what happens there?

I have re attached it & just the bottom half --the Hall device signal incase it doesn't attach correctly again, i hope it comes through, it clearly shows the magnetic field.

Hi,
yes, me too. Can detect very well stainless steel and e.g. copper /silver about the same.

My idea is that , unless item is very thin in section exposing great resistance to eddy currents e.g. thin (unusual) steel, it's unlikely that a real difference show up.

I noticed e.g. I cannot detect easy stainless steel small needles (syringe things) but can do with about same weight of steel but flat round thing.

The shape of needle makes difficault eddy currents to develop well, also the flux interaction depends much on orientation and is less compared to flat stuff.

So, if resistance is high cause of shape you notice no or weak detection... otherwise not.

Now... steel bottle tops are about the size of a coin you cannot notice big differences.

The resistivity factor, I mean, is not so relevant if object is big enough to let eddy currents develop well inside.

Unless device can sort/disc stuff using resistivity param, the things like bottle tops are detected easy.

Kind regards,
Max

[Aziz]

Hi B^C,

don't trust the graphics, you have seen anywhere. It is quite misleading.

The current direction does not change in the conventional PI and therefore the magnetic field direction does not change too.
The coil wants to maintain the same current direction after switch-off and this is done during the damping period.

Aziz

[Max]

I was just having a brain strain about this demagnetising, isn't this exactly what we are doing anyway.

Eg:

If our first TX Pulse is say of Nth Polarity & then the switch off creates a Sth Polarity, this Sth Polarity pulse would seem to be the Demagnetizing pulse?

The strength & duration of this Sth going pulse would seem to be an important issue.
If it is to weak or to strong or to short or to long in duration it would have not enough or to much of an effect, but if the field strength & duration of this Sth polarity pulse is perfected we could infact make it so it just demagnetizes the soil matrix leaving only the target signal to read.

Maybe!

Hi,
today i read an issue of uk elektor magazine (may/2007).... there's an article about old pistol soldering irons used as things to demagnitize tools... like screwdrivers etc...

Not that's like in the case of PI... but principle is always the same... the tools are inside alternate field and are moved slowly to have amplitude decay so... remains just little residual magnetization in them, like in crt degauss stuff.

I think degaussing using PI coils is possible... but at few distance from coil, maybe.

Kind regards,
Max

[B^C]

Gday Guys,

Max, it is interesting & testing would confirm things, but how to test a magnetic soil matrix, it takes time for the soil particles to align themselves once induced with a magnetic field--magnetize & then decay to realign themselves to there original state.
How to test if we can decrease the time for the soil particles to decay --realign themselves to there original state to see if it is just all snake oil or not is not so easy?????

I guess if we get a stronger signal back from a target using the same soil matrix as a test bed & the same circuit would prove something?


Hi Aziz,

I was under the same opinion, that the current does not change direction but from the Hall Devices i get a clear Nth going polarity with switch on & then the magnetic field reverses to a Sth going polarity on switch off.
The Hall devices show what polarity is inducing them & it does appear that there is in fact a reversal of the magnetic field on switch off.

If there was no polarity change the hall devices would show just that & the magnetic field would just decay away to zero but they do not show that.
They certainly indicate a reversal of the magnetic field, it may not agree with some things but it is there to see.
Interesting.

[Aziz]

B^C,

the cables to the hall sensor are inducing voltages. Check this. A twisted pair cabling would minimize this.

Aziz

[Qiaozhi]

I was under the same opinion, that the current does not change direction but from the Hall Devices i get a clear Nth going polarity with switch on & then the magnetic field reverses to a Sth going polarity on switch off.
The Hall devices show what polarity is inducing them & it does appear that there is in fact a reversal of the magnetic field on switch off.

I think Aziz is correct that the leads of the Hall effect sensor are picking up unwanted signals.

During the switch-on period a magnetic field is established in the coil, and at switch-off the magnetic field collapses. At no time does the coil current reverse. It simply reaches a maximum at switch-on, and rapidly decays at switch-off

What you appear to be discussing here is the possibility that certain soil types can become magnetized if pulses of the same polarity are used. To avoid this situation some PI detectors generate pulses of alternate polarity. I don't know whether this has any major benefit in "normal" metal detecting, but I can see that in humanitarian de-mining it could an issue.