Degaussing Magnetic Material

[Tinkerer]

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.

We are getting closer, but we are not quite there.
What we are interested in, is the target, that includes the soil.
Visualize the following:

A wire with a very small current flowing in it. It is surrounded by a magnetic field that reaches out a small distance.
Now place a target conductor just at the edge of the magnetic field.
Now increase the current in the wire. The magnetic field expands and the magnetic field lined cut across the target conductor, generating a voltage and eddy currents in the conductor.
Hold the current.
The eddy currents in the target conductor subside, but the target is still surrounded by the magnetic field.
Now switch off the current source. The magnetic field collapses. The magnetic field lines again cut across the target, but in the opposite direction as before. Therefore the eddy currents generated by the collapsing magnetic field are of opposite polarity.

You can easily try to look at the reversal of the polarity if you move a magnet past a coil. Move it in one direction you get one polarity, move it in the opposite direction you get the opposite polarity. forward and back, you get AC.

Tinkerer

[B^C]

Gday Guys,

I am using shielded cables to the Hall device as i had an issue once before with this same thing.
I have just phoned around & can purchase some better high quality shielded cable that is not affected my magnetic fields which i will go & get now. It could be an issue so i will test again to make sure.

My bet is the magnetic field does a complete polarity reversal on switch off, we shall see!
Why this would be so, i don't know--yet!
If in fact it does?

If i'm incorrect that's ok, if i'm correct that would explain a lot of strange things that happen when testing!

[Aziz]

Hi B^C,

you can shield easily electrical fields but not magnetic fields. Keep the flux area of the cabling as small as possible (twisted pair). Otherwise, the cabling itself will induce a voltage (it is somehow another coil).

Aziz

[B^C]

Gday Aziz,

Yes, i will take your advice with the cabling, i am setting things up now for some more testing.

Isn't the change of Polarity obvious with this?, let me explain.

The current increases causing a magnetic field of one Polarity in the coil, then the Current "changes direction" causing the voltage to spike of opposite Polarity. The Current has to change direction or we would have no Flyback Transient.

With the Change of direction of the Current & the Reverse Polarity of the Voltage, would this not cause the Magnetic field to Momentarily change Polarity as shown by the Hall device.

Anyway we shall see, i will set things up better & do another test.

Interesting.

[B^C]

Gday Guys,

I just finished some testing of the Polarity again to get to the bottom of this, with Aziz's suggestion i changed all the wiring etc to make sure of the readings i was getting with the Hall devices.

I got the same results as before with both my Hall devices?

Not happy with this i then wired up a new unused Hall device & it would seem that both of the older ones have internal issues as the results from the new device are completely different.
I had two Hall Devices so i could compare results to make sure of things but they must have recieved a spike or something & they do not work like the new one.

The reading now shows NO Polarity change at all at the flyback, so i would have lost my money on a bet Hahaha.

I am happy with that!

The picture now only shows a slight magnetic field at switch on & some electrical interference when switching etc.

Cool!

[Qiaozhi]

The current increases causing a magnetic field of one Polarity in the coil, then the Current "changes direction" causing the voltage to spike of opposite Polarity. The Current has to change direction or we would have no Flyback Transient.

The current in the transmit coil does not change direction.

Imagine you turn on a tap (faucet) attached to a hose pipe and water gushes into the pipe. You then turn off the tap and the water stops. At no time does the water reverse direction. It's the same with the flow of electrons.

The voltage reversal at switch-off is caused by the collapsing magnetic field.

[B^C]

Gday Qiaozhi

Yes i understand that, by changing direction i meant instead of charging it is discharging or instead of the current rising it is falling.
I should have written, it takes a change in the current to create the Flyback Transient.

I would like to get your thoughts on something you mentioned about degaussing in a previous post if possible.

You Said:

"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."


Could you elaborate?

[B^C]

Gday Guys,

Now i have partly convinced myself that there is no Polarity change as the current changes i find this animation & this is also the way i thought of it.

Have a look at the step function Animation & as the curent changes (falls) so does the Magnetic field Momentarily this is what i thought also.

http://www.williamson-labs.com/480_rlc-l.htm

Now i have to say i have some doubts again hahaha!

[B^C]

Hi Guys,

Sorry for posting one after another but there is no Edit function that i see on the Forum, it's there sometimes but not others.

The animation above goes along with the first animation that i posted about how PI works.
http://www.thomasathomas.com/How_Met...ctors_Work.htm
where there is three strokes as Tinkerer was talking about.

Nth Sth Nth

[Tinkerer]

Gday Guys,

Now i have partly convinced myself that there is no Polarity change as the current changes i find this animation & this is also the way i thought of it.

Have a look at the step function Animation & as the curent changes (falls) so does the Magnetic field Momentarily this is what i thought also.

http://www.williamson-labs.com/480_rlc-l.htm

Now i have to say i have some doubts again hahaha!

Hi All,

funny how one can see different things while looking at the same thing. We are talking about the most basic electricity theory, but, depending how one looks at it, the results come out different.

The PI coil is a conductor and as the magnetic collapses, the field lines cut across this conductor. The flyback voltage is high because this conductor is in a coil form that multiplies the voltage.

The important part to look at, is the direction or Vector of the expanding and collapsing magnetic field lines. Remember di/dt.
If we damp the Flyback, so that it does not produce an opposite voltage spike, but goes to 0V only, we get "UP-Down" only. Like the animation above shows.
However, the traditional PI has a high voltage spike that is of the opposite polarity than the TX charge pulse.
This Flyback spike goes up and down too.

Stimulated by a discussion, I think it was about "what kicks the target" some time back I posted a scope picture of a TX pulse wave form without a Flyback, lets call it a "soft landing".
So once I had a controlled "soft landing", I found that I could sample during the "soft landing" and this is where I got 10 times more sensitivity and FE discrimination as well.

So here is my idea for sale:

"SOFT LANDING PI TECHNOLOGY"

Price list:

The whole technology will cost you an arm and a leg.
To have a look at it only, will cost you an eye.
If you don't like it, you can give me the finger.

Tinkerer

Ps. the above price list is an attempt at humor in US verbatim. Translation into other languages is complicated.

[Qiaozhi]

Gday Qiaozhi

Yes i understand that, by changing direction i meant instead of charging it is discharging or instead of the current rising it is falling.
I should have written, it takes a change in the current to create the Flyback Transient.

I would like to get your thoughts on something you mentioned about degaussing in a previous post if possible.

You Said:

"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."


Could you elaborate?

What I was referring to is the fact that land mines can be triggered by a number of methods, one of which is magnetism. Theoretically, reversing the magnetic field on alternate pulses should reduce the possibility of triggering the mine. Whether this pulse reversal method is of some use in heavily mineralized soils is another story.

[B^C]

Gday Guys,

Hi Tinkerer,

I have also tried with NO flyback or little flyback & huge flyback & had some results, my circuits work differently than most as i don't have delayed reading etc or any flyback issues.
I find no need for microcontrollers-pic etc as my circuits are very stable in the worst enviroments, my GPX4500 goes crazy inside with all the electrical disturbance, i have NO issues with my circuits, quite as a mouse until a target is near, i have no trouble with mineralised ground either & have no need for ground balance circuitry.

A circuit i have at the moment cost around $30-00 Au & is more sensitive than my GPX4500 & works great & is very stable.

Like i mentioned once before, if something is better than before then it's better, if it's not it's not easy as that.

If we all think in a conventional way then we really never learn any different, it's best to test these so called theories out & see for ourselves.


Yes it is all basic electricity theory--emphasis on Theory, to me seeing is believing taking into account that most of what is written on the subject comes from way back hundreds of years ago when the people who studied it also believed in witches & thought the world was flat.
We also still drive around with inefficient engines in cars with technology that really hasn't changed since it was first designed about 200yrs ago, weve just added useless comfort items & increased efficiency about 5%.
There's one thing mankind is not, we are not smart, we just think we are, we don't even know where we come from--this is still theory also.

We have come a long way & most people now have more advanced measuring equipment in there homes than what people from by gone eras could have even imagined.

Many people follow along with what is written in the text books without testing these things themselves to find out otherwise.

Electromagnetism is one of the least understood subjects, if anyone could understand it we would move ahead fast. Even NASA has to stick there spaceships onto fireworks to get them into space haha, the chinese developed fireworks a thousand years ago or more, we just made it into solid fuel.
Were a backward lot!
Mr Tesla was smarter than most but they put him in the back yard so the wealthy could get wealthier.

Modern times have proven many things wrong, years ago they thought that current only flowed from positive to negative, recent times have proven different but so there was no confusion or so they didn't have to rewrite the text books or change equations they left them as is?

Text books & theory are for people who can't be bothered to have a look themselves, after all there only written by men & women, but who are they, do they really know, of course not, more than likely we have looked at the whole electricty thing the wrong way from the start.

Like mathematical equations, we made this up also to get an understanding of things.
The trouble is we don't know if were looking at things the right way to put the correct equations to things, so in reality what are text books. They are only peoples beliefs just like the Bible, real or not, science can't find anything to say it's not real but they also can't say it is hahaha!