NEW !!! Full differential PI front end.

[msysta]

I agree with Simon ... active damping should be possible. Active damping is used in areas like car suspensions ( upmarket models ). When you hit a pothole you want to damp the natural tendancy of the suspension to oscillate.

Moodz

I agree that there may be better results available with active damping. Maybe the flyback voltage curvature from peak to zero has different optimum values for damping resistance per derivative moment. Then one could use transistor with control from the pulse itself. Or what I've seen with oscope, the high voltage range decays very fast but range below 50 volts is slower in decay. So maybe the active period could be only in that part.

And those what I've posted earlier are wrong because I am somehow saying that flyback voltage causes the eddy current. That is not the case of course.

[moodz]

I agree with Simon ... active damping should be possible. Active damping is used in areas like car suspensions ( upmarket models ). When you hit a pothole you want to damp the natural tendancy of the suspension to oscillate.

Moodz

[simonbaker]

Hello from white and cold Finland.

What I've learned after the last post (thanks bbsailor, mikebg and others) is that the flyback voltage nulling has a theoretical minimum time so that just simply short circuiting the coil doesn't give the ultimate result in this manner. There is a way called critical damping. That is actually term describing the optimum attenuation of any oscillating system, like spring and shock absorber for example.

See this post for details

http://www.geotech1.com/forums/showthread.php?p=101304#post101304

And what comes to interaction between target eddy current and flyback voltage...

Any coil having current flow generates a magnetic field around itself. Now to be able to induct current to target object using this magnetic field there are three options:

1. move the magnetic field
2. move the target
3. tamper the current

In metal detector use choice number two is usually eliminated. And when searching, the operator is moving the magnetic field but not straghtly to charge the target but to cover larger area . This leaves us with number three.
Also it can be proven that with faster current change, a larger induction effect is achieved.
Term self induction must be known. It is the phenomenon causing the flyback voltage. In short, when current in any coil decreases, also coil's own magnetic field decreases. At the same time coil's magnetic flux decreases, generating self induction voltage which according to Lenz law objects the current reduction.

If a PI detector coil has a current that is cut off it behaves like described above, generating flyback voltage.
We must observe that coil's voltage and current are in different phase; when voltage is in maximum, current is in minimum and vice versa.
Now when maximum flyback voltage is taken to zero as fast as possible, it generates the highest possible current in coil, inducting the highest possible energy to the target.
Also when doing this fast, we are faster to set in receiving mode, listening the echo from the soil.
So there are two flys in the swatter.

This again took the conversation out from the original topic, apologies for that.

With Best Regards,
Markus S.

Happy New Year to everybody.

Critical damping may be "fastest" way for linear systems (depending on how you define fast). There still may be faster methods using non-linear techniques.

-SB

[msysta]

Hello from white and cold Finland.

What I've learned after the last post (thanks bbsailor, mikebg and others) is that the flyback voltage nulling has a theoretical minimum time so that just simply short circuiting the coil doesn't give the ultimate result in this manner. There is a way called critical damping. That is actually term describing the optimum attenuation of any oscillating system, like spring and shock absorber for example.

See this post for details

http://www.geotech1.com/forums/showthread.php?p=101304#post101304

And what comes to interaction between target eddy current and flyback voltage...

Any coil having current flow generates a magnetic field around itself. Now to be able to induct current to target object using this magnetic field there are three options:

1. move the magnetic field
2. move the target
3. tamper the current

In metal detector use choice number two is usually eliminated. And when searching, the operator is moving the magnetic field but not straghtly to charge the target but to cover larger area . This leaves us with number three.
Also it can be proven that with faster current change, a larger induction effect is achieved.
Term self induction must be known. It is the phenomenon causing the flyback voltage. In short, when current in any coil decreases, also coil's own magnetic field decreases. At the same time coil's magnetic flux decreases, generating self induction voltage which according to Lenz law objects the current reduction.

If a PI detector coil has a current that is cut off it behaves like described above, generating flyback voltage.
We must observe that coil's voltage and current are in different phase; when voltage is in maximum, current is in minimum and vice versa.
Now when maximum flyback voltage is taken to zero as fast as possible, it generates the highest possible current in coil, inducting the highest possible energy to the target.
Also when doing this fast, we are faster to set in receiving mode, listening the echo from the soil.
So there are two flys in the swatter.

This again took the conversation out from the original topic, apologies for that.

With Best Regards,
Markus S.

Happy New Year to everybody.

[Tinkerer]

Can someone please expand upon what Msysta has posted? I am most interested in the raw basics - the coil is energized and then the energy is cancelled and he says the target eddy current interracts with the flyback decay.
But what if there were a separate receive coil (perhaps at some distance away) that was shunted during the transmit pulse of the traditional PI circuit - would it then pick up the eddy current in a more "pristine" and manageable fashion? Could we do more with this signal as it would be better normalized for receive circuitry?
Kind of seems we are making little EMP devices and receptors...

I am about to find out more on this with a Surfmaster PI I have discussed in that forum section. The drive transistor was a TIP32B. That has a lower Vce rating than a TIP32C part for example, and it had a truncated flyback peak maybe because of this. But why did White's choose the B part then?

bklein,

At the thread below, you can find the circuit of the TINKERERS_V1, discriminating PI, that uses a separate coil and differential input preamplifier.

A spark gap to dampen the Flyback would be very noisy.

http://www.geotech1.com/forums/showpost.php?p=98870&postcount=1

Tinkerer

[bklein]

Can someone please expand upon what Msysta has posted? I am most interested in the raw basics - the coil is energized and then the energy is cancelled and he says the target eddy current interracts with the flyback decay.
But what if there were a separate receive coil (perhaps at some distance away) that was shunted during the transmit pulse of the traditional PI circuit - would it then pick up the eddy current in a more "pristine" and manageable fashion? Could we do more with this signal as it would be better normalized for receive circuitry?
Kind of seems we are making little EMP devices and receptors...

I am about to find out more on this with a Surfmaster PI I have discussed in that forum section. The drive transistor was a TIP32B. That has a lower Vce rating than a TIP32C part for example, and it had a truncated flyback peak maybe because of this. But why did White's choose the B part then?

[Carl-NC]

Anyway to answer quickly the questions about the 4066 driver cct ...
It does actually work

I'm surprised! You may want to check the margins, it may be just barely working.

[gadgetmaker]

Parts Help for Aziz

Aziz,

Please list part numbers and quantities you need and I will get them for you.

Karl

[Aziz]

Hi moodz,

I am just waiting to finish the list of needed expensive parts. But I have two samples of the following analog switches/muxes till now.
MAX313
MAX392
MAX333A
I will test the MAX313 and MAX392 (pin compatible). It makes the driving really simple and reduces circuit complexity.

I probably need several month to afford the rest of the parts list. So it's going really slowly.

Just keep on the good work.
Aziz

[moodz]

....well I found another spare 20 minutes and it seems there has been bit of discussion going ... all good stuff.

Anyway to answer quickly the questions about the 4066 driver cct ...
It does actually work

below is the output of channel 1 with about a 40 uS delay after first turn off the green trace is the input to the 4066 ... white trace is the sampled output ... the timing is set to intercept the decay slope at about 75 %.



and here is the breadboard with 60 turn 'boss' differential coil. Note he circuit here is built with a balanced FET driver. Because of the number of turns in the coil it is very sensitive to Earth's magnetic field. However it is also very sensitive to targets even very small ones though the sample is out at 40uS.
... Hey Aziz when I get more time we proceed OK ...

Cheers from moodz

[msysta]

Hmm.

Why not use spark gap? When current rises enough it would make an arc and null the voltage very fast

Theres at least spark gap tubes like this available.

http://commons.wikimedia.org/wiki/File:Spark-gap-tube.jpg

[gadgetmaker]

Variable Damping

Msysta,

This active damping pulse could be switched in right after the flyback pulse reaches maximum level and then (out or the damping resistance changed) at a predetermined voltage level or after a certain duration. I wonder if this would allow one to sample the signal sooner thus increasing sensitivity?

One thing that is for sure; there is a lot of fun experiments with PI technology to try!

Best to you from Arizona.

Karl

[msysta]

To my knowledge it is the decay of the coil voltage that makes the eddy currents to the target and faster the decay, stronger the eddy current. So in that sense the coil flyback energy should just be drag down as fast as possible.

Maybe a mosfet could be used there, phasing it with the clocking device through schmitt-trigger operated switch. It would act like a drain, short circuiting the coil for a derivative short moment. Or maybe a decent capacitor could do the job.

[gadgetmaker]

More on Diff circuit

SB,

If you read Moodz work he is canceling noise out and his current circuit drives only half the coil. My idea is to drive both sides, not at the same time but one at a time and then use a +/- rail on the diff amp; the problem then becomes how to handle two opposite polarity signals? You could offset the output of this with a precision dc level before gating and input to the A/D or you could use an A/D that would handle the +/- input. Now you have a completely diff circuit. I will build a prototype up when I can and send some scope traces.

I have done a lot of PIC programming in C but have never used the DSP part from Microchip; I think it is time to learn about these parts!

One of the most intersting thing to me is to do active damping and do more analysis of the data using the DSP. It always seemed to me that damping the signal with a resistor limited the response of the system. Maybe there is some descrimating information here????

Well I may be talking to myself? I will actually do some work and post some data and see I can help.

Best to all,

Karl in AZ

[simonbaker]

Moodz,

Why not try to drive both coils, one then the other by simply adding another fet and then use the two signals to cancel any dc offset during A to D conversion?

Karl

I wonder if that's similar to crazy idea i had, using dummy coil just to cancel out TX signal... but I know next to nothing about PI circuits.

http://www.geotech1.com/forums/showthread.php?t=15465

-SB