Big foot style for a PI

[Chet]

[QUOTE
When I first saw the picture spread apart I was thinking each one was IB with the Tx, but they don't need to be if one is inverted. To cancel ground both Rx coils need to see the same signal. I have some clay that I can detect, I'll try it to see how easy it might be to keep the same signal on both Rx coils. Need to make Tx for big foot.[/QUOTE]

Hi Green

I think both Rx coils need to see the same small amount of transmit signal. To do that most effectively I think they need to be individually zero balanced with the transmitter coil. Otherwise one will have considerably more transmit/flyback signal which will unbalance small target signal response between receiver coils. In other words one coil will be less sensitive to small targets than the other.

Thank you again for these tests,
Chet

[green]

Thanks for the replies. I had 1 racetrack already built, so I'll have to try some of the suggestions next time. Starting with the side by side and double racetrack. The double racetrack would have to have a lot higher signal strength since it doesn't cancel noise and EF. If I remember right I don't think it would be easy to discriminate ferrous either. Just a comparison test for signal strength. Thinking it might be easier to keep the side by side coil parallel to the ground vs the big foot for GB. Don't know if a long big foot swinging in an arc, different speed would cause a problem with EF reject. Signal strength for the 110mm round and the big foot were the same, so the big foot has to be quite long to get depth. Maybe good not bad. Picture of the coils included. The Tx is a little smaller than I would have liked. Think the Rx close together will be better than spread apart. When I first saw the picture spread apart I was thinking each one was IB with the Tx, but they don't need to be if one is inverted. To cancel ground both Rx coils need to see the same signal. I have some clay that I can detect, I'll try it to see how easy it might be to keep the same signal on both Rx coils. Need to make Tx for big foot.

[dfbowers]

Hi, Green.
Thanks for the interesting post. If you get the opportunity, can you check how your coils respond to clay bricks?
Most of the ground were I live causes a reaction in PI detectors much like a red clay brick. I'm thinking it would cause havoc with the IB feature of you coil. In all fairness, it would have to be a several bricks.
Don

[6666]

“Since each Rx is IB with the Tx should the Rx coils be in or out of phase?”

If you are going for PI IB, its best to have the coil signal balanced or slightly over balanced
because if the coils are under balanced then the target needs to push the coils past the
balance point and then the target signal starts to show, so you loose a little bit being under balanced
do you get what I mean ?
The phase needs to be so you get a positive signal.

[Chet]

Quote; “Since each Rx is IB with the Tx should the Rx coils be in or out of phase?”


Hi Green


Out of phase is my suggestion; out of phase provides ground balance and EMI cancelation.


It might be beneficial to have the receiver coils spaced above or below the transmit coil approximately 10mm. This should reduce capacitive coupling and loading on the transmit coil allowing the transmit flyback pulse to collapse faster. This spacing might also reduce the ringing effect in the receive coils.


I suggest leaving long leads on the receive coils to allow fine balancing of the null then glue the leads in place to maintain the null.


I think bunch winding of the receiver coils would be ok since there should not be a collapsing field within them.


Also the receiver coils could have two or three times more turns to achieve some signal gain.


Have another great day,
Chet

[green]

Building another coil. Have form for a racetrack coil,110mm x 170mm outside, that I tested before. Thinking could overlap two coils like a DD. end to end Rx and a Tx (big foot), side by side Rx and a Tx, separate side by side Rx with side by side Tx to IB like reply #15. Since each Rx is IB with the Tx should the Rx coils be in or out of phase? I've found a good way to learn is to predict what will happen before doing it. Any idea which one would be better and if any will be a complete flop? I'll post my predictions before testing.

[green]

Hi Green

Good results on small nuggets. In your previous scope shots; maybe a damping resistor across each of the round coils would stop the ringing?

Have another great day,
Chet

Thanks, had the same thought last night. I glued the common connection inside, so I'll have to wait for the next coil to try it, I'm going to try the damping resistors for the Rx and the MUR460 diode and damping resistor for the Tx inside the coil.

[Chet]

Hi Green

Good results on small nuggets. In your previous scope shots; maybe a damping resistor across each of the round coils would stop the ringing?

Have another great day,
Chet

[green]

A chart for gold nuggets using the 110mm round big foot coil. Dan posted awhile back comparing a target in the air and with the coil laying on the ground. He got a lower signal with the coil on the ground. I have a 11 x 10 x .5 inch zip lock bag with clay from the yard that I used to compare. The signal was slightly higher with the coil on the bag. I'm not sure if it means anything other than comparing the signal strength with a US nickel. For charting the coil is laying on a concrete block for stability. Actually I was comparing the coil on the block vs the clay. Sampling between 6 and 10 usec does effect the signal with the different time constants.

[Chet]

Hi Green

Great work! It looks like it may be possible to use the "on-time" measurements for discrimination on strong target signals. Its also interesting how the rusty piece of can is similar to a ferrite core response.

Thank you,
Chet

[green]

Hi Green

In the Post #14 chart; do the ferrite and nail charts change polarity during 'on time' when you move the target from one coil to the other coil?

Thank you again for sharing these tests,
Chet

Hi Chet
The answer is yes, The best picture isn't always in the center of the round, 50mm offset. More scope pictures.

[Chet]

Hi Green

In the Post #14 chart; do the ferrite and nail charts change polarity during 'on time' when you move the target from one coil to the other coil?

Thank you again for sharing these tests,
Chet

[green]

Hi Green

Very good results. Looks like it works better than the Racetrack coil alone. The ferrite and other target responses during on time is especially interesting. It would be interesting to see the effect of lead and aluminum targets during the on time.

Could you spread the two round coils apart until they achieve individual inductive balance above each end of the Racetrack coil. This would be a rough simulation of a DODD coil as found in a mikebg post below. The fourth coil L3 is not needed. The individual inductive balances might reduce the spikes and ringing.

Thank you for the fine work,
Chet

http://www.geotech1.com/forums/showt...0200#post70200

Including a chart comparing the three coils at there best offset. The big foot and round are close. The racetrack looses strength at shorter distances but gains at longer distances due to the larger radius.
Can't spread these coils apart because I glued them together to keep IB.
I'll scope some other targets today.

[Chet]

Hi Green

Very good results. Looks like it works better than the Racetrack coil alone. The ferrite and other target responses during on time is especially interesting. It would be interesting to see the effect of lead and aluminum targets during the on time.

Could you spread the two round coils apart until they achieve individual inductive balance above each end of the Racetrack coil. This would be a rough simulation of a DODD coil as found in a mikebg post below. The fourth coil L3 is not needed. The individual inductive balances might reduce the spikes and ringing.

Thank you for the fine work,
Chet

http://www.geotech1.com/forums/showthread.php?14097-DD-Coil-Analysis-(Sensitivity-Profile)&p=70200#post70200

[green]

Hi Green

Thank you for another great test. Since the receiver coils will be inductively balanced you might be able to increase the number of turns to increase received signal strength. And earlier sampling may be possible.

Have another great day,
Chet

Hi Chet, Finished the big foot with two 110mm round coils for Rx.and a racetrack coil around them for Tx. I changed the coil driver to current control(constant rate, 12500 amps/sec), 80 usec on time, 1amp peak current. Gives a flat response during on time for ferrite and ground. Planned on taking a sample at the beginning and end of on time to discriminate ferrous, non ferrous. Got some oscillation at the start of on time.Don't know why yet, maybe doesn't mater. Looks like ferrous signal change is opposite polarity at end of on time and first sample of off time. Lost some signal strength over mono coil, but the meter readings were more stable. With amplifier gain of 300 the 100u volt digit changed 1 count where it changed a few counts, varied with time with the mono coils. The Rx coils are 210 uh each giving 420 uh in series. Almost not able to sample at 6 to 10 usec. Think I need to use a difference amplifier and grounded center tap for the Rx. The US nickel didn't show during on time because it decays before the oscillation ends. A reply in another thread stated the big foot coil works good for pin pointing. Looking at the chart I would agree if you can see both polarities. The null in the center is quite small. Looks good on the bench, hope it's as good on the ground. The coil is IB, but I still get a spike at turn on and turn off. Scope traces are amplifier out. Forgot to turn scope frequency display off, doesn't mean anything.