PI shielding

[Tinkerer]

Tried charting the poster board graphite shield. A piece 5x10in and 10x10in. Both charted the same TC. Don't think they should, need to find why.

When close to the coil, a target with a diameter wider than the coil diameter, picks up the positive and the negative field vectors, so they null each other to some extent.

[Tinkerer]

For VLF, they use much more, like 2k Ohm region.
For PI, in general they want much less, like 1-2 Ohms. The idea is less is better.

I think we need to dig deeper into the capacitance - inductance conundrum. There are several ways to look at it.

The shield adds capacitance to the coils. The TX pulse induces eddy currents in the shield. How does it all interact together? This is what we need to spend some time on to figure out.

Induced current decays very slowly with low resistance.
Capacitive charge decays very fast with low resistance.

If we use a strategy of low resistance path to discharge the (capacitive) static charge this should work fine. OR? Static charge can be going either way. If the discharge path is good, it might also be a good charge path.

Could somebody give some elucidation on this?

[Tinkerer]

Not clear what you are suggesting. A piece of poster board 10x19 inches painted with graphite paint measured .7us TC(reply #28 and #29). Reply #29 includes target and no target recordings. I have recorded target decays with and without the shield, might change the no target recording but doesn't effect target minus no target chart calculated with Excel.

Couple reasons might not be do to conductivity. I have the unipolar Tx hooked up now. Can see a small change at amplifier out when I bring my hand near the coil but change is the same at either end of the figure8 Rx coil, polarity changes with conductive targets. Can't see change at amplifier out with a piece of #9 lead shot(TC=.7us) but do see a change at integrator out and polarity does change at opposite ends of figure8 coil. Don't see the same with my hand. What I see with my circuit, maybe different with a different circuit?

Should we maybe design a specific test procedure for that?

For example: Target always on a long (maybe 2, 3 foot) inert stick.
Hand as a target, with body (other hand) grounded to analog ground. Hmmm, analog ground? Where is the TX grounded? Maybe better to use TX ground when different.

[Altair]

Not clear what you are suggesting. A piece of poster board 10x19 inches painted with graphite paint measured .7us TC(reply #28 and #29). Reply #29 includes target and no target recordings. I have recorded target decays with and without the shield, might change the no target recording but doesn't effect target minus no target chart calculated with Excel.

Couple reasons might not be do to conductivity. I have the unipolar Tx hooked up now. Can see a small change at amplifier out when I bring my hand near the coil but change is the same at either end of the figure8 Rx coil, polarity changes with conductive targets. Can't see change at amplifier out with a piece of #9 lead shot(TC=.7us) but do see a change at integrator out and polarity does change at opposite ends of figure8 coil. Don't see the same with my hand. What I see with my circuit, maybe different with a different circuit?

Ok sorry, I was misunderstanding what you have written on previous posts. A piece of poster board 10x19 inches painted with graphite paint I think it should act like a target with a little TC, but I don't understand why your setup sense the hand if there is a shield in between. Could the shield not be completly efficent, because otherway, if hand act like a target, I don't understand why you don't see a polarity change at opposite ends of figure8 coil.

[Altair]

When close to the coil, a target with a diameter wider than the coil diameter, picks up the positive and the negative field vectors, so they null each other to some extent.

Yeah, very good explanation!

[Carl-NC]

Tried charting the poster board graphite shield. A piece 5x10in and 10x10in. Both charted the same TC. Don't think they should, need to find why.

Do they have the same graphite thickness? How big is the coil used for this test? With large coils and small targets TC is dominated by edge effects. That's why a 1" square of aluminum foil has a low TC and a large sheet of foil (same thickness) has a high TC. At some point you reach a large enough size that edge effects are minimal and TC won't vary.

[green]

Do they have the same graphite thickness? How big is the coil used for this test? With large coils and small targets TC is dominated by edge effects. That's why a 1" square of aluminum foil has a low TC and a large sheet of foil (same thickness) has a high TC. At some point you reach a large enough size that edge effects are minimal and TC won't vary.

The pieces were cut from a larger piece so thickness should be similar. The coil is two 133mm ID round coils side by side connected inverted(figure eight), Tx is oval over Rx. Charted a lot of tests today trying to understand why. I was playing with Tx damping the other day and had left damping overdamped. Tx damping seems to effect decays. I have more questions than answers now. Including two tests made today.

_8 compares decay for a ferrite core, space blanket and graphite shield. I suggested TC because decay was straight line linear-log, probably should just call it decay. Ferrite more positive than no target in the beginning, space blanket and graphite shield goes negative in the beginning. Think that makes sense, non ferrous X and R signal opposite phase?

_9 compares decay for a 1x1 inch, 4x4 inch and 8x8 inch regular strength aluminum foil square.

Maybe my circuit wont decay much faster than a .7us TC? When I charted 10 pieces of #9 lead shot they charted near what they calculate. Don't know why ferrite, space blanket and graphite shield chart a decay about the same as #9 shot.

[Carl-NC]

Maybe my circuit wont decay much faster than a .7us TC?

What is the opamp you are using, and what is the gain?

[green]

What is the opamp you are using, and what is the gain?

Including schematic and test. Didn't label IC numbers, used what I had(maybe not best choice). Can if anyone is interested.

The 33p capacitors limit the frequency to about 1MHz. Haven't tried increasing frequency since my coils test around 1MHz.

Test input. TLC555 oscillating at 9.4V battery volts divided by 200000, 20000 and 2000.

Think amplifier might not be limiting decay rate. Any guess what might be or is there something else I'm missing?

The figure8 coil isn't perfectly balanced so I use ferrite cores to trim the balance, maybe the problem?

Should have used a higher 555 frequency. Another test with higher frequency. Maybe should try increasing amplifier frequency?

See I used 2k resistors on the input in the schematic. The amplifier has 1k resistors, have been thinking of trying 2k.

[Carl-NC]

What opamp are you using?
It's possible that a ferrite core might have a slight BH gap. Can you remove the ferrite and tweak the balance by micropositioning the coil?
You could also try dropping the 33p in half to see what happens. Have you ever seen anything under 0.7us?

[green]

What opamp are you using?
It's possible that a ferrite core might have a slight BH gap. Can you remove the ferrite and tweak the balance by micropositioning the coil?
You could also try dropping the 33p in half to see what happens. Have you ever seen anything under 0.7us?

The amplifier I'm using is potted so I'm not sure. Either OPA1612, OPA1662, LT6237or LT6234. Probably a OPA1612, I have one not potted yet with a OPA1612 that I'll try with 33p and 10p caps.

The coil is potted with foam, maybe I can squeeze the coil to deform it a little.

Tried charting the graphite shield, space blanket again and 2pieces of regular aluminum foil 1.5x.125in. TC less than .7us this time. Couple things I did different. Tx coil critical damped and X signal was balanced near zero when Tx on with ferrite and a piece of aluminum foil was used to straighten decay curve when it came out of saturation. Don't know why yet and I think the 1/8in strips of aluminum foil should have a lower TC. Don't know what TC the space blanket should be? I'll post the reason TC is lower today when I figure it out. Thanks for your replies.

[green]

The amplifier I'm using is potted so I'm not sure. Either OPA1612, OPA1662, LT6237or LT6234. Probably a OPA1612, I have one not potted yet with a OPA1612 that I'll try with 33p and 10p caps.

The coil is potted with foam, maybe I can squeeze the coil to deform it a little.

Tried charting the graphite shield, space blanket again and 2pieces of regular aluminum foil 1.5x.125in. TC less than .7us this time. Couple things I did different. Tx coil critical damped and X signal was balanced near zero when Tx on with ferrite and a piece of aluminum foil was used to straighten decay curve when it came out of saturation. Don't know why yet and I think the 1/8in strips of aluminum foil should have a lower TC. Don't know what TC the space blanket should be? I'll post the reason TC is lower today when I figure it out. Thanks for your replies.

Repeated test with (2)pieces of aluminum foil, Tx critical damped and overdamped with and without added piece of aluminum foil to straighten decay curve after came out of saturation. Appears damping is causing some the problem. Need to try some more tests with different damping.

[green]

Repeated test with (2)pieces of aluminum foil, Tx critical damped and overdamped with and without added piece of aluminum foil to straighten decay curve after came out of saturation. Appears damping is causing some the problem. Need to try some more tests with different damping.

More tests different damping. Test reply #57, underdamped(580R) critical damped(1180R). Measured resonance with Tx Rd disconnected this morning(1.05MHz)calculated critical Rd(1080R)so test above was slightly underdamped not critical damped. Did a test with 4x5in space blanket, seems to have a fast TC and gives a good signal, three different Tx Rd values.

Couple things I'm thinking of trying. Tx on time 40us instead of 80us. Trying 10p caps instead of 33p caps to see if a faster amplifier would decay faster. Any other suggestions, just trying to learn something.

[green]

Tried the 10p vs 33p caps. I had two amplifiers not potted , one with OPA1612 other with OPA1662. 1662 had the transistors mounted so I used it(wanted to try 1662 since it costs less). Not sure what transistors were used in the potted amplifier and can't read the numbers on the one I just tested. I tried 3906, 3904 duals and some other number duals when I was making the amplifiers a few years back. Amplifier with 10p caps faster than with 33p caps. Amplifier with 1662 faster than potted amplifier(1612?).Don't think it should be, maybe not 1612 or different transistors. Gain is similar to potted amplifier. Need to test some targets other than space blanket to see if faster speed is really better.

[green]

Should we maybe design a specific test procedure for that?

For example: Target always on a long (maybe 2, 3 foot) inert stick.
Hand as a target, with body (other hand) grounded to analog ground. Hmmm, analog ground? Where is the TX grounded? Maybe better to use TX ground when different.

Apologize for the side track. This hand detect has got me stumped. Was playing with it yesterday. Seemed like every time I was starting to figure it out, NOT. Have been using led indicator and differentiator out on the scope. Needs a change since they are AC coupled. Looked at integrator out(DC coupled)yesterday. Hand held steady over coil seems to cause an offset that doesn't drift amplitude.
Some observations: Some times placing hand over coil B signal was opposite polarity hand over coil A(normal target). Sometimes signal polarity was the same. Sometimes no signal over B or A. Sometimes over A not B. Sometimes holding the other hand on scope ground made a difference, sometimes not.

Required signal change at integrator out to light the led depends on noise level, typically 1 to 2mV.

Were you able to detect charged PVC pipe?

Including a scope picture. Some signal change with hand over coil, think because not holding stead enough.

Hope someone has some suggestons.