PI shielding

[Tinkerer]

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.

Tried to detect static charged pipe. Could see no reaction on the scope, when sweeping over the coil. Noticed that when I picked up a piece of foam with the charged pipe, when sweeping over the coil, the rim of the coil captured the piece of foam and took it from the pipe.
Tried sweep with the left hand, 50mm above coil, right hand not grounded, see reaction on scope attached.

[Tinkerer]

Made sweep with 10x10mm kitchen foil at 50mm above coil, as a reference.

Foil on 70cm wooden stick.

[Altair]

Tried to detect static charged pipe. Could see no reaction on the scope, when sweeping over the coil. Noticed that when I picked up a piece of foam with the charged pipe, when sweeping over the coil, the rim of the coil captured the piece of foam and took it from the pipe.
Tried sweep with the left hand, 50mm above coil, right hand not grounded, see reaction on scope attached.

Are you testing with or without coil shield?

[Tinkerer]

Are you testing with or without coil shield?

Testing with shielded coil. Shield is graphite paint.

[green]

Hi Tinkerer,

A test similar to yours. Sweeping a 10x10mm square cut from regular strength aluminum foil taped to a wooden stick 50mm above coil. Sweeping my hand as close to coil without touching. Held foil on a 50mm spacer above center of coil and lifted quickly, easy test to repeat.

See you are getting maybe 300mV sweeping the foil. Good signal for a fast target. What size coil, total gain?

Testing was done across the center of one of the figure8 coils not across both.

[Tinkerer]

Hi Tinkerer,

A test similar to yours. Sweeping a 10x10mm square cut from regular strength aluminum foil taped to a wooden stick 50mm above coil. Sweeping my hand as close to coil without touching. Held foil on a 50mm spacer above center of coil and lifted quickly, easy test to repeat.

See you are getting maybe 300mV sweeping the foil. Good signal for a fast target. What size coil, total gain?

Testing was done across the center of one of the figure8 coils not across both.

I will try to repeat your test tomorrow. My coil is 8 Inches diameter. For this test running at about half power and one flyback every 50us. Will have to check the gain.
The signal shown on the scope are the positive and negative inputs to the 2 ADC's. On the scope I use signal A minus signal B and low pass filter.

[Tinkerer]

I will try to repeat your test tomorrow. My coil is 8 Inches diameter. For this test running at about half power and one flyback every 50us. Will have to check the gain.
The signal shown on the scope are the positive and negative inputs to the 2 ADC's. On the scope I use signal A minus signal B and low pass filter.

The gain on the ADA4807 opamp is about 104. dual opamp, one channel positive, other channel negative.

[Tinkerer]

Hi Tinkerer,

A test similar to yours. Sweeping a 10x10mm square cut from regular strength aluminum foil taped to a wooden stick 50mm above coil. Sweeping my hand as close to coil without touching. Held foil on a 50mm spacer above center of coil and lifted quickly, easy test to repeat.

See you are getting maybe 300mV sweeping the foil. Good signal for a fast target. What size coil, total gain?

Testing was done across the center of one of the figure8 coils not across both.

Repeated my hand test and foil tests. Same results with right hand Earthed with earthing bracelet on the wrist, connected to large earthing mat on my desk, which is earthed to mains driven power supply that has an earthing pin connected.

Another interesting observation is that the response of the foil is going positive and the response to the hand goes negative, as you can see by comparing the 2 scope pictures I posted. This was observed today in the exact same way with the right hand earthed.

[green]

Repeated my hand test and foil tests. Same results with right hand Earthed with earthing bracelet on the wrist, connected to large earthing mat on my desk, which is earthed to mains driven power supply that has an earthing pin connected.

Another interesting observation is that the response of the foil is going positive and the response to the hand goes negative, as you can see by comparing the 2 scope pictures I posted. This was observed today in the exact same way with the right hand earthed.

I use a figure8 coil so target reverses end to end. Most of the time coil A is positive with the hand(same as target), sometimes no effect and I think I remember it going negative a coupe times. Would be nice to have a more repeatable test for for shield effectiveness.

[green]

The gain on the ADA4807 opamp is about 104. dual opamp, one channel positive, other channel negative.

So you have a gain of 208 at amplifier out? What delay time did you use? I used an amplifier gain of about 450, 3.5us delay, 10us sample and 1000pps for an integrator gain of 1(average of signal over 10us). I've wondered if S/N would be better if sampling a fast decay target with an A-D vs using an integrator. The average over 10us is a lot less than taking a sample in the beginning.

[Mr.Jaick]

hi all
i'm wondering about PI coil shield resistance/conductivity
it seems like the shield needs to be low resistance / high conductivity material so the eddies generated in the shield will die quicker and not detectable.
is it true?
i'm asking because i know for a fact that the higher conductivity of a target the longer it takes for the eddy currents to die out
sooo... what's up with the coil shielding?

take this example:


adhesive copper tape
50um thickness
10mm width

will this work with proper spacing?
I want to use it between 10uS to 40uS sample delays, is it a good choice ?
can't buy scotch24

[kt315]

so the eddies generated in the shield will die quicker
---
absurd understanding for what the shield is doing.

[dbanner]

I was always under the impression that the true purpose of a shield was to eliminate capacitative effect of the ground.
Quite apart from being on mars, by the way ,the perseverance has shielding. The use a very high tc material.

[Mr.Jaick]

I know shield is there to get rid of earth capacitive coupling
but when i say eddies i mean coil pulses are gonna go through the shield anyway so the pulse will generate a tiny eddy in the shield itself, (doesn't it?)
so we want to get rid of this tiny eddy in the shield ASAP to be able to sample sooner weaker signals

[Monolith]

I know shield is there to get rid of earth capacitive coupling
but when i say eddies i mean coil pulses are gonna go through the shield anyway so the pulse will generate a tiny eddy in the shield itself, (doesn't it?)
so we want to get rid of this tiny eddy in the shield ASAP to be able to sample sooner weaker signals

To discharge induced currents fast, you want high resistance.
To discharge capacitor charge fast, you want low resistance.
To discharge inductive and capacitive charge fast at the same time, you want critical damping. Critical damping is the sweet spot where both charges are discharged at the same time.