IB Rx coil at Tx off

[Tinkerer]

Is your time scale +-10 seconds? Is hand and space blanket stationary during the 20 second sweep? If yes, what are you measuring with the coil AC coupled?

The time scale is 2 second division.

The coil is stationary, the hand and targets sweep across the coil at a measured distance controlled by a spacer.

Although this setup is definitely a PI, it is not conventional at all. I would call it a very advanced technology PI, with many innovative features.

The TX includes 8 mosfets and the timing is very tricky. It took me years to find somebody who could write the firmware.

The coil is also very unconventional. There is really not more else at the frontend, I mentioned a single opamp stage with a gain of 10.
All the rest is done in digital processing. We use a STM32F446 for the processing.

[green]

Testing Graphite shielding.
The Pdf shows the beginning with no target.
Scroll down the pages.
Then about 10 sweeps with a sheet space blanket, of about 220mmx300mm at a distance of about 70mm.
Then about 10 sweeps with a piece of space blanket, of 15x15mm at a distance of 5mm
Then about 10 sweeps with the flat hand at about 70mm
The coil is about 200mm diameter overall, TX maybe 190mm diameter
RX is about 150mm diameter.
Shielded wit 2 coats of graphite paint, fairly thick. Homebrew graphite paint with Chinese graphite powder. The shielding drain wire has a 10 Ohm resistor and a ferrite bead (1206 size) in series at the connection to the board.
Pulse repetition rate is 25,000, bi-polar, that is one positive pulse every 20us and one negative pulse every 20us.
TX voltage is 5V
Signal processing is AC coupling from RX coil, LP filter, Opamp with gain of 10, Antialiasing filter, input to 12 bit ADC.
The scope probes are connected at input to ADC.
The PICO scope is set at 12bits channel A minus Channel B, and LP filter.
I feel that the shielding is acceptable, but still try to improve.

Comments please. What could be improved and how.

Is your time scale +-10 seconds? Is hand and space blanket stationary during the 20 second sweep? If yes, what are you measuring with the coil AC coupled?

[waltr]

Good idea. Polystyrene really holds a charge well. When cutting it with a saw, the sawdust clings to everything.

During winter, the air gets very dry here, up in the mountains. Static electricity becomes a real problem. When sparks of up to 1/2 inch length fly out of your fingers, it is better not to touch any static sensitive electronic parts. In the lab, I use a humidifier that evaporates about one gallon of water per day to keep the relative humidity above 40% to mitigate the problem.

Now, how could we control and measure a static charge, so that we could test a coil with it?

I think a Theremin is a musical instrument that works with capacitance. Maybe something of that kind?

There are Static meters but they are a bit expensive. I have used them at places I work to test/measure work stations for ESD compliance.
Probably could build something that gives good relative measurements.
simple metal plate to a JFET and op-amp meter drive would do it.

[Tinkerer]

Graphite shielding test

Testing Graphite shielding.
The Pdf shows the beginning with no target.
Scroll down the pages.
Then about 10 sweeps with a sheet space blanket, of about 220mmx300mm at a distance of about 70mm.
Then about 10 sweeps with a piece of space blanket, of 15x15mm at a distance of 5mm
Then about 10 sweeps with the flat hand at about 70mm
The coil is about 200mm diameter overall, TX maybe 190mm diameter
RX is about 150mm diameter.
Shielded wit 2 coats of graphite paint, fairly thick. Homebrew graphite paint with Chinese graphite powder. The shielding drain wire has a 10 Ohm resistor and a ferrite bead (1206 size) in series at the connection to the board.
Pulse repetition rate is 25,000, bi-polar, that is one positive pulse every 20us and one negative pulse every 20us.
TX voltage is 5V
Signal processing is AC coupling from RX coil, LP filter, Opamp with gain of 10, Antialiasing filter, input to 12 bit ADC.
The scope probes are connected at input to ADC.
The PICO scope is set at 12bits channel A minus Channel B, and LP filter.
I feel that the shielding is acceptable, but still try to improve.

Comments please. What could be improved and how.

[Tinkerer]

How about a sheet of polystyrene plastic or a sheet of white foam.
Both hold a static charge.

Good idea. Polystyrene really holds a charge well. When cutting it with a saw, the sawdust clings to everything.

During winter, the air gets very dry here, up in the mountains. Static electricity becomes a real problem. When sparks of up to 1/2 inch length fly out of your fingers, it is better not to touch any static sensitive electronic parts. In the lab, I use a humidifier that evaporates about one gallon of water per day to keep the relative humidity above 40% to mitigate the problem.

Now, how could we control and measure a static charge, so that we could test a coil with it?

I think a Theremin is a musical instrument that works with capacitance. Maybe something of that kind?

[waltr]

for the capacitive effect we need to find a non-conductive material that accepts a static charge. Does anybody have suggestions?

How about a sheet of polystyrene plastic or a sheet of white foam.
Both hold a static charge.

[Tinkerer]

Prepared some SB test material.

1 sheet of cardboard with about 220x300 SB glued on it. will try tomorrow when glue has dried.
1 15mmx15mm SB on a long wooden stick.

for the capacitive effect we need to find a non-conductive material that accepts a static charge. Does anybody have suggestions?

Years ago I used the hand test to judge the shield effectiveness. without shield the hand would be detected at some distance and the hand would also introduce high frequency noise from a distance.
The shielding would reduce the sensitivity a little and would remove most of the high frequency noise.

In the meantime the sensitivity achieved has increased a lot, so that the hand can easily be detected at a greater distance, even with a shield. Adding a strong shield reduces the sensitivity significantly.

At the end we will have to compromise between sensitivity and noise, as always.

[green]

We must not forget, that the hand (and body) is also made of conductive material. Thus it is also a target.

How could we differentiate between the target response and the capacitive effect?

On your figure 8 coils, do you see a target if you put a quarter on the same position on each coil? How about putting the hand so it is over both coils? Left hand on one coil, right hand on other coil might make a loop and falsify the result.

Signal is direct coupled thru integrator out. Signal is positive over one coil and negative over the other. Hands or targets over both coils can cancel. Last stage is capacitor coupled, then absolute valued so a target over either coil is detected.

Tried another test. Cut a piece of space blanket 22.5cmx30cm and taped it to a piece of 2mm craft foam 22.5cmx30cm to replace the hand for the test. Got a negative going signal during coil decay(picture 2)when I placed the SB over one of the coils, same as placing my hand. As I was bringing my hand close to the SB edge not over the coil the signal went more negative. Think I might be increasing capacitance. Where I was touching the SB wasn't over the coil and wouldn't have caused a signal change if the SB wasn't there. Thought the SB might replace the hand for the shield test until I added the shield and tried it. SB causes a visible scope signal during coil decay where my hand doesn't. SB is a short time constant target, hand might be but if it is it's a lot shorter.

[Tinkerer]

Been reading some on shielding. https://www.geotech1.com/forums/show...061#post149061 (Then come up with a test method and we all try different shield materials?)from reply #38
(If not intended for water, just check any capacitive effect, laying hand over coil etc must not produce response or change in waveform.)from reply #3

A test using the hand to test for shield effectiveness. Coil(Rx 8in figure8, oval Tx)
With no shield, big effect when coil decays. Longer delay before sampling reduces change, wasn't expecting a change with 17us delay.
Is there a better way to test for shielding than using the hand?
Want to try some other shielding than graphite.
Any comments appreciated.

US quarter at 8inches causes 50mV change at integrator out with 5.3us delay.

We must not forget, that the hand (and body) is also made of conductive material. Thus it is also a target.

How could we differentiate between the target response and the capacitive effect?

On your figure 8 coils, do you see a target if you put a quarter on the same position on each coil? How about putting the hand so it is over both coils? Left hand on one coil, right hand on other coil might make a loop and falsify the result.

[green]

Been reading some on shielding. https://www.geotech1.com/forums/show...061#post149061 (Then come up with a test method and we all try different shield materials?)from reply #38
(If not intended for water, just check any capacitive effect, laying hand over coil etc must not produce response or change in waveform.)from reply #3

A test using the hand to test for shield effectiveness. Coil(Rx 8in figure8, oval Tx)
With no shield, big effect when coil decays. Longer delay before sampling reduces change, wasn't expecting a change with 17us delay.
Is there a better way to test for shielding than using the hand?
Want to try some other shielding than graphite.
Any comments appreciated.

US quarter at 8inches causes 50mV change at integrator out with 5.3us delay.