Author Topic: Field mill weirdness  (Read 500 times)

Offline TMaxElectronics

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Field mill weirdness
« on: April 08, 2021, 01:42:21 AM »
Quite a while back I saw a project on some old german website (last update 2005 ::)) that talked about atmospheric electricity and how to measure it using a homemade field mill.

And since I am quite a bit of a datahoarder I decided to update his project to 2021 and make it LoraWAN compatible so a whole bunch of these can be placed in a grid pattern to accurately determine for example the position of storm cells and potentially give a much more accurate warning about impending lightning strikes when throwing other atmospheric conditions in the mix (at least in theory and using a whole lot of other data from the web). Just as a proof of concept I built a little version of the mill using an esp32:


It works quite well and is sensitive enough to notice a 15V change of a plate 10cm away, but the signal is quite noise at the moment.
The weird thing is that the reading seems to significantly drift when a field is applied. I made a calibration setup that can give me ~12kV/m with almost no noise at all (shielded all the way around and powered by a super low noise PMT supply from hamamatsu), but the reading first goes to ~800 (raw reading... just so you have a rough estimate of the amount of drift. 0 is 0V/m) and then slowly drops off to ~600 after a few tens of seconds. The other thing is that there is an opposite polarity reading of ~-200 present when the field is removed, which is roughly the amount by which the output dropped before. That makes me suspect charges building up somewhere in the system that decrease the field, but I have no idea where :D

Has anybody got an idea why that could happen? I don't think it is the OpAmp as that outputs a steady reading with a fixed input current, and I have cleaned the board very well (keep in mind that the transimpedance amp has a 10MOhm feedback resistor so even flux residue messed with the reading).
The only non conductive surfaces in the vicinity are to the side (the 3d printed case) and the etched part on the top of the shielding wheel.

Offline johnf

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Re: Field mill weirdness
« Reply #1 on: April 08, 2021, 10:55:03 AM »
Your plastic is charging up and repelling the incoming field
you need an earthed metal box around the mill -this will help

putting it all through a lockin amp will remove the noise and it will be sensitive to mV

Offline TMaxElectronics

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Re: Field mill weirdness
« Reply #2 on: April 09, 2021, 12:56:04 AM »
I Had the previous case fully covered in copper tape connected to the reference electrode (not on ground potential) and that didn't really change anything. I'll try fully covering the rotor as well tomorrow but I don't thank that could cause issues as the charges on that would be shielded right?

That lock in amp concept is pretty cool, never heard of it before. Since I have the opto interrupter position output it would probably be easy enough to implement that in the esp i think. Question then is only with what signals to mix it, since the output is not really a pure sine wave.
« Last Edit: April 09, 2021, 01:00:33 AM by TMaxElectronics »

Offline T3sl4co1l

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Re: Field mill weirdness
« Reply #3 on: April 10, 2021, 03:21:37 PM »
A lock-in amp is also a correlator, or very narrow bandpass filter.  Simply correlate it with any signal that is linearly dependent to the waveform.

I suppose given the sectors are... sectors, it has a more triangular waveform?  So, another triangle would be easy enough to generate also, and would correlate very nicely with it.

The simplest form is just inverting the signal every half cycle; instead of a rampy waveform you get pairs of peaks, which average above zero.  So, filter out the ripple and there's your detection.

Well, simplest in terms of implementation -- just use some analog switches or whatever and there you go.  Mathematically a sine wave would be simpler (pure frequency mixing, hetrodyning), but often this isn't as easy to do (needs an analog multiplier).

You can also consider orthogonal pairs, namely, quadrature -- this shouldn't tell you anything about the field (Q should always be ~0, and any error should just mean timing is off), but is interesting in general: for example, measurement of impedance (real and reactive), or receiving a radio signal without a perfectly locked (coherent) reference clock.

Tim

Offline TMaxElectronics

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Re: Field mill weirdness
« Reply #4 on: April 11, 2021, 12:45:22 AM »
The output is actually kind of square-ish. Ideally it would be a perfect square wave, since the rate at which the area changes is constant. Due to field fringing at the edge of the rotor it is has softer edges though:


I am thinking of maybe capturing the spectrum of the signal and then deciding how many sine waves to mix in. I'll be doing that all digitally so I can really generate anything I want and the signal frequency is only 60Hz (3600 rpm) so the data rate is low. I'll probably just make a lut with like 0.1° resolution from the reverse fourier transform of the largest peaks in the spectrum I capture. I'm doing the signal rectification and averaging digitally already and it works kind of ok. The two transimpedance amplifiers go straight into a differential ADC.
Guess its time to learn the signals and systems stuff properly after all I guess :P

I guess the filter quality (or drop off rate) is dependent on the integration period right? The longer it gets the more the out of band noise will be averaged towards zero if I understood the theory correctly

Offline haversin

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Re: Field mill weirdness
« Reply #5 on: April 11, 2021, 02:52:23 AM »
Don't let the sensor be covered and uncovered at a frequency that a harmonic of the 60 Hz line frequency (or 50 Hz depending on where you live). The field mill will act like a synchronous rectifier and the 60 Hz AC electric field will show up as a slowly varying DC electric field on the output.

Offline TMaxElectronics

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Re: Field mill weirdness
« Reply #6 on: April 11, 2021, 01:22:44 PM »
So far I've had no issues with that. The inferference would be common mode and that gets filtered out by the circuitry before it even gets into the microcontroller. And if it is interference in the measurement direction I would actually want to see it :D

Offline TMaxElectronics

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Re: Field mill weirdness
« Reply #7 on: April 12, 2021, 01:37:39 AM »
ok it looks like I figured it out. The issue was charges going somewhere they shouldn't but not in the way one might expect. Covering the entire thing in copper tape did nothing at all. The issue was my cal setup.

I had the mill earthed and the cal reference floating, and was seeing the weird drift. I knew that I was measuring the field between the top cal electrode and the reference electrode but failed to take into account that the top electrode was effectively floaging with respect to the mill. That ment that all I was getting was garbage and drift from leakages to and from the calibration electrodes. After grounding the bottom electrode the reading got almost perfectly stable (observed drift was <1% /h). The reading is still a bit jumpy at +/- 2% but once I get it running the amp should take care of that. I also tested common mode interference and got no change in the signal as expected.

Now the only thing I am wondering is if I can somehow manage to cover the front without effecting the field. I kind of doubt it tbh but maybe some ceramic might do the trick. even though that will probably get static charges on it or get dirty and conductive again. Would be nice to be able to put these outside in the rain.

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Re: Field mill weirdness
« Reply #7 on: April 12, 2021, 01:37:39 AM »

 


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