Capacitive Feedback for Resin Height

[rylangrayston]

This is discussion that started here:
http://3dprintboard.com/showthread.p...Overhang/page3

Really deserves its own thread, so please after reading in the link above come back here to post about capacitve Feed back..

The Idea:
A foil plate laminated with an insulator grows in capacitance as more water touches it.
In this thread we are talking about what methods can be used to map capacitance to Resin/water height, as a way to replace or
at least calibrate the dripper.

A brief summary of the work that has been dose so far can be gained from these two posts:

Originally Posted by jsondag
I tested this with just a multimeter as a proof of concept. I just dangled one probe in the water, and touched the other to the foil. Seems to work, and level of salinity has no effect whatsoever. I went from a weak salt water, to more than double the salinity, with 0 change on the meter. My meter clearly doesn't have enough resolution for fine detail, I never expected it to. A RC circuit on an arduino should give very high resolution, as you're converting capacitance to charge time, and time is something micros are very good at measuring.

I used HVAC foil tape, in both a thin strip and wider one. I've found that a wider strip gave me greater movement in readings. I also found that the surface tension plays a role, as you move up and the walls get wetted, if you move down there is backlash, because you're measuring how far up the wall is wetted basically. No big deal for measuring only one way.

I'm attaching a graph of readings at half centimeter intervals. (nF on the left, cm on the bottom) and a pic of my test container.

My meter is not high quality or very precise, so the little variation from linearity is most certainly from that.

Originally Posted by rylangrayston
Hey Nice Work!

We thought this was a great Idea to and back in 2014 we did a bit of work on testing out the viability ... it certainly looks like a promising way to get feed back on vertical movement. And unlike many Other methods It could be very very Inexpensive. If you can get the readings you need to replace the dripper with an arduino, then I can certainly show you how to connect the arduino to the peachy software, to close the loop.

The its been a long time since we worked on this and its a bit foggy ...so instead of try to explain it in detail im just going to post a big file with pics and video:There is about a gig of pictures and video so here is a goodle drive link:

https://drive.google.com/folderview?...Fk&usp=sharing



basically here is what we did:

1. used a peristaltic pump with a very make shift encoder to move water from one tank to another

2. laminated copper or aluminum foil with a typical lamination sheets found at staples.

3. Placed the laminated strip in the salt water so that the water acted as one lead to the capacitor, the foil was other lead and the lamination was the dielectric.

4 Created an oscillator that changed frequency as the capacitor changed in capacitance. Scott was a big help, he both made the Relaxation Oscillator and brought much wisdom to interpreting the results.

5 Graphed the frequency of the oscillator using python mat Lib library.

7. Thought about what kind of resolutions could be achieved by mapping the frequency to actual height.



Lessons We Learned.

Altho promising there are many things that make this solution harder than it seems at first.

possible problems:

-50 or 60 Hz noise can ruin your measurements each time you cross its harmonic.

-The feed back is not liner .. it is an inverse function of frequency ... I think

- oscillators drift around in frequency due to many factors that you dont want in your measurement ie temperature

- large surface arias help to even out the random way that a meniscus wets the laminated plate.

- The capacitances achieved were very small (in the 1- 10 pF range? I think ) ... therefore to get low frequency required charging the cap thru annoyingly large resistances .. over 10 mega ohms if I remember right.

- meniscus has large hysteresis upon draining quicky..( this is not a problem at the speeds we print tho)


Our goal was to get 10 micron resolution out of this system, at best we got more in the ballpark of 50 micron "resolution" .. not bad considering we only worked on it for about 3 days, but still that 50 microns came with a host of questions about accuracy and precision.

At the time of this work we were still trying to use the micro phone input ... to get the desired resolution we would have had to go below 20 hertz ( a typical cut off for sound inputs). I think using a micro controller to measure the frequency could solve this part of the problem.

I did a rather pour job of documenting this.... the pictures and video are all I have... I dont have micro code or graphing code. although it wasent very hard to create.

In conclusion I think this could be a very valuable way to get vertical feed back, It needs more work So Im glad to see some one taking it on.
Hope this post helps you along the way.

PS, everyone give James Townly a standing ovation for playing merry little lamb first try the resistor array and the oscillator.

[jsondag]

Thanks for the info Rylan. I should have some time coming up to try some further experimentation. I'm getting 50-100pF. I think to be able to use reasonable levels of resistance I will need some wider foil tape. or perhaps to spray the entire jar with metallic paint.

Right now with 1Mohm, I think I'm going to get a 170-230uS charge time from 6-11cm (my test range), which may not give the best precision even with an arduino. I've used them for uS level timing before, when I made a flash trigger for high speed photos. I was able to get some shots of bullets mid air, etc. but even if I can measure down to the individual microsecond, that sill gives me maybe 60 steps for 5 cm of travel, which is ~0.8mm resolution.

I may try multiple strips of tape, not overlapped, electrically bridged, or just tape one strip, paint the whole jar with metallic paint, and then peel off the strip as a window.

I have a lot of projects going on right now, so it might be a few days.

[quertz]

Hey guys,

It seems to me that we have a few different ways of measuring the capacitance:

jsondags Idea which resembles this:
https://www.arduino.cc/en/Tutorial/CapacitanceMeter
and uses the time needed to charge the capacitor;

The Peachy-Team's idea which is either
using an oscillator with a known frequency and measuring the resonance amplitude or
creating an harmonic oscillator using the capacitor and determining its natural frequency.

Looking at the second solution, maybe you could recreate that oscillator and hook it up to the interrupt pin of the arduino...
According to this http://arduino.datamaster2003.com/f-measurements.htm it should be easily possible to measure kHz-frequency with it which would give us some nice accuracy and keep it away from those annoying 50 Hz.

quertz