Maximum Overhang?

[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.


capvswat.jpg2016-03-21 00.53.58.jpg

[quertz]

Wow, I really like this idea!

I think, even for those of us who already got their Peachy or will get a version without this idea (yes, I'm already assuming they will use it) this is a perfect DIY-addition to the Peachy!
They could easily provide a pin or two from the Peachy board and write a software version that uses the new sensor.
I like the idea that you can just make the tape on the outside bigger for more movement.

According to this:
http://www.nessengr.com/techdata/liq...dresistor.html
the saline will probably have a resistance around 10-100 Ohm/cm so if you use like 10kOhms for measuring the capacity you won't see a large change.

Greetings,
quertz

[jsondag]

the saline will probably have a resistance around 10-100 Ohm/cm so if you use like 10kOhms for measuring the capacity you won't see a large change.

Good point, I didn't think of the resistance in the "leads" running to the "capacitor". The salt water will act as a resistor in the lead, and will be part of the resistance. As you say though it will be negligible, especially if you place the electrode in the water close to the strip.

EDIT:
I'm assuming we are going to be using an amount of salt close to the limit of solubility, and in your link that comes out to about 4.8-10 ohms/cm for sodium chloride. If you put the contact right by the bottom edge of the strip, you're on the order of an ohm or less to the bottom of the strip. Resistance to the portions higher up will go increase, but again negligible. A wider strip also will help this as there is more cross sectional area of saline to travel through to reach the dielectric interface, lowering the resistance. I also think the current peachy will be able to handle this with maybe just a firmware update as you can either tie into an i2c for comms, or emulate a dripper. I think Rylan said there will be a way to do that at some point.

[amoose136]

Looks great. I'll have try this myself and see how few components it will take to make this measurement accurately. Does the 1.0 kit have any spare GPIO pins or are they all taken?

[jsondag]

Looks great. I'll have try this myself and see how few components it will take to make this measurement accurately. Does the 1.0 kit have any spare GPIO pins or are they all taken?

Would just take a charge resistor of high resistance, to charge the "capacitor", and a low value resistor to discharge.

The large resistance needs to be chosen to charge your capacitor slow enough to measure accurately, and fast enough to be responsive enough, and not be too affected by stray emf. This will depend on your build likely. I'd have to experiment to know a good time to shoot for.

200 ohms or something should be good for the discharge resistor.

if there are two free GPIO pins and enough free cycles on the built in micro on the peachy that's all you need. The firmware on the peachy could do that measurement and use it to correct for z-axis drift.

Or you can use a separate micro.
You can communicate with one free pin, or you could even read your dripper with the separate micro, and have it do some logic to insert or skip drips to correct for it's reading. That would require no changes to the peachy.

EDIT:
I lied, you need 3 pins. You'll need an analog pin straight onto the cap between the charge and discharge caps, to set a consistent point to trigger that the cap is charged. Digital pins aren't consistent in their trip point.

D0----~R1~--\
.....................|..............Jar
A0--------------|-----------| |------\
.....................|..........................|
D1----~R2~--/........................ GND





D0 goes high, which charges the Jar capacitor. A0 waits for it to hit a specific voltage, say 67% of supply. When it does, the micro counts how long that took, shuts off D0 (sets high impedence mode), then sets D1 low which discharges the cap. When A0 sees cap hit low (~5%?) D1 turns off (goes to high impedence).]

.... and loop

[rylangrayston]

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.


capvswat.jpg2016-03-21 00.53.58.jpg

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.

[quertz]

Wow, that's a lot of stuff.

I think we should just open a new Thread here or on the Peachy forum.

50 microns seems pretty good to me, at least to compensate for long-term trends (using the dripper for high-res height-delta, the capacitance for low-res absolute height).

quertz

[rylangrayston]

Ya good Idea quertz...
ok here is a new thread for talking about capacitive feedback of resin height:
http://3dprintboard.com/showthread.p...4282#post84282

In this thread, lets go back to talking about maximum over hang.
and remember everyone starting a new thread for a new topic is good for everyone.. it makes things much easier to find later!