suggestions for high-resolution tiltmeter (inclinometer) sensor?

[JBeale]

I tried building the circuit using the 4"/2mm vials from aliexpress.  I used the regular adhesive-backed copper tape again; I think it's probably good enough. I made the two top electrodes a little narrower than the bubble.  This time I used 1.8432 MHz 14-pin DIP can squarewave +5V oscillator from the junkbox plus a 74HC buffer to drive the circuit, and made the two 0.01 uF caps 0.002 uF instead, all the rest was the same.  My crude screw-on-end-of-lever arm calibration indicates the full output range of -220 mV to +240 mV covers just 31 arc-seconds. In quiet moments I see about 80 uV of noise which would correspond to about 5 milli-arc-seconds.  Not sure if my calibration correct but if so, that's plenty sensitive for my needs. This vial seems to take over 10 seconds to respond after a step change, much slower than the smaller and less sensitive, 1.5 inch long antique vial I used to start with.

EDIT 11-June:  A quick test indicates the vial is roughly twice as sensitive- on average- as claimed. Added output graph. My above noise estimate almost certainly optimistic. After a 7-hour run overnight, I see the output has sudden jumps as large as 6 mV, or 0.4 arc-second per my rough calibration.  The bubble will track infinitesimal level changes only if the interior is absolutely smooth. My guess is that the supplier did not waste time making the interior surface of this $12 vial any smoother than needed for a simple visual readout, so my circuit is showing the surface roughness as steps in the response.  Also possible these were cheap because they were rejects.  It's a little disappointing, but not entirely unexpected.

EDIT 2: The vial was not cleanly or securely mounted, so I cannot rule out the step changes overnight as related to thermal expansion + roughness of my own mounting surface.  I will try to make a better mount later.

[branadic]

Nice build. However, more confidence would give a V-block as available at Norelem.



-branadic-

[RoGeorge]

I see the output has sudden jumps as large as 6 mV, or 0.4 arc-second per my rough calibration.

How much movement would it be (in red external gradations) during the logging night?
Could the jumps be because of the red reading marks painted on the outside of the vial?

[JBeale]

V-block from Norelem: V-block

The V-block looks like a nice idea (EDIT: although with the cheap vials with non-constant OD, better to support only at the ends). Note my photo shows only a temporary support while I was soldering. My electrical measurements were made in a heavy-gauge steel rectangle tube, 0.12" wall thickness. It is rigid and provides electrostatic shielding all around.  Last night the vial was however simply resting flat on the unfinished interior metal surface of the tube. Differential thermal expansion could be causing it to be supported by different small bumps, tilting it slightly (?)

The steps in the output are of all different sizes and appear randomly distributed, so I don't think they are related to the exterior red marks.

It is possible to grind an interior profile into your own vial, starting with straight glass tubing, as described below. It is a similar idea as grinding your own telescope mirror. This way, you would have some control over the final surface finish.
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text: http://fer3.com/arc/m2.aspx/Making-your-own-spirit-level-vialswithout-attached-pictures-Morrow-jan-2012-g17728
photos: http://fer3.com/arc/m2.aspx/Making-your-own-spirit-level-vialsanother-attempt-send-Morrow-jan-2012-g17731
---

[rhb]

I just finished my differential screw.  For convenience, I used a 1/4"-28 bolt which I drilled through and tapped #6-32 as that was what seemed the best  local hardware store and simple tooling option  That's only 224 tpi, so not as good as using a 27-28 tpi combination, but I'll use a long handle wrench made for the purpose.  The long lever arm will make up for the coarser turn ratio.

I through drilled the bolt by using a drill press as a lathe and then tapped.  So a continuation of the doing it the hardway.  I shall *not* make the second one this way.  I'll use my mill and lathe, but I wanted to demonstrate you don't need a machine shop.  Tapping the bolt was pretty nerve wracking as I broke a #6-32 tap a few days ago and did not want to break another.

First photo is the drilling setup.  The second is the screw and the other parts.  These will now await the arrival of SS foil for the annealing process.

I don't think that a continuous V block is a good idea as the outsides of the vials are not all that precise.  I think two point support preferable.

[branadic]

The V-block looks like a nice idea. Note my photo shows only a temporary support while I was soldering. My electrical measurements were made in a heavy-gauge steel rectangle tube, 0.12" wall thickness. It is rigid and provides electrostatic shielding all around. 

While I'm still waiting for the vials I already ordered the V-blocks in different lengths and began to construct the electrical and reliable connections between the electrodes and the PCap readout circuit.

For comparison, would you like to give us some dimensions of your electrodes on the first and second vial? Hopefully post office hurries up soon, can't wait to reproduce the results.

I don't think that a continuous V block is a good idea as the outsides of the vials are not all that precise.  I think two point support preferable.

But that is exactly the way how it's done on machine levels.

-branadic-

[rhb]

I don't think that a continuous V block is a good idea as the outsides of the vials are not all that precise.  I think two point support preferable.

But that is exactly the way how it's done on machine levels.

*Not* on mine.  Take a look at the photos I posted previously of my Chinese level.

The V block is certainly fine if the vials are made of accurately round constant OD tubing.  But my vials are out of round by 0.002-0.004".  I've not attempted to check diameter along the length.

I think how you do it is as much dependent upon what you charge as anything.  At $800+ one would not be happy with the Chinese construction.  But at $50 it's pretty clever.  I rather doubt that levels with V block supports use $12 Chinese vials.

[branadic]

But at $50 it's pretty clever.  I rather doubt that levels with V block supports use $12 Chinese vials.

Maybe that is the big difference between your and my construction. I won't use Chineese vials. Thus, a V-block with grinded faces is the more precise way that I can go with. In the groove I will integrate a spring made of a piece of nickel sheet, that connects to the common electrode. I will then use two seperate bridges made of plastic or Macor with another nickel spring inside, that connects to the hot electrodes. This is what I currently design in CAD.

-branadic-

[rhb]

Maybe that is the big difference between your and my construction.

In part difference in application.  I'm interested in a cheap 2 axis tiltmeter with a $50 BoM that would provide  remote data collection via LoRa to an Internet connected relay node. I think that's probably doable using the Chinese vials set in thin set on location.  The cost of alternatives is a significant impediment in a lot of geophysical applications.

Such a system would make real time monitoring of subsidence due to ground water pumping practical.  At present the best option for such problems is airborne synthetic aperture radar or Lidar, both of which limit how often you can afford to collect data. There are a lot of locales where this is a huge issue.

[metrologist]

I was looking at a diff screw using 1/4-20 and M8x1.25 or I think 6-32 and M5x0.8 would be too much.

All roughing done except the ends of the vial carrier. I will machine standard clevis rod ends and use vertical diff screw and the pivot might just be a vertical screw with nut underneath for coarse adjust if not pointed pivots coming in from each side (the the Chinese level).

I really wish I had a surface grinder. I "leveled" this with shims and then measured my lathe bed had 1 thou of twist. I'm surprised I got it that close using a high quality carpenter level.

[StillTrying]

After a 7-hour run overnight, I see the output has sudden jumps as large as 6 mV, or 0.4 arc-second per my rough calibration.  The bubble will track infinitesimal level changes only if the interior is absolutely smooth. My guess is that the supplier did not waste time making the interior surface of this $12 vial any smoother than needed for a simple visual readout, so my circuit is showing the surface roughness as steps in the response.

Your very first plots seemed to show a stop in the drift just before the jumps, perhaps they all need a very small amount of vibration now and and again to keep the bubble moving.
A small buzzer or speaker in the box that does a quick buzz every minute or so!

[rhb]

The pointed pivots in the Chinese level are, I think, quite clever.  Even if they are not exactly coaxial, if they are tightened with the level approximately true using the pivot and the differential screw, then the angular adjustment range is so small that it doesn't matter.  There is a 15x price difference between a cheap Chinese level and a Starrett or other top tier make.

When you look at the V bed construction example that was posted you can see why they are so pricey.  Vials ground inside and out and a *very* close fit pivot.  The pivot holes are jig borer work on that design.  If the vial arm is 5" long on a 0.0001" in 10"  level the pivot has to fit to less than half that.

The 1/4-28 & 6-32 differential screw I made this morning was pretty simple to do.  Getting the drill bit centered under the bolt is the hardest part. With a lathe it's  quite trivial.  It's a little less than 1/3 the sensitivity of the 27-28 combination Conrad suggested.  BTW The 27 tpi thread is the US recommended standard for thin wall tubing.  The 5/8" -27 thread is the standard for microphone stands.

Yesterday I saw a steel version of this:

https://www.ebay.com/itm/Mic-Stand-Adapter-5-8-27-Male-Threaded-to-Hot-Shoe-for-Microphone-Mount-/272625442310

But now I can't find it.  There are also brass versions.  So one of the knurled nuts could be brazed in place and the center drilled and tapped 28 tpi.  On the other hand 27 tpi taps and dies are under $10 each in smaller sizes.

[metrologist]

Yes, but the imperial/metric combos result in 1270 and 4064 TPI. At a 5" pivot, that would yield 8 or about 2.5 divisions per turn. I may start with an M8x1.25 screw and single-point thread the end for 1/4-20, or I cheap out and just drill the M8 screw and braze in a 1/4-20 that is turned down and pressed in.

Your screw does not look concentric. How do you plan to attach it to your carrier tube?

[rhb]

I'm hoping it's "concentric enough".  If not I'll use my lathe and do it properly.  However, I could also use a captive nut if it's close.  I think it's more a problem of not being coaxial, rather than not being concentric.  So bending the 6-32 screw a little may solve it.

I was a little disappointed in the result.  I've done better.

[JBeale]

For comparison, would you like to give us some dimensions of your electrodes on the first and second vial? Hopefully post office hurries up soon, can't wait to reproduce the results.

I adjusted them relative to the size of the bubble, with the goal to make the top electrode just a little bit less wide than the bubble. You can see in the photo the bubble is still visible around the edges of the top electrodes.

 For the second, longer vial (the 4"/2mm one), I think the top 2 electrodes were 5 or 6 mm wide, and 37 mm long, and there is a gap of maybe 3 mm between the two top electrodes. The bottom electrode runs nearly the length of the capsule, except leaving the very ends free so the end mounts can contact the glass directly, and it wraps around a little bit more than 1/2 the circumference of the cylinder.  At first I tried just 1/2 circumference and then decided it didn't look right, but I don't think the exact dimensions of the bottom electrode affects the result much.  If you wanted to make the bottom electrode more narrow, so a continuous V-groove mount could contact the glass directly all the way across, I think that would still work nearly the same.

[rhb]

I concluded that what mattered was the amount of bubble between the electrodes, which led me to bubble width top and  bottom  electrodes .  But that is merely an analysis leading to a conjecture.  Either of which may well be wrong.

[JBeale]

Looks to me like my 4"/2mm vial has a "sticky" bubble, where both the leading and trailing edge of the bubble gets separately stuck in spots, and then suddenly moves to catch up. I suspect the true surface tilt of my garage slab is a smoothly varying function of time, mostly driven by ambient temperature. When measuring the overall bubble position overnight into the morning for 12 hours, I am seeing a jerky and sometimes perhaps even non-monotonic output at the sub-arc-second level.  The sudden steps are as large as 10 mV or 2/3 arcsec.  Each step occurs over a period of 10 to 15 seconds, which seems to be roughly the response time of this vial. (EDIT: by 2/3 arcsec I mean the fraction, in other words 0.667 arcsec)

 I think $12 may simply be too cheap to expect better performance.  You could add a buzzer or vibrator motor to "tap the vial" but I'm not convinced you then necessarily get to the desired minimum-energy point, perhaps just another random metastable sticky spot.  I notice this
https://www.leveldevelopments.com/products/vials/ground-vials/5726101-ground-vial-15x95mm-sensitivity-0-02mmm/ and they claim to do their own manufacturing in the UK (not the Far East). Apparently 15 mm diam x 96 mm long is a standard size. They are offering vials with sensitivity of 4"/div at $30 and 10"/div at $25, so I'm going to give them a try.

[metrologist]

The sudden steps are as large as 10 mV or 2/3 arcsec.

That would be visual. I imagine there could be some units better than others. Mine seems to oscillate as it settles. For example on the desk, if I move to the other side, the bubble will move a couple+ divisions and then retract slowly to rest. Maybe I'm not seeing it in RT, so I should record it.

[JBeale]

I do not observe the stickyness visually on my vial either. I think it happens only with extremely slow level changes (for example, exactly the ground motion I am trying to measure). The largest step was 2/3 asec which for visual reference is still only 1/6 of one division, and when I say "sudden" it is over a period of about 10 seconds.  I easily get more motion than that just from shifting my weight on the concrete floor of my garage, so I don't think it would stand out visually.  Maybe I should not say the level vial is bad, it's just that the capacitive readout is very good. Based only on electronic noise, it resolves bubble position to 10 microns, which is around 5 milli-arcsec using a vial of this sensitivity. That is 100x smaller angle change than this 2/3 asec step. Likely could do even better with a slower lowpass filter than the few-second time constant I have now.

[branadic]

Have you verified it's not your readout circuit? You could install a differential capacitor instead of the electrodes to make sure it's stable and you are not hunting the white rabbit.

-branadic-

[JBeale]

Or I could just tilt the level up 90 degrees. I think that would move the bubble into the end and away from both top electrodes, so I would see a stable output. I'll try that later today.

[branadic]

This is another way to go, you're right. I wonder how temperature influences your circuit and thus output voltage, beside temperature influence of the bubble itself.
My vials are still resting in Frankfurt, since four days now. 
Hopefully the vials I ordered from UK will arrive earlier. I almost finished CAD design and the required parts today.

-branadic-

[metrologist]

I do not observe the stickyness visually on my vial either.

I mistook your comment as 2 to 3 arcsec... You did not see this with the other vial, and it seems within expectation from my pov. I'm just going to use mine visually, unless I come across some obscure reason to need more sensitivity, such as building a "repeat-o-meter" to verify my surface plate, but there may be better way do to that a-la the methods described earlier.

[JBeale]

I ran the circuit overnight with the vial turned 90 degrees (pointing straight up), so the electrodes see just the fluid with no bubble. There was no hint of the discrete steps that I saw with the vial resting level, the noise looks random. The typical peak-to-peak noise level over a period of minutes was about 30 uV, and the total DC drift overnight was 0.8 mV. I believe the mV and larger size steps must be from the bubble motion. 

[metrologist]

Seems logical, you'd have surface tension and such. Maybe the other vial uses a different fluid - I think it's typically alcohol?