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Suppose one has a large metal pipe made of mild steel approximately 6-feet tall and 8-inches in diameter. Suppose this pipe has an regular pattern of perforations on its surface. Imagine the pipe is standing upright on the ground, mounted on a platform.

(Spoiler alert: I have such a pipe!)

I'd like to create a sensor or sensor network of some kind to sense when a human touches or nearly touches the pipe.

My initial inquiry went towards capacitive touch sensors and I've tried breakout boards using the MPR121 and AT42QT1010 chips, and have also tried a simple circuit like the one depicted here using the Arduino Capacitive Sensor library.

Nothing has worked.

I've connected the boards via wire directly to the pipe, and I've tried using electrodes (aluminum - about 6" square and another 2" square) insulated in a ziploc bag mounted to the inside surface of the pipe. What I've learned (I think) is that the massive amount of metal itself is such a large capacitance source (if that's the right terminology) that the differential when a human touches it is too minimal. The relative capacitance increase is too little.

Questions:

  • Is that right?
  • Are there techniques to improve this approach?
  • If this approach is futile, what are some other approaches I could use?

Bonus: If there's a way to get any level of resolution of where a person is touching the pipe, that's actually my perfect application, but I'm starting to think that's a pipe dream. (...sorry.)

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  • \$\begingroup\$ Perhaps you could turn it into a giant "Theremin". \$\endgroup\$ – Trevor_G Oct 31 '17 at 21:22
  • \$\begingroup\$ Yes, significant self-capacitance makes sensing difficult. However, an AC Maxwell bridge circuit, excited by an AC source might be possible. Environmental variations (temperature, humidity etc) would be confounding factors - do you have control of these? \$\endgroup\$ – glen_geek Oct 31 '17 at 21:25
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    \$\begingroup\$ First thing I imagined is: (1) Hook up an electric fence charger to the pipe; (2) use a "loud scream" detector circuit; (3) done. ;) \$\endgroup\$ – jonk Oct 31 '17 at 21:41
  • \$\begingroup\$ If you can expect an ambient 50 Hz or 60 Hz from the power line nearby, I'm almost wondering about a circuit that phase locks to the ambient 50/60 Hz; another circuit that self-adjusts its tuning, using the pipe as part of the circuit, until the self-oscillation of the pipe and circuit match the phase-locked frequency; and then sits there waiting by supplying both the phase locked frequency and the tuned frequency to a cheap MC1496 mixer. Anyone coming nearby will imbalance it. Just a crazy thought. \$\endgroup\$ – jonk Oct 31 '17 at 21:57
  • \$\begingroup\$ @Trevor, I had that same thought but I have no experience with oscillators, etc. \$\endgroup\$ – Bill Nov 1 '17 at 0:42
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What capacitive sensors do is usually just measuring the detuning (i.e. the change in resonant frequency) that happens when a finger / human becomes the opposite electrode of a metal surface, which is normally only opposing earth "far away".

With earth being far away from the metal patch and the finger being relatively close and large, and thus "bringing ground closer to the metal patch", these changes in resonant frequency are usually easily detectable.

Now, if we wanted to use your tube as metal patch, that would certainly still work, but the fact that its resonant frequency might be very low (compared to what standard capsense controllers do) will probably be a show stopper here; also, even a whole human will not change the effective distance to the second electrode of that pipe-ground capacitor much.

I'd go and try to use a simple Doppler radar module, mounted atop, instead. Approaching humans move; sense that.

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  • \$\begingroup\$ I will look into a doppler module. That sounds promising! \$\endgroup\$ – Bill Nov 1 '17 at 0:44
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Yes, You are right, the self capacity of pipe the size You described is extremely large relative to the capacity change caused by human finger approach, there is no way You could detect this kind of change using self capacity technology. Usually the capacitive touch button size is about the human finger diameter.

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  • \$\begingroup\$ and how would a conducting foil in close proximity to the conducting tube be any better? \$\endgroup\$ – Marcus Müller Oct 31 '17 at 21:29
  • \$\begingroup\$ For sure You would have to make small layer of dielectric material between the foil and the pipe. But You will have multiple small sensors instead of a extreme big one [the pipe]. \$\endgroup\$ – KJA Oct 31 '17 at 21:32
  • \$\begingroup\$ no, you wouldn't, because foil-dielectric-tube-dielectric-many other foils would make excellent capacitive coupling between these sensors. (it's called capacitive sensing for a reason). \$\endgroup\$ – Marcus Müller Oct 31 '17 at 21:33
  • \$\begingroup\$ I think this is not correct, If You create an array of small sensors placed on the pipe (with a proper isolation), You could treat the pipe as a reference (ground) and measure the capacity change of a single sensor relative to the pipe. An example could be the Tesla door handle (the sensor) and the car body [GND] ( the pipe). You scan each sensor one by one. \$\endgroup\$ – KJA Oct 31 '17 at 21:38
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    \$\begingroup\$ the door handle is a few sensors, on mostly non-conductive parts; your "really well isolated copper foil" on a metallic surface only becomes a better capacitor. Think about this: for the capacitive sensing to work well, the finger has to significantly change the amount of energy in the electric field around the electrode (static case) or be subject to measurably much induced current (AC case). Now, in your scenario, nearly all of the field energy will be between copper foil and tube – especially because you put them close together, with a high \$\epsilon\$ dielectric in between. \$\endgroup\$ – Marcus Müller Oct 31 '17 at 21:42

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