capacative sensing

[hrc]

posting for another engineer who is looking for techincal information on how capacative sensors work. Links to discussions, white papers or sites.

Thanks

 

[analogkid2digitalman]

Google?

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[itsmoked]

Capacitive sensors always work by running an oscillator. Typically the oscillator is RC based. The frequency, of course, changes as the capacitance shifts. The sensor then has some more circuitry that converts the frequency to some standard/expected output. [4-20mA, 0-5V, etc]

One of the toughest parts is getting the RC happy under a widely varying ambient and converting the generally non-linear RC into a linear result.

 

[Skogsgurra]

Well, almost always. Have used a PIC outpin driving a few capacitvie pads via 47 kohm resistors and sensing delay from Vcc to lower threshold with PIC inpins. "Keyboard" (PCB with pads) overlaid with insulating plastic sheet. Capacitance used to be in the 10 pF range when untouched and around 50 pF when touched. Worked well in dry and clean environment. Never used in wet and dirty places.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

I knew someone would send a Mark-48 into my "always".

Wow that's a cheap keyboard!! How'd it work? Would you do it again? Why would water and dirt "break it" (should be distinguishable)? What sheeting did you use?? How much of a pain in the butt was production? Did the huge freakin 50/60hz on the finger confuse everything??? Huh Huh? Tell me!

 

[Skogsgurra]

Sorry Smoked. It was used for a couple (three actually) test boxes that we used to do encoder checks with. So it never went further than coarse handcrafted units. We used ordinary overhead film for the protection. Hum (50/60 Hz) was not a problem since we had a metal box and kept the box in one hand while pressing with fingers on the other hand. So Human being, Box and Keypads were all on the same "hum potential", if there was any.

The water and dirt could esily seep in under the protective film. I guess that this could be taken a lot further. First thing would be to use some adhesive between cover and PCB to seal off humidity.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Hmmm.. I see... Okay. Thanks. The sealing sounds like a royal pain in a production situation.. Up to the point of requiring a keyboard overlay of a quality generally left to the keyboard people at which point, maybe one should just use the standard membrane keyboard. (yuck!)

Though maybe not, because the capacitive, (as apposed to the capacAtive method), allows no moving parts >-> failure. I have seen so many membrane keyboards take down industrial equipment!

 

[hrc]

In reponse to the first...google....been there...all we get is the mft sites of sensors.

What is needed is something that describes (white paper?) on contactless sensing using capacitive sensing. Itsmoked touched on it but need more detailed information if possible

 

[itsmoked]

You are not going to find any white papers... Discriptions sure, whitepapers no.

The best you will find are touch sensor IC maker's data sheets. (last link below)

http://www.edn.com/article/CA498777.html

http://www.imagineeringezine.com/e-zine/capacitance-7.html

http://www.flashwebhost.com/circuit/touchcpo.php

http://www.radiolocman.com/electrical-engineering/circuit-cache.html?di=18801

http://kitsrus.com/pdf/k137.pdf

http://www.qprox.com/products/qt1xxx.php

http://www.qprox.com/downloads/datasheets/qt110_104.pdf