Capacitive Touch Screens - How exactly do they work?

[Bad_Astronaut]

I think there is a lot of misleading information about capacitive touchscreens online. For instance, the Wikipedia “Touchscreen” article states:

A capacitive touchscreen panel consists of an insulator such as glass, coated with a transparent conductor such as indium tin oxide (ITO).[19][20] As the human body is also an electrical conductor, touching the surface of the screen results in a distortion of the screen’s electrostatic field, measurable as a change in capacitance. Different technologies may be used to determine the location of the touch. The location is then sent to the controller for processing.

The article implies that the ITO is on the outside of the screen, but it seems to me that it must be on the inside, forming one plate of the capacitor. Your finger forms the other plate, with the glass screen as the dielectric. Also, it’s obvious that your finger is not electrically connected to anything on the screen, since it works with an insulator between them (eg. a plastic cover).

Also how is the capacitance actually sensed - as a change in voltage?

My third question relates to “touch screen gloves.” Most descriptions I’ve seen suggest that the problem with normal gloves is that they are insulators, but again this is not the problem (a plastic sheet is an excellent insulator). I think the problem is simply that they keep the conductor (your finger) too far away from the screen - capacitance is inversely proportional to distance. The dielectric constant of the material determines the critical distance. Making the glove’s fingertip conductive extends the conductor right up to the surface of the screen.

Anyone have the straight dope on capacitive touch screens?

[voltaire]

I don’t, but HowStuffWorks does - including a nice diagram.

[Bad_Astronaut]

@voltaire

I don’t, but HowStuffWorks does - including a nice diagram.

Ya I saw that, it’s pretty hand-wavey though. It doesn’t address the specific things I’m wondering about.

[Kevbo]

@Bad_Astronaut

Also, it’s obvious that your finger is not electrically connected to anything on the screen, since it works with an insulator between them (eg. a plastic cover).

This is correct. Your finger is(hopefully) connected to your body. Your body serves as a small “earth”, the AC signal on the screen has to charge and discharge the capacitance of your body, and this is a load on the electronics driving the screen.

The e field coupling is not proportional to distance, but to the square, so it falls off very fast with distance. The protective sheets need to be thin, but they also have fairly high dielectric constant, so are electrically thinner than a similar layer of air of fleece glove would be.

[beowulff]

Knock yourself out:
http://www.freescale.com/files/sensors/doc/white_paper/PROXIMITYWP.pdf

[Bad_Astronaut]

@beowulff

Knock yourself out:
http://www.freescale.com/files/sensors/doc/white_paper/PROXIMITYWP.pdf

Thanks, this is interesting. According to this, an AC signal is constantly being generated behind the screen:

When an object is brought close to a metal electrode, for instance a finger from our highly dielectric and conductive
human subject, an electric path is formed, producing a change in the e-field current. Normally, the sensor measures
the AC impedance of the generated e-field and translates that measurement into a DC output voltage.

I wonder if modern phones and tablets work the same way - the pdf discusses single on/off buttons rather than position-sensing screens.

@Kevbo

The e field coupling is not proportional to distance, but to the square, so it falls off very fast with distance.

The e-field itself is proportional to distance squared, but the capacitance is proportional to linear distance. I believe the sensing circuit sees the capacitance as it’s load.