Naked Science Forum

Non Life Sciences => Geek Speak => Topic started by: gurpal on 22/07/2009 15:59:36

Title: How do touch-screen computers work?
Post by: gurpal on 22/07/2009 15:59:36

how do touch screen computers work?
(how does touch screen work?)

Title: Re: How do touch-screen computers work?
Post by: Chemistry4me on 23/07/2009 06:05:40

With a touch sensor, a controller, and a software driver.

http://www.touchscreens.com/intro-anatomy.html

Title: Re: How do touch-screen computers work?
Post by: techmind on 25/07/2009 12:12:43

There are two main touchscreen technologies, known as resistive and capacitive.

Resisitive types (as used on some of the Palm handheld PDAs) have two transparent sheet-conductors in front of the display, the top one being slightly flexible and being separated from the one below by some tiny spacers. When you touch the screen you mechanically make the top layer bend to touch the lower layer, making an electrical connection. When the controller detects this connection it applies a uniform horizontal current to the top sheet and measures the voltage on the lower sheet (potential divider) then applies a uniform vertical current to the lower sheet and measures the voltage on the top sheet. In this way the position of the contact between the sheets is determined.
Resistive screens are cheap, but mechanically wear out, impair the optical clarity of the display, and can only measure a single point of touch at a time.

Capacitive touch screens (such as used on the iPhone) use an X-Y matrix of transparent electrodes over the front of the screen and apply AC voltages to one or more (eg vertical) electrodes in sequence and measure the capacitively-coupled signal in the (eg horizontal) electrodes. When a finger is placed near the grid, the finger absorbs some of the signal and reduces the coupling to the receiving-electrodes.
Depending on the complexity of your controller, you can potentially detect multiple simultaneous touches with this approach.
Because there's no mechanical parts, capacitive touch screens are very robust and don't wear out. They can also be designed to operate through toughened glass/polycarbonate so can even be made vandalproof too. They have less visual degradation on the screen clarity than resisitive, but do tend to be a bit more expensive because they're more complex.

Title: How do touch-screen computers work?
Post by: Turveyd on 27/08/2009 12:14:20

I thought the I-Phone has CCD's like in your camera underneath the screen,  but working on a thermal wave length so when you touch the glass they see the heat from your finger,  why you can't use stylus's.

Title: How do touch-screen computers work?
Post by: techmind on 28/08/2009 23:55:08

No, the iPhone definitely uses a capacitive touch-screen.

If it helps convince you, you could verify that you could operate the iPhone touchscreen with your finger, through a very thin (perhaps half a millimetre or less) opaque plastic sheet laid on top of the screen.


Resistive screens work well with a stylus as they work by pressure alone.

Capacitive screens typically have quite a coarse mesh of sense-electrodes, probably something like 10 by 16 on the iPhone, and use interpolation to get the fine precision. For the interpolation to work well, you need an object, such as a fingertip, which has size comparable to the grid-spacing. Capacitive technologies also require a conducting object (the blood etc inside your finger is very conductive even if the skin isn't!) and which is relatively "earthy". Capacitive screens don't consequently work very well with stylus (you might find you can get a poor and 'lumpy' response if you try to use a metal-bodied pen)...

Title: Re: How do touch-screen computers work?
Post by: syhprum on 30/07/2017 17:24:48

I find my 5 year old neighbours children with their tiny fingers have no trouble but my fat ones are a problem