Principle of touch screens

Hello Kaustubh!

I don't know how many types of Touch screens are there in the market today and I only know some. Most are resistive and capacitive. In the
resistive type, there are also several types. They are analog resistive type, matrix resistive type and the newest technology touch
screen^×Near Field Imaging (NFI).

Resistive touch screens are made of two active switch layers. The top switch layer is a flexible material, usually polyester. One side is
coated with an abrasion and chemical resistant anti-glare or high-gloss hardcoat, and the other side is sputtered with lndium Tin Oxide (if
you're familiar with this compound).

The bottom switch layer can be made from either ITO-coated polyester, the same material as the top layer, or ITO-coated glass. Ground
shields and polarizing filters can be applied behind this layer. The top and bottom switch layers, when laminated together, make up the
touch screen. Resistive, as the name implies, will create a resistance when pressed together (because of the touch). This resistance will
then be transfered to its electronics enough to be converted into pulses for its microprocessor to understand. I will not elaborate this

There are also calle 8-wire or 5-wire touch screens. The 5-wire touch screen has all four bus bars on one layer. The opposite layer is used
only as a sense, or pick-off layer. As one can image, depositing all four bus bars on the same layer without some compensation would make
the touch screen virtually unusable since all four sides would be at the same potential. Instead, the bus bars are patterned or deposited
using a resistive material. Whatever the design, the bus bars are deposited in a manner as to artificially linearized the voltage potential
across the touch screen surface.

How reliable? Very reliable in normal condition unless the layers will get stuck due to wear and tear. But some touchscreens continues to
function, even with surface is damaged. Some can function even if you wear gloves. I hope you get some useful info here. Regards.
In one type of the DCS system (older version but still in use in many refineries & plants) the touchscreen effect is achieved by employing
combination of infra-red LEDs and photodiodes. The row of LEDs and photodiodes are put on the opposite side of the screen a few millimeters
above the screen surface. When a user "touch" the screen, the finger will block one of those invisible light rays (actually one horizontal
and one vertical ray....or maybe a few adjacent rays are blocked). This location of "shadow" (in x & y coordinate) is interpreted as the
location of the touch.
The act of pulling one's finger out at any location is then interpreted as selecting the area.
This technology of touchscreen allows the user to use it without really needing to touch the screen (if he doesn't want to), hover one's
finger (or any opaque object like pencil) just above the screen at the desired location and pull out to select.
The reliability... very good. At our institute, the unit that we had for more than 7 years has never had any problem with the touchscreen.

Wan Zul
Institut Teknologi Petroleum PETRONAS