How touchscreens work

09/01/1998


A touchscreen is a computer input device that enables users to make a selection by touching the screen, rather than typing on a keyboard or pointing with a mouse. Computers with touchscreens have a smaller footprint, can be mounted in smaller spaces, have fewer movable parts, and can be sealed. Touching a screen is more intuitive than using a keyboard or mouse, which translates into lower training costs.

3 components in common
All touchscreen systems have three components. To process a user's selection, a sensor unit and a controller sense the touch and its location, and a software device driver transmits the touch coordinates to the computer's operating system. Touchscreen sensors use one of five technologies: resistive, capacitive, infrared, acoustic wave, or near field imaging.

Resistive touchscreens typically include a flexible top sheet and a glass base separated by insulating dots. Each layer is coated with a transparent metal oxide on its inside surface. Voltage applied to the layers produces a gradient across each. Pressing the top sheet creates electric contact between resistive layers, essentially closing a switch in the circuit.

Capacitive touchscreens are also coated with a transparent metal oxide, but the coating is bonded to the surface of a single sheet of glass. Unlike resistive touchscreens, where any object can create a touch, capacitive touchscreens require contact with a bare finger or conductive stylus. The finger's capacitance, or ability to store an electric charge, draws some current from each corner of the touchscreen, where voltage has been applied.

Infrared touchscreens are based on light-beam interruption technology. Instead of placing a layer on the display surface, a frame surrounds it. The frame has light sources, or light-emitting diodes (LEDs), on one side, and light detectors, or photosensors, on the opposite side, creating an optical grid across the screen. When any object touches the screen, the invisible light beam is interrupted, causing a drop in the signal received by the photosensors.

Acoustic wave touchscreens use transducers mounted at the edge of a glass screen to emit ultrasonic sound waves along two sides. The ultrasonic waves are reflected across the screen and received by sensors. When a finger or other soft-tipped stylus touches the screen, the sound energy is absorbed, causing the wave signal to weaken. In surface acoustic wave (SAW) technology, waves travel across surface of the glass, while in guided acoustic wave (GAW) technology, waves also travel through the glass.

Near field imaging (NFI) touchscreens consist of two laminated glass sheets with a patterned coating of transparent metal oxide in between. An ac signal is applied to the patterned conductive coating, creating an electrostatic field on the surface of the screen. When a finger--gloved or ungloved--or other conductive stylus comes into contact with the sensor, the electrostatic field is disturbed.

Elizabeth Morse is communication coordinator at Dynapro (Vancouver, British Columbia, Canada), a hardware, software and touchscreen manufacturer.


Checklist for Touch Screen Selection
Besides price, consider the following before choosing a touchscreen technology:
1. How will the operator touch the screen? (Check one.)
__ Bare finger__ Gloved hand__ Other stylus

2. Describe the environment. (Circle all that apply.)
Moisture, dust, grease, chemicals, abrasives.
Temperature fluctuations, humidity fluctuations.
Vibration, shock.

3. Does the touchscreen require a NEMA seal?

4. Could it be vandalized?

5. Will it be used:

6. Is the environment protected?

7. Describe the stability required. (Check as needed.)
__ a. Periodic calibration to align the touchscreen to the display surface.
__ b. Drift-free alignment required.

8. Touchscreen attributes (Circle applicable number.)

a. Is low or high image clarity required? Low 1 2 3 4 5 High

b. How fine of resolution is required? Grainy 1 2 3 4 5 Fine

c. How fast does it respond to touches? Slow 1 2 3 4 5 Quick

d. How much force is needed to operate? Low force 1 2 3 4 5 High Force

e. How long is it expected to last? _________________________

f. What are the power requirements? ______________________





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