Touch Screen TECHNOLOGIES Page 1

TECHNOLOGIES

There are a variety of touchscreen technologies. They are:-

1. Resistive.

2. Surface Acoustic Wave (SAW).

3. Capacitive.

4. Infrared.

5. Optical imaging.

6. Dispersive Signal Technology (DST).

7. Acoustic Pulse Recognition (APR).

Capacitive technology can further be implemented following through techniques-

a) Surface capacitance.

b) Projected capacitance.

c) Mutual capacitance.

d) Self capacitance.

Different technologies may be used determine the location of touch by different means. Then the location is sent to the controller for processing that particular application.

Now we discuss the working principles of each techniques stated above.

1. Resistive: Resistive touchscreen are composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. When contact is made to the surface of the touchscreen, the two sheets are pressed together. On these two sheets there are horizontal and vertical lines that when pushed together, register the precise location of the touch. Because the touchscreen senses input from contact with nearly any object (finger, stylus/pen, palm) resistive touchscreen are a type of "passive" technology.

Fig: 1 Operation of Resistive touchscreen technology

For example, during operation of a four-wire touchscreen, a uniform, unidirectional voltage gradient is applied to the first sheet. When the two sheets are pressed together, the second sheet measures the voltage as distance along the first sheet, providing the X coordinate.

When this contact coordinate has been acquired, the uniform voltage gradient is applied to the second sheet to ascertain the Y coordinate. These operations occur within a few milliseconds, registering the exact touch location as contact is made.

Resistive touchscreen typically have high resolution (4096 x 4096 DPI or higher), providing accurate touch control. Because the touchscreen responds to pressure on its surface, contact can be made with a finger or any other pointing device.

2. Surface Acoustic Wave (SAW): Surface Acoustic Wave technology uses ultrasonic waves that pass over the touchscreen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touch screen panels can be damaged by outside elements. Contaminants on the surface can also interfere with the functionality of the touchscreen.