Touch Screen TECHNOLOGIES page 2

Fig: 2 Operation of SAW touchscreen technology

1. Capacitive: A capacitive touchscreen panel is one which consists of an insulator such as glass, coated with a transparent conductor such as Indium Tin Oxide (ITO). As the human body is also a 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. These are mentioned below.

a) Surface capacitance:

In this basic technology, only one side of the insulator is coated with a conductive layer. A small voltage is applied to the layer, resulting in a uniform electrostatic field. When a conductor, such as a human finger, touches the uncoated surface, a capacitor is dynamically formed. The sensor's controller can determine the location of the touch indirectly from the change in the capacitance as measured from the four corners of the panel. As it has no moving parts, it is moderately durable but has limited resolution, is prone to false signals from parasitic capacitive coupling and needs calibration during manufacture. It is therefore most often used in simple applications such as industrial controls and kiosks.

b) Projected capacitance:

Projected Capacitive Touch (PCT) technology is a capacitive technology which permits more accurate and flexible operation, by etching the conductive layer. An X-Y grid is formed either by etching a single layer to form a grid pattern of electrode or by etching two separate, perpendicular layers of conductive material with parallel lines or tracks to form the grid (comparable to the pixel grid found in many LCD displays).

The greater resolution of PCT allows operation without direct contact, such that the conducting layers can be coated with further protective insulating layers, and operates even under screen protectors, or behind weather and vandal-proof glass. Due to the top layer of a PCT being glass, PCT is a more robust solution versus resistive touch technology. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger. There are two types of PCT: Self Capacitance and Mutual Capacitance.

c) Mutual Capacitance:

In mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi touch operation where multiple fingers, palms or stylus can be accurately tracked at the same time.

d) Self Capacitance:

Self capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.