Your Smart Phone Applies Geometry to Interpret Touch

Touch sensitive screens first appeared on industrial control panels over 20 years ago, but they are now common on personal computers and smart phones.

There are two problems to be solved when designing the system: sensing when the screen is touched and working out where it was touched.

The most common technology is the resistive one, which uses the simple idea of a switch. The outer surface of the screen is covered with three thin layers of different materials.

The first one is metallic, the second a fine gauze made of an insulator and the third a flexible conducting plastic. Finally, the top is coated with a scratch-proof film to prevent damage.

All these layers are so thin that they are almost transparent in ordinary light. That way, it is possible to see the graphics on the screen.

Terminals are connected at one corner of the screen and a small voltage applied between the conducting plastic layer and the metallic film.

Since the two layers are separated by the insulating gauze, no current will flow, but when the screen is touched, the flexible plastic bends and makes contact with the metallic layer below through the open spaces in the gauze.

When the two layers make contact, current flows -- just like in a switch -- and the computer senses that the screen has been touched.

To determine the point that was touched, voltage is measured at the other three corners of the screen.

Since the plastic layer is not a good conductor, the voltage will be proportional to the distance from the respective corner to the point of contact.

It is possible to work out the coordinates of the point of contact by applying triangulation geometry.

Imagine a drawing compass is mounted at the first corner of the screen and then opened to draw an arch whose radius is equal to the distance calculated from the voltage.

Repeat the same drawing from the second and third corners. The three arches will meet at one point on the screen -- the point of contact. This is how the system evaluates the coordinates.

Another common type of touch screen technology applies the principle of capacitance. An electrical capacitor is simply two layers of conductors separated by an insulator.

Thus, the basic design is also in the form of layers. However, the middle layer is made of a continuous insulating material instead of the gauze.

When voltage is applied to the two conducting layers, a uniform electric field is formed between them. Touching the screen distorts this field, thereby changing the voltage.

The change in voltage depends on the location of the distortion. Therefore, by measuring from three corners of the screen, it is possible to triangulate the coordinates of the point of touch.

The major advantage of the capacitance touch screen is that it has no moving parts, making it more durable than the resistive type.

For this reason, it is the preferred design for industrial control panels, touch-sensitive ATM screens and laptop touchpads.

However, while the resistive type responds when touched by any material, the capacitive design can only be operated by bare fingers. Even wearing gloves will inhibit its operation.