A touch screen is a monitor with touch sensors, which enables it to be used as an input device. The user is thus able to control the computer with his finger. The technology is nowadays widely used in tablets or smartphones, but also in automats. In these cases resistive or capacitive touch sensors are most commonly used. But what is a capacitive or resistive sensor? And which one should be used?
Technology
There are two kinds of touch sensors, resistive and capacitive. Resistive touchscreens react to pressure, we find this kind of displays in most of train stations, while capacitive screens react to differences in the electrostatic field, Apple uses this technology for its iPhone.
The capacitive touch sensor (left) reacts to the electromagnetic field of a finger. The resistive touch sensor (middle) reacts to its pressure. The two electrically conductive layers connect at that point (right).
Beyond those we can find infrared sensors, which are resistant to vandalism as they can be used behind an armored glass. They are, however, more sensitive than other types and can be easily triggered by near contact. This makes them less than ideal for usage in lifts.
Touch sensors with surface acoustic wave are possible as well. In this case ultrasound waves are sent from the x and y coordinates, when touching the differences in the waves' motions will be registered.
Comparison
Displays with resistive sensors reflect more sunlight and, therefore, are difficult to see outside. However, they are more accurate and enable the user to work at pixel level (for example, with a pen).
Capacitive displays react to the slightest contact with the finger, but not to contact with an object or fingernails. This makes them less accurate than resistive touchscreens, as it is difficult to reach pixel accuracy with the finger. Besides, they are more expensive than resistive sensors in general. They can be used behind a sturdy glass, however, which makes them ideal for displays used in public places, as in lifts.
Interface
Earlier, touch sensors were connected to the device via a serial connection. The term serial comes from the fact that bits are sequentially sent by the connection. The connection via I²C was common as well. An I²C, IIC, or Inter Integrated Circuit, is a bidirectional connection between two integrated circuits. Nowadays, serial connection and IIC have been largely replaced by USB.
USB, or Universal Serial Bus, is the most widely used interface between computers and peripheral devices used today. It is easy to handle and swift. USB 2.0 achieves a speed of up to 480 MB/s
.
Installation
We advise to install capacitive displays in places where vandalism is an issue, as they can be installed behind a thick, armored glass. Resistive displays are not suitable for places with these issues as the sensors are in the first layer and can be easily damaged.
In places where optimal electromagnetic compatibility is needed, for example in the MRI unit of a hospital, a resistive display would be the right choice.
Single- or Multi-Touch
Some capacitive touch sensors are able to register several touches at the same time, this is called multi-touch technology. It enables the user to zoom in or turn a picture using more than one finger. This technology is already used in most of smartphones and tablets, however, it is not necessary in easy-to-operate devices. For example, in a lift where a single-touch display would be enough.