Most modern computers or cell phones are now operated using touchpads or touchscreens. The touch-sensitive surfaces replace the computer mouse and the keyboard. Behind all of these simple operations for the user, is a sophisticated technology that combines hardware and software. Let’s take a look at how do the touchpad and touchscreen work?
All touchpads have a touch-sensitive surface and a controller that measures the signals and passes them to the operating system. The operating system converts our finger movement into the movement of a mouse and transfers them to the screen. Tapping the surface of the touchpad corresponds to a mouse click, pulling two fingers apart from the center, you can enlarge the image on the screen and with three or four fingers that wipe the surface, pages and images can be scrolled through. The hardware that generates the signals can be based on various physical principles.
The most common are resistive and capacitive touchpads.
The principle behind Resistive touchpads
Resistive touchpads require the pressure that is exerted on the surface by a finger or another object. The touch-sensitive surface of the touchpad consists of two conductive indium tin oxide (ITO) layers, which are separated by small spaces. The lower layer is applied to a firm and stable base, while the upper layer is covered with stretchable polyester from the outside. If you touch the polyester layer, the upper ITO layer is pressed onto the lower one. To determine the position of the pressure point, a DC voltage is alternately applied first on one layer and one millisecond later on the other conductive layer. Since the two layers are briefly connected to each other at the pressure point, a current flows here. The position of the pressure point can then be uniquely determined based on the voltage changes caused there, by the controller. It then forwards the coordinates to the operating system. With this principle, two layers are always necessary for the measurement, one is connected to the voltage, the other transmits the position in one direction.
Resistive touchpads are considered pioneers in touch technology but are generally not multi-touch capable. This means that you cannot operate them with multiple fingers. If you press the surface with two or more fingers, only the contact area of the two ITO layers is widened and the fingers cannot be detected individually. The main disadvantage of this technique, however, is that a coordinate is always acquired using the upper, flexible layer. The bending, pressing, and stretching leads to wear and tear in the ITO coating, which changes the electrical properties. Over time, this leads to the fact that the determination of the coordinate becomes less precise. However, resistive touchpads can be manufactured comparatively inexpensively and can be operated with any object. This is particularly important for doctors who often have to operate their medical equipment with rubber gloves. Resistive touchpads are mainly used in older cell phones or in some tablet PCs and in medicine. However, resistive touchpads are now being developed that are also multi-touch capable and are becoming increasingly important in the industry.
The principle behind Capacitive touchpads
In contrast to resistive technology, capacitive touchpads do not require pressure. They are made up of a two-layer coordinate network of electrodes, which are arranged in one layer as columns and in the other as rows. There is an insulating material between the electrodes called dielectric. A circuit is attached to the lower side, which constantly measures the capacitance at the crossing points of the electrodes. At the top, an insulating protective layer, usually made of glass, ensures that the electrodes are not damaged and the finger can slide well over the surface. This property makes the capacitive touchpad much more robust than the resistive touchpad. Since a finger is electrically conductive, charges can flow off it, as soon as it touches the surface of the touchpad. This changes the electrostatic field between the electrodes and leads to a measurable change in the capacitance. The capacitance changes when the finger moves across the surface which is recorded at the various electrode intersections by a microcontroller. It is forwarded to the operating system that converts these signals into a click or a movement. Capacitive touchpads are multi-touch capable, as they constantly measure the capacity in the entire coordinate network and can register the inputs of individual fingers separately. They differ from resistive touchpads. The disadvantage of the capacitive principle, however, is that only conductive objects can be used, to operate the touchpad. Other objects such as pens, fingernails, or gloves have no effect. The capacitive principle is used for smartphones and tablet PCs and around 99 percent of all laptops also contain capacitive touchpads.
Large touchpads on which you can draw with a stylus like on paper or tablet PCs are sometimes based on an inductive principle. However, this requires a special stylus with an integrated coil. This pen influences a circuit board under the touchpad surface, which uses antenna coils to determine the coordinates of the stylus.
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A somewhat more expensive technique is the use of infrared LEDs, as used in newly developed, very large touchscreens. There, a row of small infrared light-emitting diodes forms a grid of light rays at the edge of the screen, which is read out on the other side with photodetectors. If a finger or another object comes between the LED and the detector, the beam path is interrupted and there is a measurable signal drop at the photosensor. This allows the controller to locate the point of contact and passes it on to a software as a signal. Since the surface of the screen is not coated, this technology offers high light transmission and is very suitable for touch screens.