How do resistive and capacitive screens differ?

How do the touchpad and touchscreen work?

Most modern computers or cell phones nowadays are operated using touchpads or touchscreens. The sensitive surfaces replace the computer mouse on the PC and the keyboard on telephones or tablet PCs. Behind all these operations, which are simple for the user, lies a sophisticated technology that combines hardware and software.

All touchpads are designed according to the same pattern: They have a touch-sensitive surface and a controller that measures the signals on the surface and forwards them to an operating system. The operating system then translates our finger movements into a mouse movement and transfers it to the screen. Tapping on the surface of the touchpad corresponds to a mouse click, if you pull two fingers apart from the center, you can enlarge the picture on the screen and with three or four fingers, swiping over the surface, you can scroll through pages and pictures. The hardware that generates the signals can be based on various physical principles.

The most common are resistive and capacitive touchpads.

Resistive principle

Touch-resistant principle

Resistive touchpads require the pressure exerted on the surface by a finger or other object. The touch-sensitive surface of the touchpad consists of two conductive indium tin oxide (ITO) layers that are separated by small spacers. The lower layer is applied to a firm and stable base, while the upper layer is covered on the outside with stretchable polyester. If you touch the polyester layer, the upper ITO layer is pressed onto the lower one. In order to determine the position of the pressure point, a DC voltage is applied alternately to one conductive layer and milliseconds later to the other. These tensions run perpendicular to one another and each fall evenly from one edge to the opposite edge. 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 determined unambiguously on the basis of the voltage changes caused thereby. The controller forwards the coordinates to the operating system. With this principle, two layers are always necessary for the measurement: the voltage is applied to one, the other transmits the position in one direction.

Resistive touchpads are pioneers in touch technology, but are generally not multi-touch capable. This means that you cannot use it 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 recorded individually. The biggest disadvantage of this technique, however, is that a coordinate is always recorded using the upper, flexible layer. The constant bending and stretching leads to microscopic cracks in the ITO coating, which changes the electrical properties. Over time, this leads to the determination of the coordinate becoming less precise. However, resistive touchpads can be produced comparatively inexpensively and can be operated with any object. This is especially 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. In the meantime, however, resistive touchpads are being developed that are also multi-touch capable and are gaining in importance in industry.

Capacitive principle

Touch capacitive principle

In contrast to resistive technology, capacitive touchpads do not require any pressure. They are made up of a two-layer coordinate network of electrodes that are arranged as columns in one layer and as rows in the other. An insulating material, a so-called dielectric, is located between the electrodes. A circuit is attached to the lower side that constantly measures the capacitance at the crossover points of the electrodes. On the top, an insulating protective layer, usually made of glass, ensures that the electrodes are not damaged and that the finger can slide easily over the surface. This property makes the capacitive touchpad far more robust than the resistive touchpad. Since a finger is electrically conductive, charges can flow off 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. As the finger moves across the surface, the capacitance changes at the various electrode intersections. The changes are all recorded by a microcontroller and forwarded to the operating system. This translates the signals into a "click" or movement on the screen. Capacitive touchpads are multi-touch capable, as they continuously measure the capacity in the entire coordinate network and can register the inputs of individual fingers separately. This is how 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. But 99 percent of all laptops also contain capacitive touchpads.

More techniques

Graphics tablets - large touchpads on which you can draw like on paper with a stylus - or tablet PCs are sometimes based on an inductive principle. However, this requires a special input pen with an integrated coil. This pen influences a circuit board under the touchpad surface, which uses antenna coils to determine the coordinates of the input pen.

Infrared touch screen

The use of infrared LEDs, as used in newly developed, very large touchscreens, is a somewhat more expensive technique. On the edge of the screen, a row of small infrared light-emitting diodes forms a grid of light rays, which are read on the other side with photo detectors. If a finger or another object comes between the LED and the detector, the beam path is interrupted and there is a measurable drop in signal at the photo sensor. This enables the controller to locate the point of contact and forwards it as a signal to software. Since the surface of the screen is not coated, this technology offers a high level of light transmission and is very suitable for touch screens. However, the limited service life of the light-emitting diodes and a coarse spatial resolution are the greatest disadvantages of this technology.