Today, most modern computers or mobile phones operate using touch pads or touch screens. The sensitive surface replaces the computer mouse on the PC and the keyboard on the phone or tablet. Behind all these user-friendly operations is a clever technology that combines hardware and software.
All touch pads have the same design: they have touch sensitive surfaces and controllers that measure signals on the surface and transmit them to the operating system. The operating system then converts our finger movements into mouse movements and transmits them to the screen. Tapping the touch pad corresponds to mouse clicking. You can pull two fingers apart from the center. You can enlarge the image on the screen and wipe the surface, web pages and pictures with three or four fingers to scroll. The hardware that generates the signal can be based on different physical principles. The most widely used are resistive and capacitive touch pads.
Resistive touch pads require pressure from fingers or other objects on the surface. The touch sensitive surface of the touch panel is composed of two conductive indium tin oxide (ITO) layers separated by small spacers. The lower layer is painted on a firm and stable base, while the upper layer is painted from the outside with stretchable polyester. When touching the polyester layer, press the top ITO layer down to the bottom. In order to determine the position of the pressure point, a DC voltage is alternately applied to another conductive layer after one millisecond.
These stresses are perpendicular to each other, and each stress falls evenly from one edge to the opposite edge. Since the two layers are briefly connected to each other at the pressure point, the current flows here. Based on the resulting voltage change, the position of the pressure point can then be clearly determined. The controller forwards the coordinates to the operating system. According to this principle, measurement always needs two layers: one is voltage and the other is one direction.
Resistive touch pad is considered to be the pioneer of touch technology, but it usually does not have multi touch function. This means that you cannot handle them with multiple fingers. If you press the surface with two or more fingers, only the contact surface of the two ITO layers will be expanded, and the fingers cannot be detected alone. However, the biggest disadvantage of this technology is that the coordinates are always detected through the upper flexible layer.
Constant bending and stretching lead to micro cracks in ITO coating, which changes the electrical properties. Over time, this will make the determination of coordinates inaccurate. However, resistive touch pads can be produced relatively cheaply and can be operated with any object. This is particularly important for doctors who often need to use rubber gloves on medical equipment. Resistive touch pad is mainly used in older mobile phones or some tablet computers and medicine. However, at the same time, resistive touch panels are being developed, which also have multi touch capability and become more and more important in industry.
Compared with resistance technology, capacitive touch pads do not require pressure. They consist of a network of two layers of electrodes arranged in columns in one layer and in rows in the other. Between the electrodes is an insulating material, the so-called dielectric. There is a circuit on the lower side, which continuously measures the capacitance at the intersection of electrodes. At the top, an insulating protective layer, usually made of glass, ensures that the electrode is not damaged and that the fingers can slide on the surface. This feature makes the capacitive touchpad more robust than the resistive touchpad. Since the finger is conductive, the charge can flow out of it once it touches the surface of the touch pad. This changes the electrostatic field between the electrodes and leads to a measurable change in capacitance. When the finger moves on the surface, the capacitance at the intersection of different electrodes changes.