Thesis - Teknisk Design

Among the weaknesses of these devices it must be highlighted the fact thatthe detection of the touch in this kind of touch screens is conditioned by the amountof pressure exerted by the user. The meaning of this is that there is a threshold on theamount of pressure for detection of touch, so if you do not reach it, the touch will notbe considered. This fact makes the use of resistive touch screen a little bit frustrating,for example, when you have to repeat the same movement for the device to detectit. Due to the layout of the layers to achieve this technology, resistive touch screenshave the worst visibility and the least amount of emitted light of the available touchscreens in the market. Finishing, the last weakness of resistive touch screensintroduced in this report is related to its components. The resistive layers are not hardenough, that makes the touchable layer easy to scratch, turning the touch screen’slive short (Dhir, A. 2004).SURFACE ACOUSTIC WAVEWith the recent investigation in the field of surface acoustic waves (SAW) a new typeof touch screen appears using this technology. Acoustic waves are produced in thesurface of the touch screen. When this is touched, part of the wave is absorbed. Thisis used to estimate where the solid interfere with the wave and set the position by thereceivers (Dhir, A. 2004).The main point of using this technology is the ability to detect information fromx-axis. It also offers image clarity and resolution, as is not needed more than onelayer. For SAW touch screen any solid object can be detected when touching thescreen. Last but not least, even if the touch screen is damaged with scratches it willcontinue working properly as it does not interfere with the waves.After the last statement it can be deduced that this type of screens hasinterferences when there is dust in its surface or water (if it is outdoors and rainy). Asthe glass is the surface in contact with the environment it is easier to be break (Dhir,A. 2004).CAPACITIVE TOUCH SCREENSWhen looking at capacitive touch screens a huge variety of possible detectors arefound. The basic operation is pretty similar for all capacitive touch screens. There isan insulator, usually a glass, coated with a transparent conductor (usually Indium tinoxide ITO), which is charged with a small voltage. There are different ways andlayouts that will be discussed further. This electrostatic field is distorted by the touch orthe approach of an electrical conductor such as bare fingers. This distortion ismeasured as a change in the capacitance of the coated glass (Dhir, A. 2004).Taking detectors into account there are two main spread technologies thatneed more explanation. The Figure 2.1 gives an idea the different classes ofprojected capacitance touch screens and the proprieties of each of them.Surface capacitanceThis technology is based mostly in the change of uniformity in electrostatic fields. Tocreate this uniform electrostatic field a small voltage is applied to the top layer. Theelectrostatic field is distorted by the touch of a conductor such as fingers, whichdynamically formed a capacitor. The surface touched by the conductor is the10