Touch screens are becoming increasingly critical in more and more embedded applications, such as industrial control systems, transportation systems and others. In 2008 the market research firm iSupply forecast that the worldwide market for all touch screens would nearly double, from $3.4 billion to $6.4 billion by 2013.
Especially in embedded systems, new technologies like projected capacitive touch screens are accelerating the growth of touch screens by enhancing their durability, reliability and overall performance.
In particular, projected capacitive technology can be applied to touch screens in embedded applications where the harshness of the environment might wreak havoc on older touch technologies, such as resistive touch screens.
Inherent Issues with Resistive Touch Screens
Resistive touch screens, the most common touch panel technology today, have several well known problems. Some of these problems can be minimized by adding additional contact points, but ultimately the weaknesses of resistive touch screens are an inherent drawback of their construction.
Resistive touch screens are built around two parallel layers of conductive Indium Tin Oxide (ITO). Typically, the bottom layer is printed on a stiff material like glass. The top layer, which is the layer closest to the user, is printed onto a flexible material like a thin sheet of plastic film.
When the user touches the top layer, the plastic film bends until it contacts the bottom ITO layer. The contact between the two conductive layers alters the resistance of the ITO layers. This change in resistance is used by the touch panel's controller to determine the location where the two layers touch (Figure 1 below).
The types of resistive touch screens are characterized by the number of connections on the ITO layers. The most basic type, a four-wire resistive touch screen, has two contact points on the top layer and two on the bottom layer.
These contacts are connected to dedicated inputs on analog-to-digital converters (ADC). Pressing the top layer changes the resistance of the two ITO layers. These changes in resistance, as measured by the ADCs, define the location of the operator's touch.
Because the top layer is a flexible film, it is susceptible to damage. The sheet can be torn by the user, it can deform over time, it can warp in high or low temperatures and it can be damaged by cleaning materials.
Basically, anything that alters the resistance of the ITO layers changes the touch screen's performance and, in some cases, can cause permanent damage. This is why resistive touch screens have to be recalibrated often and may even fail after suffering only minor damage.
Some of these problems can be mitigated by adding more contact points. For example, an eight-wire resistive touch screen includes an injected reference voltage on both ITO layers. These reference levels help the system self-calibrate, temporarily overcoming problems caused by drift and warp.
A five-wire resistive touch screen uses different connection points and drives the lines differently, allowing it to provide even better correction for drift, warp and some level of physical damage.
But no matter what scheme is used to drive a resistive touch panel, the root cause of the technology's problems is the requirement that the top layer be made of a material flexible enough to be easily depressed by the user. These flexible materials are inherently susceptible to physical damage, which affects the performance and effective life of the resistive touch screen.
