Increasingly we read in test reports and journals of capacitive and resistive touch screens, but very few of us know what the differences, the advantages and the disadvantages are.
Besides these two methods, there are other optical methods for detection of contact points, which are more often used in larger devices such as PCs and touch-tables.
Touchscreens consist in most cases of the display, that is just like normal monitors used for image output, and the overlying touchscreen, which recognizes the contact points and passes them to the computer.
The touchscreen consists of three components. The touch sensor, the controller and a software driver.
The touch sensor of resistive and capacitive touchscreens is made from a touch-sensitive layer, which in most cases is made of glass or polyester and has a slight electrical current. Once this area is touched, a change in voltage as well as their position can be determined.
The controller is usually integrated in the monitor with a built-in PC card which can capture the user input on the touch screen and sends it to the appropriate operating system.
The touchscreen driver, which receives the data sent from the controller, is installed on the system software. Depending on the driver touch signals are sometimes interpreted as a simple mouse clicks, so that old systems can handle this new input medium.
Resistive touchscreens responsive to pressure can be found in navigation devices or in handheld games consoles like the Nintendo 3DS.
A resistive touch screen consists of two opposing layers, separated by insulating spacers from each other. Both layers are coated with transparent, conductive indium tin oxide (ITO). While the most posterior layers are made of a solid material such as glass, the front layer, which is touched by the user, is made of a flexible, pressure-sensitive material.
On both layers a low voltage is applied. Due to the spacer there is no electricity flow in the idle state. Once an object or finger presses on the screen, creating a contact between the layers, the current flows. By the changed tension the distance of the touchpoint to the corners, where the power supply is located , can be calculated. So, with the distances to all 4 corners, the exact position of the touch can be determined.
The main advantage of resistive touch screens is that they can be operated with many objects. So it is no problem to operate on the display with worn gloves, which does not work with capacitive displays.
Through the use of input pencils, the device can be controlled precisely, which is of great advantage for small displays.
For most manufacturers, the low production costs, are the biggest advantage, which explains the widespread use of this display type.
Besides these advantages, a few aspects argue against this type of display. Resistive touch screens have their limits in their multi-touch capabilities. This means that the user normally has only one point of contact available and therefore functions like zoom with two fingers are not possible. Another aspect, that speaks against the use of pressure-sensitive displays, is the high abrasion. Because the upper layer consists mostly of plastic, it is easily scratch-prone and can rip after prolonged use. Thereby the identification of contact points is no longer possible and the display must be replaced in this case.
Capacitive touchscreens are among the second group of the most widely used techniques for touchscreens. They although work with a touch-sensitive surface, but this approach differs greatly from that of the resistive touch screen.
The capacitive touch screen consists mainly of a glass surface, as applied to the resistive technology, a very thin layer of ITO and was placed and put under tension. At the corners of the display electrodes are attached, which generate a steady flow of current through an electrical field.
If the display is touched with a capacitive object, say an object that can absorb and transport a charge, a charge transport occurs. The new set of streams can now be measured at the electrodes in the corners and the position of the contact point can be determined.
A further development of the capacitive touch screen is the Near Field Imaging (NFI). Here an ITO layer with an attached, barely recognizable sensor grid of micro-fine wire is used between two glass panes. The voltage is applied to the fine grid patterned sensor so that the electric field is established. If the surface is then touched by a capacitive object the electric field is disturbed and the position of the contact point can be determined.
Through the sensor grid, it is possible to use a up to 18mm thick layer of glass as Surface. In addition, the contact point can be determined better than on normal capacitive screens.
Although the capacitive screen has a very high resistance in contrast to the resistive, which is exposed through its plastic surface to rapid wear, deep craters can still destroy the display.
The main advantage, besides the resistance, are the multi-touch capability of this technique. By IFN-technique it is not only possible to determine the position of a contact point very accurately, it is also possible to define several contact points.
These abilities but are quite expensive, so capacitive display found only in small devices, which work closely with several points of contact.
Another disadvantage is clear from the name. Because touch-points can only be generated by capacitive objects it is not possible to operate the display with gloves, because they can not absorb or transport any charge. This phenomenon observed many smartphone owners in the winter, when they try to use their phone in the cold. But capacitive displays are not only used in smartphones. ATMs are also machines in with touchscreens are used.
Besides the methods to identify contact points by electric fields optical systems also exist. These are mostly used for large touch surfaces, in which normal touch-screens would be too costly, or where a very large number of touch points are to be realized.
These optical systems use one or more cameras that capture the touch and movement on the display screen and determine from these data the exact position of the touch point.
While there is a huge set of various constructions, I would like to mention in this post but only two.
Rear Diffused Illumination (DI)
The Rear DI method is based on the modified contrast value that comes from touching a surface. In this process, the screen is evenly illuminated with infrared light (IR) from behind. Alternatively, there is the front DI process in which the screen is illuminated from the front. Both methods are essentially the same.
Once an object touches the display the infrared light is reflected at this point more than at other parts of the display screen.
The altered IR-light ratios are captured by a digital camera to the computer and passed on. It may happen, depending on the structure of the diffuser, that objects which float above the surface of the display are also detected. So contact points are detected even if there is no contact with the surface. This so-called hand-shadow effect occurs most often when users hold very close to her palm on the display surface. This may occur misinterpretation of touch points can restrict the use of a Rear DI displays strong, so a precise configuration is very important in this method.
On the computer a program using the camera image calculates the positions of the contact points and their size. In many cases the open source software community core vision is used. This software passes the touch points that occurred to the programs or the operating system.
The Rear DI display is mostly used in multi-touch tables and walls which is due to the use of multiple cameras and multiple projectors, which project the screen contents, easily expandable.
Two camera method
The two-camera method is one of the simplest methods to identify a touch on a surface. In this case only two cameras are used. The cameras are mounted on the upper left and right corner of the screen and monitor the area directly above the display (See “Image 4”). Through trigonometric calculationsthe position of the point of contact can be determined.
This method is used for example for devices with large display, such as the All-in-one PC from MSI, which should however be quite cheap and space saving.
However, as shown in the picture right, it can happen (through this special design) that there are several touch-points but the computer can only detect two of them because touchpoints are hidden in the “shadow” of other ones. Due to the limited number of perspectives the number of simultaneous touch points, due to possible occlusions, is also severely limited. Thus in this method only two touch points can be determined exactly.
There are approaches that want to achieve, by adding more cameras in different perspectives, to identify an unlimited number of touch points. One of the most promising projects in this area is the zero-touch project.
Which technique is best for each project / product now depends on its environment, so I will not make any definitive assessment of the various technologies.
All these techniques have their advantages and disadvantages and raison d’etre.
Upcoming blog entries
Making the Game – Part 4: Alternative NUI controls ( Motion Control and Accelerometer )
Making the Game – Part 5: Multi-Touch Framework (Adobe Flash / Air)
Making the Game – Part 6: Multi-touch devices and their limits
Image 1, 2: http://www.tci.de/service/dokumentation/touchtechnologien/resistiv/
Image 3: http://wiki.nuigroup.com/Diffused_Illumination
Image 4: Moeller, Hamilton, Lupfer, Webb, Kerne, ZeroTouch: A Zero-Thickness Optical Multi-Touch Force Field, ACM 978-1-4503-0268-5/11/05 – Link: http://dl.acm.org/citation.cfm?id=1979710