The majority of touch screens of today use one of two technologies – Projected capacitive or analogue resistive. More and more designs, also in embedded computing, choose the projective capacitive technology, PCAP touch, to turn the surface in front of the display into a modern user interface.
An important reason for the trend is that PCAP touch is widely used in smart phones and tablets. Industrial computers and embedded systems thereby make use of new and extended ways of user input and control by touching and pointing, with one finger or more, rotate, zoom and moving objects on the display. Consequently industrial embedded products are adapted to modern user behavior and benefit from the look and feel of the latest mobile gadgets.
The analogue resistive touch screen technology was launched about 20 years ago. It’s the cheaper alternative of the two today. The pressure sensitive surface reacts to touch from a variety of objects such as a finger, with or without thick gloves, pencils and tools. For these reasons one or another product will definitely use the resistive touch technology, now and in the future, but the trend is in favor of PCAP touch. Let’s have look at the advantages!
Protective shielding and extended operating life
PCAP touch is the alternative with the highest light transmittance in comparison and the best image clarity. Almost none of the light is blocked when the PCAP touch screen is mounted in front of the display. The resistive technology suffers from reduced light transmittance in comparison with PCAP touch and therefore decreases the sought-after brightness of the underlying TFT-LCD.
A direct contact between the finger and the PCAP touch sensitive surface is not needed for operation. A glass plate may be mounted as an outer transparent protective layer without interfering with the touch functionality. The choice of material and thickness of the plate may even protect the system from vandalism in public places. It’s worth mentioning that the operating life of the touch screen generally is well over the operating life of the display.
Introducing multi-touch gesture control
Yet another advantage of PCAP touch over the resistive technology is the possibility of using so-called multi-touch gesture in the user interface. It’s the input method familiar to more and more people since it’s widely used in smart phones and tablets. The picture at the beginning of the text gives you an idea of what multi-touch gestures are, or actually more correctly, dual-touch gestures since no more than two fingers are involved.
The principal of the PCAP touch technology is based on two electrode leader layers, in rows and columns, mounted with a separator in between. An electric field is created on one of the electrode leader layers. The capacitance coupling is measured for every x/y electrode intersection, one by one, as the controller scans the surface.
Scanning the capacitive coupling
A finger touching the surface will affect the capacitive coupling between the two layers in the touched spot. The coordinates of the touching finger will be calculated by the controller from scanning the surface. Triangulation using the information detected in a number of x/y intersections improves the precision. The touch and movements of the fingers on the screen are translated to gestures and input by the operating system and application software.
No calibration needed
Calibration of the PCAP touch screen and display is made once and for all in production of the product. Coordinate drift thereafter is non-existent since the touch coordinates are measured scanning the physically fixed grid of electrode intersections. The finger touch on the screen is detected with precision, year after year and in a wide range of operating temperatures.
Bonding the display and touch screen
The PCAP touch screen is attached to the display either by edge bonding or optical bonding. The downside with bonding only the edges of the touch screen and display is the inevitable air-gap between the two surfaces in between the bonded frame. Dust and condensation of water in that gap decrease image clarity.
Light travelling through the internal layers will induce reflections as a consequence of the air gap. This leads to glare and decreased optical properties not the least in direct sunlight. Edge bonding is the less costly bonding option of the two. Touch screen and display edge bonded are quite easily dismantled during service and repairs to replace one or the other.
When light travels through material layers and pass from one material to another with different refractive index (for instance glass to air or air to glass) reflections are generated.
The refractive indexes of the glass and adhesive are matched to reduce reflections and increase image quality.
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