Why you need to use capacitive touch panels in your GUI design
More developers of embedded systems are deploying projected capacitive (PCAP) touch display technology as it continues to emerge as the operator interface technology of choice for a broad range of applications. It started with mobile consumer electronics, where smartphones have shown that a PCAP interface can be intuitively easy to learn and use, efficient with regards to system resources, very engaging for the user and give the finished product a sleek and sophisticated look and feel.
Now embedded designers are discovering that the broad range of performance characteristics for PCAP touch panels makes the technology a great fit for many embedded applications. In fact, the deployment of touch displays, including PCAP units, in a wide variety of applications is exploding.
Market research firm DisplaySearch reported that the touch panel market grew by more than 29 percent in 2009 over the previous year. The future outlook for PCAP, also according to DisplaySearch, reflects PCAP becoming the number one touch technology in terms of revenue in 2010 and will pass resistive touch in unit shipments by the year 2013.
How PCAP Works
The previous generations of touch panels in embedded applications mostly involved resistive touch technology, which features a flexible outer layer of a thin plastic film. This layer must be flexible so that touching the outer layer will create contact with a rigid inner layer where the electrical contact is registered.
Unfortunately for many embedded systems, this thin plastic outer layer makes resistive touch panels less durable due to the likelihood of scratching or somehow damaging the thin plastic.
Temperature extremes or repeated usage of the same location can warp and deform the plastic layer, rendering the operator interface unreliable in most cases. In addition, the flexibility of the outer layer causes the accuracy of a resistive touch panel to drift out of calibration and, as a result, manual recalibration is required periodically.
In contrast to resistive touch, PCAP displays feature two robust layers of glass. Between the glass layers is a conductive material, Indium Tin Oxide (ITO). The entire unit is sealed and protected from the outside world.
The two ITO layers on the two sheets of glass form an X/Y grid of capacitors. The electric fields of these capacitors are projected from the top layer of glass. Placing a finger on or near the outer layer couples these electric fields and conducts a charge away from the panel.
The controller for the PCAP panel is constantly scanning through the drive electrodes along the X axes of the screen while it monitors the sensing electrodes along the Y axes (Figure 1 below). When a change in capacitance on the screen occurs, the controller is able to filter out extraneous noise and other transients, and identify this event as a finger touch.
Figure 1. PCAP panels constantly scan through the drive electrodes along the X axes of the screen while it monitors the sensing electrodes along the Y axes
The sensing electrodes and the controller must be capable of some highly precise measurements as well as very aggressive noise filtering and complex algorithms since the change in capacitance is in the femto-farad range (10-15 F). The controller must amplify, condition and measure these signals. The grid made by the ITO traces along the X and Y axes allows the controller to determine the location of the touch on the screen.
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