Touchscreens’ runaway market appeal has given rise to a mature worldwide supply chain in just a few years, encompassing the manufacturing plants in Taiwan and Japan where the transparent sensors are fabricated; the U.S. and European manufacturers of the controller chips that translate changes in resistance or capacitance into finger-down locations; and module makers and system integrators that add the clear cover, laminate the transparent films and integrate the electronics.
Legacy, resistive touchscreen technology uses two conductive polymers on separate layers that can be deformed to touch each other wherever a finger or stylus touches the top layer. Resistive controllers—which are available off the shelf from Analog Devices, Texas Instruments, STMicroelectronics and other mixed-signal chip makers—are relatively simple and very accurate, but they do not usually recognize multiple touches. Divergent architectures have been developed for resistive touchscreens, using varying numbers of connecting wires (such as four-wire or eight-wire) to make the task simpler or more accurate for specific applications.
“Touchscreen technology is extremely diverse, with many methods specialized for different applications; but in general resistive is the legacy technology, while projected capacitive has recently become the industry leader,” said Jenny Colegrove, vice president for emerging display technologies at DisplaySearch.
Projected capacitive touchscreens rule the roost in high-end mobile devices. Smart appliances and security keypads have no real need for capacitive touchscreens, however, and for some applications even resistive touchscreens are overkill. “Resistive touchscreens are still popular because of [the technology’s] maturity and low price point, whereas projected capacitive still has some issues with yields and the lamination process for large-scale screens,” said IHS iSuppli’s Alexander. “As long as you have a variety of screen sizes, application environments, price points and use cases for these devices, you are still going to need a variety of touchscreen technologies available.”
Projected capacitive technology drives one plate of a transparent capacitor with a signal, then measures the results at the adjacent plate with an analog-to-digital converter. The capacitive sensors are usually cast in a diamond pattern, with one diamond-shaped capacitor plate on each side of the glass or both on the same side, using whisker-sized jumpers. A smartphone uses a couple of hundred capacitive sensors and tablets up to 10 times as many, making it possible for a smart controller chip to discern any number of touches at resolutions fine enough to detect even the smallest child’s finger. Several controller makers, including Cypress and Integrated Device Technology (IDT), are proposing proprietary patterns that eliminate the need for jumpers.
Multitouch gesture capability began with two fingers to zoom, three to scroll and four to swipe; it’s now become a free-for-all, as multitouch variations proliferate to enable finer manipulation of on-screen objects. High-quality transparent sensor patterns support smart gesture recognition, but the smarts originate in controller algorithms that debounce and condition the signal from multiple fingers. The touchscreen controller sends the information to the application processor, which in turn identifies gestures of varying complexity, such as tap- to-select, brush-to-scroll and pinch-to-zoom.
Touchscreens are evolving toward allowing multiple finger touches to manipulate objects on-screen in much the same way objects might be handled on a real desktop.
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