Until now, capacitive has reigned as the undisputed leader of touchscreen technologies, as it allows for reliable touch detection on hard surfaces and multiple touch detection for gesture recognition. However, two breakthroughs in the touchscreen space could threaten capacitive’s perch at the top.
Rohm is laying claim to the first resistive touch controller that can handle multi-touch, albeit with a maximum of two touches registered at once. This was previously only possible with more expensive capacitive touch-based products.
Two touches are enough for simple gesture recognition, including pinching, spreading, and rotating, features that Rohm includes in its BU21023/BU21024 series (Fig. 1). In addition, a built-in calibration function minimises the effects of panel variations during production, as well as fluctuations in touchscreen device characteristics caused by temperature variations or time-based degradation.
“The new controllers integrate a dedicated analogue circuit and a CPU that uses Rohm’s proprietary algorithm in order to realise precise two-point coordinate and gesture detection,” says Raimund Wagner, product manager for Rohm Semiconductor GmbH.
Wagner explains that resistive touchscreens have a four or five-wire interface, enabling a simple connection with the controller—an advantage over the complex customised interfaces required for capacitive technology. The simpler interface also boosts EMI immunity.
“We expect that resistive touch will continue to be used in the low-end consumer applications, while high-price applications will tend to use capacitive touch solutions,” says Wagner. “In addition, resistive touch solutions are superior for use in ‘difficult environments’ like automotive, industrial, or other EMI critical areas. Also, outdoor applications or products that are operating in high humidity environments will prefer resistive touch solutions.”
According to Rohm, resistive touchscreens account for over 75% of the touchscreen market, largely due to their lower cost and higher precision when compared to capacitive technologies. Another advantage is that they will operate with fingernails or if someone is wearing gloves, unlike capacitive touchscreens that require a conductor or dielectric.
The second breakthrough comes via Peratech, which is based in Richmond, UK. Its polymer composite material, which changes resistance when mechanically compressed, now comes in a transparent version suitable for touchscreen sensors.
Quantum tunnelling composite (QTC) can be printed into thin, transparent sheets, which Peratech calls QTC Clear (Fig. 2). QTC, which is force-sensitive, can be used to replace resistive touchscreens or enhance capacitive ones to create a 3D input.
“QTC technology is completely unique—it’s not resistive or capacitive,” says Philip Taysom, joint CEO of Peratech. “The material changes its resistance from around 500 M? to less than 10 k? with just one or two microns of deflection,” he adds, noting that unlike resistive technology, it can be used with hard substrates such as glass.
The QTC Clear layer is only 6 to 8 microns thick, sandwiched between two layers of indium tin oxide (ITO), which is in turn sandwiched between two hard sheets, typically glass. QTC technology can be used effectively with stylus or gloved finger input. Due to the material’s properties, virtually no current flows unless a force is applied. In contrast, the capacitive screen’s frequent high current requirements may induce EMI issues.
QTC produces a proportional response when the user exerts increasing pressure. When pressing the screen with a finger, the QTC layer senses the varying force over the whole area of the touch, providing data that can be interpolated to produce very high resolution touchscreens. If the user drags their finger, the direction and force of the touch can be measured.
Taysom cited drawing and painting programs as an example of applications that could take advantage of QTC’s force-sensitive nature. Probably more exciting, though, is a joystick effect for gaming that’s produced by rocking your finger from side to side.
Essentially, the polymer layer is a giant variable resistor, explains Taysom. It can be read by any analogue-to-digital converter that can accept a resistive input, which is then fed directly to the application processor. “Dramatically less computing power is required—the traditional touchscreen controller is completely removed,” he says.
QTC Clear is available under license from Peratech. In fact, it’s already been licensed to a leading touchscreen manufacturer, according to Taysom.