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Taking touchscreens into the third dimension

Incorporating a touchscreen has almost become a prerequisite in modern designs. While some end users will continue to prefer designs with buttons and knobs, touchscreens have become sufficiently robust and low cost to make them a viable alternative.

Yet, according to Fanie Duvenhage, director of Microchip's Human Machine Interface Division: "Capacitive sensing is quite steep learning curve for people to go through on their first design."

Unsurprisingly the technology is not standing still. While simple buttons and sliders based on capacitive sensing are becoming the norm, more exotic variations are waiting in the wings.

Microchip has taken it to a new dimension. Literally. Using its GestIC technology, user interfaces can be taken into the third dimension, allowing gesture recognition at distances of up to 15cm from the sensor. The chip containing the GestIC technology is the MGC3130, which features a low noise analogue front end, integrated digital signal processing unit, frequency hopping against noise and recognition of XYZ positional data – most specifically hand gestures.

"There is also preprocessed gesture recognition [on the chip]," said Duvenhage. "You could do normal waves in specific direction, or you can do more complicated gestures, like a circle or the equivalent of what we call an 'air wheel' where you can adjust something by rotating your hands clockwise or counterclockwise."

A host of consumer applications could be interested in this including laptops, tablets, smartphones, game controllers and remote controllers.
Duvenhage said: "It basically uses an E-field sensing technique; what a lot of people refer to as mutual capacitance sensing. We generate an E-field and measure the relative heights of different electrodes or pads – you are measuring the capacitive coupling between the two."

Sensor technology has its own challenges, particularly with relation to manufacturability, transparency and cost. But often, especially for these proximity sensing applications, the sensors could be a PCB trace, some conductive paint or the ITO layers used in the display construction, which can then be optimised for sensitivity.

So while Duvenhage acknowledges progress in sensor technology, he believes that the controller manufacturers have had the greater part to play in the increased functionality. "There is the signal acquisition part that is continuing to evolve and provide people with a much better signal-to-noise ratio, but there is also a lot of signal processing that happens after that. A lot of time in the control side is spent on the algorithms that we use to avoid noise, interpret signal, and interpolate between points that we acquire."

Broader applications
Another company pushing boundaries is Cypress Semiconductor. Patrick Prendergast, product marketing manager for TrueTouch at Cypress, says his company was one of the first to develop standalone controllers for this purpose, rather than being a peripheral function on general purpose microcontrollers.

The company has a host of new technologies in capacitive sensing, one being DualSense, which includes the ability to keep TrueTouch controlled touchscreens working accurately even when wet.

Controllers featuring DualSense are capable of both mutual and self capacitance. Mutual capacitance allows for multiple fingers to be accurately tracked on a smart phone, for example. Mutual capacitance sensing essentially provides a bit map image of the touchscreen, but with capacitance as the units, rather than greyscale. It makes it possible to identify exactly what is noise and where the fingers are on the screen. Self capacitance gives a simpler XY profile of what the capacitance looks like.

DualSense, according to Prendergast, uses self and mutual capacitance together to give much better water rejection and finger tracking performance. "Water makes mutual capacitance unpredictable in how the interface is going to interact with fingers, so having self cap there, and some of our other proprietary self cap sensing methods, we are able to subtract out the effect of the water and accurately track fingers."

Another development aimed at improving the accuracy and usability of touchscreens is Cypress' auto mode switching. This high-sensitivity function is featured in some of the Samsung S4 devices and the Sony Ericsson Xperia SP. It replaces the need to go into the device menu to select a higher sensitivity by detecting the capacitance automatically and adjusting accordingly. Prendergast said: "A glove gives you a much lower capacitive signal than a finger. We can detect that, switch to a higher sensitivity mode and report the glove signal back without having to force the user to switch modes."

It also removes the need for the user to remove their gloves. While this is good news if you regularly use a phone in cold climates, it also opens up the adoption of touchscreen technology in a host of industrial applications which require operators to wear gloves constantly – food and chemical plants for example.

The secret behind this is shape and pattern recognition. "We have a very high sensitivity analogue front end with very high noise immunity to displays and other environmental noise," claimed Prendergast. "When we couple those things with our pattern and shape recognition technology we are able to classify different types of touch objects – by doing that we enable this automode switching."

Of less immediate appeal to the user, but of more significance to the designer, is the SmartSense auto-tuning capability being introduced for the Cypress CapSense range of controllers. It is a feature that allows the designer to incorporate a single chip/firmware combination and ship it with all variations on the core product.

Prendergast said: "What will happen is that you will have different capacitance on all of the PCBs and so, typically, our competitors will have to tune for each of the PCBs independently. With Smartsense, you can plug this one firmware in and it will handle all of those combinations without changing." The sensitivity using SmartSense is no different from not using it, but it just uses one firmware and one tuning. "It is a big thing for our customers because it allows better yield," claimed Prendergast.

Despite advances being made by companies such as Cypress and Microchip, Duvenhage does not see the touchscreen technology taking over the interface market completely. He said: "You are starting to see a lot of voice recognition, since the algorithms are improving. And even remote controls, which we have designed with a touchpad, still features traditional pushbutton controls because that is what people are used to. There is going to be room for plenty of human interfaces."

Tim Fryer

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