Advances in capacitive touch panels

Author : Mike Logan, andersDX

03 April 2017

Although capacitive touchscreens can normally respond when the user is wearing thin latex or rubber gloves, demand is now growing for touchscreens to offer an acceptable user experience for wearers of thicker gloves, such as cut-resistant industrial gloves worn to comply with safety regulations, or ordinary leather or synthetic gloves, to allow wearers to interact with industrial automation or machinery, smart signage, access-control panels, or outdoor point-of-sale equipment, to name a few.

Often, these systems are installed behind thick cover glass for protection, for outdoor use or other applications where harsh treatment is expected. In the past, typical project requirements have called for cover glass up to about 1.8mm thick, whereas a growing number of customers today are looking for a touchscreen to operate behind glass up to 4mm or even 6mm thick.

A large part of the solution is in the performance of the touch controller, rather than the touch sensor itself. Moore’s Law improvements in processing performance enable the latest touchscreen controller ICS to deliver a better combination of sensitivity and response time than preceding generations of devices. The latest knowhow for laying-out ITO (indium-tin oxide) electrode patterns, also allows design tools to determine patterns that are better-optimised for touchscreens to be placed behind thick cover glass or to accommodate users wearing gloves. It may soon be possible to create touchscreens that can be tuned for optimum performance with a certain type of glove, such as application-specific industrial gloves.

System approach recommended
Whether engineers designing touch-enabled user interfaces into their latest projects can satisfy application requirements for thick cover glass, or end-user demand for response when wearing gloves     depends on achieving a suitable combination of ITO pattern, controller performance, and cover-layer properties including optical clarity and thickness.

Of course, there are several interdependencies between the components, materials and settings, and these should be considered from a system perspective at an early stage of the project. The ITO pattern that forms the array of sense and drive nodes, which sets up the capacitances for touch detection, influences the signal-to-noise ratio and, together with the touch-controller firmware, determines the touch threshold. Both factors influence the permissible cover-glass thickness, and potential performance if the user is wearing thick gloves.

The graphical display behind the touch sensor can couple noise into the capacitive touch-sensing layers and therefore the ITO pattern can determine the sensors susceptibility to EMI.  Including a ground shielding layer in the touch-sensor stack-up can reduce noise emanating from the LCD and entering the sensor’s circuitry.

In addition to considering the ITO pattern design, the firmware of the selected controller IC can be adjusted to suit various types of cover lenses to optimise system performance. If the sensitivity needed to allow touch response with the anticipated type of gloves cannot be achieved using glass of the intended thickness, the designer may consider trading-off some physical resilience by specifying thinner glass in exchange for increased sensitivity. Alternatively, a specialised type of glass, such as Gorilla Glass, could be considered instead. Gorilla Glass is both tough and thin: a combination that is widely appreciated in the mobile industry. Thanks to the economies of scale of the mobile industry, it is available at relatively low cost for standard screen sizes up to about 7 inches.

Firmware in conjunction with special algorithms designed to filter out unwanted EM noise are critical to achieving the desired touchscreen performance and one should always be prepared to make allowances to fine tune the FW if necessary, should various elements of the display module, including cover glass, touch sensor, bezels, backlights, and adhesive layers require modification. Further fine-tuning of the FW settings may be needed when the touchscreen is integrated with the circuitry that comprises the remainder of the overall system.

Early decisions
Generally, touchscreen performance parameters are wider today than at any time in history. High performance and almost seamless user experiences, including good response to users wearing gloves or fast-acting single- or sometimes multi-touch response from behind thick and robust cover glass. Today’s touchscreens can be designed to operate reliably in extremely cold environments, in wet or humid conditions, or where harsh treatment or frequent heavy impacts against the cover glass can be expected.

The key to successful design is to establish an achievable specification at the beginning, usually working with an experienced technical partner to identify the best combination of controller chipset, touch panel, cover glass and other key components. Making the right choice early is important to ensure the required features can be delivered, and can help simplify fine-tuning and avoid unexpected problems later in the project.

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