Display
PCAP touch technology in the industrial environment
Intuitive touchscreen controls on tablets and smartphones are standard today and easy to use for people both young and old. This is all thanks to projected capacitive touch technology (PCAP). Great progress is owed not only to the simple and fluid operation that gesture control offers, but also to the design opportunities. Dominic Spirgat, product sales manager, displays and boards at Rutronik asks are they also suitable for any industrial application?
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here devices need to be operated with thick gloves on, there is presently no alternative
to the resistive touchscreen, because they work in a manner akin to a mechanical button, with two conductive layers, usually glass and film, separated by spacers. Applying mechanical pressure using a finger or a stylus will trigger a touch signal. The surface usually consists of a film that is sensitive to scratches and can therefore be easily damaged. Most costly glass/glass versions are less susceptible to scratches, but they are easy to break. At present, the simplest four-wire versions are most commonly in use, especially in the industrial environment. They are subject to aging over time and only allow a certain number of touch cycles. Compared to these, the five-wire versions ensure more operating cycles. They only need to be calibrated once and offer greater precision, but this touch system also ages by virtue of the underlying technology.
Alongside the quick and easy integration of resistive touch technology into a display, the ability to use them while wearing gloves is one of the key advantages of this technology. However, it is not possible to comfortably implement intuitive gesture control.
PCAP technology
PCAP touch technology uses a matrix of capacitive sensors on the surface of the touchscreen that generate an electrical field in the room. This can penetrate even sealed glass and plastic surfaces, enabling operation even behind a glass cover or
protective panel, which allows the application to be protected from vandalism using reinforced glass, while the touch display can be seamlessly integrated into the customer's housing without revealing edges that have the potential to collect grime. This not only allows for smart designs but also enables the application to be cleaned even with aggressive disinfectants and detergents without damaging the display or the rest of the application. Depending on the requirements and the location at which the application is to be deployed, the extra glass panel can also be matte (anti-glare) to prevent reflections from the sun or bright lighting. Fingerprints can also be removed with ease if the glass has an “anti-fingerprint” surface (AFP). Using various sizes, forms and rear printing of the cover glass, it is possible to tailor any application very specifically and adapt it to the company's corporate design using the company's own logo. Regardless of design choices, however, the functionality of the touch system should always come first, because the PCAP sensor penetrates different materials to different degrees. The most suitable material is glass, because it offers the best penetration. In the food industry in particular, however, alternatives are sought to avoid the risk of splintering, usually polycarbonate or acrylic glass. Aside from the fact that these do not splinter, they are also highly robust and transparent. However, PCAP sensors do not penetrate them as well as glass, so a polycarbonate or acrylic cover should not be more than roughly 2mm thick.
Optical bonding Air is also a very poor conductor for PCAP sensors, and so a gap between the touch sensor and cover material will generally have a negative impact on touch reliability and reduce sensitivity. To avoid this, it is recommended that the two layers be joined using “optical bonding”, or by laminating the entire surface with an adhesive layer. There are two
commonly-used methods employed here. The first is a liquid adhesive that hardens upon exposure to UV light, the second a high-transparency adhesive layer that has adhesive on both sides.
Optical bonding can be used not only to join the cover glass with the touchscreen, but also to join the touch sensor itself with the TFT layer. The main optical advantage here is that light refraction is largely avoided at the transitions between the various materials, keeping the display easily readable even in sunlight. The mechanical benefits lie in the greater robustness of the “unit” as a whole, because the full-surface application of the adhesive is similar to the manufacture of a composite glass which, in avoiding gaps, also prevents the entry of dust, other particles and water condensation over time that can occur with conventional frame bonding techniques. Optical bonding is still relatively costly, especially compared to the conventional technique of bonding the touch panel to the display frame, which usually uses a kind of double-sided adhesive tape. This is because full-surface bonding is much more complex. The adhesive must be highly transparent to avoid affecting the optical properties such as brightness and contrast, and UV radiation must not cause discoloration. There must be no trapped air during the bonding process. The adhesive itself must also be able to withstand mechanical forces such as thermal expansion caused by temperature changes as well as shock and vibration. Compared to resistive touch technology, PCAP is still susceptible to interference by electrical fields. The
housing must also be taken into account in the early phases of design to prevent external interference from causing unintended activations, coordinate miscalibration or even unresponsiveness in parts of the touch surface. Examples of the source of problems include the metallic housing of the TFT itself as well as the metallic frame of the application housing. This is why manufacturers of touch sensors specify appropriate minimum safe distances. Wires in the application should be laid so that they do not pass near the PCAP's terminals. A well-conceived earthing approach is generally a fundamental requirement for the PCAP to work correctly.
Statements have shown that even if the
trend in industry is moving towards PCAP touch technology, it is not the right choice for every application. Depending on where the application is to be deployed and the control concept in use, other technologies (resistive, SAW or IR touch technology) may be superior and offer a better cost/benefit ratio. Traditional resistive touch technology, for example, still has a place in this world thanks to its ease of integration and the ability to operate it without problems while wearing gloves. But if multi-touch, gesture detection and intuitive operation are essential, more engineering work will be required. And once a PCAP application has been correctly planned and implemented, it will provide optimum functionality and reliability for the entirety of its life.
www.rutronik.com
24 September 2016
Components in Electronics
www.cieonline.co.uk
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