PCAP touch panels for extreme outdoor applications

With many years of experience in developing and producing PCAP touch panels, SCHURTER Electronics Ltd offers high-quality, customer-specific input systems for every application. Its engineers develop and design the optimum solution for usage under particularly demanding conditions in close cooperation with its customers.

PCAP touch panels for use in extreme environmental conditions need a special design, and such systems must withstand far higher demands for safe everyday operation in outdoor applications. How can this be achieved?

Outdoor applications vary in many ways from indoor applications. Outdoor applications must satisfy higher temperature and weather resistance specifications for usage in extreme weather conditions. To fulfil these challenges, there are weatherproof and functionally stable PCAP touch panel solutions specially qualified for outdoor applications.

Outdoor touch panel applications are exposed to direct sunlight. Long-term UV exposure, therefore, damages the material and leads to yellowing, clouding and bubbling or even cracking of the touch sensors. These optical defects reduce not only the light transmission of the touch panel but also its appearance. The wavelength of UV light (100nm to 400nm) is outside the visible range. The glass of a PCAP touch panel blocks UV wavelengths of lower than 300nm, therefore, also UVB and UVC radiation. Longer wavelengths are blocked by more than 95% via the integration of UV filters. These UV filter materials are qualified according to ASTM G154 and MIL-STD-810H standards with 1000 hours each in an accelerated UV ageing test.

Another qualification test is the so-called weathering or Florida test. In this test, the effect of the whole spectrum of sunlight (295-800nm), including rain sprinkling on the touch panel, is simulated. Within a short time, this test allows a statement to be made about possible UV damage after months or years of outdoor application. The UV filter function protects not only the PCAP sensors and the bonding but also the integrated display, therefore, preventing yellowing.

Printing on the front glass is frequently done with ceramic ink. However, ceramic printing provides just a limited selection of colours, thicker ink layers and high baking temperatures. When utilising UV-based organic printing, a larger colour selection can be realised with decreased layer thickness and identical light tightness. This printing technique also passes the standards mentioned. The qualification demonstrates that the ink does not change under UV influence and the adhesion of the print stays stable over a longer period. Therefore, printing with UV inks provides optimum stability for PCAP touch panels for outdoor applications.

Harmful gases generated by air pollution can lead to the corrosion of components and contact surfaces in outdoor applications. Sulphur and nitrogen, for example, form sulphuric and nitric acid when they react with moisture. These harmful gases can create oxidation and corrosion of the silver conductive paths of the PCAP touch panels. To protect the touch sensors, they are designed to be impervious to harmful gases.

Touch panels in outdoor applications are exposed to large temperature fluctuations every day. In harsh sunlight, they

must withstand temperatures of up to +80C. Therefore, touch applications in outdoor areas need operating temperatures from -40C to +85C. Qualified materials are offered for these applications that fulfil the storage temperature range of -50C to +90C. Via environmental simulation and long-term tests, these PCAP sensor materials and optical adhesives have been successfully qualified for outdoor applications.

As well as the stability of the sensors, a controller is needed that also fulfils this operating temperature range. The PCAP system must also be able to automatically compensate for capacity changes caused by strong temperature changes in the environment. Special algorithms ensure this in the firmware of the controller used. Depending on the application and its specific requirements, PenMount, ILITEK or EETI controllers are preferred.

Up until now, liquids on the touch panel surface have caused false triggering or malfunctions in the application. Due to the further development of special evaluation electronics-based self-capacity measurement, the functional reliability of PCAP and CapKey input systems is now ensured even under running water. This indicates that physical buttons can be dispensed with, and the buttons or sliders can be incorporated behind the glass as a CapKey solution.

The hardware and firmware of the CapKeys have also been optimised for conductive substances, such as salt water. This allows an operation without malfunctions in new application areas. Even cleaning agents or soapy water do not cause the CapKeys to be triggered incorrectly. Water-resistant PCAP touch solutions can be employed in many applications where liquids could cause malfunctions, such as outdoor applications, marine equipment, medical devices and sanitary applications.

Particularly in applications with extreme environments, the realisation of an optimal HMI solution needs a high degree of know-how and experience. All specifications and their special demands in the application must be verified and qualified.