Published in PC & Industrie 10/2020
Article as .pdf (German) >>
You might think that the conventional keyboard as an input element has its golden age behind it. In the telecommunications sector, it still ekes out an existence in landline telephones, but in the mobile sector, it has had to give up almost completely to smart display technologies. The integration of multi-touch screens enables mobile devices, where the use of a mouse and keyboard is rather impractical, to exploit their full potential.
The graphic visualization options create interactive communication experiences for the user in line with the corporate design of the company. Logical visualizations, as well as clear designs, make even complex menu structures easy to use. The demand to make the interface between man and machine uncomplicated and intuitive is also driving development in the area of gesture control, eye tracking and voice control further and faster. User-related input errors are to be eliminated, and the safety of operation is to be increased. This also makes touch displays very popular for applications in home automation, kiosk systems or plant and machine control.
Google, Siri or Alexa
It remains to be seen whether humans will still need a haptic input interface in the current sense in five or ten years. For a long time now, we no longer need to press a play button to listen to music or, thanks to modern smart home integration, to turn on the light switch. A small request to Google, Siri or Alexa will suffice. More AI-driven technologies will set the trend in the future.
While Generation Y and older grew up with a variety of hardware systems that today’s generations know only by hearsay and consider the use of a keyboard an everyday occurrence, future generations may smile at this as a nostalgic use. Just think of the further development of calculating machines in the last 50 years or the development of the first mobile telephones – all not so long ago.
In the status quo, however, input keyboards have their absolute justification and should be seen as an alternative or supplement to touch systems and co. depending on the area of application. Especially in devices with a small number of functions, which at the same time require good visibility and are in constant user interaction, proven input components such as membrane keyboards, silicone keypads and pushbuttons remain the preferred choice – also in terms of cost. After all, the financial outlay for programming a graphical user interface between man and machine is a not inconsiderable factor. It is true that often not everything that is technically possible is also economical.
Membrane keypads and silicone keypads are currently used in almost all industries due to their closed surface, resistance to chemicals and their impermeability to liquids and dust. In the medical sector, for example, membrane keypads can be found in nurse emergency call systems, controls for hospital beds, input keypads for diagnostic equipment and defibrillators, as well as other interfaces for patient care. Many of these input devices are also operated by inexperienced users, such as elderly patients, so that a complex touch input solution is rather disadvantageous.
The keyboard is also evolving
Due to constant further development, however, the membrane keypad should not be seen as a pure switching solution. Rather, it is a combination of sophisticated design, special coatings and equipment extras such as integrated LEDs or an integrated touch panel. In this way, membrane keypads can serve with a user-friendly and intuitive interface thanks to their diverse design options. Through the use of tools, it can take on any shape.
Complementary to the common screen printing process, digital printing is particularly suitable for the realization of color gradients and for prototypes and small series. Using specially embossed films, it is even possible to achieve metal-look effects, such as brushed stainless steel or anodized aluminum. Another advantage over display solutions is that the printed surface, as well as optionally integrated LED status displays, can also be seen from a 180° rotated viewing angle.
The flat structure of a membrane keypad consists of a foil composite made up of at least four foils: The decorative foil, the top and bottom switching foils and a spacer foil. The decorative foil, as the top layer of the keyboard, is made of polyester or polycarbonate and is printed on the reverse side to ensure the highest possible protection against abrasion. Compared with other flexible materials, polyester in particular impresses with a high life cycle of over 1 million switching cycles, very high abrasion resistance and resistance to chemicals.
For applications in the medical sector and areas with high hygienic requirements, the front film can be equipped with an antibacterial coating. This antimicrobial polyester kills or inhibits bacterial growth and provides constant protection against bacterial contamination. The functional elements of the membrane keypad are the lower switching foil with the conductive tracks, LEDs, switching points and the connection lug located on it, and the upper switching foil with the contact surfaces.
Depending on the application-specific requirements, the options for backlighting vary. Especially in darkened work environments, illuminated keypads are necessary to easily find important functions. Key embossing can provide additional support here. LEDs can be integrated very easily into the existing switching foil or, if required, an additional LED switching foil can be added. Special adhesives and sealing materials are used to ensure the function of the LED even in harsh environments with, for example, strong vibrations.
The backlighting of a membrane keypad can be achieved by using LGF (Light Guide Film). In this case, the typical flat design and flexibility are retained. With LGF technology, LEDs are inserted at the edge into a very thin (0.1 – 0.2 mm), highly light-refracting film. Due to the high light refraction, only very few LEDs are required for homogeneous illumination, depending on the size of the membrane keypad. The backlighting of each key can be in different colors.
In contrast to EL backlighting, LGF technology does not require a high-frequency AC power source. A special feature are letterings with disappearing effect. This is a labeling that is not recognizable when the keypad is not illuminated and is only visible when the keypad is illuminated. This allows a clear ON / OFF mode to be signaled.
Another advantage of the membrane keypad is the tactile feedback to the user. Snap dials provide a noticeable tactile feel when pressed and, depending on the design, an audible “click” sound. This direct feedback reduces file entry errors. Metal snap domes are available in a wide variety of sizes, shapes and actuation forces and are preferred for applications with a high number of actuations. Membrane switches without metal snap domes can also have tactile feedback by using an embossed design.
Embossing can be applied to both the decorative foil and the top switching foil. There is the possibility of key, dome and edge embossing with which very ergonomic key shapes can be realized. Embossed keys also have the effect of finger guidance. Keys are given a particularly noble look by a high-quality epoxy coating, which gives the key a three-dimensional appearance. The combination of snap domes and key embossing is common.
In addition to the proven variants consisting of different, superimposed foil layers, there is also the option of replacing the upper polyester foil with a silicone surface. This silicone cover with keys can be laminated onto the circuit package of a normal membrane keypad as a pure actuation layer. The resulting hybrid of a classic membrane keypad and a silicone switching mat gives scope to combine the advantages of the individual input solutions in one keypad. With the silicone surface, a three-dimensional design can be realized, a point at which the membrane keyboard is naturally severely limited. This makes it possible to have protruding keys with a free design, guide elements such as offset rings around a key, Braille inscriptions or cursor keys. At the same time, the surface is visually and haptically enhanced.
Combination of membrane keypad and touchscreen
If more functionality or a more complex operating menu is desired, a combination of membrane keypad and modern touch screen solutions can be used.
An additional keypad next to the touch screen promotes user-friendly menu navigation by allowing input commands or preset parameters to be called up easily and clearly. Through appropriate programming, the fixed keys can also assume dynamic functions, matching the respective menu. For the version with touch screens or display screens, viewing windows are integrated into the membrane keypad.
Depending on the requirements, these can be individually specified as matt, scratch-resistant, UV-resistant and colored. The connection of membrane keypad and touch screen is realized by a highly transparent adhesive layer with a light emissivity of >99%. This process prevents the formation of Newtonian rings.
A similar combination is also possible with silicone keypads. In this case, the key layout is molded directly onto the plastic front frame. This creates an optimum bond between the two components. The display and electronics are combined with the resulting frame in a further processing step. The advantage of the multi-component injection molding technique is that only one component needs to be designed. This leads to significantly reduced investment and effort in production and quality assurance.
Silicone Rubber Keypad
The single, classic silicone switching mat is still one of the most widely used switching elements in the electronics and automotive industries. When operating a remote control alone, it is used millions of times every day. However, for medical applications, where good cleanability of input elements is required, the silicone switching mat can also hold its own thanks to its closed surface.
As a central component, it triggers a direct switching function on the circuit board underneath and determines the haptics, the visual and tactile impression and, via the integrated contact elements, the electrical behavior. In addition, the switching mat often performs a sealing function on the printed circuit board. Its mechanical service life of up to 10 million switching cycles makes it a durable and virtually maintenance-free input solution. It adapts to your design and is extremely flexible and space-saving. Key caps made of metal and plastic, different key shapes and individual key colors can easily be realized in one mat.
The illumination of individual keys or the entire mat is possible by LEDs in combination with transparent silicone or integrated light guides. A very precise 2K injection molding process can also be used to realize multicolor keyboard lighting. The process allows the integration of a light channel in each individual key. This light channel is absolutely isolated from the rest of the transparent key and achieves precise focusing of the light beam. Safety mats with this technology can now not only be backlit, the individual keys can also display status indicators.
The force-displacement behavior
The force-displacement behavior of a silicone safety mat can be customized to meet the requirements of the specific application. Actuating forces from 20g to 500g and actuating travels from 0.3 to 5.0mm are possible. The realization options for the electrical contacts range from low-cost, yet safe carbon printing to high-quality metal contact pills with a current-carrying capacity of up to 2A. The silicone switching mat can particularly show its advantages when large quantities are required. The high one-off costs for the tools make small series very cost-intensive. However, compared to equipment with individual probes, they are significantly cheaper even in medium series.
Vandal-proof input key
In public areas or local traffic, as well as in industrial machine control systems, vandalism-protected input pushbuttons continue to play a role, ensuring precise and reliable operation, as well as quick retrieval. The pushbuttons often take on the functions of a pure command transmitter, so that signals such as start and stop or forward and reverse are clearly communicated. With engraved or printed pushbutton covers, colored Satus LEDs and various housing colors, pushbuttons can also be given an individual look. Multicolor illumination even makes it possible to display multicolor status indications.
The pushbuttons are made of robust plastic or metal and are built directly into a front panel and housing. Appropriate seals give them a protection class of up to IP68 / IP69K. The service life of up to 1 million cycles also makes the pushbuttons very reliable. Piezo switches, which are made of metal, offer a longer service life. Due to the omission of a mechanical stroke, wear and contamination are virtually eliminated.
Lifetimes of over 50 million cycles make them very reliable and virtually maintenance-free. They are used in areas where vandalism damage is to be expected or where they are exposed to heavy loads due to environmental influences.
Miniaturization and design
For each of these proven electromechanical operating units, the increasing demand for miniaturization and design orientation plays an important role. Pushbuttons with low installation depths, membrane keyboards with extensive features and a simultaneously flat design reflect the trend. The end devices are to become flatter, thinner and lighter, but miniaturization also reaches its limits in the input area; after all, the devices still have to be operable by humans. Touchscreens and voice control, coupled with further developments in the field of artificial intelligence (AI), are becoming increasingly present, precisely because they are already an essential part of our everyday lives.
In the status quo, however, innovative control elements with a combination of the proven electromechanical control units and modern touch panels continue to provide for exciting further developments, also with regard to an increasing addition of gesture controls.
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