Specialist article on membrane keyboards
Also published in PC & Industry: Operation + Visualization 06/2021: Article as .pdf >>
The motto of making a human-machine interface understandable and intuitive often requires a combination of proven electromechanical components in combination with multi-touch displays. Where previously complex keyboard layouts were created to operate a machine, today complex menu structures can be visualized using logical and clear designs on a touch display. With a combination of modern display technology and proven keyboard technology, the advantages of both worlds are played out effectively and economically. The membrane keyboard in particular has proven to be a useful addition.
An additional keypad next to the Touch display promotes user-friendly menu navigation by allowing recurring input commands or preset parameters to be called up quickly, easily and clearly. Through the haptic feedback of a membrane keyboard user-related input errors can be avoided, which increases the security of operation. Integrated snap-action discs ensure not only a noticeable tactile sensation when a key is pressed, but also an audible clicking, acoustic feedback. The metal snap disks are available in a wide range of sizes, shapes and actuation forces and are selected according to the application. Membrane keyboards without metal snap-action discs are used in environments where actuation noise is a nuisance. The key embossing alone ensures safe finger guidance and tactile feedback.
Window in membrane keyboards
There are various realization techniques for the execution with touch screens. The simplest variant is to punch out the display area, another is to integrate a viewing window into the membrane keyboard. Depending on the requirements, these windows can be individually specified with a glossy, matt, anti-reflective, scratch-resistant, UV-resistant and colored finish. The film and display are connected by a highly transparent adhesive layer with a light emission level of >99%. By gluing both input elements together, a closed surface is created with high resistance to chemicals and impermeability to liquids and dust. This makes the combined input solution interesting for applications in the food, laboratory and medical sectors.
For applications in the medical field and environments with high hygienic requirements, the front foil can be a membrane keyboard also with one antibacterial coating be equipped. For subsequent easy assembly, the user interface can be combined with an aluminum front panel. Bolts or threads can optionally be integrated directly into the front panel. Another option is to connect the membrane keyboard to an injection-moulded housing or injection-moulded frame. The molded part can be designed in such a way that a display can be clipped in directly, thus simplifying further processing steps.
Design options of membrane keyboards
Product design is an increasingly decisive purchasing factor. The individual foil layers are precisely punched, glued and can then be inserted almost seamlessly into the correspondingly exposed housing contours. Even when the black display is switched off, the membrane keyboard takes on significant design aspects. In addition to the common screen printing process, digital printing is particularly suitable for the realization of color gradients and for prototypes and small series. In contrast to the pure LC display, a printed surface can also be seen from a viewing angle rotated by 180°.
Specially embossed foils can even be used to achieve metallic effects such as brushed stainless steel or anodised aluminium. A high-quality epoxy coating gives the keys a particularly elegant, three-dimensional look.
It is also possible to replace the upper polyester film with a silicone surface. This silicone cover with keys can be laminated onto the circuit pack of a normal membrane keyboard as a pure actuation layer. The resulting hybrid of classic membrane keyboard and silicone keymat gives scope to combine the advantages of the individual input solutions in one keyboard. This enables protruding keys with free shapes and guide elements, such as separate rings around a key, Braille inscriptions or cursor keys. At the same time, the surface is visually and haptically enhanced.
lighting options
Depending on the needs of the application, backlight options vary. For example, an illuminated "ON" button can also be easily found in darkened work environments. LEDs can be easily integrated into the existing switch membrane or, if necessary, integrated into an additional LED switch membrane. Special adhesives and sealing materials are used to ensure the function of the LED even in harsher environments with strong vibrations, for example. In addition, the LEDs achieve sufficient brightness even in daylight, which distinguishes them above all for their function as status displays.
The backlighting of a membrane keyboard can also be done by using LGF (Light Guide Film). The typically flat design and flexibility are retained. Each key can be backlit in different colors, and the LGF technology does not require a high-frequency AC voltage source. A special feature are labels with a disappearing effect. This is an inscription that cannot be seen when the keyboard is not illuminated.
EMC shielding of membrane keyboards
A reliable and trouble-free function of the input devices is more important than the design, especially in sensitive measuring environments. Especially when several electrical sources are used in one place, malfunctions and disturbances of the individual devices can occur if there is insufficient shielding against electrical or electromagnetic effects.
The EMC shielding of a membrane keyboard is realized, for example, by an aluminum or copper-coated polyester foil. The shielding film is integrated into the keyboard in such a way that full-surface shielding is guaranteed. This shielding effect prevents the device from malfunctioning due to the ingress of unwanted signals, as well as interference in its own electromagnetic environment. Circumferential seals prevent short circuits caused by penetrating moisture. The membrane keyboard can thus achieve a protection class of up to IP67. In connection with integrated displays, a segmented EMC shielding of the keyboard and conductors is used.
