Electromechanical Input Systems – But Which Ones?

Selecting a keypad means comparing the features of the many different technologies available, including membrane or silicone keys or even custom designs that combine one or more technologies. By Steve Appenrodt, Product Sales Manager for Switches at Rutronik.

When it comes to input systems or human-machine interfaces, the focus is currently centered on touch-screens. Despite touch screens’ advantages, keyboards are still the better alternative for many applications, and there are a number of modern solutions which go beyond the conventional keyboard commonly used for computers and laptops.

In contrast to electromechanical input systems, the advantage of touch-screens is that their service life does not depend on the amount of wear and tear electromechanical contacts are subjected to. A further highly interesting point of comparison, in terms of design and hygiene, is when the outer surface of the input system is shielded fully, perhaps by glass. However, not every application demands real "touch" operation. Sometimes, depending on the application, this type of operation is not even desired, as it is impossible to represent switching action through a tactile pressure point and stroke. Conventional electromechanical solutions are therefore first choice, particularly when cognitive operation is not possible.

But what does "conventional" actually mean? The term basically refers to keyboards consisting of individual components (discrete keys) and flat surface input systems (membrane keypads, silicone keypads).

Keyboard Consisting of Individual Components

The first thing that comes to mind when you hear the word "keyboard" is usually a computer or a laptop. This type of keyboard is characterised by a relatively small variety of keys, while certain keystroke and actuation force requirements must be met. This can best be achieved by employing individual keys.

The use of discrete keys, however, places users at a significant disadvantage. A large number of individual components need to be soldered onto a circuit board and each solder joint is critical in achieving maximum keyboard performance and reliability. Each key head needs to be labeled individually, resulting in higher material and stock management costs. And last, but definitely not least, designers need to come up with a solution for fixing the comparatively bulky component safely into or onto the device.

Membrane Keypad

Membrane keypads can often be found on sports equipment (ergometers, cross trainers or treadmills), vending machines, and robust portable devices, such as mobile measuring instruments and robotic lawnmowers. All that is visible to the user is a plastic film, possibly consisting of a window behind which graphic and numeric displays or LEDs can be found. This front membrane (also referred to as a graphic overlay) is usually embossed in order to enhance finger guidance and operating reliability. The most common types of embossed finishes for keys include rim, dome, and pillow. An embossed front membrane is also an ideal solution if Braille is required – for example if the keypad is to be used in a public building.

The synonym "graphic overlay" indicates unequivocally a specific advantage: the graphic design options are nearly unlimited! In contrast to keyboards consisting of individual components, which only allow for round or square keys, membrane keypads enable any shape of key at no additional cost. They also give total freedom to the designer in terms of colors for company logos and corporate design or for the optical layout of the surface. The membrane is always printed on the rear, meaning little consideration must be paid to actual wear resistance.

The base material for a front membrane is a clear polyester or polycarbonate film – materials that are resistant to both temperatures and all forms of attack from substances. They ensure, for example, the keypad is resistant to oil in production areas or workshops or guarantee the UV resistance of colors if the keypad is installed outdoors. In addition to the various embossing and printing options, designers also have the choice of various structures: shiny, matt or textured finish, or even a combination of these, that is, a matt film with high-gloss key head is possible.

The various application areas are further extended by additional features such as electrical shielding, interchangeable labeling, varied illumination effects, etc.

A self-adhesive layer on the rear seals the membrane keypad. Keypad installation is thus pretty simple: peel off the protective paper, lay the cable, stick it in place – done!

By the way, with membrane keypads it is also possible to achieve an effect that is otherwise only seen with sensor keys or touch-panels. When the keystroke is kept relatively short, there is no tactile stroke or pressure point – evaluation is nonetheless straightforward… since they are "real" electromechanical contacts neither auxiliary energy nor controllers are necessary. This means lower costs, simpler design, and greater energy efficiency.

However, there is one disadvantage - the third dimension. When a keypad not only needs to be long and wide, but also thick in regard to the key shape and/or stroke, there are certain physical limits to what is possible with a film. Silicone keys, in contrast, can be several centimeters thick.

Silicone Keypad

Silicone keypads are used for applications like remote controls in the multimedia industry. As can be expected, silicone rubber offers larger strokes and therefore achieves a characteristic tactile feedback. Actuation strokes of up to 4mm can be realised quite easily.

To ensure everything functions not only mechanically but also electrically, there is a conductible disk (or sometimes several) beneath each key. The disk is traditionally made of carbon (carbon pill), alternatively of various conductible materials or even features a gold-plated surface. The latter displays less resistance which helps to reduce power consumption and increase the service life of the application – an obvious advantage, particularly for battery-operated devices. The counterpart is an electrode pattern on a printed circuit board. The conductive pill creates a short circuit between two contact areas on the PCB while pushing the silicone rubber key.

The inherent properties of silicone rubber, such as a wide temperature range and simple - and thus cost-effective - mouldability, further add to its appeal.

Silicone is widely used for switch technology mainly due to the reliability and long life of the keypads. Needless to say, key labelling is a critical factor for a sustainable solution, because what is the point of a durable keypad if you can no longer read the keys? Wear resistant screen printing colors, various coatings (from lacquering to PU coating), and laser printing are suitable solutions. In combination with various rubber colors (several colors are possible on a single keypad) and backlighting, designers are offered a wide and inspirational choice of options when creating silicone keypads.

And if it has to be something completely different: a keypad disguised as steelwork with a stainless steel front panel and key buttons appears extremely robust, while the substructure is an economical flat keypad.

Combining Various Technologies

Somewhat different are keypads that combine the features of the two above mentioned technologies or other technologies. Conceivable applications may include, for example, medical technology in the care sector, where excellent haptic feedback and easy-to-clean devices are required. In this case, a silicone keypad could comprise cutouts for a display and the surface, excluding the keys, can be covered with a PE film.

Individuality is Key

Touch principle-based input systems open up completely new avenues, while offering real benefits. However, the majority of applications can be operated effectively and efficiently using the contact establishing methods described here.

The decider for each solution is its individual adaptation to the respective application. This relates to the dimensions as well as to the environmental requirements or design brief. To ensure specific requirements and costs are met in the most efficient manner possible, field application engineers and product managers at Rutronik work in close cooperation with the specialists for membrane keypads and silicone keypads at knitter-switch to support customers worldwide. The German manufacturer has been at the forefront of switch technology for roughly 50 years, with customers profiting from tried-and-tested, state-of-the-art products, extensive experience, and exceptional knowledge. knitter-switch develops all its solutions in Germany, offering uncomplicated on-site support and effective coordination. Production in Asia enables innovative products at competitive prices.

In Summary: Which Keypad is Best for Which Application?

• Silicone keypads offer a certain overall key height, possibly longer switch life, and lower costs.
• When graphic design is thrown into the mix, a membrane keypad is definitely the best choice – the same applies when simple installation and effective sealing are demanded.
• A keypad consisting of discrete keys is only recommended for special applications, e.g. computer keyboards. If you want to type efficiently for long periods of time without "finger fatigue", rubber or membrane keyboards/keypads are not the best choice.

Rutronik