The Transparent Membrane Switch

My first exposure to membrane-switch technology was in the mid 80’s, when I was hired by an electronics company to design and manage the setup of a touch screen manufacturing facility. Initially, I didn’t realize that this strange, see-through device was actually a non-tactile membrane switch. If you haven’t used a touch screen yet, I would be very surprised—they appear everywhere, from pocket organizers to ATMs. Or perhaps you’ve seen restaurant employees entering order information on touch screen monitors. What the touch screen does is allow the user to interact directly with the display, rather than make selections with a keyboard. Because you physically touch the correct command, the speed of data entry increases dramatically while the chance of errors decreases. The most impressive feature of touch screens is that the functions shown and the location of these functions are constantly changing as the display itself changes. The initial display may show a range of user-selectable command options. When one of these options is touched, a new display appears with a new range of commands. Another benefit was illustrated by a touch-screen I helped develop for a medical-device manufacturer. Because the equipment the touch screen controlled was destined for use world wide, the device required instructions in multiple languages. Rather than using permanent decals or graphic overlays printed in each language to label controls, the touch screen could be programmed to display labels and instructions in the appropriate language.

Several technologies exist for building touch screens:

The transparent membrane switch falls under the “resistive” category, but it also might be helpful to understand the other options and their benefits and limitations.

Capacitive

The touch of a conductive stylus (usually a human finger) on this type of touch screen changes the capacitance of the screen, allowing a computer to determine the location of the touch. The construction consists of a rigid piece of glass covered on one side with a transparent conductor that itself is covered with a protective coating. Capacitive touch screen provide very good light transmission, but won’t work if the stylus is not conductive (for example, the user is wearing gloves).

Infrared

Small LEDs hidden the bezel around the display emit light beams in both the X and Y directions over the display surface. When the user’s finger or stylus breaks the light beams, an X and Y location is determined. This type of touch screen has no moving parts and is very durable. Once calibrated, its ability to identify touch coordinates will not drift. However, its resolution (ability to detect changes in touch position) is limited by the number of LEDs in the horizontal and vertical directions.

Acoustic Wave

This construction uses one piece of rigid glass in front of the display, and acoustic waves are transmitted across the surface of the glass (or through the glass itself). It requires a soft stylus—such as a finger—to absorb the energy of the wave. The controller that drives it determines touch coordinates by recognizing a change in the wave frequency at the touch location. This type of touch screen also offers excellent transmission and is unaffected by scratches and other surface damage. But it may be unsuitable for high-noise areas or locations where it is subject to vibrations.

Force Sensing

With this technology, pressure from touching the display is registered by strain sensors mounted at each corner of a rigid, four-sided piece of glass. The different strain levels recorded by the sensors are used to determine touch location. This type of touch screen can by used over any type of monitor to make it touch sensitive. It offers excellent light transmission, but tends to be very expensive.

Resistive

This is the technology used in membrane switch touch screens. Like all membrane switches, the resistive touch screen includes a transparent, flexible membrane layer and a transparent static layer. The flexible layer is polyester with a conductive coating. When pressed (using either a finger or a non-conductive stylus), the conductive coating makes ohmic (resistive) contact with a conductive coating on the static layer, which can be made of rigid polyester or any other transparent, rigid material. Adhesives that keep the layers aligned and in close proximity to one another are located only on the periphery of the transparent area. However, small insulators are interspersed between the layers across the display area to control actuation force and prevent the layers from making contact when the screen is not being touched. One drawback to resistive touch screens is that they incorporate two layers separated by an air-filled gap, all of which combine to reduce light transmission and make displayed functions appear more diffuse. Also, the conductive coating used, indium tin oxide (ITO), can act like a mirror in certain lighting conditions, inhibiting the readability of the display. However, resistive technology is generally the least expensive touch screen option. Additionally, the top layer is a continuous film, which simplifies sealing against harsh environmental conditions. Also, the flex cable running to the controller is an integral part of the touch screen and easily shaped to support various applications. Finally, touch screens can be produced as part of a larger decorative panel or keyboard assembly, simplifying the overall production process. Resistive touch screens can take one of two forms: digital and analog.

Originally posted at Membraneswitchnews.com by Alan Burk

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