Over the past few years, touch screen technology has revolutionized the way we view data input devices. With the introduction of high-resolution digital displays, touch screens now allow users to be truly interactive with electronic devices.
Originally designed for use in industrial applications, touch screens are now being used everywhere from warehouse inventory controls to restaurant order entry. More and more we see touch screens being adapted to our personal lives.
Consider the Personal Data Assistant (PDA). Many of us use them on a daily basis. In fact once we become comfortable with the system we often rely on our PDAs to help us keep track of not only phone numbers and addresses but also our finances and daily schedules as well.
Touch screens help provide a good method by which we can utilize nested programs within electronic devices. No longer are we confined to the limited number of discrete buttons found on conventional membrane switches. With touch screens used in combination with high-resolution digital displays and nested programming we have literally millions of switch options available at the touch of a finger.
There are five basic technologies used to build touch screens. This guide focuses on the resistive type, AKA the "transparent membrane switch", but gives a brief product comparison to the other touch screen technologies.
The five touch screen technology types are:
The ASTM subcommittee on membrane switches defines a membrane switch as: "A momentary switch device in which at least one contact is on, or made of, a flexible substrate". Resistive touch screen technology satisfies all of the requirements necessary to be classified as a membrane switch. If we take a closer look at the construction shown in figure 1, we see that a resistive touch screen is made up of a membrane layer and static layer separated by tiny dielectric dots that help to keep the switch open until a force is applied to the membrane layer.
The membrane, or flexing layer, is made of thin transparent, conductively coated polyester. When pressed, using finger or stylus, the transparent conductive coating makes ohmic (resistive) contact with the opposing transparent conductor, the static layer, on the other side of the switch.
The Static layer can be made of the same flexible material as the membrane layer or a clear rigid material, such as glass or polycarbonate that is also conductively coated.
Adhesive material is applied only on the periphery of the transparent area for sealing purposes. However, there has to be small dielectric insulators that prevent the membrane and static layers from making contact when not being touched. The insulators or dots as they are called also help control the actuation force necessary to close the switch.
In the past one of the drawbacks to resistive touch screen technology has been that there are several layers and air gaps to look through, each contributing to light transmission loss and diffusion. This problem has been greatly reduced by using a process called optical bonding, where the static layer is bonded, using optically clear PSA, directly to a transparent rigid backer, thus eliminating one of the air gaps. In even more recent designs the flexible static layer is eliminated and replaced with rigid conductive-coated glass or polycarbonate. This not only solves the problem of light transmission loss but also reduces the cost seen in the optical bonding process.
Despite minor drawbacks resistive touch screen technology is generally considered the most popular type of touch screen. This growing popularity comes from the tact that resistive type touch screens are the least expensive of all touch screen technologies. In addition to low cost the resistive touch screen design is totally sealed from outside contamination. This is important where Nema 4 conditions must be met, especially in situations where gloves are frequently worn such as: Industrial Controls, Medical, and Test & Diagnostics applications. Resistive touch screens can also be combined with a decorative graphic layer and standard membrane switch keyboard components to create a more user-friendly data input device.
There are two types of resistive touch screen technologies: Digital and Analog. The following sections provide a detailed explanation of the construction for both types.
Sometimes referred to as the X-Y Matrix touch screen. The ITO coating is etched (or selectively applied) to form rows on one layer and columns on the opposite layer. When assembled the rows and columns form a matrix of switches. Each switch is a permanent discrete switch location and the size cannot change. The resolution is dependent on the number of rows and columns. Digital touch screens are as easy to interface with as a conventional membrane switch keyboard and are popular when finger size touch zones are required. See figure 2 for an example of a 6 row, 4-column digital touch screen that once assembled will have 24 individual touch locations.
The analog touch screen is a construction used on pen recognition type computers or when a much higher resolution of switches is required than is possible with the digital design. Resolution of the analog touch screen is limited only by the lighted panel behind the touch screen and can easily reach 1400 dpi or better. Also you only need a 4 or 5 pin connector whereas the digital requires one pin for each row and each column. The analog membrane layer carries the X (or Y) axis and the non-flexing layer (static layer) carries the opposite axis.
Both axes must remain perfectly linear. Since the membrane layer is flexing there is a possibility for fracturing of the ITO (conductive coating), especially along the edge of the transparent area, leading to failure and calibration loss. However most manufacturers guarantee at least 1 million actuations in any given touch area. The example shown in figure 3 is a 4-wire analog touch screen. When the two layers are placed on top of each other you basically have a single switch, i.e., no matter where you touch between the bus bars you close the same switch. However, the computer will detect location based on the voltage drop sensed in the X (membrane layer) and Y (static layer). That information is processed by the CPU and shown as a location on the digital display.
This section gives a brief description of 4 other touch screen technologies with comparisons made to the resistive type of touch screen.
Capacitive Touch Screen: The touch of a conductive stylus (usually a human finger) on this type of touch screen changes the capacitance of the screen allowing the CPU to determine location on the screen. The construction consists of a rigid piece of glass with a transparent conductor on one side. A thin protective coating (hardcoat) is over this conductive surface.
Advantage: Light transmission is very good.
Disadvantage: Will not work with non-conductive stylus, High cost
Infrared Touch Screen: Small LEDs emit light beams in both the X and Y directions over the entire surface of the display. The LEDs and sensors are hidden in the bezel perimeter so that when the user's finger or stylus breaks the light beams, the CPU determines both X and Y location.
Advantage: This design does not have any moving parts and is very
durable and once calibrated will not drift.
Disadvantage: Resolution limited to spacing of the LEDs in both the
horizontal and vertical directions, High Cost
Acoustic Wave Touch Screen: This construction uses one piece of rigid glass in front of the display. Horizontal acoustic waves are transmitted across the surface of the glass or within the glass. This technology requires a soft stylus like a finger to absorb the energy of the wave (will not work with a non-elastic stylus). The controller will recognize a change in the wave frequency for both the X and Y direction and determines the touch location.
Advantage: light transmission is excellent.
Disadvantage: scratches and other surface damages will affect the Input
results so they do not work well in high traffic, indoor,
public settings, High Cost
Force Sensing Touch Screen: Pressure from touching the display results in a mechanical movement, which is converted to electrical signal and then touch location by the CPU. A strain gauge is mounted to each of the four corners of a rigid piece of glass. When the glass is touched each of the gauges are strained differently which equates to a touch location. The strain gauges can be placed in a platform that the monitor sits on. Using this platform any monitor can be placed on the platform and be transformed into a "touch" monitor.
Advantage: Light transmission is excellent.
Disadvantage: Custom strain gauge touch screen designs are very expensive.
There are additional components that help create a complete custom resistive touch screen. In addition to the membrane and static layers mentioned above, components such as adhesives, spacers, bezels, shielding, and plastic housings are also essential to meeting the product design and customer specifications.