There are a number of popular touch technologies. No one technology is right for all applications. Please take a moment to review the strengths and weaknesses of each type, before you decide which sensor is right for you. As always, when in doubt, please call the factory for advice.

A resistive touch sensor has two layers that are not in contact with each other

• an outside flexible layer, coated on the inside with a conductor, e.g. ITO, (Indium Tin Oxide),
• non-conductive separator "dots", e.g. mica or silica, small enough to not be noticable,
• an inside supporting layer of glass, coated on its face with conductor (ITO).

Around the outside of the viewing area is a printed circuit pattern. When the outside flexible layer is pressed against the inside layer, the resulting voltages can be measured in more than one direction. By comparing these voltages to the starting voltage, the point where the touch happened can be calculated.

Resistive sensors are simple and relatively inexpensive. The flex required to push the surfaces together can eventually fail the flexible layer. The flex requires some pressure, so force is required to make contact. The presence of two layers in addition to the LCD can reduce brightness by 10-20%. On the plus side, a new flexible glass outer layer is now available to improve wear and chemical resistance. Overall, the technology is simple and predictable, so resistive sensors remain the most common form of touch sensor.

The active surface of a resistive sensor is slightly larger than the LCD it serves. To avoid false touch where the LCD is mounted in the cabinet, the bezel should be slightly outside the active area, or the seals inset. With these precautions, resistive touch can provide face seals that can accept a pressure wash.

A capacitive touch sensor is a simple supporting sheet of glass with a conductive coating on one side. Around the outside of the viewing area is a printed circuit pattern. This pattern sets a charge across the surface, which is disturbed by the finger touching the screen. There are various technologies that detect the touch differently: some can even detect near-touches without making contact.

All capacitive sensing methods look for electrical disturbance, so the electrical characteristics of the touching object are important. One finger is much like another, but a glove, a stylus or other types of objects may change the response or not respond at all.

Capacitive sensors are built right on a single glass layer. They don't break down by from flexing, but they may be subject to abrasion of the ITO coating. The single layer transmits light well, so brightness reduction is only 5-15%. The contact requires no pressure, so the force required to make touch is insignificant.

The active surface of a capacitive sensor is larger than the LCD it serves. The pressure from the cabinet bezel seals represents a continuous "touch", but it does not change so the calibration process can compensate for mounting the screen. Of course, if the screen shifts, it will have to be re-calibrated. A firmly-mounted capacitive sensor can provide an excellent face seal for very harsh conditions.

Infrared touch consists of

• a glass face in front of the LCD to touch down on,
• emitters/sensors along all four edges of the viewing area, built into a raised bezel.

The sensor simply detects when the beam is broken/changed when a finger or other object enters the space.

Infrared sensors are almost completely insensitive to what the object is that breaks the beam, so they respond the same to a finger, glove or stylus. The "calibration" is based on where the sensors are, so calibration is hardly an issue. Contact pressure is insignificant, and the LED emitters are inherently a long-life device. The glass protective layer loses as little as 5-10% of the LCD's light.

The raised bezel assembly increases overall thickness and must remain exposed. This makes infrared less desirable for today's compact cabinets and may rule it out for dirty or washdown applications. However, infrared may be the very stable product required for some kiosk or gaming applications.

The surface acoustic wave method sends ultrsonic pulses across the surface of a glass layer and detects where they are disturbed. The parts are

• a glass layer over the front of the LCD with a flexible bond to the LCD,
• piezo-electric crystals bonded to the glass, to put out a sound pulse travelling across the glass face,
• reflectors to redirect the sound back to the crystals, where the sound wave is re-converted to an electric charge.

When the finger touches the glass, some of the sound is absorbed or redirected. The separate sensors see this effect differently, so the position of the touch can be calculated.

SAW is an interesting technology with some special advantages and issues. Unlike resistive or capacitive, there is no coating on the glass: the brightness is hardly affected and wear is not an issue. However the glass must not be rigidly mounted or the mounts will absorb all the sound energy, leaving nothing to get back to the sensor. The very flexible seals required where it meets the cabinet are generally too flexible for washdown applications. In dirty applications, these seals may cake with dirt and eventually need to be cleaned or replaced. On the positive side, we have seen high temperature steelmaking slag embed itself in the glass with no serious impact on the operation of the touch sensor.

In the past, the sensors were large, overhanging the LCD dimensions, and the crystals were mounted on the front, a nuisance for the face seals. Most sensors are now made with rear-face crystals and so-called "beveled" glass: the overhang is reduced, but still present, because the crystal must clear the edge of the LCD. This presents special problems for the LCD/touch mounting system. Our unique "screw adjust" CM open frames let us deal with those adjustments easily in the field.