A resistive touch screen typically uses a display overlay consisting of layers, each with a conductive coating on the inner surface. The conductive inner layers are separated by special separator dots, evenly distributed across the active area. Finger pressure causes internal electrical contact at the point of touch, supplying the electronic interface (touch screen controller) with vertical and horizontal analog voltages for digitization. For CRT applications, resistive touch screens are generally spherical (curved) to match the CRT and minimize parallax. The nature of the material used for curved (spherical) applications limits light throughput such that two options are offered: Polished (clear) or antiglare. The polished choice offers clarity but includes some glare. The antiglare choice will minimize glare, but will also slightly diffuse the light throughput (image). Either choice will demonstrate either more glare (polished) or more light diffusion (antiglare) than associated with typical non-touch screen displays. Despite the tradeoffs, the resistive touch screen technology remains a popular choice, often because it can be operated while wearing gloves (unlike capacitive technology). Note that resistive touch screen materials used for flat panel touch screens are different and demonstrate much better optical clarity (even with antiglare). The resistive technology is far more common for flat panel applications.
A capacitive touch screen includes an overlay made of glass with a coating of capacitive (charge storing) material deposited electrically over its surface. Oscillator circuits located at corners of the glass overlay will each measure the capacitance of a person touching the overlay. Each oscillator will vary in frequency according to where a person touches the overlay. A touch screen controller measures the frequency changes to determine the X and Y coordinates of the touch. Because the capacitive coating is even harder than the glass it is applied to, it is very resistant to scratches from (SIC) sharp objects. It can even resist damage from sparks. A capacitive touch screen cannot be activated while wearing most types of gloves (non-conductive).
An infrared touch screen surrounds the face of the display with a bezel of light emitting-diodes (LEDs) and diametrically opposing phototransistor detectors. The controller circuitry directs a sequence of pulses to the LED’s, scanning the screen with an invisible lattice of infrared light beams just in front of the surface. The controller circuitry then detects input at the location where the light beams become obstructed by any solid object. The infrared frame housing the transmitters can impose design constraints on operator interface products.
SAW (Surface Acoustic Wave)
A SAW touch screen uses a solid glass display overlay for the touch sensor. Two surface acoustic (sound) waves, inaudible to the human ear, are transmitted across the surface of the glass sensor, one for vertical detection and one for horizontal detection. Each wave is spread across the screen by bouncing off reflector arrays along the edges of the overlay. Two receivers detect the waves, one for each axis. Since the velocity of the acoustic wave through glass is known and the size of the overlay is fixed, the arrival time of the waves at the respective receivers is known. When the user touches the glass surface, the water content of the user’s finger absorbs some of the energy of the acoustic wave, weakening it. The controller circuitry measures the time at which the received amplitude dips to determine the X and Y coordinates of the touch location. In addition to the X and Y coordinates, SAW technology can also provide Z axis (depth) information. The harder the user presses against the screen, the more energy the finger will absorb, and the greater will be the dip in signal strength. The signal strength is then measured by the controller to provide the Z axis information. Today, few software applications are designed to make use of this feature.
Touch Screen Controllers
Most manufacturers offer two controller configurations–ISA Bus and Serial-RS232. ISA bus controllers are contained on a standard printed circuit plug-in board and can only be used on ISA or EISA PCs. Depending on the manufacturer they may be interrupt driven, polled or be configured as another serial port. Serial controllers are contained on a small printed circuit board and are usually mounted in the video monitor cabinet. They are then cabled to a standard RS232 serial port on the host computer.
Most touch screen manufacturers offer some level of software support which include mouse emulators, software drivers, screen generators and development tools for Windows, OS/2, Macintosh and DOS. Most of the supervisory control and data acquisition (SCADA) software packages now available contain support for one or more touch technologies.