Technologies:
There are a variety of touch screen technologies that have different methods of sensing touch. The following is a list of technologies used in touch screens:
1- Resistive: Resistive touch screen displays are composed of multiple layers that are separated by thin spaces. Pressure applied to the surface of the display by a finger or stylus causes the layers to touch, which completes electrical circuits and tells the device where the user is touching. (3)
2- Surface Acoustic Wave: Surface Acoustic Wave (SAW) touch screens rely upon sound waves; thus, "sound wave reflectors" are placed along the edges of the glass. Two transducers are placed in two of the corners, and two receivers are mounted in the opposite corners. A sound wave travels parallel to the edges of the glass. When the sound wave encounters the reflectors, the wave is transmitted from the transducers to the receivers. A touch point is detected when a drop in the amplitude of the sound wave occurs. (6)
3- Capacitive: Capacitive touch screen displays relay on the electrical properties of the human body to detect when and where on a display the user touching. Because of this, capacitive displays can be controlled with very light touches of a finger and generally cannot be used with a mechanical stylus or a gloved hand. (4)
4- Surface Capacitance: Surface Capacitive touch screens have a conductive coating on the front surface. Wires are attached to each corner, and a small voltage is applied to each of the corners. When the screen is touched, a small current flows to the touch point, causing a voltage drop. The drop is sensed by the four corners, allowing the sensor to pinpoint the exact touch point. (5)
5- Projected Capacitance: Projected Capacitive touch screens enables touches to be sensed through a protective layer in front of a display, allowing touch monitors to be installed behind store windows or vandal-resistant glass. DirectTouch consists of a 7.8 mm sensor with tempered glass outer layer, and ThruTouch works through a customer-installed outer layer. The complete system resists impacts, scratches, and vandalism and is also unaffected by moisture, heat, rain, snow and ice, or harsh cleaning fluids, making it ideal for outdoor applications. The solid-state touch screen and controller provide increased levels of reliability and longer life expectancy, resulting in a drift-free response and a low-maintenance unit that requires no recalibration. (7)
6- Mutual Capacitance: This is common PCT approach, which makes use of the fact that most conductive objects are able to hold a charge if they are very close together. In mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 16-by-14 array, for example, would have 224 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or stylus can be accurately tracked at the same time. (1)
7- Self-Capacitance: Self-Capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing. (1)
8- Infrared Grid: Infrared touch screens are based on light-beam interruption technology. Instead of an overlay on the surface, a frame surrounds the display. The frame has light sources, or light emitting diodes (LEDs) on one side and light detectors on the opposite side, creating an optical grid across the screen. When an object touches the screen, the invisible light beam is interrupted, causing a drop in the signal received by the photo sensors. (8)
9- Infrared Acrylic Projection: A translucent acrylic sheet is used as a rear projection screen to display information. The edges of the acrylic sheet are illuminated by infrared LEDs, and infrared cameras are focused on the back of the sheet. Objects placed on the sheet are detectable by the cameras. When the sheet is touched by the user the deformation results in leakage of infrared light, which peaks at the points of maximum pressure indicating the user's touch location. Misrosoft's PixelSense tables use this technology. (1)
10- Optical Imaging: A relatively-modern development in touch screen technology, two or more optical sensors are placed around the edges (mostly the corners) of the screen. Infrared backlights are placed in the sensor's field of view on the other sides of the screen to project a uniform field of infrared light slightly above the glass surface. A touch shows up as a shadow and each pair of optical sensors can locate the touch or even measure the size of the touching object. This technology is growing in popularity, duo to its scalability, versatility, and affordability, especially for larger units. (9)
11- Dispersive Signal Technology: Dispersive Signal Technology (DST) uses one pane of glass with a transducer placed in each corner of the glass. When a person touches the glass, mechanical energy (bending waves) is "dispersed from the touch location. The transducers, or sensors, detect the waves, then read then in order to triangulate the touch point. (10)
12- Acoustic Pulse Recognition: Acoustic Pulse Recognition (APR) is a patented touch technology from Elo. It shares similarities with 3M's DST; however, APR can be integrated with both small-and large- format displays. The touch technology also shares some commonalities with SAW. Like DST, APR utilizes one pane of glass with one transducer in each corner. When touch occurs, mechanical energy (bending waves) radiates from the touch location and is detected by the transducers. The transducers pinpoint the touch location by generating a unique sound for that location on the glass. (11)
There are a variety of touch screen technologies that have different methods of sensing touch. The following is a list of technologies used in touch screens:
1- Resistive: Resistive touch screen displays are composed of multiple layers that are separated by thin spaces. Pressure applied to the surface of the display by a finger or stylus causes the layers to touch, which completes electrical circuits and tells the device where the user is touching. (3)
2- Surface Acoustic Wave: Surface Acoustic Wave (SAW) touch screens rely upon sound waves; thus, "sound wave reflectors" are placed along the edges of the glass. Two transducers are placed in two of the corners, and two receivers are mounted in the opposite corners. A sound wave travels parallel to the edges of the glass. When the sound wave encounters the reflectors, the wave is transmitted from the transducers to the receivers. A touch point is detected when a drop in the amplitude of the sound wave occurs. (6)
3- Capacitive: Capacitive touch screen displays relay on the electrical properties of the human body to detect when and where on a display the user touching. Because of this, capacitive displays can be controlled with very light touches of a finger and generally cannot be used with a mechanical stylus or a gloved hand. (4)
4- Surface Capacitance: Surface Capacitive touch screens have a conductive coating on the front surface. Wires are attached to each corner, and a small voltage is applied to each of the corners. When the screen is touched, a small current flows to the touch point, causing a voltage drop. The drop is sensed by the four corners, allowing the sensor to pinpoint the exact touch point. (5)
5- Projected Capacitance: Projected Capacitive touch screens enables touches to be sensed through a protective layer in front of a display, allowing touch monitors to be installed behind store windows or vandal-resistant glass. DirectTouch consists of a 7.8 mm sensor with tempered glass outer layer, and ThruTouch works through a customer-installed outer layer. The complete system resists impacts, scratches, and vandalism and is also unaffected by moisture, heat, rain, snow and ice, or harsh cleaning fluids, making it ideal for outdoor applications. The solid-state touch screen and controller provide increased levels of reliability and longer life expectancy, resulting in a drift-free response and a low-maintenance unit that requires no recalibration. (7)
6- Mutual Capacitance: This is common PCT approach, which makes use of the fact that most conductive objects are able to hold a charge if they are very close together. In mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 16-by-14 array, for example, would have 224 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or stylus can be accurately tracked at the same time. (1)
7- Self-Capacitance: Self-Capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing. (1)
8- Infrared Grid: Infrared touch screens are based on light-beam interruption technology. Instead of an overlay on the surface, a frame surrounds the display. The frame has light sources, or light emitting diodes (LEDs) on one side and light detectors on the opposite side, creating an optical grid across the screen. When an object touches the screen, the invisible light beam is interrupted, causing a drop in the signal received by the photo sensors. (8)
9- Infrared Acrylic Projection: A translucent acrylic sheet is used as a rear projection screen to display information. The edges of the acrylic sheet are illuminated by infrared LEDs, and infrared cameras are focused on the back of the sheet. Objects placed on the sheet are detectable by the cameras. When the sheet is touched by the user the deformation results in leakage of infrared light, which peaks at the points of maximum pressure indicating the user's touch location. Misrosoft's PixelSense tables use this technology. (1)
10- Optical Imaging: A relatively-modern development in touch screen technology, two or more optical sensors are placed around the edges (mostly the corners) of the screen. Infrared backlights are placed in the sensor's field of view on the other sides of the screen to project a uniform field of infrared light slightly above the glass surface. A touch shows up as a shadow and each pair of optical sensors can locate the touch or even measure the size of the touching object. This technology is growing in popularity, duo to its scalability, versatility, and affordability, especially for larger units. (9)
11- Dispersive Signal Technology: Dispersive Signal Technology (DST) uses one pane of glass with a transducer placed in each corner of the glass. When a person touches the glass, mechanical energy (bending waves) is "dispersed from the touch location. The transducers, or sensors, detect the waves, then read then in order to triangulate the touch point. (10)
12- Acoustic Pulse Recognition: Acoustic Pulse Recognition (APR) is a patented touch technology from Elo. It shares similarities with 3M's DST; however, APR can be integrated with both small-and large- format displays. The touch technology also shares some commonalities with SAW. Like DST, APR utilizes one pane of glass with one transducer in each corner. When touch occurs, mechanical energy (bending waves) radiates from the touch location and is detected by the transducers. The transducers pinpoint the touch location by generating a unique sound for that location on the glass. (11)
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