Touch Screen Technology

Touch screen technology has the potential to replace most functions of the mouse and keyboard. The touchscreen interface is being used in a wide variety of applications to improve human-computer interaction. As the technology advances, people may be able to operate computers without mice and keyboards. Because of its convenience, touch screen technology solutions has been applied more and more to industries, applications, products and services, such as Kiosks, POS (Point-of-Sale), consumer electronics, tablet PC, moderate to harsh Machine Control, Process Control, System Control/Office Automation and Car PC, etc.

The touch panels themselves are based around four basic screen technologies: Resistive, Capacitive, Infrared (IR), and Surface Acoustical Wave (SAW). Each of those designs has distinct advantages and disadvantages. Note: many of these are designed to comply with specific National Electrical Manufacturers Association (NEMA) standards to meet various installation requirements. For more information about NEMA standards, visit www.nema.org.

Resistive Touch Screen Technology

Offering excellent durability and resolution, resistive technology is used in a variety of applications and environments. The Analog Resistive touch screen is a sensor consisting of two opposing layers, each coated with a transparent resistive material called indium tin oxide (ITO). The ITO used has a typical sheet resistivity between 100 and 500 ohms per square. The layers are separated by a pattern of very small transparent insulating dots. Silver ink bus bars (~50mW/sq) make an electrical connection to the surface of the ITO at the outside edges, spanning the desired axis of the given layer. Silver ink traces (~50mW/sq) connect the bus bars to an electromechanical connector used for interfacing to the sensor. The cover sheet has a hard, durable coating on the outer side, and a conductive coating on the inner side. When touched, the conductive coating makes electrical contact with the coating on the glass, and a touch is registered by the analog controller.

Resistive touchscreens deliver cost-effective, consistent and durable performance in environments where equipment must stand up to contaminants and liquids, such as in restaurants, factories, and hospitals. Disadvantages of Resistive technology include only 75% optical transparency and the fact that a sharp object can damage the resistive layers.

The Analog Resistive technology is perfect for PDAs, web phones, and other handheld consumer applications.

Capacitive Touch Screen Technology

The touchpad contains a two-layer grid of electrodes that are connected to a sophisticated full-custom mixed signal integrated circuit (IC) mounted on the reverse side of the pad. The upper layer contains vertical electrode strips while the lower layer is composed of horizontal electrode strips. The IC measures "Mutual capacitance" from each of the horizontal electrodes to each of the vertical electrodes. A human finger near the intersection of two electrodes modifies the mutual capacitance between them, since a finger has very different dielectric properties than air. When a user touches the screen, some of the charge is transferred to the user, and makes the potential difference on the screen. After the panel controller recognizes that, the controller will send the X-Y axis information to the PC port.

The advantage is that capacitive technology transmits almost 90% percent of the light from the screen. The superior efficiency gives capacitive better than resistive technology.

Surface Acoustic Wave (SAW) Technology

The Surface Acoustic Wave (SAW) technology is one of the most advanced touch screen types. The technology is based on two transducers (transmitting and receiving) placed for the both of X and Y axis on the touch panel. The other important element of SAW is placed on the glass, called reflector. The controller sends electrical signal to the transmitting transducer, and transducer converts the signal into ultrasonic waves and emits to reflectors that are lined up along the edge of the panel. After reflectors refract waves to the receiving transducers, the receiving transducer converts the waves into an electrical signal and sends back to the controller. When a finger touches the screen, the waves are absorbed, causing a touch event to be detected at that point.

Compared to Resistive and Capacitive technologies, SAW technology provides superior image clarity, resolution, and higher light transmission. Because the panel is all glass, there are no layers that can be worn, giving this technology the highest durability factor and also the highest clarity. Disadvantages of Surface Acoustic Wave (SAW) technology include the facts that the touch screen must be touched by finger, gloved hand, or soft-tip stylus (something hard like a pen won't work) and that the touchscreen is not completely sealable, can be affected by large amounts of dirt, dust, and / or water in the environment.

The Surface Acoustic Wave technology is recommended for ATMs, Amusement Parks, Banking and Financial Applications, public information kiosks, computer based training, or other high traffic indoor environments.

Touch Screen Functional Description

Touching the top surface compresses the flexible top layer to the supported bottom layer causing electrical contact of the two layers between the span of insulating dots. Determining a touch location requires two measurements, one to obtain an X-axis coordinate and one to obtain a Y-axis coordinate. A single axis measurement is taken by applying a drive voltage across the ITO of one layer via the silver ink bus bar and trace connections. The voltage applied to this layer produces a voltage gradient across the ITO. The voltage linearly changes from the minimum drive voltage at one end to the maximum drive voltage at the other end. The opposing layer, via a path through its ITO and silver ink connections, is used to measure the voltage at the point of contact on the voltage driven layer. This process is repeated, alternating functions of the two layers to obtain a measurement on the other axis.

Measurements are made using a 10-bit analog to digital converter (ADC). A 10-bit ADC can resolve 2-to-the-10th power or 1024 different input values in each the horizontal and vertical direction. The four-wire system resolution is, however, less than 1024 due to losses in the drive voltage that occur before it reaches the touch screen ITO.
Touch point coordinates are reported to the host computer or microcontroller through a serial communications port.

TouchScreen Technology Comparison (pdf - 123 KB)

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