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.
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.
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.
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.
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)