There
are a number of types
of touch screen technology:
Resistive
A
resistive touch screen
panel is composed of several
layers. The most important
are two thin metallic electrically
conductive and resistive
layers separated by thin
space. When some object
touches this kind of touch
panel, the layers are connected
at certain point; the panel
then electrically acts
similar to two voltage
dividers with connected
outputs. This causes a
change in the electrical
current which is registered
as a touch event and sent
to the controller for processing.
When measuring press force,
it is useful to add resistor
dependent on force in this
model -- between the dividers.
Surface
Acoustic Wave (SAW)
Surface
Acoustic Wave technology
uses ultrasonic waves that
pass over the touch screen
panel. When the panel is
touched, a portion of the
wave is absorbed. This
change in the ultrasonic
waves registers the position
of the touch event and
sends this information
to the controller for processing.
Surface wave touch screen
panels can be damaged by
outside elements. Contaminants
on the surface can also
interfere with the functionality
of the touchscreen.
Capacitive
A
capacitive touch screen
panel is coated with a
material, typically indium
tin oxide that conducts
a continuous electrical
current across the sensor.
The sensor therefore exhibits
a precisely controlled
field of stored electrons
in both the horizontal
and vertical axes - it
achieves capacitance. The
human body is also an electrical
device which has stored
electrons and therefore
also exhibits capacitance.
When the sensor's 'normal'
capacitance field (its
reference state) is altered
by another capacitance
field, i.e., someone's
finger, electronic circuits
located at each corner
of the panel measure the
resultant 'distortion'
in the sine wave characteristics
of the reference field
and send the information
about the event to the
controller for mathematical
processing. Capacitive
sensors can either be touched
with a bare finger or with
a conductive device being
held by a bare hand. Capacitive
touch screens are not affected
by outside elements and
have high clarity, but
their complex signal processing
electronics increase their
cost.
Infrared
An
infraed touch screen panel
employs one of two very
different methods. One
method used thermal induced
changes of the surface
resistance. This method
was sometimes slow and
required warm hands. Another
method is an array of vertical
and horizontal IR sensors
that detected the interruption
of a modulated light beam
near the surface of the
screen. IR touch screens
have the most durable surfaces
and are used in many military
applications that require
a touch panel display.
Strain
Gauge
In
a strain gauge configuration
the screen is spring mounted
on the four corners and
strain gauges are used
to determine deflection
when the screen is touched.
This technology can also
measure the Z-axis. Typically
used in exposed public
systems such as ticket
machines due to their resistance
to vandalism.
Optical
Imaging
A
relatively-modern development
in touch screen technology,
two or more image sensors
are placed around the edges
(mostly the corners) of
the screen. Infrared backlights
are placed in the camera's
field of view on the other
sides of the screen. A
touch shows up as a shadow
and each pair of cameras
can then be triangulated
to locate the touch. This
technology is growing in
popularity, due to its
scalability, versatility,
and affordability, especially
for larger units.
Dispersive
Signal Technology
Introduced
in 2002, this system uses
sensors to detect the mechanical
energy in the glass that
occur due to a touch. Complex
algorithms then interpret
this information and provide
the actual location of
the touch. The technology
claims to be unaffected
by dust and other outside
elements, including scratches.
Since there is no need
for additional elements
on screen, it also claims
to provide excellent optical
clarity. Also, since mechanical
vibrations are used to
detect a touch event, any
object can be used to generate
these events, including
fingers and styli. A downside
is that after the initial
touch the system cannot
detect a motionless finger.
Acoustic
Pulse Recognition
This
system uses more than two
piezoelectric transducers
located at some positions
of the screen to turn the
mechanical energy of a
touch (vibration) into
an electronic signal. This
signal is then converted
into an audio file, and
then compared to preexisting
audio profile for every
position on the screen.
This system works without
a grid of wires running
through the screen, the
touch screen itself is
actually pure glass, giving
it the optics and durability
of the glass out of which
it is made. It works with
scratches and dust on the
screen, and accuracy is
very good. It does not
need a conductive object
to activate it. It is a
major advantage for larger
displays. As with the Dispersive
Signal Technology system,
after the initial touch
this system cannot detect
a motionless finger.
Frustrated
Total Internal Reflection
This
optical system works by
using the principle of
total internal reflection
to fill a refractive medium
with light. When a finger
or other soft object is
pressed against the surface,
the internal reflection
light path is interrupted,
making the light reflect
outside of the medium and
thus visible to a camera
behind the medium.[1]
Graphics
tablet/screen hybrid technique
This
new technique is definitionally
not really a touchscreen,
but has the same properties,
in addition to having much
more accuracy. It is a
graphics tablet that incorporates
an LCD into the tablet
itself, allowing the user
to draw directly "on" the
display surface. It should
not be mixed up with tablet
pc hybrids.
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