back to Touch Screens
RESISTIVE TOUCH
SCREENS
Resistive touchscreens are used in more applications than
any other touch technology–for example, PDAs, point-of-sale,
industrial, medical, and office automation, as well as consumer
electronics.
All variations of resistive touchscreens have
some things in common:
- They are all constructed similarly in layers-a back
layer such as glass with a uniform resistive coating plus
a polyester coversheet, with the layers separated by tiny
insulating dots. When the screen is touched, it pushes
the conductive coating on the coversheet against the coating
on the glass, making electrical contact. The voltages
produced are the analog representation of the position
touched. An electronic controller converts these voltages
into digital X and Y coordinates which are then transmitted
to the host computer.
- Because resistive touchscreens are force activated,
all kinds of touch input devices can activate the screen,
including fingers, fingernails, styluses, gloved hands,
and credit cards.
- All have similar optical properties, resistance to
chemicals and abuse.
- Both the touchscreen and its electronics are simple
to integrate into imbedded systems, thereby providing
one of the most practical and cost-effective touchscreen
solutions.
Four-Wire Resistive
 Four-wire
resistive technology is the simplest to understand and manufacture.
It uses both the upper and lower layers in the touchscreen
"sandwich" to determine the X and Y coordinates.
Typically constructed with uniform resistive coatings of
indium tin oxide (ITO on the inner sides of the layers and
silver buss bars along the edges, the combination sets up
lines of equal potential in both X and Y.
In the illustration below, the controller first applies
5V to the back layer. Upon touch, it probes the analog voltage
with the coversheet, reading 2.5V, which represents a left-right
position or X axis.
It then flips the process, applying 5V to the coversheet,
and probes from the back layer to calculate an up-down position
or Y axis. At any time, only three of the four wires are
in use (5V, ground, probe).
The primary drawback of four-wire technology is that one
coordinate axis (usually the Y axis), uses the outer layer,
the flexible coversheet, as a uniform voltage gradient.
The constant flexing that occurs on the outer coversheet
with use will eventually cause microscopic cracks in the
ITO coating, changing its electrical characteristics (resistance),
degrading the linearity and accuracy of this axis.
Unsurprisingly, four-wire touchscreens are not known for
their durability. Typically, they test only to about 1 million
touches with a finger-far less when activated by a pointed
stylus which speeds the degradation process. Some four-wire
products even specify 100,000 activations within a rather
large, 20 mm x 20 mm area. In the real world of point-of-sale
applications, a level of 100,000 activations with hard,
pointed styluses (including fingernails, credit cards, ballpoint
pens, etc.) is considered normal usage in just a few months'
time.
Also, accuracy can drift with environmental changes. The
polyester coversheet expands and contracts with temperature
and humidity changes, thereby causing long-term degradation
to the coatings as well as drift in the touch location.
While all of these drawbacks can be insignificant in smaller
sizes, they become increasingly apparent the larger the
touchscreen. Therefore, Elo normally recommends four-wire
touchscreens in applications with a display size of 6.4"
or smaller.
However, the relative low cost, inherent low power consumption,
and common availability of chipset controllers with support
from imbedded operating systems, makes Elo AT4 four-wire
touchscreens ideal for hand-held devices such as PDAs, wearable
computers, and many consumer devices.
Eight-Wire Variation
Eight-wire resistive touchscreens are a variation of four-wire
construction. The primary difference is the addition of
four sensing points, which are used to stabilize the system
and reduce the drift caused by environmental changes. Eight-wire
systems are usually seen in sizes of 10.4" or larger
where the drift can be significant.
As in four-wire technology, the major drawback is that
one coordinate axis uses the outer, flexible coversheet
as a uniform voltage gradient, while the inner or bottom
layer acts as the voltage probe. The constant flexing that
occurs on the outer coversheet will change its resistance
with usage, degrading the linearity and accuracy of this
axis.
Although the added four sensing points helps stabilize
the system against drift, they do not improve the durability
or life expectancy of the screen. Therefore, Elo does not
recommend eight-wire touchscreen solutions.
Five-Wire Resistive
As we have seen, four- and eight-wire touchscreens, while
having a simple and elegant design, have a major drawback
in terms of durability in that the flexing coversheet is
used to determine one of the axes. Field usage proves that
the other axis rarely fails. Could it be possible to construct
a touchscreen where all the position sensing was on the
stable glass layer? Then the coversheet would serve only
as a voltage probe for X and Y. Microscopic cracks in the
coversheet coating might still occur, but they would no
longer cause non-linearities. The simple buss bar design
is not sufficient and a more complex linearization pattern
on the edges is required.
In the five-wire design, one wire goes to the coversheet
(E) which serves as the voltage probe for X and Y. Four
wires go to corners of the back glass layer (A, B, C, and
D). The controller first applies 5V to corners A and B and
grounds C and D, causing voltage to flow uniformly across
the screen from the top to the bottom. Upon touch, it reads
the Y voltage from the coversheet at E. Then the controller
applies 5V to corners A and C and grounds B and D, and reads
the X voltage from E again.
So, a five-wire touchscreen uses the stable bottom layer
for both X- and Y-axis measurements. The flexible coversheet
acts only as a voltage-measuring probe. This means the touchscreen
continues working properly even with non-uniformity in the
coversheet's conductive coating. The result is an accurate,
durable and more reliable touchscreen over four- and eight-wire
designs.
Six- and Seven-Wire Variations
There are some manufacturers who claim improved performance
over five-wire resistive with additional wires.
The six-wire variation adds an extra ground layer to the
back of the glass. It is not needed for improved performance,
and in some cases is not even connected to the companion
controller.
The seven-wire variation adds two sense lines, like with
the eight-wire design, to decrease drift due to environmental
changes. Elo's patented AccuTouch "Z border" electrode
pattern is a better solution to prevent drift.
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