use of our innate ability to point and touch, the touch screen has
become the most user friendly human interface available. It has found
widespread use in all levels of consumer and industrial applications
including ATM's, kiosks, games, retail systems, factory automation,
processing plants, and military and aerospace applications. Of the
different touch screen technologies available (four-wire resistive,
capacitive, and infrared), the four-wire resistive analog touch screen
has gained wide acceptance because of its low cost, durability, and
reliability, and it is the type we will be discussing in this article.
touch screen consists of a controller, normally occupying a single PCB
slot in the system hardware, and a transparent sensor positioned over a
liquid crystal display (LCD), or other flat panel display, functioning
as the operator interface. Typical controller functions include
switching voltages between sensor layers, performing voltage
measurements, and analog-to-digital (A/D) conversions. The sensor
consists of two opposing substrates, each coated with a transparent
electrically conductive material, separated by a thin gap. When the
operator presses the touch screen with a finger or stylus, the top
layer flexes and comes into electrical contact with the lower layer.
The controller then obtains two voltage measurements representing the
point of contact's X and Y coordinates. System programming associates
this position with a specific function graphically depicted on the flat
panel display (e.g., start, stop, abort, etc.) and responds accordingly.
the conventional four-wire configuration, each substrate is sputtered
on one side with Indium Tin Oxide (ITO), a uniform transparent coating
that functions as a voltage divider and provides a linear voltage drop
over the active area of the screen. This is the region of the sensor
that the operator touches. Two bus bars formed of conductive material,
such as silver, are coated on each substrate outside the active area
and are coupled to the opposite ends of the ITO. The two layers are
positioned with their ITO coatings facing and their conduction lines
running perpendicular to each other. Tiny, transparent, insulating dots
maintain a gap between the two substrates.
Touch screen operation
entails applying a voltage gradient across the resistive surface of one
layer and measuring the voltage on the other layer which acts as a
wiper arm on a potentiometer. After completing the first voltage
measurement, the roles of the two layers are reversed by controller
switching and a second measurement is taken. When the operator touches
a point on the screen, the two measured voltages represent the point's
X and Y coordinates. In Figure 1, we see a standard four-wire
configuration where the two layers are designated as X substrate and Y
substrate. The controller is applying a voltage and ground to the
opposite bus bars of the Y substrate and measuring the voltage on one
of the bus bars on the X substrate. The ITO coating is shown as an
infinite number of resistors across the active area. The voltage
applied is typically 5 volts, and the switching speed between the two
layers is about 5 microseconds.
touch screen's life and positional accuracy are dependent upon two key
factors?maintaining the uniformity of the ITO coating on each substrate
and minimizing positional drift due to property changes. It is the
integrity of the ITO coating that determines the linearity of the
voltage drop across the active area. Excessive use or abuse, slight
scratches in the ITO's resistive surface during manufacture, or a cut
with a thin blade will change the resistance of the affected area and
ITO linearity. Touch screen stability is thereby compromised resulting
in incorrect association of measured voltages with graphical switch
representations on the display. Touching an area associated with a Stop
or Abort switch, for example, could conceivably activate a Start or
Continue function instead.
Because the properties of the silver and
ITO coatings on the substrate vary at different rates, positional drift
occurs near the edges of the touch screen creating a constant need for
recalibration. Referring to Figure 2, when 5 volts are applied across a
substrate, we would see 2.5 volts when making a voltage measurement
anywhere along the middle of the active area. When measurements are
taken at the top and bottom of the active area, we would see voltages
that are just below 5 volts and just above zero volts, respectively.
This is attributable to the voltage drops that occur over the bus bars.
As environmental conditions change, such as an increase in temperature,
the resistance of the individual coatings will change at different
rates resulting in a change in the voltage distribution across all of
the coatings. If we repeat the above measurements, we would find that
the voltages at the top and bottom of the active area have changed. In
fact, had we originally taken voltage measurements at several points
going from the center to the top or bottom of the substrate, we would
now observe different voltage readings for most of these points.
minimize the drift and linearity problems inherent in the conventional
four-wire touch screen, manufacturers have developed alternative
designs including five-wire and eight-wire configurations. In the
five-wire configuration, only the bottom substrate uses the ITO coating
as a voltage divider, and it is bounded on the perimeter by a frame of
four resistive patterns. The top layer serves as a pickup. It has a
conductive coating that is positioned facing the ITO on the bottom
layer and a fifth wire for voltage measurements. Pressing the top layer
with a finger causes it to flex at the point of contact and bring its
conductive coating into electrical contact with the ITO beneath it. As
the controller switches the voltage and ground between the resistive
patterns, voltages are obtained from the fifth wire representative of
the X-Y coordinates.
Immediate advantages, claimed by
proponents of the five-wire touch screen, are the additional flexing
and abuse that the upper layer is capable of enduring since ITO
linearity concerns and positional drift due to different coating
properties do not apply to the conductive material.
touch screen is similar to the conventional four-wire configuration
with the exception that it adds two sense lines to each substrate?one
to each bus bar?at the edges of the touch screen as shown in Figure 3.
Using these sense lines to obtain a reference voltage, the controller
can ensure that the proper voltage is applied across the ITO by
compensating for voltage drop variations in the bus bars. Although
manufacturers of the eight-wire design contend that touch screen
stability is increased as a result of eliminating positional drift, it
seems, however, that the ITO linearity problem remains unaddressed.
third design has been recently introduced that effectively addresses
both problems?positional drift and ITO linearity. It essentially
converts the touch screen's active area from a resistive to a
conductive area by eliminating the ITO coating as the voltage divider.
Figure 4 provides a functional illustration of the new design that has
been named RuggedTouch* by its manufacturer, CAM Graphics Co., Inc.,
located in Amityville, New York. It is a four-wire configuration that
uses a proprietary durable resistor, with properties similar to silver,
that has been reduced in size to a thin strip and deposited outside the
active area of each substrate. A second
resistive strip, for reasons we will address later, is also deposited
in the inactive area on the opposite side of the substrate; both
resistors function as voltage dividers in place of the ITO. Rows of
thin conductive ITO strips are formed on each substrate running across
the active area and are coupled on either side to the resistors. Since
the ITO strips function as conductors only, the resistance of each
substrate's active area is negligible, and the touch screen's life and
positional accuracy are no longer dependent upon maintaining the
resistance uniformity of each layer. Furthermore, significantly
reducing the size of the resistive material and placing it out of
harm's way in the inactive area of the screen has resulted in extending
the screen's life. Constant flexing and minor damage, which would
usually necessitate recalibration or replacement of conventional touch
screens, do not degrade the RuggedTouch's stability and performance.
drift has also been minimized due to the similar properties of the
resistor and the silver bus bars. Since they will have similar
variations in resistance due to changing electrical and environmental
conditions, their voltage distribution remains unaffected.
the RuggedTouch employs a unique circuit configuration, it is designed
to be a drop-in replacement for standard four-wire resistive analog
touch screens. The controller still occupies a single PCB slot with no
requirements for hardware or software modifications in the host system.
The value of the RuggedTouch resistor can be controlled to individual
customers' specifications. While typical resistive analog touch screens
are limited by the availability of standard ITO coatings and the
geometry of the touch screen, the deposited resistor can be configured
for axis resistance values from a few ohms to 20,000 ohms.
RuggedTouch operation also remains unchanged. As shown in Figure 4, the
two layers representing the X substrate and Y substrate are positioned
with their ITO conduction lines facing and running perpendicular to
each other. The controller applies a voltage and ground to the left and
right resistive strips on the Y substrate and measures the voltage on
the X substrate. The controller then switches the voltage and ground to
the X substrate and measures the voltage on the Y substrate thereby
obtaining the two voltages representing the X-Y coordinates.
second resistive strip in each substrate, provides an alternate current
path in the event any of the ITO conductive strips are severed. In the
conventional touch screen, a thin cut would alter the resistance of
the ITO coating affecting the positional accuracy of the screen.
In the RuggedTouch, however, ITO resistance is not a consideration, and
the voltage can be read at either one of the severed ends of the
conductive strip. Conceivably, a damaged area on the RuggedTouch that
is considerably larger than a razor cut but smaller than the contact
area of the operator's finger on the screen will have no adverse effect
on the operation and may even go unnoticed. Even if the damaged area
were larger, the operator would just touch another area that lies
within the graphical switch.
A key benefit derived from this feature
is reduced system down time. This is of particular significance for
situations that present hostile environments for touch screens such as
kiosks, games, and industrial applications (workers have been known to
use screwdrivers and other sharp tools instead of their fingers). With
the exception of damages considerably larger than the type we have
discussed thus far, the RuggedTouch can continue to function without
having to close down an ongoing operation. Repair actions can be
postponed until the operation is completed or even until the next
A tradeoff usually accompanies product
improvement, and, more often than not, it's an increase in price. For
RuggedTouch, however, it's a case of better and cheaper. Eliminating
the ITO as the resistive element, has resulted in a considerable
reduction in the rejection rate during production. The resulting
decrease in production cost and increase in production yield has
allowed the manufacturer to pass on a 10 to 20 percent savings in price
to the customer.
* Patent Pending