This afternoon after a big bowl of
fufu , I stepped into a meeting and the moderator upon seeing my ipad ,
enquired “Nehemiah, how does touch screen work?” so for those who want to find
out, here goes….
It was not so long ago that we would
tap away on a PalmPilot with a tiny stylus, or exercise our thumbs on a
BlackBerry micro-keyboard. Then, in January 2007, along came the Apple iPhone,
and everything changed. Suddenly, people were wiping their fingers across
screens, pinching images and performing other maneuvers that had not previously
been part of the smartphone interface.
What is Gorilla Glass, and what makes it so special ?
Now we not only take touch input for
granted, we expect to be able to use multitouch (using more than one finger on
the screen at a time) and gestures as well. What made this touch screen
revolution possible, and where is it likely to take us?
Many
paths to touch
To begin with, not all touch is
created equal. There are many different touch technologies available to design
engineers.
According to touch industry expert
Geoff Walker of Walker Mobile, there are 18 distinctly different touch
technologies available. Some rely on visible or infrared light; some use sound waves
and some use force sensors. They all have individual combinations of advantages
and disadvantages, including size, accuracy, reliability, durability, number of
touches sensed and -- of course -- cost.
As it turns out, two of these
technologies dominate the market for transparent touch technology applied to
display screens in mobile devices. And the two approaches have very distinct
differences. One requires moving parts, while the other is solid state. One
relies on electrical resistance to sense touches, while the other relies on
electrical capacitance. One is analog and the other is digital. (Analog
approaches measure a change in the value of a signal, such as the voltage,
while digital technologies rely on the binary choice between the presence and absence
of a signal.) Their respective advantages and disadvantages present clearly
different experiences to end users.
Resistive
touch
The traditional touch screen
technology is analog resistive. Electrical resistance refers to how easily
electricity can pass through a material. These panels work by detecting how
much the resistance to current changes when a point is touched.
This process is accomplished by
having two separate layers. Typically, the bottom layer is made of glass and
the top layer is a plastic film. When you push down on the film, it makes
contact with the glass and completes a circuit.
The glass and plastic film are each
covered with a grid of electrical conductors. These can be fine metal wires,
but more often they are made of a thin film of transparent conductor material.
In most cases, this material is indium tin oxide (ITO). The electrodes on the
two layers run at right angles to each other: parallel conductors run in one
direction on the glass sheet and at right angles to those on the plastic film.
When you press down on the touch
screen, contact is made between the grid on the glass and the grid on the film.
The voltage of the circuit is measured, and the X and Y coordinates of the
touch position is calculated based on the amount of resistance at the point of
contact.
This analog voltage is processed by
analog-to-digital converters (ADC) to create a digital signal that the device's
controller can use as an input signal from the user.
What's so special about Gorilla
Glass?
Many vendors are quick to trumpet
the use of Corning's Gorilla Glass in their products. The glass is used as a
protective outer layer for many devices, from smartphones to large flat panel
televisions. But what makes Gorilla Glass different?
The answer lies in the composition
of the glass itself. Most display glass is an alumina silicate formulation,
which is made up of aluminum, silicon, and oxygen. The glass also contains
sodium ions spread throughout the material. And this is where the difference
starts.
The glass is put in a bath of molten
potassium at about 400 degrees. The sodium ions are replaced by potassium ions
in a process that's a bit like soaking a pickle in salty brine. It's a
diminishing process: More of the sodium ions are replaced by potassium at the surface
of the glass, and then fewer and fewer are exchanged as you go further into the
glass.
Why change from sodium to potassium?
Sodium (Na) has an atomic number of 11, while potassium (K) has an atomic
number of 19. If you remember your high school chemistry, this indicates that
the potassium atoms are significantly larger than the sodium atoms. (The atomic
radius of a neutral sodium atom measures out as 180 picometers and potassium at
220 picometers, so potassium measures out as more than 20% larger.)
Imagine that you have a box packed
tightly with tennis balls. What would happen if you took out the top layer of
tennis balls and replaced them -- one for one -- with larger softballs? The
softball layer would be squeezed together much more tightly and it would be
harder to get one out.
That's what happens with glass when
the potassium ions take the place of the sodium ions. The potassium ions take
up more space and create compression in the glass. This makes it more difficult
for a crack to start, and even if one does start, it is much less likely to
grow through the glass.
The concept of strengthening glass
through ion exchange is not new; it has been known since at least the 1960s.
And other companies offer glass that has been strengthened by this type of process.
Corning's Gorilla brand of strengthened glass has gained considerable market
share, however, and has a very visible presence in the marketplace.
One of the big advantages of
resistive touch panels is that they are relatively inexpensive to make. Another
is that you can use almost anything to create an input signal: finger tip,
fingernail, stylus -- just about anything with a smooth tip. (Sharp tips would
damage the film layer.)
This technology has a lot of
disadvantages, however. First, the analog system is susceptible to drift, so
the user may have to recalibrate the touch panel from time to time. (If you
owned a PalmPilot or other PDA, you may remember having to occasionally go
through the recalibration process on their PalmPilot.) Next, the ITO material
used for the conductors is brittle and not well suited for bending. Over time,
repeated use can cause the ITO to crack, which disrupts the flow of electricity
and can result in a dead spot on the touch screen.
In addition, there needs to be a gap
between the two sensor planes that must be bridged in order to make contact
between the two. Just about the only material suitable for this gap is air, but
this presents some problems of its own.
First, the gap adds to the combined
thickness of the display and touch module. As the consumers demand thinner and
thinner devices, a single millimeter can be a big deal.
Another problem has to do with the
optical properties of the different layers. If you look at a drinking straw in
a glass of water, it will look as though it is slightly bent where it enters
the water, even though it is straight. This is because light can bend, or
"refract," when it makes the transition from one material to another.
If the materials have the same index of refraction, the light won't change its
path, but if the index of refraction is different, the light will bend.
The space between the plastic and
glass layers of a resistive touch panel is filled with air, and the air has a
different index of refraction than the other layers, which makes the light bend
as it passes from one layer to another. This can create visible artifacts that
can impact the display quality.
The air gap is especially a problem
when you view the display under high ambient light conditions, such as outdoors
in bright sunlight. The outside light passes through the top layer, then bends
when it hits the air gap, and can then reflect between the glass and plastic
layers before exiting out the front of the display again. This bouncing light
can reduce the image's contrast, making the display look washed out and
impossible to see.
But probably the biggest problem
with resistive panels in consumers' eyes is that they can sense only one touch
at a time. If you touch the panel in two places at once, the combined effect
will produce one coordinate for the touch point, and that will be different
from either of the two actual points. There are ways to create resistive panels
that can sense multiple touches at one time, but these can be expensive and
complex, such as creating a matrix of separate contact pads on one of the
layers.
Projected
capacitance
Fortunately, there's a better way.
Many mobile devices now rely on a technology known as "projected
capacitance," often referred to in the industry as "p-cap" or "pro-cap."
According to various sources, resistive touch has rapidly lost market share to
pro-cap and is forecast to continue to decline.
This chart shows the shares of the
different touch screen technologies for all applications worldwide, on a unit
basis. Pro-cap technology is on its way to becoming the dominant
technology.
Pro-cap is a solid-state technology,
which means that it has no moving parts (unlike the resistive touch
technology). Instead of being based on electrical resistance, it relies on
electrical capacitance.
When you apply an electrical charge
to an object, the charge can build up if there is no place for the electrons to
flow. This "holding" of electrons is known as
"capacitance." You have probably experienced this effect first-hand.
When you walk across a carpet in rubber-soled shoes in the winter time,
electrons can build up in your body. If you should then reach for a light
switch or some other conductive object that does not have a similar built-up
charge, those electrons can flow from your body to the object, producing a
spark of electricity.
If you apply a charge to a
conductor, and then bring another conductor near it, the second conductor will
"steal" some of the charge from the first one, just as the light
switch did when your finger approached it. If you know what the charge was to
start with, you can tell when the amount of the charge has changed. This is the
principle behind pro-cap touch screens.
Early capacitance touch technologies
required that you actually touch a conductive layer. This approach left the
conductor exposed to wear and damage. Today's projective capacitance technology
relies on the fact that an electromagnetic field "projects" above the
plane of the conductive sensor layer. You can cover the touch module with a
sheet of thin glass, for example, and it will still sense when a conductor
comes near.
Pro-cap touch screens use two layers
of conductors, separated by an insulator (such as a thin sheet of glass, though
other insulating layers can be used). The conductors typically are made of
transparent ITO, just as with the resistive designs. The conductor layers never
have to bend, however, so its brittle nature is not a problem with pro-cap
screens.
The conductors in each layer are
separate, so that the capacitance of each one can be measured separately. As
with a resistive panel, the conductors run at right angles to each other, so
that the device can sense an X and a Y position when touched. The difference is
that the separate conductors are scanned in rapid sequence, so that all the
possible intersections are measured many times per second.
When you touch the screen with your
finger, it steals a little of the charge from each layer of conductors at that
point. The electrical charge involved is tiny, which is why you don't feel any
shock when you touch the screen, but this little change is enough to be
measured. Because each conductor is checked separately, it is possible to
identify multiple simultaneous touch points.
Pro-cap technology is not without
its challenges. The system of conductors is susceptible to electrical noise
from electromagnetic interference (EMI). This can be a problem for display
devices such as LCD and OLED panels that rely on an active matrix backplane of
transistors to rapidly switch the individual subpixels on and off. The touch
screen controller must be able to filter out this background noise and figure
out which signals are from actual touch points.
The controller is often asked to
make other decisions as well. Comparing results from adjacent coordinates can
help determine if the touch is hard or soft, or if it is the result of the palm
of the hand resting on the screen and thus should be ignored. Some smartphones
rely on the touch screen to signal when the phone is being held next to the user's
face, so that the screen can be turned off to save power.
All these tasks require significant
processing power, which makes the controller more expensive. In addition, the
touch screen only works when you apply a conductor; the ball of your finger works,
but not your fingernail. Some pro-cap screens will work even if you're wearing
thin surgical gloves, but they won't work if you have thick winter gloves on.
(The exception is if the gloves themselves are conductive; you can buy gloves
with conductors woven into the fingertips so that they can conduct the charge
from the screen to your finger.)
In spite of these shortcomings,
pro-cap technology has become the dominant choice for mobile devices. And there
are improvements on the way that could make them even better.
Can't
be too thin or too light
Consumers have made it clear that
they want smartphones and other mobile devices to be as thin and lightweight as
possible. As a result, design engineers are always looking for technology
improvements that let them remove layers and materials from their products. And
touch screens are not immune to such scrutiny.
The traditional structure for adding
pro-cap touch to a display is to purchase a separate module. You would start
with an LCD panel that is made up of two glass layers that contain the liquid
crystal material; the top glass sheet is covered with a polarizing layer.
Above that goes the pro-cap touch
module, which is made by coating both sides of a glass sheet with a conductor
(typically ITO), which is then patterned to create the electrodes. This glass
sheet is then laminated to the polarizer layer of the LCD panel described in
the previous paragraph.
Finally, a protective cover glass is
placed on top of the touch panel so that the top electrodes are not exposed.
This cover can also have decorations (such as logos or icons for fixed
controls) and be designed to protect the display from damage.
If you've been counting, you'll
realize that it all adds up to four different sheets of glass in the stack --
which means that even today's thin smartphones aren't as thin as some might
prefer. If manufacturers could eliminate one of these sheets, they'd reduce the
space required for glass and the weight of the glass in the display by 25%.
Those are savings worth pursuing.
A method that is gaining momentum is
called the "one-glass solution" (OGS); it eliminates one of the
layers of glass from the traditional pro-cap stack. The basic idea is to
replace the touch module glass by a thin layer of insulating material. In general,
there are two ways to achieve this.
One approach to OGS is called
"sensor on lens." (In this case, the "lens" refers to the
cover glass layer.) You deposit an ITO layer on the back of the cover glass and
pattern it to create the electrodes. You add a thin insulator layer to the
bottom of that, and then deposit a second ITO layer on the back of that,
patterning it to create electrodes running at right angles to the first layer.
This module then gets laminated onto a standard LCD panel.
The other approach is called
"on-cell" pro-cap. (Here the "cell" refers to the LCD
display.) A conductive layer of ITO is deposited directly onto the top layer of
glass in the LCD panel, and then patterned into electrodes. A thin insulating
layer is applied, and then the second ITO layer is patterned with the second
layer of electrodes. Finally, the top polarizing layer is applied on top, and
the display is completed by adding the cover glass.
This may not make much difference to
the end user, but it can make a huge difference for the companies in the supply
chain -- including which companies are actually included.
When the touch technology is
deposited on the cover glass using the sensor on lens approach, you end up with
a separate touch module that can be sold to the LCD display assemblers. This
would mean more revenues for the touch technology manufacturers who would
supply these modules.
On the other hand, the on-cell
alternative means that the LCD panel manufacturers can add these touch layers
onto their own panels. The display assemblers would then just have to purchase
a simple cover glass to complete the display. The touch module makers would be
cut out of the process.
For now, it appears that the sensor
on lens approach has an advantage over on-cell solutions. The on-cell approach
means that LCD makers would have to make two separate models of each panel: one
with touch and one without. This could add cost to an industry that is already
running on razor-thin margins. Also, on-cell touch is limited to the size of
the LCD panel; sensor on glass modules can be larger than the LCD panel,
providing room for the dedicated touch points that are part of many smartphone
designs.
LCD
vs. OLED
In case you've been wondering where
OLED displays fit into all this: An OLED display stack is somewhat different
from an LCD stack. It only requires one substrate (glass) layer as opposed to
LCD's two, and the OLED material layer is much thinner than the LCD layer. As a
result, the finished display can be half as thick as an LCD panel, saving
weight and thickness -- which is important in a smartphone design.
(A number of smartphones today use a
form of active-matrix OLED display called Super AMOLED; these include several
Samsung devices such as the Galaxy S III and the Motorola Droid Razr M).
As a practical matter, glass is
still used as the encapsulating layer, so OLEDs generally have two layers of
glass. In addition, not all OLEDs are RGB -- some use white emitters instead to
try to reduce the differential aging problem, and add a color filter layer to
the stack.
In spite of all this, as far as
touch screen technologies are concerned, OLEDs are more like LCDs than they are
different: Both have active matrix TFT backplanes, and both tend to have a
cover glass layer for protection. So essentially the same stack configurations
are available to OLED panels.
What's
next for touch
No matter which solution wins out,
it is clear that pro-cap technology is the best method for touch screens on
mobile devices -- at least for the foreseeable future. Still, there are some
changes already showing up in touch screen technology.
For example, some panel makers are
creating "in-cell" touch panels, where one of the conductive layers
actually shares the same layer as the thin film transistors (TFTs) used to
switch the display's sub-pixels on and off. (These transistors are fabricated
directly on the semiconductor backplane of the display.) This approach not only
reduces the electromagnetic noise in the system, but also uses a single
integrated controller for both the display and the touch system. This reduces
part counts and can make the display component thinner, lighter, more energy
efficient and more reliable.
This approach only makes sense for
very high volume products, such as a smartphone from a major vendor that is
expected to sell millions of units, because the panel will have to be made
specifically for that unique model. The first products using
"in-cell" touch technology have already appeared on the market, such
as the new Apple iPhone 5, but it looks as
though it will take years before this approach will become a widespread
solution.
Some device manufacturers are also
adding stylus support to their products. The new higher-resolution displays
make it useful for some users to have access to a pointing or writing device
that has a finer tip than a finger. Some devices rely on an "active"
stylus that can be sensed by the pro-cap system, such as the Samsung Galaxy Note. Others,
such as the Amazon Kindle, are choosing
single-point infrared optical sensing that can detect the position of any
pointed object on the screen.
Meanwhile, system designers are
developing new ways to interact with mobile devices via touch, such as expanded gesture sets and three-dimensional proximity sensing.
Even as other modes of interaction -- such as speech recognition for voice
input -- become more sophisticated, touch is likely to remain the primary way
we control our devices.
By Alfred Poor for computer world
Alfred Poor is a speaker, writer, and display technology
expert. He is a contributing editor with Information Display, the magazine for
the Society for Information Display, and contributed to the Handbook of Visual Display
Technology published 2010 by Springer-Verlag and Canopus Academic Publishing.
