Hardware Reference
In-Depth Information
Touch devices fall into two categories: opaque and transparent. A typical
opaque touch device is the touchpad on a notebook computer. A typical transpar-
ent device is the screen of a smart phone or tablet. We will only consider the latter
here. They are usually called
touch screens
. The major types of touch screens are
infrared, resistive, and capacitive.
Infrared screens have infrared transmitters, such as infrared light emitting
diodes or lasers on (for example) the left and top edges of the bezel around the
screen and detectors on the right and bottom edges. When a finger, stylus, or any
opaque object blocks one or more beams, the corresponding detector senses the
drop in signal and the hardware of the device can tell the operating system which
beams have been blocked, allowing it to compute the (
x
,
y
) coordinates of the fin-
ger or stylus. While these devices have a long history and are still in use in kiosks
and other applications, they are not used for mobile devices.
Another old technology consists of
resistive touch screens
. These consist of
two layers, the top one of which is flexible. It contains a large number of horizontal
wires. The one under it contains vertical wires. When a finger or other object
depresses a point on the screen, one or more of the upper wires comes in contact
with (or close to) the perpendicular wires in the lower layer. The electronics of the
device make it possible to read out which area has been depressed. These screens
can be built very inexpensively and are widely used in price-sensitive applications.
Both of these technologies are fine when the screen is pressed by one finger
but have a problem when two fingers are used. To describe the problem, we will
use the terminology of the infrared touch screen but the resistive one has the same
problem. Imagine that the two fingers are at (3, 3) and (8, 8). As a result, the
x
=
3
and
x
8 horizontal
beams. Now consider a different scenario with the fingers at (3, 8) and (8, 3),
which are the opposite corners of the rectangle whose corners are (3, 3), (8, 3), (8,
8), and (3, 8). Precisely the same beams are blocked, so the software has no way of
telling which of the two scenarios holds. This problem is called
ghosting
.
To be able to detect multiple fingers at the same time—a property required for
pinching and expanding gestures—a new technology was needed. The one used on
most smart phones and tablets (but not on digital cameras and other devices) is the
projected capacitive touch screen
. There are various types but the most common
one is the
mutual capacitance
type. All touch screens that can detect two or more
points of contact at the same time are known as
multitouch screens
. Let us now
briefly see how they work.
For readers who are a bit rusty on their high-school physics, a
capacitor
is a
device that can store electric charge. A simple one has two conductors separated by
an insulator. In modern touch screens, a grid-like pattern of thin ''wires'' running
vertically is separated from a horizontal grid by a thin insulating layer. When a fin-
ger touches the screen, it changes the capacitance at all the intersections touched
(possibly far apart). This change can be measured. As a demonstration that a mod-
ern touch screen is not like the older infrared and resistive screens, try touching
=
8 vertical beams are interrupted as are the
y
=
3 and
y
=
