touch screen

Touch screen technology

Touch screen technology

Touch screen – an information input device, which is a screen that responds to touch.

History

The touchscreen was invented in the United States as part of programmed learning
research. The PLATO IV computer system, which appeared in 1972, had a touch
screen on a grid of infrared rays, consisting of 16 × 16 blocks. But even such low
accuracy allowed the user to choose the answer by clicking on the right place on the
screen.
In 1971, Samuel Hurst (the future founder of Elographics, now Elo TouchSystems)
developed an elograph – a graphic tablet that operated on a four-wire resistive principle
(U.S. Patent 3,662,105). In 1974, he managed to make the elograph transparent; in
1977, he developed a five-wire screen. Together with Siemens, Elographics managed
to make a convex touch panel that was suitable for picture tubes of that time. At the
1982 World's Fair, Elographics introduced a touch-screen TV.
In 1983, the HP-150 computer with an IR-touch screen was released. However, in those
days, touch screens were used mainly in industrial and medical equipment.
Consumer devices (telephones, pocket PCs, etc.) included touch screens as a
replacement for a tiny keyboard when devices with large (full front panel) LCD screens
appeared. The first handheld gaming console with a touchscreen is the Nintendo DS,
the first mass-media device that supports multi-touch is the iPhone.

Application.

Touch screens are used in payment terminals, information kiosks, equipment for trade
automation, handheld computers, mobile phones, game consoles, and operator panels
in the industry.

Advantages and disadvantages of handheld devices

Advantages
 The simplicity of the interface.
 The device can combine small sizes and a large screen.
 Speed dial in a relaxed atmosphere.
 The multimedia capabilities of the device are seriously expanding.
Disadvantages
 No tactile feedback
 High power consumption.
 Particularly thin screen models, even with minor damage, run the risk of being
cracked or even broken.
 Hygiene of Screen.
Advantages and disadvantages of stationary devices
Advantages

In information and vending machines, operator panels and other devices that do not
have active input, touch screens have established themselves as a very convenient way
of human interaction with the machine.

Touch screen technology 1

Advantages:

 Increased Reliability.
 Resistance to harsh external influences (including vandalism), dust and moisture
resistance.
Disadvantages
 (For capacitive screens). No tactile feedback.
 When working with a vertical screen, the user is forced to keep his hand on
weight. Therefore, vertical screens are suitable only for occasional use like
ATMs.
 On a horizontal screen, hands obscure the view.

Even with a sharp pen, parallax limits the accuracy of positioning operator actions on
touch screens without a cursor. At the same time, using the cursor creates additional
difficulties for the operator, reducing ergonomics.
When using the screen with not completely clean hands, use is difficult due to the
the difficulty of moving the fingers, as well as the resulting fingerprints and spots, if the
screen does not have special coatings to neutralize them.
These disadvantages do not allow using only the touch screen in devices with which a
person works for hours. However, in a well-designed device, the touch screen may not
be the only input device – for example, at the cashier’s workplace, the touch screen can
be used to quickly select goods, and the keyboard to enter numbers.

Principles of operation of touch screens

There are many different types of touch screens that work on different physical
principles.

Resistive Touch Screens – Four-wire screen

The principle of operation of a 4-wire resistive touch screen

The principle of operation of the 5-wire resistive touch screen
The resistive touch screen consists of a glass panel and a flexible plastic membrane.
Both the panel and the membrane are coated with a resistive coating. The space
between the glass and the membrane is filled with micro-insulators, which are evenly
distributed over the active area of ​​the screen and reliably insulate conductive surfaces.
When the screen is pressed, the panel and membrane are closed, and the controller,
using an analog-to-digital converter, detects the change in resistance and converts it
into touch coordinates (X and Y). In general terms, the reading algorithm is as follows:
A voltage of + 5V is applied to the upper electrode; the lower one is grounded. The left
and right are short-circuited and the voltage on them is checked. This voltage
corresponds to the X-coordinate of the screen.
Similarly, + 5V and ground are applied to the left and right electrodes, and the Y-
coordinate is read from the top and bottom.
There are also eight-wire touch screens. They improve tracking accuracy, but do not
increase reliability.

Five-wire screen

The five-wire screen is more reliable due to the fact that the resistive coating on the
membrane is replaced by a conductive one (the 5-wire screen continues to work even
with a slotted membrane). A resistive coating with four electrodes at the corners is
applied to the rear window.

Initially, all four electrodes are grounded, and the membrane is “pulled” by a resistor to
+ 5V. The voltage level on the membrane is constantly monitored by an analog-to-digital
converter. When nothing touches the touch screen, the voltage is 5 V.

As soon as the screen is pressed, the microprocessor picks up the change in
membrane voltage and starts calculating the contact coordinates as follows:
A voltage of + 5V is applied to the two right electrodes, the left ones are grounded. The
voltage on the screen corresponds to the X-coordinate.
The Y-coordinate is read by connecting to the + 5V of both upper electrodes and to the
“ground” of both lower.
Features

Resistive touch screens are cheap and pollution resistant. Resistive screens respond to
touch with any smooth, hard object: hand (bare or gloved), pen, credit card, pick. They
are used wherever vandalism and low temperatures are possible: for the automation of
industrial processes, in medicine, in the service sector (POS-terminals), in personal
electronics (PDAs). The best samples provide accuracy of 4096 × 4096 pixels.
The disadvantages of resistive screens are low light transmission (not more than 85%
for 5-wire models and even lower for 4-wire), low durability (no more than 35 million
clicks at one point) and insufficient vandal resistance (the cover-up is easy to cut).
Matrix Touch Screens
Design and principle of operation

The design is similar to resistive, but simplified to the limit. Horizontal conductors are
applied to the glass, vertical to the membrane.

When you touch the screen, the conductors touch. The controller determines which
conductors are closed and transfers the corresponding coordinates to the
microprocessor.

Features

They have very low accuracy. Interface elements have to be specially arranged taking
into account the cells of the matrix screen [8]. The only advantage is simplicity,
cheapness and unpretentiousness. Typically, matrix screens are polled line by line
(similar to a matrix of buttons); This allows you to set up multi-touch. Gradually replaced
by resistive.
Capacitive Touch Screens
Design and principle of operation

Capacitive touch screen operating principle

A capacitive (or surface-capacitive) screen uses the fact that an object of large capacity
conducts alternating current.

The capacitive touch screen is a glass panel coated with a transparent resistive material
(usually an alloy of indium oxide and tin oxide is used). The electrodes located at the
corners of the screen supply a small alternating voltage (the same for all angles) to the
conductive layer. When you touch the screen with your finger or other conductive
object, a current leakage appears. Moreover, the closer the finger to the electrode, the
lower the screen resistance, which means that the current strength is greater. Current in
all four corners is recorded by sensors and transmitted to the controller, which
calculates the coordinates of the touch point.
In earlier models of capacitive screens, direct current was used – this simplified the
design, but when the user contacted the ground poorly, it led to malfunctions.
Capacitive touch screens are reliable, about 200 million clicks (about 6 and a half years
of clicks with an interval of one second), do not let in liquids and tolerate non-conductive
pollution perfectly. Transparency at 90%. However, a conductive coating located directly
on the outer surface is still vulnerable. Therefore, capacitive screens are widely used in
machines that are only installed in a weatherproof room. Do not respond to gloved
hand.

It is worth noting that due to differences in terminology, surface and projection-
capacitive screens are often confused. According to the classification used in this
article, the screen, for example, the iPhone is projective-capacitive, and not capacitive.
Projection Capacitive Touch Screens
Design and principle of operation

Projection Capacitive Touch Screen

On the inside of the screen is a grid of electrodes. The electrode together with the
human body forms a capacitor; Electronics measures the capacitance of this capacitor
(gives a current pulse and measures the voltage).

Samsung was able to install sensitive electrodes directly between the sub-pixels of the
AMOLED screen, which simplifies the design and increases transparency.
Features

The transparency of such screens is up to 90%, the temperature range is extremely
wide. Very durable (bottleneck – sophisticated electronics that handle clicks). PESE can
use glass up to 18 mm thick [10], which leads to extreme vandal resistance. They do
not respond to non-conductive contaminants, conductive are easily suppressed by
software methods. Therefore, projection-capacitive touch screens are widely used in
personal electronics and in machines, including those installed on the street. Many
varieties support multi-touch.

Touch screen technology 3

Touchscreens for surface acoustic waves
Design and principle of operation

The screen is a glass panel with piezoelectric transducers (PES) located at the corners.
At the edges of the panel are reflective and receiving sensors. The principle of operation
of such a screen is as follows. A special controller generates a high-frequency electrical
signal and sends it to the probe. A probe converts this signal into a SAW, and reflective
sensors reflect it accordingly. These reflected waves are received by appropriate
sensors and sent to the probe. The probes, in turn, receive reflected waves and convert

them into an electrical signal, which is then analyzed using a controller. When you touch
the screen with your finger, part of the energy of the acoustic waves is absorbed. The
receivers record this change, and the microcontroller calculates the position of the touch
point. It reacts to touch with an object that can absorb a wave (finger, gloved hand,
porous rubber).
Features
The main advantage of a screen on surface acoustic waves (SAW) is the ability to track
not only the coordinates of the point, but also the pressure (here, rather, the ability to
accurately determine the radius or area of ​​pressure), due to the fact that the degree of
absorption of acoustic waves depends on the pressure at the point touch (the screen
does not bend at the touch of a finger and does not deform, therefore, the force of
pressing does not entail qualitative changes in the processing by the controller of data
on the coordinates of the impact, which captures only the area b, overlapping the path
of acoustic pulses). This device has a very high transparency, since the light from the
imaging device passes through glass that does not contain resistive or conductive
coatings. In some cases, glass is not used at all to combat glare, and emitters, receivers
and reflectors are attached directly to the screen of the display device. Despite the
complexity of the design, these screens are quite durable. According to the statement
of, for example, the American company Tyco Electronics and the Taiwanese company
GeneralTouch, they withstand up to 50 million touches at one point, which exceeds the
resource of a 5-wire resistive screen. Surfactant screens are used mainly in slot
machines, in guarded help systems and educational institutions. As a rule, surfactant
screens are distinguished into ordinary ones – 3 mm thick, and vandal resistant – 6 mm.
The latter withstand the fist of an average man or the fall of a metal ball weighing 0.5 kg
from a height of 1.3 meters (according to Elo Touch Systems). The market offers
options for connecting to a computer both through the RS232 interface, and through the
USB interface. At the moment, controllers for SAW touch screens are more popular,
supporting both types of connection – combo (data from Elo Touch Systems).
The main disadvantage of a SAW screen is a malfunction in the presence of vibration or
when exposed to acoustic noise, as well as when the screen is dirty. Any foreign object
placed on the screen (for example, chewing gum) completely blocks its work. In
addition, this technology requires touching with an object that necessarily absorbs
acoustic waves, that is, for example, a plastic bank card is not applicable in this case.
The accuracy of these screens is higher than matrix ones, but lower than traditional
capacitive ones. As a rule, they are not used for drawing and entering text.
Infrared touch screens

The principle of operation of the infrared touch panel is simple – the grid formed by
horizontal and vertical infrared rays is interrupted when you touch the monitor with any
object. The controller determines where the beam was interrupted.
Features
Infrared touch screens are afraid of pollution and therefore are used where image
quality is important, for example, in e-books. Due to its simplicity and maintainability, the
circuit is popular with the military. Often on this principle do the keyboards of intercoms.
This type of screen is used in Neonode mobile phones.
Optical touch screens

The glass panel is equipped with infrared illumination. At the glass-air boundary, total
internal reflection is obtained, at the glass-foreign object boundary, light is scattered. It
remains to capture the scattering picture, for this there are two technologies:
In projection screens, a camera is placed next to the projector. This is how, for example,
Microsoft PixelSense works.
Or they make photosensitive an additional fourth sub-pixel of the LCD screen.
Infrared touch screens

The principle of operation of the infrared touch panel is simple – the grid formed by
horizontal and vertical infrared rays is interrupted when you touch the monitor with any
object. The controller determines where the beam was interrupted.
Features

Infrared touch screens are afraid of pollution and therefore are used where image
quality is important, for example, in e-books. Due to its simplicity and maintainability, the
circuit is popular with the military. Often on this principle do the keyboards of intercoms.
This type of screen is used in Neonode mobile phones.
Optical touch screens

The glass panel is equipped with infrared illumination. At the glass-air boundary, total
internal reflection is obtained, at the glass-foreign object boundary, light is scattered. It
remains to capture the scattering picture, for this there are two technologies:
In projection screens, a camera is placed next to the projector. This is how, for example,
Microsoft PixelSense works.
Or they make photosensitive an additional fourth sub-pixel of the LCD screen.
Features

Allow you to distinguish between pressing your hand from pressing any objects, there is
a multi-touch. Large touch surfaces are possible, right down to the blackboard.
Strain gage touch screens

React to screen deformation.

 

The accuracy of strain gauge screens is low, but they
withstand vandalism perfectly. The main application is ATMs, ticket machines and other
devices located on the street.
DST touch screens
Main article: Dispersive Signal Technology
The touch screen DST (Dispersive Signal Technology) responds to glass deformation.
You can tap the screen with your hand or any object. A distinctive feature is the high
reaction rate and the ability to work in conditions of severe screen contamination.

Induction Touch Screens

The induction touch screen is a graphic tablet with an integrated screen. Such screens
respond only to a special pen.

They are used when a reaction is required precisely by pressing with a pen (and not
with a hand): high-end art tablets, some tablet PC models.
1. Supported with restrictions, maximum 2 touch points.
2. If you need only a glass panel, without any transparent conductive films –
conditionally 95%. If you don’t even need it (you can use the standard screen
coverage) – conditionally 100%
3. High – down to the pixel (accurately tracks a sharp pen). Medium – up to a few
pixels (sufficient for finger taps). Low – in large blocks of the screen (drawing is
not possible; very large interface elements are required).
4. Limited by the reliability of electronics
5. Limited by sensor fouling.
6. Limit – equipment of limited access (personal electronics, industrial equipment).
Premises – shared access in a secure room. Street – general access on the
street.

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