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Touch Screen Technologies
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History
Touch screens emerged from academic and corporate research labs in the second half of the 1960s.
One of the first places where they gained some visibility was in the terminal of a computer-assisted learning terminal that came out in 1972 as part of the PLATO project.
Touch screens became widely used in kiosk and point of sale systems in banks and stores.
In 1983, the first touch screen computer, the HP-150, reached the market.
Introduction of advanced touch screen technologies leading to the commercialization of tablet PCs, PDAs, and touch-screen phones.
Touch Screen
Technologies
Touch Screen Technologies
Resistive
Capacitive
Surface Acoustic
Resistive
2 Resistive Surfaces (Idium-Tin-Oxide)
Metallically-coated
Insulating Space
Touch compresses and forms closed circuit
Most widely used due to its simple structure
2 Types
1. Matrix (digital)
Striped electrodes on substrates such as glass or plastic face each other
2. Analogue
Transparent electrodes without any patterning facing each other
Low production costs
Overview of the Position Measurement System
Phase Lock Loop
Crystal Oscillator
Conversion to
X & Y Coordinates
2 Parallel Sheets
If there is no pressure applied –electronically separated
Applied pressure –impedance between the 2 sheets is lowered at the touch point.
Measurement of X & Y Coordinates:
Top sheet carries a voltage gradient by applying a voltage between the electrodes of top sheet
Bottom sheet serves as a slide in a linear potentiometer.
Voltage Gradient
9/10V
7/10 V
•Equal voltage drop across each resistor in voltage gradient
•Dependent upon resistance value
Linear potentiometers are sensors that produce a resistance output proportional to the displacement or position
Resistance value changes with rotation of screw
Linear Potentiometers
R1
R2
R total
Conduction Current
Electrons move along
Displacement Current
Electrons are completely displaced
Resistive
Measurement of the touch point resistance is valuable
Value varies depending upon force applied
Capacitive
Conductive lower coating (Indium-Tin-Oxide)
No top coating, only rigid protective cover
Finger serves as second conducting layer
Ohm’s Law relates current to voltage in DC circuit in the form of V = iR
Capacitive touch screen uses Alternating Current (AC).
The current is continuous across the ITO surface Remember Sinusoid waves from lab
Impedance is equivalent to resistor in AC circuitV = iZ; where Z = (1/jwC)
J = sqrt(-1) w= 2pF where F = Freq.
C = Capacitance = (erA)/d
Human body achieves capacitance and conducts current
Touch Event Voltage drop at the point of touching
Affects strength of current across ITO surface
Voltage gradient across surface
Conductive ITO surface allows for continuous current across the surface
Electronic circuits located underneath ITO surface measure the resulting distortion in the sine waves produced by voltage drop as a result of the touch event.
Surface Acoustic Wave
Based upon emission and absorption of sound waves
Materials used:
Transducers
One glass screen
Reflectors
Sensors
Twotransducersare placed along the X and Y axes and generate sound waves.
Surface Acoustic Wave
The waves propagate across the glass and are reflected back to the sensors.
When screen touched, a portion of the wave is reflected back to the sensors immediately.
The sensor is able to tell if the wave has been disturbed by a touch event at any instant, and can locate it accordingly.
Surface Acoustic Wave
Surface Acoustic Wave
How can the sensors tell?
Waves travel at the speed of sound
Speed of Sound = 343 m/s
Based on the time it takes for the wave to return to the source, the sensor can tell if it was disturbed or not.
If it was, based on the time it takes to get back to the source, the sensor can calculate the distance.
These calculations will generate (X,Y) coordinates
Surface Acoustic Wave
Benefits
100% clarity because of the lack of metallic layers in the screen
Able to interact with the use of multiple mediums
Stylus, finger, glove …
Negatives
Screen can become contaminated and cease to operate correctly.
Commercial
Applications
Current Uses
Kiosks
ATMs, Self Checkout Counters, Airport Check-in, etc.
PDAs
Tablet PCs
Mobile Phones
Handheld Gaming Consoles
PDAs and Tablet PCs
Apple Newton and Palm Pilot
First PDAs to use touch screens
Apple Newton introduced in 1993
Palm Pilot introduced in 1997
IBM ThinkPad 750P and 360P
Introduced in 1993
PDAs and Tablet PCs were the first consumer devices to utilize touch screen technology
Mobile Phones
Apple iPhone
Introduced in 2007
Uses multi-touch technology
Widely popularized the use of touch screen technology
Touch Screen Cell Phones
Every major carrier now has a touch screen phone available
Touch screen market for mobile phones is projected to reach $5 Billion by 2009
Where Is Technology Heading?
According to USA Today:
Advanced touch screen phones expected to increase from 200,000 shipped in 2006 to 21 million units by 2012.
Regular touch technology has already been incorporated into 38 million phones as of 2006, and is estimated to be in nearly 90 million phones by 2012.
Uses of Multi-Touch
Enhanced dining experience at restaurant
Concierge service at hotels
Concept mapping
Use as interactive Whiteboards
Better multi-media experience
Microsoft Surface
1) Screen
2) Infrared Cameras
3) CPU
4) Projector
Microsoft Surface (cont.)
No mouse or keyboard
Multiple contact points
Several simultaneous users
Non-digital objects used as input
Not limited by traditional touch
Multi-Touch Collaboration Wall
Invented by Jeffrey Han
Made by Perceptive Pixel
Original for military use
Currently used by CNN
Available at Neiman Marcus
LED: DC Circuit
Battery
LED
Switch
Resistor
LED
A semi-conductor diode
Emits light when current moves in forward direction (p-n junction)
Source: http://en.wikipediawiki/Image:PnJunction-LED-E.PNG
LED cont.
As DC circuit, when switch closes, the LED light up
•As DC circuit, when switch closes, the LED light up
Source: http://en.wikipediawiki/Image:Rectifier_vi_curve.GIF
How Does it Work?
Utilizes Frustrated Total Internal Reflection
The surface of the table is a thin diffuser, which has infrared light and a projector reflected on its underside.
Infrared cameras pick up on any objects such as fingers, paintbrushes over the surface, when touching the display.
Frustrated Total Internal Reflection
Light encounters an interface (boundary) with lower index of refraction, light becomes refracted (bent).
Refraction depends on the angle of incidence
After a certain angle it undergoes TIR
A finger, with a lower index of refraction, when it comes in contact with that surface can cause the light to escape
Image Processing
Infrared camera captures image on the screen
The image processing subtracts background image to reduce noise
It uses a Gaussian smoothing filter to reduce noise
Introduced a cutoff filter to make the image black and white
Grouped the white pixels together
Then the program finds the center and relative size of the pixel groups to drive the applications.