06-07-2012, 10:11 AM
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 circuitV = 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.