touch – input mode. overview input mode is rapidly developing and becoming a popular method of...
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![Page 1: Touch – input mode. Overview Input mode is rapidly developing and becoming a popular method of naturalising user input Touch screens –Generally used for](https://reader030.vdocuments.mx/reader030/viewer/2022032703/56649d035503460f949d6b7d/html5/thumbnails/1.jpg)
Touch – input mode
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Overview
• Input mode is rapidly developing and becoming a popular method of naturalising user input
• Touch screens– Generally used for selection in situations where a
mouse is impractical and buttons are unreliable
• Pen / stylus input– Used as touch screen but at higher resolution (or
smaller screen)– Used for freeform input such as writing
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Technologies
• There are currently four popular methods:– Resistive– Capacitive– Surface Acoustic Wave (SAW)– Infra red
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Resistive technology
• V=Voltage (Volts)– A difference in electrical potential
• I=Current (Amps)– A flow of electric charge which (by convention) flows from a
higher to a lower voltage potential
• R=Resistance (Ohms)– Resists the flow of electric current
• I = V/R – Current flow proportional to Voltage and inversely proportional to
resistance
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Potential divider
21
22
221
2
22
21
21
21
RR
RVV
RRR
VV
IRV
VVV
RR
VI
RRRR
VI
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Potentiometer
V2 is proportional to the position of the arrow. At the top V2=V and at the bottom V2=0
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Resistive touch sensor• Uses layers
– back layer such as glass– a uniform resistive coating
on the glass– a polyester coversheet,
with the layers separated by tiny insulating spacers.
• When the screen is touched, the conductive coating on the coversheet connects with the coating on the glass.
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Uses potentiometer principle
• Uniform PD on back plate, front plate used as probe to get voltage V2(X)
• Uniform PD on front plate, back plate used as probe to get voltage V2(Y)
• Use A to D converter for Cartesian coordinates
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Properties• Force activated so can use finger even with a
glove, stylus or any other (non sharp) prodding device
• Contacts are make or break so not pressure sensitive
• Conductive coating reduces display brightness• Continual flexing of outer layer causes
microscopic cracks so affects linearity of resistance (other processes can overcome this)
• Tolerates dust/dirt, rain and insects
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5 wire solution
• Only the top resistive surface is flexed so only it cracks
• To overcome the loss of resistive linearity in the top layer, only use the top layer as a probe
• The bottom layer is used for X and Y detection
• Top layer still cracks but as it is only a probe, non-linearity is not a problem
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Capacitive technology
• uniform conductive coating on a glass panel• electrodes around the panel's edge distribute low voltage
uniformly across the conductive layer creating a uniform electric field
• a finger touch draws current from each corner and the controller measures the ratio of the current flow from the corners and calculates the touch location
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Properties• More sensitive than resistive (just touch -
no need to press)• Must touch with a bare finger or
conductive stylus• Can be gasket sealed for outdoor
operation• Very durable• Reduces display brightness• Tolerates dust/dirt, rain and insects
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Infrared technology
• Simplest of all– Row of infrared LEDs in Y plane and X plane– Row of IR detectors on opposite edges– Lack of detection on IR sensor indicates X,Y
coordinate of the finger
• Lowest resolution– Beam spreads so sensors need to be wider
apart the further the beam has to travel
• Lasers could solve this
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Properties
• Hybrid of resistive and capacitive– Can operate with any object e.g. gloved finger
as long as it is not smaller that the beam resolution
– No need to press– Does not reduce display brightness– Intolerant to dust/dirt and rain– Very durable– Intolerant of insects on the screen
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SAW technology
• Glass overlay with transmitting and receiving piezoelectric transducers on X and Y axis
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• Ultrasonic waves distributed across the surface of the glass by an array of reflectors
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• Surface wave directed into a receiver by more reflectors
• Strength of wave is dependant on to distance travelled
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• Finger or other ultrasonic energy absorber effectively reduces the vibration of the glass so the receiver picks up a weaker signal
• Same process is repeated for the Y direction
• Based on the timings of the weak signal, coordinates are calculated
• If more pressure is applied, more signal attenuation occurs so Z value is also measurable
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Properties
• Screen glass is the touch sensor so no loss of picture brightness
• Location and pressure (X,Y & Z)
• Very resilient as just glass – no layer bonding
• Can not be gasket sealed
• Tolerates dust and dirt but not rain (no seal and rain will disrupt waves)
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Benefits of touch screens
• Replaces keyboard and mouse
• Intuitive
• Soft keys – reconfigurable so only relevant options are displayed
• Very durable (but need careful choice of technology based on environment)
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Applications• Industrial control
– Sealed units with no moving parts fair better than keyboard, push buttons, thumbwheels etc
• Industrial vehicles such as tractors
• Consumer– POS, ticketing, photo selection– Kiosks, advertising, information servers– Amusement machines, burning cigarettes and
spilt drinks don’t ruin the input device…