Touch screens

Touch – input mode
• 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
• There are currently four popular methods:
– Resistive
– Capacitive
– Surface Acoustic Wave (SAW)
– Infra red
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
Potential divider
R  R1  R2
I 
I 
R1  R2
V  V1  V2
V2  IR2
V2 
R1  R2
V2  V
R1  R2
V2 is proportional
to the position of
the arrow. At the
top V2=V and at
the bottom V2=0
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
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
• Force activated so can use finger even with a
glove, stylus or any other (non sharp) prodding
• Contacts are make or break so not pressure
• 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
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
• The bottom layer is used for X and Y
• Top layer still cracks but as it is only a
probe, non-linearity is not a problem
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
• 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
• Very durable
• Reduces display brightness
• Tolerates dust/dirt, rain and insects
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
• Hybrid of resistive and capacitive
– Can operate with any object e.g. gloved finger
as long as it is not smaller that the beam
– No need to press
– Does not reduce display brightness
– Intolerant to dust/dirt and rain
– Very durable
– Intolerant of insects on the screen
SAW technology
• Glass overlay with transmitting and
receiving piezoelectric transducers on X
and Y axis
• Ultrasonic waves distributed across the
surface of the glass by an array of
• Surface wave directed into a receiver by
more reflectors
• Strength of wave is dependant on to
distance travelled
• Finger or other ultrasonic energy absorber
effectively reduces the vibration of the
glass so the receiver picks up a weaker
• Same process is repeated for the Y
• 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
• 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
• Can not be gasket sealed
• Tolerates dust and dirt but not rain (no seal
and rain will disrupt waves)
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)
• 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…