Touch – input mode

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

Technologies

• 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

resistance

Potential divider

21

22

221

2

22

21

21

21

RR

RVV

RRR

VV

IRV

VVV

RR

VI

RRRR

VI

Potentiometer

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 glass.

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

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

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

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

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

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

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

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 reflectors

• 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 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

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)

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)

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…