1. Touch – input mode

2. 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

3. Technologies • There are currently four popular methods: • Resistive • Capacitive • Surface Acoustic Wave (SAW) • Infra red

4. 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

5. Potential divider

6. Potentiometer V2 is proportional to the position of the arrow. At the top V2=V and at the bottom V2=0

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. SAW technology • Glass overlay with transmitting and receiving piezoelectric transducers on X and Y axis

16. Ultrasonic waves distributed across the surface of the glass by an array of reflectors

17. Surface wave directed into a receiver by more reflectors • Strength of wave is dependant on to distance travelled

18. 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

19. 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)

20. 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)

21. 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…