touch sensor technology for hand-held and mobile...

Touch Sensor Technology forHand-held and Mobile devices

Ankur Garg 200601139Pranjal Mishra 200601219

Overview INTRODUCTION

HISTORY

TYPES OF TOUCH SENSORS

APPLICATION OF TOUCH SENSORS MOBILES HANDHELD DEVICES i-PODS FINGER PRINT SCANNER

INTERFACING WITH HAND-HELD DEVICES

CONCLUSION

INTRODUCTION

A touch screen sensor is generally a clear glass panel with a touchresponsive surface. The touch sensor/panel is placed over a displayscreen so that the responsive area of the panel covers the viewablearea of the screen.

The sensor normally has an electrical current or signal goingthrough it and touching the screen causes a voltage or signalchange.

This voltage change is used to determine the location of thetouch on the screen.

HISTORY Do you think Touch Sensors are gifts of 21st century ??

First Touch Sensor was developed in 1971 by DoctorSam Hurst (Prof. at the University of Kentucky)

This sensor was called the "Elograph" and waspatented by The University of Kentucky ResearchFoundation.

In 1974, the first true touch screen incorporating atransparent surface came on the scene developed byDr. Sam Hurst

HISTORY

Touch Sensors have been used extensively in industries since 1970’s because they have simple user interface, are easy to use and have a less response time.

With the improvement in technology and advent of hand-held devices, Touch Sensors can now be integrated into small devices such as Mobile phones, i-pods, finger print scanners, gaming PDA’s etc.

Why are Touch Sensors used??

Touch sensors are mostly used in the devices where space is ata premium. They provide a friendly user interface which is notonly easy to use but also attractive.

Touch sensors are completely sealable which helps to create adevice resistant to water spills and dusts to a larger extent.

Touch sensors provide fast response which eliminates the delay caused by mechanical buttons in general devices.

Types Of Touch Sensors Capacitive

Resistive

Surface Acoustic Wave

Infrared

Optical

We will describe touch sensors and their working usingparticular devices which are presently in use and are verycommon.

Application of Touch Sensors…

Mobile Devices

Hand-Held Devices

Mobile Devices

Touch Sensors represents a significant breakthrough in Mobile devices, enabling lighter, smaller and thinner handsets.

Touch Sensors are very easy to use, power efficient, compact and can be integrated easily in Mobile Devices.

I-PhoneI-Phone uses capacitive touch sensors.

Instead of a physical keypad, I-phoneuses virtual buttons and controls thatappears on its screen.

Once we touch the screen, thefeatureless rectangle becomes aninteractive surface and the inputs areread and interpreted using capacitivetouch sensors.

Capacitive Touch Sensors…

Capacitive Touch Sensorsconsist of an all-glass touchscreen with a transparentmetallic conductivecoating, as seen in thefigure.


Capacitive touch-screensuse a layer of capacitivematerial to hold an electricalcharge; touching the screenchanges the amount ofcharge at a specific point ofcontact.

Capacitive Touch Sensors…When a finger touches the touchscreen it draws a minute amount ofcurrent at the point of contact,creating a voltage drop.

This happens because the tissueof the human body containsconductive electrolytes covered bya layer of skin which is a lossydielectric. It is this conductiveproperty of fingers that makescapacitive touch sensing possible.


An electrode pattern isprinted along the edges of thescreen which is responsiblefor creating a low voltage fieldover the conductive layer.

The current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computed

Most of the electric field lines inthis system are concentrateddirectly between the plates ofcapacitors but some field linesspills over into the area outside theplates, and these are called fringingfields. These fringing fields shouldbe directed into an active sensingarea accessible to a user.

Capacitive Touch Sensors…The current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computedThe current flow from each corner is proportional to the distance to the touch point. By measuring the distance the exact location of contact can be computed


The current flow from eachcorner is proportional to thedistance from the touch point.By measuring the distance theexact location of contact canbe computed.


Touch strength can be increased by1. Pressing harder2. Increasing area of touched

surface3. Increasing capacitance

When D is decreased

1. Capacitance is increased2. Touch strength is

increased

Problems with Capacitive Sensors… Most of the time, these systems are good at detecting thelocation of exactly one touch. If we try to touch the screen inseveral places at once, the results can be erratic.

Some sensors simply disregard all touches after the first one.The main reasons for the same are:

Many systems detect changes along an axis or in a specificdirection instead of each point on the screen.

Some screens rely on system-wide averages to determinetouch locations.

Some systems take measurements by first establishing abaseline. When we touch the screen, we create a newbaseline. Adding another touch causes the system to take ameasurement using the wrong baseline as a starting point.

Then how i-Phone uses multiple touch?

An APPLE i-PHONE does notuse the exact type of sensorsdescribed earlier.

For providing Touchcommands that require multipletouches, the i-Phone uses a newarrangement of existingtechnology but in a differentpattern.

The touch-sensitive screenincludes a layer of capacitivematerial arranged according to acoordinate system

Then how i-Phone uses multiple touch?

The circuitry can sense changesat each point along the grid. In otherwords, every point on the gridgenerates its own signal whentouched and relays that signal to theprocessor.This allows the device todetermine the location andmovement of simultaneous touchesin multiple locations.

The iPhone's screen detects touch through one of two methods: Mutual capacitance or Self capacitance.

Multiple Touch – Mutual Capacitance

The intentional capacitancethat occurs between two charge-holding objects or conductors, inwhich the current pass throughone into the other is mutualcapacitance.

Sensing line and the driving lines are closely spaced together, theair or material separating them acts as a dielectric, and the lines actas capacitor plates.

i-Phone touch sensor hasDriving lines carrying current,and sensing lines, detect thiscurrent at nodes.

Multiple Touch – Self Capacitance

i-Phone self capacitance has atransparent electrode layerwhich detects any change incapacitance using the capacitivesensing circuit.

The capacitive sensing circuit isconnected to each electrodewhich enables the determinationof the exact location of touch.

Disadvantages of Capacitive Touch Sensor

Capacitive Touch Sensors can only detect

a touch by a conducting object and hence

cannot be used with stylus or with gloves.

In the presence of high electric fields,

charges can develop over the capacitive

touch sensor and may result in false

detection.

These drawbacks can be overcome by using a resistive TouchSensor

Resistive Touch Sensors…

Mobile Devices


Resistive touch screens consistof a glass or acrylic panel that iscoated with electrically conductiveand resistive layers made withindium tin oxide (ITO).

The thin layers are separated byvery small invisible spacers calledmicrodots.

The total resistance of each layer varies from vendor to vendor,but typical screens are in the 100 to 900-ohm range

When contact is made on thesurface of the touch screen, thetwo sheets are pressed together.

To Measure the Y-coordinateof point of contact, voltage isapplied across the screen in Ydirection.

The contact of layers creates a voltage divider which is sensed bysensors in X direction and the Y coordinate is determined.

The complete process is repeated with the X direction beingdriven, and reading is taken from Y electrodes to determine the Xcoordinate.


Resistive touch Sensors are not very clear. Modern resistivetouch sensors can achieve 80 % Clarity.

The layers of resistive touch sensor can be damaged by verysharp objects.

Addition of Resistive touch sensors reduce the brightness, colorsaturation and contrast of the display and hence a strongbacklighting is required to compensate for transmissionlosses.This, in turn, increases the current consumption and thepower consumption.

process then repeats with the X direction being driven, and a reading is taken from one of the Y electrodes.

Disadvantages of Resistive Touch Sensor

Surface acoustic wave (SAW) technologyuses ultrasonic waves that pass over thetouch screen panel.

When the front surface of the touchscreen is touched, a portion of themechanical wave is absorbed, thuschanging the received signal.

The signal is then compared to a storedreference signal, the change is recognized,and the coordinate is calculated.

Surface Acoustic Wave Touch Sensors

This process happens independently for both the X and Y-axes.

Comparisons…Type: Resistive Acoustic Wave: Capacitive

Activation: Pressure sensitive Wave aborption Human body electricity

Antiglare protection: Minimal Medium Clear, Light-Etch, Etched

Clarity: Medium Best Minimal, Medium, Best

Damaged by: Very sharp objects Glass - breakable Glass coating wears out

Can handle dirt: Good Poor Best

Made with: Hardened acrylic plastic Glass with coatings Glass with coatings

Works with: Finger, glove, stylus Finger, glove,soft stylus Finger

Durability (MTBF): 15 million touches 30 million touches 60 million touches

Resolution: 1 million touch points 1 million touch points 1 million touch points

Warranty: 1 Year 3 Years 5 Years

SUMMARY: Best Price Best Clarity Most Durable

i-Podi-Pod also uses capacitive touch sensors.

Instead of a physical keypad, I-pod uses Clickwheel a touch sensitive ring used to navigatethrough all of iPod's menus .

It provides two ways to input commands: bysliding a finger around the wheel and bypressing buttons located under and in themiddle of the wheel.

Hand-Held Devices

I-PodThere is a membrane embedded withmetallic channels under the plastic cover ofthe click wheel.

At the points where the channelsintersect, a positional address is created,like coordinates on a graph.

The metal channels that form the grid conductelectricity. When a finger gets close to the grid, thecurrent tries to flow to the finger to complete thecircuit. The piece of nonconductive plastic the Click Wheelcover acts as insulator, so the charge builds up at thepoint of the grid that's closest to the finger causingchange in capacitance

Determining the speed of touch…

The change in capacitance ismeasured by a click wheel controllerchip which sends signal tomicroprocessor on sensing change incapacitance

The greater the change incapacitance at a particular point, thecloser the finger must be to that point

As the finger is moved around thewheel, the charge builds-up and movesaround the wheel with the finger.

The faster the finger moves aroundthe wheel, more compacted streamof signals are send, as themicroprocessor receives the signals, itperforms the corresponding action.

When the finger stops movingaround the wheel, the controllerstops detecting changes incapacitance and stops sending signalsto the microprocessor andcorresponding action is stopped.

Determining the speed of touch…

Finger Print Scanners

Hand-Held Devices

Scanners generate an image ofthe ridges and valleys that make upa fingerprint.

The scanner device reads thesensor output and determineswhether it is characteristic of aridge or a valley.

Because it requires a real fingerprint-type shape, rather than thepattern of light, this makes the system harder to trick.

A capacitive touch sensor is used to generate the image.

Interfacing with Handheld Devices

A basic touch screen has three main components: a touchsensor, a controller, and a software driver. Working of thesensors has been discussed and now lets see the controllerdevices which are used as an interface between the embeddedboard and the sensor.

A high resolution analog to-digital converter (ADC) is used inthe controller chips to convert the sensed analog voltage to adigital code.

Analog interface circuit are not used because the capacitivesensor may be affected by subtle noise, crosstalk, and coupling.

Interfacing with Handheld Devices

An Atmel Corp low-cost touch sensor IC -AT42QT1040 is available in a 3mm x 3mm x0.85mm 20 pin package, making it suitable foruse in mobile phones and other handhelddevices where PCB space is at a premium.

The AT42QT1040 draws only 31µA from a 1.8Vdc supply,allowing capacitive sensing to be added with minimal impact onbattery lifetime.

Similar to AT42QT1040 there are a number of cheap and smallIC’s available which can be used with standard Touch Sensors tothe micro-controller to provide a reliable interface.

Trade off…

Though the Touch sensors help in optimizing the premium spaceof handheld devices, the device drains more power to providebetter clarity.

Touch Sensors are durable in terms of life span but require morecare while handling.

Even though the Touch sensors are attractive and easy to use butone has to pay higher price to avail this luxury.

Conclusion…

The Touch Sensor Technology is beingdeveloped for future purposes of massuse at cheaper prices. An example is a e-book reader and notes keeper which candisplay pages and allows the users tomake notes, highlight line using stylus.

This is useful for kids for learning asthe interface is lively and allows instantaccess to different books. And also thiscan save them from heavy school Bags!!

References

http://www.embedded.com/216400720?cid=NL_embedded

www.analog.com/library/analogDialogue/archives

http://www.touchscreens.com/introduction.html

http://www.searchcio-midmarket.techtarget.com

http://www.wikipedia.org/wiki/Touchscreen

http://www.electronics.howstuffworks.com/iphone1.htm

http://www.magictouch.com/assistivetech.html

http://www.focus.ti.com/lit/an/slyt209a/slyt209a.pdf

http://www.soton.ac.uk/~rmc1/robotics/artactile.htm

touch sensor technology for hand-held and mobile...

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