Capacitive Sensors Final

8/3/2019 Capacitive Sensors Final

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CAPACITIVE TOUCH SENSORS

A substitute for mechanical sensors


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INTRODUCTION

Capacitive sensing, a rising technology .

Phenomenal growth of touch mechanism.

Apples i-phone: great popularity among themasses.

Touch screen, touch pad, touch lamp, capacitivehumidity sensor, capacitive pressure sensor etc.

are some of its applications.


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SOME CONCEPTS

(1)Capacitance

(2)Relationship b/w capacitance anddistance

(3)Sensor


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CAPACITANCE

Electric charge stored fora given electric potential.A group of plates whichcan store electric charge

is a capacitor.

C= q/V,

Where, q: Charge incoulombs

V: Potential in volts

C: Capacitance in farads.


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HOW CAPACITANCE VARIESWITH DISTANCE

For a parallel plate capacitor,

C= K.E.A / d

K: relative permittivity, E: absolutepermittivity, A: area of the plates, d:distance between the plates.

C varies as 1/ Gap.


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SENSOR

It measures a physical quantity and converts itinto a signal which can be read by an observeror by an instrument.

Rate of change of sensors output with themeasured quantity: sensitivity.

MEMS: A sensor on microscopic scale.

Other examples of sensors: Touch-sensitiveelevator buttons and lamps which dim orbrighten by touching the base


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SENSING CAPACITANCE


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OPERATING PRINCIPLE ANDCONSTRUCTION

Components: Electrode Assembly, OscillatorCircuit, Evaluation Circuit and Output Circuit.

Read device of capacitive sensor includes asignal source supplying an electrical readsignal for driving the capacitive sensor; and a

discrete-time sense circuit for generating anelectrical output signal.


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WORKING PRINCIPLE

Use of a thin, transparent, capacitive sensor touchscreen that designers can place over any viewablesurface for input and navigation.

As shown in above figure, a finger on top of a grid of

conductive traces changes the capacitance of thenearest traces.

The capacitive sensor's surface receives touchinformation from contact with the user and sends thisinformation to the controller board.


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FOCUSING THE ELECTRICFIELD

Electric field is emitted from every surface when avoltage is applied to the conductor.

The electric field from a capacitive sensor needs to becontained within the space between the probes sensingarea and the target for accurate gaging.

Any conductor beside or behind the probe form anelectric field with the guard instead of the sensing area.


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EFFECTS OF TARGET MATERIAL

Capacitive sensors measure all conductors:

brass, steel, aluminum, or even salt-water, withequal accuracy.

The electric field from the probe sensing area

seeks a conductive surface.

Nonconductors can be measured by passingthe electric field through them to a stationary

conductive target behind. Fringing can be used to measure

nonconductive targets without a conductive

background target.


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WHY CAPACITIVE SENSINGRTS TECHNOLOGY:

Current generations of cell phones are facingissues due to a crowded keypad and touchscreen combination.

Packed with sophisticated communicationssubsystems, PDAs have evolved into full-fledged computers with Internet connectivity.

Designers are migrating toward the extra UIspace resistive touch screens (RTS) offer.


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DRAWBACKS OF RTS Optics of the underlying display are severely

impaired. Poor durability.

Device's main hardware input is eliminated, if

the RTS panel breaks, rendering the deviceuseless.

Requires a top bezel to protect the edges of

the screen. Have poor electrostatic discharge (ESD)

resistance.

Prone to condensing humidity damage.


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CAPACITIVE SENSING,

A BETTER ALTERNATIVE.

A thin, transparent capacitor sensing screen isa better alternative.

(1)Mechanical simplicity.

(2)Operates even when the sensor is placedbeneath a durable surface like polycarbonateor acrylic.

(3)Consistent optical quality.(4)Can sense through curved surfaces without

functionality loss and critical spacing between

sensor layers is not required.


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RTS AND CAPACITIVE SENSORS

Fig. 1. A resistive touch panel is a mechanical sensorwith two layers of material, typically separated by air.

Fig. 2 The capacitive sensor module comprises fourregions, active and viewing areas, opaque PET inactiveborders on three sides, and tail region housing thecapacitive sensing electronics.


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APPLICATION IN MOBILE KEYPAD

A mobile phone keypad best illustrates an

efficient manner to size capacitive buttons toimprove accessibility.

Equally sized buttons are not equallyaccessible.

Corner buttons are the easiest to access forseveral reasons.

Upper buttons should be slightly larger than the

lower corner buttons. Lower corner buttons can be smaller because

they have two edges nearby .


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KEYPAD LAYOUT OF MOBILE

Figure: In this mobile phone keypad layout, withthe capacitive approach, buttons can be resizedto make each button equally accessible.


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MOBILE HANDSET DESIGN LINE The success of the capacitive sensor

scroll wheel used in devices such as the

iPod has caused other consumer productdevelopment teams to evaluatecapacitive sensors to enhance the userinterface and aesthetic look of theirproducts.

Capacitive touch sensor interfaces aren'tlimited to MP3 players, but can bedeveloped for any product currently usingtraditional mechanical switches, such asthe menu control buttons found ontoday's latest mobile handsets.

These advanced feature menu controlswitches could easily be restyled with ahighly reliable, attractive, and cost-effective capacitive touch sensors.


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OTHER APPLICATIONS


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THANK YOU

Documents

Capacitive Sensors Final