how point work

7/27/2019 How Point Work

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PART 5 INPUT /OUTPUT DEV ICES

How PointingDevices Work

C H A P T E R

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THERE is nothing natural or intuitive about a keyboard. No child is born knowing how to

type, and even when the skill is learned, theres little sense to itno one can give a sensible ex-planation of why the alphanumeric keys are arranged the way they are.

For many, the keyboard is actually a barrier to learning how to use a computer. Even for the

experienced typist, theres nothing instinctive in pressing F5 to print a file. Engineersnot one

of them touch typists, Ill betat Xerox Corporations Palo Alto Research Center (PARC) de-

veloped a concept first explored by Douglas C. Engelbert of the Stanford Research Center. The

concept was apointing device, something a computer user could move by hand, causing a corre-

sponding movement onscreen. Because of its size and tail-like cable, the device was named for

the mouse. Apple Computer made the mouse a standard feature of its Macintosh computers, and

Windows has made a mouse standard equipment on PCs, as well.The mouse is not the only pointing device thats been invented. The joystick used with

games essentially accomplishes the same task, but doesnt feel quite right in all situations.

Digitizing tablets are popular with artists and engineers who must translate precise movements of a

pen into lines on the screen. Touch screens, using your finger or a special light pen to control

the software, are too tiring to use for any length of time. The most successful of pointing innova-

tions have been eraserhead pointing devices, so called because they look like a pencils eraser

stuck between the G and H key; the touch pad, which is a digitizing table without the precision;

and trackballs. All three are popular on laptops, used where theres no space for a conventional

mouse.The mouse and its cousins can never replace the keyboard, but they can supplement the keys

by doing tasks such as moving and pointing to onscreen objects, tasks for which the cursor keys

are ill-suited. Were just reaching the point where we control our PCs simply by speaking to

them.

Mice are still an integral part of our systems. The mechanical mouse is the most popular

pointing device for graphic interfaces represented by Windows and the Macintosh OS. You con-

trol your PC by pointing to images instead of typing commands. Heres how the mouse translates

the movements of your hand into the actions onscreen.

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The Mechanical Mouse

PART 5 INPUT /OUTPUT DEV ICES234

On the rims of each encoder are tiny metal contact points. Two pairs of contactbars extend from the housing of the mouse and touch the contact points oneach of the encoders as they pass by. Each time a contact bar touches a point,an electrical signal results. The number of signals indicates how manypoints the contact bars have touchedthe more signals, the fartheryou have moved the mouse. The more frequent the signals, thefaster youre moving the mouse. The direction in which therollers are turning, combined with the ratio betweenthe number of signals from the vertical and horizontalrollers, indicates the direction that the mouse ismoving.

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As the ball rotates, it touches and turnstwo rollers mounted at a 90-degree angleto each other. One roller responds to back-

and-forth movements of the mouse,which correspond to vertical movementsonscreen. The other roller senses side-ways movements, which correspond toside-to-side movements onscreen.

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Each roller is attached to a wheel, knownas an encoder, much as a cars drive trainis attached by its axles to the wheels. Asthe rollers turn, they rotate the encoders.

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Want to know how a trackball works? Turn these pages

upside-down and youll get some idea. A trackball is sim-

ply a mouse mounted so that the ball is rotated with your

fingers instead of on the surface of your desk.

A Mouse on Its Back

As you move a mechanical mouse by draggingit across a flat surface, a ballmade of rubberor rubber over steelprotruding from the un-derside of the mouse turns in the direction ofthe movement.

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Signals are sent to the PC over themouses tail-like cable. Windows con-verts the number, combination, and fre-quency of signals from the two encodersinto the distance, direction, and speednecessary to move the onscreen cursor.

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Tapping either of buttons atop themouse also sendssignal to the PC,which passes the

signal to the soft-ware. Based on hmany times youclick, and the postion of the onscrepointer at the timthe click, the soft-ware performs thtask you want toaccomplish.

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How a Touchpad Works

PART 5 INPUT /OUTPUT DEV ICES236

The capacitances are most affected at the center of the finger. By reading thecapacitances of adjoining intersections, the touchpad can identify the fingerscenter, and feeds that location to Windows to position the onscreen arrow. Thecapacitances are measured about 100 times a second. Changes in those mea-surements caused by moving the finger are translated into cursor movement.

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The crossing electrodes do nottouch, but a positive electrical chargebuilds up in one set and a negativecharge in the other. This creates anelectric fieldbetween the layers.Integrated circuits for the horizontaland vertical electrodes sample the

strength of that fields electrical po-tential, or mutual capacitance. Thesize and shape of the electrodes andthe non-conductive, dialecticmaterialseparating them influence theamount of capacitance.

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Capacitance is also affected bythe surrounding electromagnefield from other objects, includa finger, which has very differdialectic properties from air. Eif the finger doesnt actually

touch the pad, the fingertips fipenetrates the grid of electrodchanging the capacitances whelectrodes cross over and undone another nearest the finge

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ElectrodesIntegrated circuit Center of field

Beneath the top rub-ber layer of a touch-pad are two morelayers, each of whichcontains a row of elec-

trodes, one row goinghorizontally and theother vertically.

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GH

How a Pointing Stick Works

A microcontroller monitors the amounts ofelectricity passing through all the resistorsand uses that information to translate thefinger pressure into onscreen cursormovement. There are minimums and max-imums to how much the currents can vary,which prevents runaway pointer move-ments caused by casually touching thepointing stick or by pressing it too hard.

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The portion of a pointing stick thatlooks like a pencil eraser is typicallyembedded among the G, H, and Bkeys on a keyboard.

When a finger applies lateralpressure to the eraserhead, itdoes not move. Instead, theforce is passed on to a combition of four force-sensing restorsplaced to measure forwabackward, and sideways force

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The resistors are made of two elec-trical contacts separated by a filmthat resists the flow of electricity.Pressure from the finger is passedto one of the contacts, squeezing itagainst the film and creating a bet-ter connection between the contactsso that more electricity flows be-tween the contacts.

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Forcesensing

resistors

Direction offinger pressure

ContactsResistive

film

Directions of force

Microcontroller

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