Making Things Talk

HOW TO LOCATE (ALMOST) ANYTHING 265

Determining Distance

Electronic locating systems like GPS, mobile phone location, and sonar seem magical at

first, because there’s no visible evidence as to how they work. When you break the job

down into its components, it becomes relatively straightforward. Most physical location

systems are based on the same principle. They determine distance from several known

and fixed locations by measuring the energy of an electromagnetic or acoustic wave

coming from the object to be located. Then they combine those measurements to

determine a position in two or three dimensions.

For example, a GPS receiver determines its position on

the surface of the planet by measuring the strength of

received radio signals from several geosynchronous satel-

lites. Similarly, mobile phone location systems measure

the signal strength of the phone at several cell towers.

Sonar and infrared ranging sensors work by sending out

an acoustic signal (sonar) or an infrared signal (IR rangers)

and measuring the strength of that signal when it’s

reflected off the target.

Distance ranging techniques can be classified as active

or passive. In active systems, the target has a radio, light,

or acoustic source on it, and the receiver just listens

for the signal from the target. In passive systems, the

target doesn’t have to have any technology on board. The

receiver emits a signal, and the signal bounces off the

target. Mobile phone location is active, because it relies on

the phone sending out a radio signal. Sonar and infrared

ranging are passive, because the sensor has to emit a

signal in order to measure the reflection. GPS is an active

locating technology, because although the receiver doesn’t

emit a signal, it has an electronic receiver onboard to

receive satellites’ signals.

Sometimes distance ranging is used for acquiring a

position, and other times it’s used for refining it. In the

following examples, the passive distance rangers deliver a

measurement of physical distance, but the radio ranging

tell you only when another radio is in transmission range of

your radio, and whether it’s near or far within the range.

Passive Distance Ranging

Ultrasonic rangers like the Devantech SRF02 and infrared

rangers like the Sharp GP2D12, shown in Figure 8-1, are

examples of distance rangers. The Devantech sensor

sends an ultrasonic signal out and listens for an echo;

it’s basically a sonar device. The Sharp sensor sends

out an infrared light beam, and senses the reflection of

that beam. These sensors only work in a short range.

The Sharp sensor can read about 10 cm to 80 cm, and

the Devantech sensor reads from about 15 cm to 6.4 m;

these are useful only for very local measurements. Passive

sensors like these are handy, though, when you want to

measure the distance of a person in a limited space, and

you don’t want to have to put any hardware on the person.

They’re also handy when you’re building moving objects

that need to know their proximity to other objects in the

same space as they move.

Figure 8-1

Devantech SRF02 and Sharp GP2D12 sensors. The Devantech

sensor can read a range from 15 cm to 6 m. The Sharp sensor can

read a range from 10 cm to 80cm.

Making_Things_Talk.indb 265 11/13/09 4:55 PM