Global Positioning System Reference
In-Depth Information
frequency of ultrasound also has the advantage of making ranging measurements
more accurate; since the wavelengths are much shorter object resolution is easier.
Typical consumer products using ultrasound generally operate at 40kHz. In
common with IR systems, the consumer components are very cheap and require
no licenses. Ranges of tens of meters are similar to IR, and systems are very
suitable for indoor use. There are professional applications of ultrasonic ranging,
for example, detecting the position of flaws in solid components; imaging
ultrasound (operating at much higher frequencies) is now common in medical
imaging.
Nature has evolved ultrasonic ranging systems that are used in air by flying
bats and in water by dolphins. Bats use their ears and heads to measure angles and
use ultrasonics for navigation and finding prey (some of which use
countermeasures).
The disadvantages of ultrasonics for positioning are the same as for IR,
especially the need for a clear line of sight. A major benefit over IR is, however,
the extreme ease with which accurate distance measurements can be made in air.
This is possible because the slow pulse flight time allows them to be timed by
digital processors that can sample the received pulses at a relatively modest rate
well within the capabilities of the cheapest microcontrollers, as the following
example illustrates:
Range of object being detected
= 5m
Pulse round-trip flight time
= 30ms
Sample speed of cheap microcontroller
= 10 million samples/s
Pulse resolution (time)
= 0.1μs
Pulse resolution (distance)
= 3mm
Processor samples for pulse round trip
= 3333
It can therefore be appreciated why similar indoor UWB radio systems are
expensive and still exotic given the timing circuits and systems are 6 orders of
magnitude more exacting!
Ultrasonic signals are easy to produce since all that is required is the
production of a 40kHz carrier that can then be connected to a transducer, which is
in essence a small loudspeaker consisting of a tuned element made of a ceramic
piezo resonator. Receivers are very similar to transmitters and produce an
electrical output signal that requires only modest amplification. Since they are
tuned resonators their bandwidths are relatively narrow. More sophisticated
professional transducers are also available.
7.3.1
Ultrasonic Distance Measurements
A number of consumer products use ultrasonic ranging. Early Polaroid instant
cameras used an ultrasonic rangefinder to adjust the camera focus. Today, there
are many cheap ultrasonic tape measures available. These send out pulses of
