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4.1.6 Active ranging Chapter 4
Active ranging sensors continue to be the most popular sensors in mobile robotics. Many
ranging sensors have a low price point, and, most importantly, all ranging sensors provide
easily interpreted outputs: direct measurements of distance from the robot to objects in its
vicinity. For obstacle detection and avoidance, most mobile robots rely heavily on active
ranging sensors. But the local freespace information provided by ranging sensors can also
be accumulated into representations beyond the robot’s current local reference frame. Thus
active ranging sensors are also commonly found as part of the localization and environmen-
tal modeling processes of mobile robots. It is only with the slow advent of successful visual
interpretation competence that we can expect the class of active ranging sensors to gradu-
ally lose their primacy as the sensor class of choice among mobile roboticists.
Below, we present two time-of-flight active ranging sensors: the ultrasonic sensor and
the laser rangefinder. Then, we present two geometric active ranging sensors: the optical
triangulation sensor and the structured light sensor.

4.1.6.1 Time-of-flight active ranging
Time-of-flight ranging makes use of the propagation speed of sound or an electromagnetic
wave. In general, the travel distance of a sound of electromagnetic wave is given by

⋅
d = c t (4.6)
where
d = distance traveled (usually round-trip);

c = speed of wave propagation;
t = time of flight.
v
It is important to point out that the propagation speed of sound is approximately
0.3 m/ms whereas the speed of electromagnetic signals is 0.3 m/ns, which is 1 million
times faster. The time of flight for a typical distance, say 3 m, is 10 ms for an ultrasonic
system but only 10 ns for a laser rangefinder. It is thus evident that measuring the time of
flight with electromagnetic signals is more technologically challenging. This explains
t
why laser range sensors have only recently become affordable and robust for use on mobile
robots.
The quality of time-of-flight range sensors depends mainly on

• uncertainties in determining the exact time of arrival of the reflected signal;
• inaccuracies in the time-of-flight measurement (particularly with laser range sensors);
• the dispersal cone of the transmitted beam (mainly with ultrasonic range sensors);