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6.5 Proximity Sensors
ambient lighting or is absorbed by dark materials (i.e., the environment has
toomuchnoise).
In more sophisticated IR sensors, different IR bands can be selected or
modulated to change the signal-to-noise ratio. This typically ensures that
an object in range doesn’t absorb the light, causing the sensor to miss the
presence of the object. Nomad robots have an IR sensor option.

6.5.3 Bump and feeler sensors

TACTILE Another popular class of robotic sensing is tactile, or touch, done with bump
and feeler sensors. Feelers or whiskers can be constructed from sturdy wires.
Bump sensors are usually a protruding ring around the robot consisting of
two layers. Contact with an object causes the two layers to touch, creating
an electrical signal. In theory, the sensitivity of a bump sensor can be ad-
justed for different contact pressures; some robots may want a “light” touch
to create a signal rather than a “heavier” touch. In practice, bump sensors are
annoyingly difficult to adjust. In the “Hors d’Oeuvres, Anyone?” event of
the 1997 AAAI Mobile Robot Competition, humans were served finger food
by robot “waiters.” The robot waiters were various research robots with serv-
ing trays attached. Humans were supposed to communicate to the Colorado
School of Mines’ Nomad 200 robot waiter (seen in Fig. 6.10) that they were
done eating by kicking the bump sensor. The sensitivity of the bump sensor
was so low that it often required many kicks, producing a very comical scene
with Bruce Lee overtones.
Placement of bump sensors is a very important issue. The bump sensors
on a Nomad 200 base clearly protect the robot only from low obstacles not
perceivable by sonar. The Denning mobile robot platforms built in the 1980’s
used a bump sensor that looked much like a thick piece of gray tape. Den-
ning mobile robots look like a fat version of the Nomad 200’s, and the bump
sensor is wrapped around the cowling of the robot at the waist level. Un-
fortunately, in certain turning configurations, the wheels extend beyond the
cowling. In those situations, the bump sensor is totally useless in preventing
the expensive synchro-drive mechanism from being damaged in a collison.
Feeler sensors are whiskers or antennae, only not as sensitive as those on
animals. Contact of a whisker and an object will trip a binary switch, whereas
there is reason to believe that an insect or animal can extract much more
information. Bump and feeler sensors are actually tactile sensors since they
require the robot to be touching something in order to generate a reading.