Page 232 - Introduction to AI Robotics

6.5 Proximity Sensors

a. b. 215

Figure 6.8 Maps produced by a mobile robot using sonars in: a.) a lab and b.) a
hallway. (The black line is the path of the robot.)

need to consider the operating environment for the sensor and its impact on
the signal.
The impact of the problems of specular reflection and cross talk become
easier to see with plots of sonar returns overlaid with the perimeter of the
area they were taken in; see Fig. 6.8. Some walls are invisible, others are too
close. Clearly sonar readings have to be taken with a grain of salt.
The 30 cone also creates resolution problems. While sonars often have
excellent resolution in depth, they can only achieve that at large distances if
the object is big enough to send back a significant portion of the sound wave.
The further away from the robot, the larger the object has to be. Most desk
chairs and table tops present almost no surface area to the sensor and so the
robot will often not perceive their presence and run right into them.
In practice, another problem leads to spurious sonar readings: power. The
generation of a sound wave requires a significant pulse of energy. If the robot
is operating at low power levels, the correct waveform will not be gener-
ated and the return signal worthless. This problem is often difficult to debug