Page 123 - Introduction to Autonomous Mobile Robots

Chapter 4
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nominal values based on successful, perpendicular reflections of the sound wave off of an
acoustically reflective material. This does not capture the effective error modality seen on
a mobile robot moving through its environment. As the ultrasonic transducer’s angle to the
object being ranged varies away from perpendicular, the chances become good that the
sound waves will coherently reflect away from the sensor, just as light at a shallow angle
reflects off of a smooth surface. Therefore, the true error behavior of ultrasonic sensors is
compound, with a well-understood error distribution near the true value in the case of a suc-
cessful retroreflection, and a more poorly understood set of range values that are grossly
larger than the true value in the case of coherent reflection. Of course, the acoustic proper-
ties of the material being ranged have direct impact on the sensor’s performance. Again,
the impact is discrete, with one material possibly failing to produce a reflection that is suf-
ficiently strong to be sensed by the unit. For example, foam, fur, and cloth can, in various
circumstances, acoustically absorb the sound waves.
A final limitation of ultrasonic ranging relates to bandwidth. Particularly in moderately
open spaces, a single ultrasonic sensor has a relatively slow cycle time. For example, mea-
suring the distance to an object that is 3 m away will take such a sensor 20 ms, limiting its
operating speed to 50 Hz. But if the robot has a ring of twenty ultrasonic sensors, each
firing sequentially and measuring to minimize interference between the sensors, then the
ring’s cycle time becomes 0.4 seconds and the overall update frequency of any one sensor
is just 2.5 Hz. For a robot conducting moderate speed motion while avoiding obstacles
using ultrasonics, this update rate can have a measurable impact on the maximum speed
possible while still sensing and avoiding obstacles safely.

Laser rangefinder (time-of-flight, electromagnetic). The laser rangefinder is a time-of-
flight sensor that achieves significant improvements over the ultrasonic range sensor owing
to the use of laser light instead of sound. This type of sensor consists of a transmitter which
illuminates a target with a collimated beam (e.g., laser), and a receiver capable of detecting
the component of light which is essentially coaxial with the transmitted beam. Often
referred to as optical radar or lidar (light detection and ranging), these devices produce a
range estimate based on the time needed for the light to reach the target and return. A
mechanical mechanism with a mirror sweeps the light beam to cover the required scene in
a plane or even in three dimensions, using a rotating, nodding mirror.
One way to measure the time of flight for the light beam is to use a pulsed laser and then
measure the elapsed time directly, just as in the ultrasonic solution described earlier. Elec-
tronics capable of resolving picoseconds are required in such devices and they are therefore
very expensive. A second method is to measure the beat frequency between a frequency-
modulated continuous wave (FMCW) and its received reflection. Another, even easier
method is to measure the phase shift of the reflected light. We describe this third approach
in detail.