Page 120 - Introduction to Autonomous Mobile Robots

Perception

• interaction with the target (e.g., surface absorption, specular reflections); 105
• variation of propagation speed;
• the speed of the mobile robot and target (in the case of a dynamic target);

As discussed below, each type of time-of-flight sensor is sensitive to a particular subset
of the above list of factors.

The ultrasonic sensor (time-of-flight, sound). The basic principle of an ultrasonic
sensor is to transmit a packet of (ultrasonic) pressure waves and to measure the time it takes
d
for this wave packet to reflect and return to the receiver. The distance of the object caus-
c
ing the reflection can be calculated based on the propagation speed of sound and the time
of flight . t
⋅
ct
d = -------- (4.7)
2
The speed of sound c in air is given by

c = γRT (4.8)

where
γ = ratio of specific heats;
R = gas constant;
T = temperature in degrees Kelvin.

c
In air at standard pressure and 20° C the speed of sound is approximately = 343 m/s.
Figure 4.6 shows the different signal output and input of an ultrasonic sensor. First, a
series of sound pulses are emitted, comprising the wave packet. An integrator also begins
to linearly climb in value, measuring the time from the transmission of these sound waves
to detection of an echo. A threshold value is set for triggering an incoming sound wave as
a valid echo. This threshold is often decreasing in time, because the amplitude of the
expected echo decreases over time based on dispersal as it travels longer. But during trans-
mission of the initial sound pulses and just afterward, the threshold is set very high to sup-
press triggering the echo detector with the outgoing sound pulses. A transducer will
continue to ring for up to several milliseconds after the initial transmission, and this gov-
erns the blanking time of the sensor. Note that if, during the blanking time, the transmitted
sound were to reflect off of an extremely close object and return to the ultrasonic sensor, it
may fail to be detected.