Page 121 - Introduction to Autonomous Mobile Robots

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wave packet Chapter 4
transmitted sound

threshold
analog echo signal
threshold

digital echo signal
integrator time of flight (sensor output)
integrated time
output signal

Figure 4.6
Signals of an ultrasonic sensor.

However, once the blanking interval has passed, the system will detect any above-
threshold reflected sound, triggering a digital signal and producing the distance measure-
ment using the integrator value.
The ultrasonic wave typically has a frequency between 40 and 180 kHz and is usually
generated by a piezo or electrostatic transducer. Often the same unit is used to measure the
reflected signal, although the required blanking interval can be reduced through the use of
separate output and input devices. Frequency can be used to select a useful range when
choosing the appropriate ultrasonic sensor for a mobile robot. Lower frequencies corre-
spond to a longer range, but with the disadvantage of longer post-transmission ringing and,
therefore, the need for longer blanking intervals. Most ultrasonic sensors used by mobile
robots have an effective range of roughly 12 cm to 5 m. The published accuracy of com-
mercial ultrasonic sensors varies between 98% and 99.1%. In mobile robot applications,
specific implementations generally achieve a resolution of approximately 2 cm.
In most cases one may want a narrow opening angle for the sound beam in order to also
obtain precise directional information about objects that are encountered. This is a major
limitation since sound propagates in a cone-like manner (figure 4.7) with opening angles
around 20 to 40 degrees. Consequently, when using ultrasonic ranging one does not acquire
depth data points but, rather, entire regions of constant depth. This means that the sensor
tells us only that there is an object at a certain distance within the area of the measurement
cone. The sensor readings must be plotted as segments of an arc (sphere for 3D) and not as
point measurements (figure 4.8). However, recent research developments show significant
improvement of the measurement quality in using sophisticated echo processing [76].
Ultrasonic sensors suffer from several additional drawbacks, namely in the areas of
error, bandwidth, and cross-sensitivity. The published accuracy values for ultrasonics are