Page 228 - Introduction to AI Robotics

6.5 Proximity Sensors

Figure 6.5 Polaroid ultrasonic transducer. The membrane is the disk. 211

A robotic sonar transducer is shown in Fig. 6.5. The transducer is about the
size and thickness of a dollar coin, and consists of a thin metallic membrane.
A very strong electrical pulse generates a waveform, causing the membrane
on the transducer to produce a sound. The sound is audible as a faint click-
ing noise, like a crab opening and closing its pinchers. Meanwhile a timer
ECHO is set, and the membrane becomes stationary. The reflected sound, or echo,
vibrates the membrane which is amplified and then thresholded on return
signal strength; if too little sound was received, then the sensor assumes the
sound is noise and so ignores it. If the signal is strong enough to be valid, the
timer is tripped, yielding the time of flight.
The key to how useful the data is requires understanding how the sound
wave is generated by the transducer. In reality, the sound beam produces
multiple secondary sound waves which interact over different regions of
space around the transducer before dissipating. Secondary sound waves are
SIDE LOBES called side lobes. Most robot systems assume that only sound from the main,
or centermost, lobe is responsible for a range measurement. The width of
the main lobe is often modeled as being 30 wide at about 5 meters away.
However, in practice, reactive robots need to respond to obstacles in the 0.3