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measure distances with a resolution of about a foot (Koenigsburg, 1982). More specifically, a desired
resolution of 1 mm requires a timing accuracy of 3 ps (Vuylsteke et al., 1990). This capability is somewhat
expensive to realize and may not be cost effective for certain applications, particularly at close range
where high accuracies are required.
Surface Interaction—When light, sound, or radio waves strike an object, any detected echo represents
only a small portion of the original signal. The remaining energy reflects in scattered directions and can
be absorbed by or pass through the target, depending on surface characteristics and the angle of incidence
of the beam. Instances where no return signal is received at all can occur because of specular reflection
at the object surface, especially in the ultrasonic region of the energy spectrum. If the transmission source
approach angle meets or exceeds a certain critical value, the reflected energy will be deflected outside the
sensing envelope of the receiver. Scattered signals can reflect from secondary objects as well, returning
to the detector at various times to generate false signals that can yield questionable or otherwise noisy
data. To compensate, repetitive measurements are usually averaged to bring the signal-to-noise ratio
within acceptable levels, but at the expense of additional time required to determine a single range value.

Ultrasonic TOF Systems
Ultrasonic TOF ranging is today the most common noncontact technique employed, primarily due to
the ready availability of low-cost systems and their ease of interface. Over the past few decades, much
research has been conducted in investigating applications in mobile robotics for world modeling and
collision avoidance, position estimation, and motion detection. Several researchers have assessed the
effectiveness of ultrasonic sensors in exterior settings (Pletta et al., 1992; Langer & Thorpe, 1992; Pin &
Watanabe, 1993; Hammond, 1994). In the automotive industry, BMW now incorporates four piezocer-
amic transducers (sealed in a membrane for environmental protection) on both front and rear bumpers
in its Park Distance Control system (Siuru, 1994).
The Polaroid ranging module is an active TOF device developed for automatic camera focusing and
determines the range to target by measuring elapsed time between transmission of an ultrasonic waveform
and the detected echo (Biber et al., 1980). Probably the single most significant sensor development is
from the standpoint of its catalytic influence on the robotics research community and industrial appli-
cations; this system is the most widely found in the literature (Koenigsburg, 1982; Moravec & Elfes, 1985;
Everett, 1985; Kim, 1986; Arkin, 1989; Borenstein & Koren, 1990). Representative of the general charac-
teristics of a number of such ranging devices, the Polaroid unit soared in popularity as a direct conse-
quence of its extremely low cost (Polaroid offers both the transducer and ranging module circuit board
for less than $50), made possible by high-volume usage in its original application as a camera auto-focus
sensor.
The most basic configuration consists of two fundamental components: (1) the ultrasonic transducer,
and (2) the ranging module electronics. A choice of transducer types is now available. In the original
instrument-grade electrostatic version (Fig. 19.64), a very thin metalized diaphragm mounted on a
machined backplate forms a capacitive transducer (Polaroid, 1981). A smaller diameter electrostatic trans-
ducer (7000-Series) has also been made available, developed for the Polaroid Spectra camera (Polaroid,
1987). A ruggedized piezoelectric (9000-Series) environmental transducer introduced for applications that
may be exposed to rain, heat, cold, salt spray, and vibration is able to meet or exceed guidelines set forth
in the SAE J1455 January 1988 specification for heavy-duty trucks. The range of the Polaroid system runs
from about 0.3 m (1 ft) out to 10.5 m (35 ft), with a half-power (−3 dB) beam dispersion angle of
approximately 12° for the original instrument-grade electrostatic transducer. A typical operating cycle is
as follows.
• The control circuitry fires the transducer and waits for an indication that transmission has begun.
• The receiver is blanked for a short period of time to prevent false detection due to residual transmit
signal ringing in the transducer.
• The received signals are amplified with increased gain over time to compensate for the decrease
in sound intensity with distance.

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