-Monta
Figure 6.4: The ISR Genghis series of legged robots localize x-y
position with a master/slave trilateration scheme using two 40 kHz
ultrasonic “pingers.” (Adapted from [ISR, 1994].)
6.2.1 IS Robotics 2-D Location System
IS Robotics, Inc. [ISR], Somerville, MA, a spin-off company from MIT's
renowned Mobile Robotics
Lab, has introduced a beacon system based on an inexpensive ultrasonic
trilateration system. This
system allows their Genghis series robots to localize position to within
12.7 millimeters (0.5 in) over
a 9.1×9.1 meter (30×30 ft) operating area [ISR, 1994]. The ISR system
consists of a base station
master hard-wired to two slave ultrasonic “pingers” positioned a known
distance apart (typically
2.28 m — 90 in) along the edge of the operating area as shown in Figure
6.4. Each robot is equipped
with a receiving ultrasonic transducer situated beneath a cone-shaped
reflector for omnidirectional
coverage. Communication between the base station and individual robots
is accomplished using a
Proxim spread-spectrum (902 to 928 MHz) RF link.
The base station alternately fires
the two 40-kHz ultrasonic pingers
every half second, each time trans-mitting
a two-byte radio packet in
broadcast mode to advise all robots
of pulse emission. Elapsed time
between radio packet reception and
detection of the ultrasonic wave
front is used to calculate distance
between the robot’s current posi-tion
and the known location of the
active beacon. Inter-robot commu-nication
is accomplished over the
same spread-spectrum channel
using a time-division-multiple-ac-cess
scheme controlled by the base
station. Principle sources of error
include variations in the speed of
sound, the finite size of the ultrasonic transducers, non-repetitive
propagation delays in the
electronics, and ambiguities associated with time-of-arrival detection.
The cost for this system is
$10,000.