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6 Common Sensing Techniques for Reactive Robots
Proprioception is often only an estimate. This is due to the impact of the
environment on the actual movement of the robot. Arkin in his PhD thesis 8
showed that the same wheeled robot, HARV, traveled different distances for
the same encoder count on a sidewalk, grass, and wet grass. The texture of
the different surfaces caused the wheels of HARV mobile to slip to varying
degrees. A robot on a tiled floor might slip twice as much as on dry grass.

6.4.1 Inertial navigation system (INS)

INERTIAL NAVIGATION Aircraft, submarines, and missiles use inertial navigation systems (INS).INS
SYSTEMS (INS) measure movements electronically through miniature accelerometers. As
long as the movements are smooth, with no sudden jarring, and the sam-
ples are taken frequently, an INS can provide accurate dead reckoning to 0.1
percent of the distance traveled. 52 However, this technology is unsuitable for
mobile robots for several reasons. The cost of an INS is prohibitive; units run
from $50,000 to $200,000USD. The cost is due in part to having to stabilize the
accelerometers with gyroscopes, as well as the nature of precision electron-
ics. Mobile robots often violate the constraint that motion must be smooth.
A hard bump or sudden turn can exceed the accelerometers’ measurement
range, introducing errors. INS sytems are typically big; smaller devices have
less accuracy. Sojourner, the Mars rover, carried an INS system. In one trek,
it would have stopped 30 cm from a rock it was to sample if it had just used
proprioception. Instead, by using exteroception, it got within 4 cm.

6.4.2 GPS

GPS, or Global Positioning System, is becoming more common on robots,
PRECISION especially those used to automate farm equipment (an effort called precision
AGRICULTURE agriculture). GPS systems work by receiving signals from satellites orbiting
the Earth. The receiver triangulates itself relative to four GPS satellites, com-
puting its position in terms of latitude, longitude, altitude, and change in
time. GPS isn’t a proprioceptive sensor per se since the robot must receive
signals from the satellites, external to the robot. However, they are not exte-
roceptive sensors either, since the robot isn’t computing its position relative
to its environment. Since they tend to be used in place of dead reckoning on
outdoor robots, only GPS will be covered here.
Currently the only sets of GPS satellites that a receiver can triangulate it-
self against are the Navstar “constellation” maintained by the United States
Air Force Space Command or the Russian counterpart, GLONOSS, main-