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6 Common Sensing Techniques for Reactive Robots
cess: one that requires a global world model. Early work in reactive systems
used robots with only a few simple sensors, a sonar or sonar ring for range
and a camera for color, texture, or motion affordances. As a result, there
was a philosophy of one sensor per behavior. Behaviors could share a sen-
sor stream, but without knowing it. This philosophy led to the approach
taken by Brooks that sensor fusion at the behavioral level was a mirage. In-
stead sensor fusion was really multiple instances of the same behavior with
different sensor inputs being combined together. To an outside observer, it
would look like some complicated process was being enacted inside the ro-
bot, but in fact it would be a simple competition with an emergent behavior.
SENSOR FISSION Brooks dubbed this sensor fission in part as a take off on the connotations of
the word “fusion” in nuclear physics. In nuclear fusion, energy is created by
forcing atoms and particles together, while in fission, energy is creating by
separating atoms and particles. Fig. 6.3a shows a diagram of sensor fission.
Murphy reported on studies from cognitive psychology and neurophys-
iology showing that behavioral sensor fusion does occur in animals, and
therefore should be part of a robot’s behavioral repertoire. 99 The gist of the
studies was that sensor fusion does occur in behaviors. The sensor pathways
throughout the brain remain separate and can be routed to multiple behav-
iors in the superior colliculus. Only when the sensor signals routed to the
portion of the brain associated with a particular behavior arrive at that loca-
tion does there appear to be any transformation. The transformation appears
to be a new common representation. Any or all of these sensor streams can
be active and influence the resulting behavior. For example, consider the
predation behavior in cats. If a cat hears a noise and sees a movement, it will
react more strongly than if it has only a single stimulus. This type of sensor
ACTION-ORIENTED fusion is called action-oriented sensor fusion to emphasize that the sensor data
SENSOR FUSION is being transformed into a behavior-specific representation in order to sup-
port a particular action, not for constructing a world model. Fig. 6.3b shows
a diagram of action-oriented sensor fusion.
Sensor fission and action-oriented sensor fusion cover competing and com-
plementary sensing. Sensor fission is by definition a competitive method,
though complementary sensors may be used to support a particular instance
of a behavior. Action-oriented sensor fusion is not restricted to either com-
peting or complementary sensors, since the behavior makes a local transfor-
mation anyway. This leaves the category of coordinated sensors untouched.
SENSOR FASHION Arkin filled in the apparent gap by calling coordination sensor fashion, an allit-
erative name intended to imply the robot was changing sensors with chang-
ing circumstances just as people change styles of clothes with the seasons. A