Logical sensors

Logical sensors

LOGICAL SENSORS A powerful abstraction of sensors is logical sensors, first introduced by Henderson
and Shilcrat.65 A logical sensor is a unit of sensing or module that
supplies a particular percept. It consists of the signal processing from the
physical sensor and the software processing needed to extract the percept; it
is the functional building block for perception. A logical sensor can be easily
implemented as a perceptual schema.
An overlooked aspect of a logical sensor is that it contains all available
alternative methods of obtaining that percept. For example, a percept commonly
used for obstacle avoidance is a polar plot of range data. The logical
sensor for the percept might be named range_360 and return a data structure
or object specifying the polar plot. The logical sensor would go further
and list all the possible ways the robot had for constructing a polar plot of
that form. The robot might be able to use sonar, a laser, stereo vision, or texLOGICAL
EQUIVALENCE ture. Each of those modules would be logically equivalent; that is, they would
return the same percept data structure so they can be used interchangeably.

However, they wouldn’t necessarily be equivalent in performance or update
rate. As will be seen in this chapter, the sonar is liable to produce a noisy
percept in a second or two, while stereo vision may take minutes. Even different
stereo vision algorithms may produce different results on the same
data stream. Therefore, the logical sensor contains a selector function which
specifies the conditions under which each alternative is useful and should be
selected.
Notice that a logical sensor can be implemented as a perceptual schema,
where the methods are the alternative means of generating the percept and
the coordinated control strategy contains the knowledge as to when a particular
method is appropriate. Also note that each individual method can
be implemented as a perceptual schema, leading to the recursive, buildingblock
effect.
In reactive systems, the term logical sensor has degenerated somewhat
from its original usage and is essentially equivalent to a perceptual schema.
“Logical sensor” is often used to connote information hiding, where the particular
sensor and processing algorithm is hidden in the “package.” This is
useful because a robot might use the same physical sensor in two different
ways. An avoid behavior might use a polar plot of sonar range data, while a
panic-stop behavior might use the minimum of all the incoming sonar data.
Since the perceptual schema use the raw sonar data differently, it is as if they
were different sensors.