Page 137 - Designing Sociable Robots

breazeal-79017 book March 18, 2002 14:5

118 Chapter 8

where E emotion is the activation level of its affiliated elicitor process; B emotion is a DC bias
that can be used to make some emotion processes easier to activate than others. P emotion
adds a level of persistence to the active emotion. This introduces a form of inertia so that
different emotion processes don’t rapidly switch back and forth. Finally, δ t is a decay
term that restores an emotion to its bias value once the emotion becomes active. Hence,
unlike drives (which contribute to the robot’s longer-term “mood”), the emotions have
an intense expression followed by decay to a baseline intensity. The decay takes place on
the order of seconds.

Emotion Arbitration
Next, the emotion processes compete for control in a winner-take-all arbitration scheme
based on their activation level. The activation level of an emotion process is a measure of
its relevance to the current situation. Each of these processes is distinct from the others and
regulates the robot’s interaction with its environment in a distinct manner. Each becomes
active in a different environmental (or internal) situation. Each motivates a different observ-
able response by spreading activation to a specific behavior process in the behavior system.
If this amount of activation is strong enough, then the active emotion can “seize” temporary
control and force the behavior to become expressed. In a process of behavioral homeostasis
as proposed by Plutchik (1991), the emotive response maintains activity through feedback
until the correct relation of robot to environment is established.
Concurrently, the net [A, V, S] of the active process is sent to the expressive components
of the motor system, causing a distinct facial expression, vocal quality, and body posture
to be exhibited. The strength of the facial expression reflects the level of activation of
the emotion. Figure 8.4 illustrates the emotional response network for the fear process.
Affective networks for the other responses in table 8.1 are defined in a similar manner. By
modeling Kismet’s emotional responses after those of living systems, people have a natural
and intuitive understanding of Kismet’s “emotional” behavior and how to influence it.
There are two threshold levels for each emotion process: one for expression and one for
behavioral response. The expression threshold is lower than the behavior threshold. This
allows the facial expression to lead the behavioral response. This enhances the readability
and interpretation of the robot’s behavior for the human observer. For instance, if the
caregiver shakes a toy in a threatening manner near the robot’s face, Kismet will first
exhibit a fearful expression and then activate the escape response. By staging the response
in this manner, the caregiver gets immediate expressive feedback that she is “frightening”
the robot. If this was not the intent, then the caregiver has an intuitive understanding of why
the robot appears frightened and modifies behavior accordingly. The facial expression also
sets up the human’s expectation of what behavior will soon follow. As a result, the caregiver