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158 Chapter 10

Expressive versatility Kismet’s face currently supports four different functions. It reflects
the state of the robot’s emotion system, called emotive expressions. It conveys social cues
during social interactions with people, called expressive facial displays. It synchronizes with
the robot’s speech, and it participates in behavioral responses. The face system must be quite
versatile as the manner in which these four functions are manifest changes dynamically with
motivational state and environmental factors.

Readability Kismet’s face must convey information in a manner as similar to humans
as possible. If done sufficiently well, then naive subjects should be able to read Kismet’s
facial expressions and displays without requiring special training. This fosters natural and
intuitive interaction between Kismet and the people who interact with it.

Believability As with much of Kismet’s design, there is a delicate balance between com-
plexity and simplicity. Enforcing levels of abstraction in the control hierarchy with clean
interfaces is important for promoting scalability and real-time response. The design of
Kismet’s face also strives to maintain a balance. It is quite obviously a caricature of a hu-
man face (minus the ears!) and therefore cannot do many of the things that human faces
do. However, by taking this approach, people’s expectations for realism must be lowered
to a level that is achievable without detracting from the quality of interaction. As argued in
chapter 5, a realistic face would set very high expectations for human-level behavior. Try-
ing to achieve this level of realism is a tremendous engineering challenge currently being
attempted by others (Hara, 1998). It is not necessary for the purposes here, however, which
focus on natural social interaction.

10.2 Levels of Face Control

The face motor system consists of six subsystems organized into four layers of control. As
presented in chapter 9, the face motor system communicates with the motor skill system
to coordinate over different motor modalities (voice, body, and eyes). An overview of the
face control hierarchy is shown in figure 10.1. Each layer represents a level of abstraction
with its own interfaces for communicating with the other levels. The highest layers control
ensembles of facial features and are organized by facial function (emotive expression, lip
synchronization,facialdisplay).Thelowestlayercontrolstheindividualdegreesoffreedom.
Enforcing these levels of abstraction keeps the system modular, scalable, and responsive.
The Motor Demon Layer
The lowest level is called the motor demon layer. It is organized by individual actuators and
implements the interface to access the underlying hardware. It initializes the maximum,
minimum, and reference positions of each actuator and places safety caps on them. A