Page 167 - Designing Sociable Robots

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148 Chapter 9

The Motor Skills System

Given the current task (as dictated by the behavior system), the motor skills system is
responsible for figuring out how to carry out the stated goal. Often this requires coordinating
multiple motor modalities (speech, body posture, facial display, and gaze control). Requests
for these modalities can originate from the top down (i.e., from the emotion system or
behavior system) as well as from the bottom-up (e.g., the vocal system requesting lip and
jaw movements for lip synchronizing). Hence, the motor skills level must address the issue
of servicing the motor requests of different systems across the different motor resources.
The motor skills system also must appropriately blend the motor actions of concurrently
active behaviors. Sometimes concurrent behaviors require completely different sets of actu-
ators (such as babbling while watching a stimulus). In this case there is no direct competition
over a shared resource, so the motor skills system should command the actuators to execute
both behaviors simultaneously. Other times, two concurrently active behaviors may com-
pete for the same actuators. For instance, the robot may have to smoothly track a moving
object while maintaining vergence. These two behaviors are complementary in that each can
be carried out without the sacrifice or degradation in the performance of the other. However,
the motor skills system must coordinate the motor commands to do so appropriately.
The motor skills system is also responsible for smoothly transitioning between sequen-
tially active behaviors. For instance, to initiate a social exchange, the robot must first
mutually orient to the caregiver and then exchange a greeting with her. Once started, Kismet
may take turns with the caregiver in exchanging vocalizations, facial expressions, etc. After
a while, either party can disengage from the other (such as by looking away), thereby termi-
nating the interaction. While sequencing between these behaviors, the motor system must
figure out how to transition smoothly between them in a timely manner so as not to disrupt
the natural flow of the interaction.
Finally, the motor skills system is responsible for moving the robot’s actuators to convey
the appropriate emotional state of the robot. This may involve performing facial expressions,
or adapting the robot’s posture. Of course, this affective state must be conveyed while
carrying out the active task(s). This is a special case of blending mentioned above, which
may or may not compete for the same actuators. For instance, looking at an unpleasant
stimulus may be performed by directing the eyes to the stimulus, but orienting the face
away from the stimulus and configuring the face into a “disgusted” look.

Motor Skill Mechanisms
It often requires a sequence of coordinated motor movements to satisfy a goal. Each motor
movement is a primitive (or a combination of primitives) from one of the base motor systems
(the vocal system, the oculo-motor system, etc.). Each of these coordinated series of motor