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the actions that the agent can perform in order to achieve the goals. Automated plan-
ning can be described as a search for a solution on a problem space where, the states
are represented using a set of predicates, functions and types, and the actions are de-
scribed with a set of preconditions and effects that model the state transitions. An ac-
tion is applicable only if all its preconditions hold in the current state and executing
the action changes the current state by adding and deleting the action effects. A prob-
lem is specified as an initial state (true literals in the starting state) and a set of goals.
Also, an optimization metric (as in our case valence, arousal and/or total time) can be
defined. Our domain has been designed based on the previous concepts of drive, emo-
tion, preference, activation and personality traits to represent each agent of the virtual
world. Now, we will define the different concepts composing the model, in automated
planning terms.
3.1
Drives
As already said, we use five drives: hunger, thirst, tiredness, dirtiness and boredom.
Drives are represented in the domain through functions. The ideal value for all drives is
established at zero. So, when a drive has a value of zero, its need is totally satisfied. Any
other value means the intensity of the need and the distance to the ideal value. The value
of each drive is increased as time goes by to represent the need rise. To reduce it, the
agent has to carry out some action. For instance, the agent must
eat
to reduce the drive
hunger
. Given that the drives increase with time, every time an action is executed, one
or more drives will be decreased, but the rest will be increased. Thus, the planning task
becomes hard if we want all drives to be fulfilled (below a given threshold).
3.2
Objects
Objects describe the different elements of the virtual world. Objects may be of two
kinds: resources (or physical objects) and rooms. Resources represent objects needed
to carry out the actions to cater for needs; for instance, food, balls, books, etc. Rooms
describe physical spaces, where the agents may move and where resources are placed.
Both kinds of objects are represented as planning types and several instances of them
will be present in each problem. Also resources may be of two kinds: fungible resources
and non-fungible resources.
3.3
Personality Traits
Personality traits describe the agents personality and are based on the Big Five fac-
tors model (openness to experience, conscientiousness, extraversion, agreeableness and
neuroticism). Openness to experience involves active imagination, aesthetic sensitivity,
preference for variety and intellectual curiosity. Openness is modeled as a higher prefer-
ence for new experiences, i.e., an agent with high openness (open-minded) tends to use
and prefer new objects to known objects, while an agent with low openness will tend
to prefer known objects to new objects. Neuroticism represents the degree of emotional
stability of the agent. The bigger the neuroticism is, the smaller the emotional stability
