Figure 4 shows a sample top-level external behavior used by a team agent. The agent's top priority is to locate the ball. If the ball's location is known, it moves towards the ball or goes to its position (i.e., to assume its role), depending on its internal state. It also responds to any requested communications from teammates. It is when it goes to its position that it makes use of the anticipation mechanism presented here.
Figure 4: An example of a top-level external behavior for a robotic
soccer player.
The referenced ``Handle Ball'' and ``Play Position'' behaviors may be affected by the agent's current role and/or formation. Such effects are realized by references to the internal state either at the level of function arguments (args1, args2), or within sub-behaviors. None of the actions in the condition-action pairs here are action primitives; rather, they are calls to lower level behaviors.
The definition of a position includes home coordinates, a home range, and a maximum range, as illustrated in Figure 5.
Figure 5: Different positions with home coordinates and home and max
ranges.
The position's home coordinates are the default location to which the agent should go. However, the agent has some flexibility, being able to set its actual home position anywhere within the home range. It is this flexibility that is exploited by the anticipation mechanism. When moving outside of the max range, the agent is no longer considered to be in the position. The home and max ranges of different positions can overlap, even if they are part of the same formations.
A formation consists of a set of positions and a set of units. The formation and each of the units can also specify inter-position behavior specifications for the member positions, as illustrated in Figure 6(a). In this case, the formations specify inter-role interactions, namely the positions to which a player should consider passing the ball [16]. Figure 6(b) illustrates the units, the roles involved, and their captains. Here, the units contain defenders, mid-fielders, forwards, left players, center players, and right players.
Figure 6: (a) A possible formation (4-3-3) for a team of 11
players. Arrows represent passing options.
(b) Positions can belong to more than one unit.
Since the players are all autonomous, in addition to knowing its own role, each one has its own belief of the team's current formation along with the time at which that formation was adopted, and a map of teammates to positions. Ideally, the players have consistent beliefs as to the team's state, but this condition cannot be guaranteed between synchronization opportunities. Another offshoot of the player's autonomy is that each is free to leave its position unilaterally, leaving the team to adjust behind it. Thus, players are not bound by their positions when presented with unexpected action opportunities.
Our team structure allows for several features in our robotic soccer team. These features are: (i) the definition of and switching among multiple formations with units; (ii) flexible position adjustment and position switching; (iii) and pre-defined special purpose plays (set plays) [17].