The final implemented improvement facilitated by our flexible teamwork structure is the introduction of set-plays, or pre-defined special purpose plays. As a part of the locker-room agreement, the team can define multi-step multi-agent plans to be executed at appropriate times. Particularly if there are certain situations that occur repeatedly, it makes sense for the team to devise plans for those situations.
In the robotic soccer domain, certain situations occur repeatedly. For example, after every goal, there is a kickoff from the center spot. When the ball goes out of bounds, there is a goal-kick, a corner-kick, or a kick-in. In each of these situations, the referee informs the team of the situations. Thus all the players know to execute the appropriate set-play. Associated with each set-play-role is not only a location, but also a behavior. The player in a given role might pass to the player filling another role, shoot at the goal, or kick the ball to some other location.
For example, Figure 7 illustrates a sample corner-kick set-play. The set-play designates five roles, each with a specific location, which should be filled before the ball is put back into play. Based on the home positions of the current formation, each individual agent can determine the best mapping from positions to set-play locations, i.e. the mapping that requires the least total displacement of the 5 players. If there is no player filling one of the necessary formation roles, then there must be two players filling the same role, one of which must move to the vacant role. In the event that no agent chooses to do so, the set-play can proceed with any single set-play-role unfilled. The only exception is that some player must fill the set-play-role responsible for kicking the ball back into play. A special-purpose protocol is incorporated into the set-play behaviors to guarantee such a condition.
Once the set-play-roles are filled, each player executes the action associated with its set-play-role. As illustrated by the player starting the corner-kick in Figure 7, a player could choose among possible actions, perhaps based on the opponent positions at the time of execution. No individual player is guaranteed of participating in the play. For example, the uppermost set-play position is there just in case one of the other players misses a pass or shoots wide of the goal: no player will pass directly to it. Each player leaves its set-play-role to resume its former role either after successfully kicking the ball, or after a pre-specified, role-specific amount of time.
Figure 7: A sample corner-kick set-play. The dashed circles show the
positions in the team's current formation and dashed arrows indicate
the set-play-roles--black circles--that they would fill. Solid
arrows indicate the direction the ball is to be kicked as part of each
set-play-role.
We found that the set-plays significantly improved our team's performance. During the RoboCup-97 competitions, several goals were scored as a direct result of set-plays.