As described in section 2, total balance of the whole system is a key issue to the robot design. In order for the system to facilitate various kinds of behaviors, a more complicated mechanical system and its sophisticated control techniques are necessary. We should start with a simpler one and then step up. The simplest case is to use just a car-like vehicle which has only two DOFs (degrees of freedom, for example forward and turn), and pushes the ball to the target (dribbling).
The target can be just a location, the goal (shooting), and one of the teammates (passing). In the case of location, a dribbling skill to carry the ball to the location might be sufficient. In the latter case, the task is to kick the ball into the desired direction without caring about final position of the ball. To discriminate it from a simple dribbling skill, we may need more DOFs to realize a kick motion with one foot (or what we may call arm). In the case of passing, velocity control of the ball might be a technical issue because one of the teammates to be passed to is not stationary but moving.
In Levels II and III, requirements include an obstacle avoidance behavior and the coordination of this behavior with a ball-carrying (or passing/shooting) behavior. To smoothly switch two behaviors, the robot should slow down, which increases the possibility that the opponent will take the ball. To avoid these situations, the robot can quickly switch behaviors, which causes instability of the robot motion. One might use the omni-directionally movable vehicle based on a sophisticated mechanism [Asama et al. 1996]. For example, the vehicle could move to any direction anytime. In addition to the motion control problem, there are more issues to be considered such as how to coordinate these two behaviors (switching conditions) [Uchibe et al. 1996].