If a robot is to accurately direct the ball towards a target position, it must be able to approach the ball from a specified direction. Using the ball prediction from the vision system, the robot aims at a point on the far side of the target position. The robots are equipped with two methods of doing so:

- Ball Collection:
- Moving behind a ball and knocking it towards the target.
- Ball Interception:
- Waiting for the ball to cross its path and
then intercepting the moving ball towards the target.

When using the ball collection behavior, the robot considers a line from the target position to the ball's current or predicted position, depending on whether or not the ball is moving. The robot then plans a path to a point on the line and behind the ball such that it does not hit the ball on the way and such that it ends up facing the target position. Finally, the robot accelerates to the target. Figure 3 illustrates this behavior.

**Figure 3:** **Ball Collection-**The robot computes the line from the
ball to the target (line a) as well as the line through the ball and
perpendicular to this line (line b). Whenever the robot is on the
same side of line b as the target, it aims for an intermediate target
to the side of the ball so that it avoids hitting the ball away from
the target. Otherwise, the robot aims for a point directly behind the
ball along line a. Once there, it accelerates towards the target.

When using the ball interception behavior (Figure 4), on
the other hand, the robot considers a line from *itself* to the
target position and determines where the ball's path will intersect
this line. The robot then positions itself along this line so that it
will be able to accelerate to the point of intersection at the same
time that the ball arrives.

**Figure 4:** **Ball Interception-**The robot computes the intersection
of the line between itself and the target position (line a) and the
ball's line of trajectory (line b). The robot then positions itself
at a fixed distance (*D*) behind the intersection point, either moving
forwards or backwards to get there. Knowing the time *T* required to
accelerate from a stopped position to distance *D*, and also knowing the
ball's velocity, the robot accelerates towards the final target when
the ball is time *T* away from the interception point.

In practice, the robot chooses from between its two ball handling routines based on whether the ball will eventually cross its path at a point such that the robot could intercept it towards the goal. Thus, the robot gives precedence to the ball interception routine, only using ball collection when necessary. When using ball collection, it actually aims at the ball's predicted location a fixed time in the future so as to eventually position itself in a place from which it can intercept the ball towards the target.

Tue Sep 30 19:12:38 EDT 1997