LEGO MINDSTORMS Robots Manual page 166

If tag sends out a tag message but doesn't receive a reply, it doesn't do anything. The lack of a reply can mean two things: either the other robot did not receive the IR tag message, or the robot
bumped into an obstacle, not the other robot. In the original RoboTag by Matthew Miller, the arena contained no obstacles, so it was safe to assume that the other robot didn't "hear" the tag
message. The tagging robot would then spin in place shouting "Tag!" repeatedly, hoping to get in range of the other robot's IR port.
In our design, tag doesn't do anything if a reply is not received. This opens up the possibility of adding physical obstacles to the arena. If the robot bounces into something that doesn't respond to
the tag message, it must be an obstacle.
The next behavior is avoid, which helps the robot to avoid the edge of the playing arena. It is triggered by the light sensor. This behavior backs up and turns to move away from the edge, much
like tag.
int avoidCommand;
task avoid() {
while(true) {
if (LIGHT_SENSOR < averageLight - 3) {
// Back away from the border.
avoidCommand = COMMAND_FLOAT;
Wait(20);
avoidCommand = COMMAND_REVERSE;
Wait(50);
// Turn left or right for a random duration.
if (Random(1) == 0) avoidCommand = COMMAND_LEFT;
else avoidCommand = COMMAND_RIGHT;
Wait(Random(200));
avoidCommand = COMMAND_NONE;
}
}
}
The highest-level behavior is tagged, which is triggered when the IR port receives notification that the robot has been tagged. This behavior tells the robot to send an IR acknowledgement, play
a sad sound, and sit still for eight seconds.
int taggedCommand;
task tagged() {
while(true) {
if (Message() == MESSAGE_TAG) {
taggedCommand = COMMAND_STOP;
SendMessage(MESSAGE_ACKNOWLEDGE);
PlaySound(4);
Wait(800);
ClearMessage();
taggedCommand = COMMAND_NONE;
}
}
}
Arbitration
As mentioned earlier, an additional task is needed to link the robot's behaviors to its motors. In this implementation, a task called arbitrate examines the output command variable of each
behavior. If it is not COMMAND_NONE, it is used to set the current motor command.
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