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FIGURE 12.1
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This subroutine locates and nds the beacon.
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LOCATING A GOAL
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MainProgram: while true gosub SetEnvironment gosub FindBeacon wend End
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FIGURE 12.2 A while-loop causes the program to test the algorithm repeatedly.
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12.1.2 THE MAIN PROGRAM The main program sets up an environment with obstacles and then starts the goal-seeking behavior (Fig. 12.2). The subroutine SetEnvironment (see below) sets the environment and places the robot and beacon at random positions. In order to test the algorithm we need to run the program several times to see if any obstacle arrangement can baf e the code and cause the robot to fail to reach the goal. We could do this by manually running the program many times. A better way, though, is to have the main program repeat the sequence of creating a random environment and locating the goal in an endless loop. 12.1.3 CREATING A CLUTTERED ROOM The subroutine in Fig. 12.3 clears the screen then draws three circles and three squares (experiment with more or less). The size and location of each object is chosen randomly. This makes the environment full of obstacles at random positions that can be hard to circumnavigate. The robot is located at a random position on the left side of the screen and the beacon (a red circle) at a random position on the right side.
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SetEnvironment: ClearScr // Draw three circles and three squares for i=1 to 3 SetColor Black LineWidth 4 x = random(450) + 100 y = random(300)+100 size = random(50)+50 Circle x,y,x+size,y+size x = Random(450)+100 y = Random(300)+100 size = random(100)+50 Rectangle x,y,x+size,y+size next // place robot rLocate 25,Random(350)+100 rInvisible Red // place beacon bx =750 by = Random(350)+100 Circle bx-10,by-10,bx+10,by+10,red,red Return
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FIGURE 12.3 Creates a cluttered room and places the robot and beacon at random positions.
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12.1.4 FACING THE BEACON The rBeacon(color) function in RobotBASIC is used to locate a beacon of a speci ed color. It returns zero (false) if the beacon is not directly in front of the robot. If the beacon is directly ahead of the robot, the function returns the distance to the beacon. You can consider the number returned as a nonzero number and therefore is equivalent to being true. This means that you can use the function to test if the beacon is directly ahead of the robot or not. However the function can also be used to return the distance to the beacon. This can be useful in many situations, especially to determine when the robot has reached the point under the beacon (see later). The function is usable to look for any color you specify as a parameter. Normally, the robot will see colors on the screen as objects to be avoided. If you want the robot to assume that objects of the beacon color are in the air and thus cannot cause collisions, you need to issue the rInvisible color statement listing the appropriate color. This statement tells the robot that the color being used as a beacon is not an obstacle. Figure 12.4 shows how to create a function that turns the robot until the beacon is directly in front of it. The expression not rBeacon(Red) is the same as saying:
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rBeacon(Red) = false or rBeacon(Red) = 0
12.1.5 MOVING TOWARD THE BEACON The subroutine ForwardTillBlocked, shown in Fig. 12.5, moves the robot forward until it encounters an object or it reaches the beacon. The expression in the while-loop checks for an obstacle with the three front infrared sensors and the front and side bumpers. The subroutine CheckFound determines if the robot has reached the beacon and sets the variable BeaconFound to true or false to indicate the current status. If the beacon has been found the while-loop is exited with a Break statement.
FaceBeacon: while not rBeacon(Red) rTurn 1 wend Return
FIGURE 12.4
This subroutine turns the robot toward the beacon.
ForwardTillBlocked: while not (rFeel() & 14) AND not (rBumper() & 14) rForward 1 gosub CheckFound if BeaconFound Then break wend Return
FIGURE 12.5 This code moves the robot forward until it reaches an object or the beacon.
LOCATING A GOAL
12.1.6 GOING AROUND AN OBSTACLE If the robot encounters an object, it needs to go around it. It is certainly possible to develop many different ways to go around an object, but we already have one from Chap. 8. All our robot needs to do is follow the edge of the object as if it were a wall. However, if the code from Chap. 8 is used as it was written the robot would just continue to follow around the object forever. We need a way to tell it to stop when it has reached the other side. The robot has no easy way to determine when it has reached the other side of the object. In fact, if there are other objects close by, the robot might not even be able to get to the other side without causing a collision. An easy solution is to simply let the robot follow the wall for a little while and stop. If you study Fig. 12.1 you will see that if the robot stops too early it will just try to face the beacon again and start over. Obviously, the robot does not have to follow the wall until it gets to the other side; it only has to follow it for a reasonable length of time. The question is How long is reasonable If the object is small, then a short time is best because we don t want to go all the way around the object. If we always use a short time though it is conceivable that some combination of objects could occur that would trap the robot. This might happen if the robot does not go far enough around the object to get a clear (or at least clearer) path to the beacon. In such situations, the robot might simply continue to retrace its steps repeatedly moving toward the goal until blocked, following the wall but not far enough, moving toward the goal again, but essentially in the same situation as before. One way to solve such a problem is to introduce some randomness into the robot s behavior. If you examine the code in Fig. 12.6 you will notice that it is the same as the code in Chap. 8, but in place of a while-loop we are now using a for-loop. The while-loop in Chap. 8 caused the robot to follow the wall forever. The for-loop causes this code to be executed between 20 and 270 times. These numbers were
GoAround: If BeaconFound Then return rTurn -random(150) if TurnDir > 0 FN = 6 else FN = 12 endif for i=1 to 20 + random(250) while (rFeel() & FN) or (rBumper() &4) rTurn -TurnDir wend rForward 1 while not rFeel() rTurn 5*TurnDir rForward 1 wend next Return
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