C-2 Turns (Pivoting)

Next, you'll code an app to make your robot turn 90° right. Then you'll modify the app so the robot turns 90° left and turns 180° around.

Save Copy of App With New Name

In your Arduino code editor, use the "Save As" command to save a copy of your existing driving_test app as a different app named: pivot_test

If necessary, here are instructions for how to save a copy of an app with a new name.

Once you saved the new app name, modify the block comment near the beginning of the app code to change Driving Test to Pivot Test.

Turn Right

The most common types of turns needed for robot navigation are: turn 90° right, turn 90° left, and turn 180° around.

The RedBotMotors class defines a method named pivot() that can be used to turn the robot either clockwise or counter-counterclockwise. The pivot() method turns the robot by driving both motors in opposite directions.

There are other ways to turn your robot, but the pivot() method results in a perfectly tight turn because the robot's axis of rotation is centered between its wheels. Pivoting is the best way to turn the robot when space is limited (as it will be in your robot demonstration environment).

When the motors.pivot() method is used in your code, the motors will start and will keep pivoting continuously. You'll use a delay() statement to allow the motors to pivot for a certain amount of time before turning the motors off with the motors.brake() method.

First, modify the "Press to Start" code within the if statement in the loop() function to remove the existing code statements that make the robot drive forward and then backward.

Next, you'll add new code so that when the robot's button is pressed, the robot will drive forward for 1.5 seconds (about 2 feet), turn 90° right, and then drive forward for another 1.5 seconds. Add this new code within the if statement in the loop() function (after the noTone() statement):

    motors.drive(200);
    delay(1500);
    motors.brake();

    motors.pivot(100);
    delay(650);
    motors.brake();

    motors.drive(200);
    delay(1500);
    motors.brake();

The motors.pivot() method requires one parameter inside its parentheses:

• The motor power, which can be any integer (whole number) between -255 and 255. A positive power pivots the robot clockwise (to the right), and a negative power pivots the robot counter-clockwise (to the left). A larger absolute power produces a faster pivot speed (-255 and 255 are the fastest speeds, while -1 and 1 are the slowest speeds). In this case, the power will be 100.

PIVOT SLOWLY: Pivot the robot at a lower motor power to avoid wheel slippage. In general, try using a power of 100 for pivoting, depending on the surface.

The second delay() of 650 milliseconds (0.65 seconds) is an estimate of how much time it will take your robot to pivot 90 degrees. You may have to change this value after testing your robot.

Upload App to Robot

Follow the steps to upload the app to your robot:

1. Connect Robot to Computer

2. Turn on Robot Power

3. Select Correct Board and Port

4. Upload App to Robot

Unplug the USB cable from the robot, and place the robot on the floor. Be sure an area of about 3 feet square in front and to the right of the robot is clear of any obstacles.

• Press the D12 button on your robot's circuit board. Your robot should beep and then drive forward for 1.5 seconds (about 22 inches). Then the robot should pivot 90° right, and then drive forward for another 1.5 seconds (about 22 inches).

Test out your robot's app several times to see how close the pivot is to a 90° right turn.

In the app code, the second delay() of 650 milliseconds (0.65 seconds) is an estimate of how much time it will take your robot to pivot 90 degrees. You may have to change this value based on your testing:

• If your robot is pivoting less than 90°, increase the pivot delay() time slightly (such as: 700 ms).

• If your robot is pivoting more than 90°, decrease the pivot delay() time slightly (such as: 600 ms).

If you do change the pivot delay() time, upload the modified app to your robot, and test it again with the new value. You may need to change the value several times to fine-tune your results.

Later in this tutorial, you'll learn how to use the wheel encoders to measure how far the wheels have turned, in order to make the robot pivot by a specific angle.

Turn Left

Next, you'll modify the app code so your robot will pivot 90° left. To do this, you simply need to make the robot pivot counter-clockwise by using a negative motor power.

Modify the motors.pivot() statement so the motor power is -100 (instead of positive).

You don't need to change the pivot delay() time because the amount of time to pivot 90 degrees should be the same whether the robot is pivoting clockwise (to the right) or counter-clockwise (to the left).

Upload the modified app to your robot.

Unplug the USB cable from the robot, and place the robot on the floor. Be sure an area of about 3 feet square in front and to the left of the robot is clear of any obstacles.

• Press the D12 button on your robot's circuit board. Your robot should beep and then drive forward for 1.5 seconds (about 22 inches). Then the robot should pivot 90° left, and then drive forward for another 1.5 seconds (about 22 inches).

Test out your robot's app several times to see how close the pivot is to a 90° left turn.

Turn Around

Next, you'll modify the app code so your robot will pivot 180° to turn around. To do this, you simply need to allow the robot to pivot for twice the amount of time as a 90° pivot.

For a 180° turn, it doesn't matter if the robot pivots clockwise or counter-clockwise. So you can either leave the motor power in the motors.pivot() statement as -100 (counter-clockwise) — or you can change the power back to 100 (clockwise).

Modify the pivot delay() time so its value is twice the amount that was needed for a 90° turn. For example, if the time required for a 90° turn was 650 milliseconds, try 1300 ms for a 180° turn.

Upload the modified app to your robot.

Unplug the USB cable from the robot, and place the robot on the floor. Be sure a path of about 3 feet in front of the robot is clear of any obstacles.

• Press the D12 button on your robot's circuit board. Your robot should beep and then drive forward for 1.5 seconds (about 22 inches). Then the robot should pivot 180° around, and then drive forward for another 1.5 seconds (about 22 inches), returning approximately to its starting point.

Test out your robot's app several times to see how close the pivot is to a 180° turn. You may have to adjust the pivot delay() time in the app code, and re-upload the app to test again.