LMICSE: Lego Mindstorms in Computer Science Education

Grading Requirements

For each of the tasks in this lab:

• Demonstrate to the instructor or lab TA that you robot successfully completes its mission, and have them initial the lab activity sheet.
• Print off the code for that task, making sure that you have commented that code appropriately.
• Turn in the initialed checkoff sheet and the code for each of the tasks by 5 pm on the Friday following lab.

Prerequisite Knowledge

This lab assumes very little, but it would help if you were familiar with the general structure of a simple Java program.

Materials

To complete this lab you will need the basic Turtle robot, without any bumpers or sensors. Power the robot on. If the RCX beeps twice and the LCD screen says something like 7.6 0, with an icon of a little person between the x.y number and the single digit, you're all set. If not, you'll have to download the Lejos firmware - the software which makes the robot understand LeJOS. To do that, see your instructor or lab TA.

Basic Turtle Methods

Turtles know how to do a number of things (its methods), and in this lab we introduce a few of them. Basically, we can ask a Turtle to move forward, backwards, turn for an amount of time, or stop moving all together. The details:

forward(int n)  : Move forward for n milliseconds.

backward(int n) : Move backward for n milliseconds.

turn(int n)     : Turn counterclockwise (clockwise if negative) for
                  n milliseconds.

stop()          : Stop the motors and wait for them to stop.

Task One  

Task 1 is to download and run a file in an existing project. Download it from this link, and store it in your home directory. It will be a compressed file (a "zip" file) which you can uncompress by double clicking on it. After you have uncompressed it, startup the BlueJ program and open that project. You should see a window like the following:

The two icons (Task1 and Turtle) represent two Java classes. Turtle is a class definition provided by your instructor which defines the Turtle robot. You never need to (or should!) change this code. Task1 is a simple example program which makes the turtle move in a "U" like shape.

Let's run this program. Four steps:

1. Get Ready: Plug the Lego infrared tower into a usb port on the computer. A good place in our lab is the back of the monitor (if there is one, unplug the zip drive). Place the robot near the tower with its infrared port facing the tower and turn it on (press the on-off button).
2. Compile: We need to make should that the files have been compiled, i.e., translated into a form the robot can understand. For each of the two classes, hold down the "control key" and click the mouse on the class icon. (we call this a "control-click"). A popup menu appears. Choose "MindstormsTool" and then "Compile". (Don't just choose "Compile", that translates it to run the computer itself, which is not what we want.)
3. Download: Then control-click on the Task1 icon, and choose "MindstormsTool" and then "download". A status window will pop up showing the download progress. Yes, it does take a while!
4. Run It: Take the robot over to one of the robot tables, put it down and press the run button. It should start moving.

Ok, if that worked, let's start examining the code. (If not, talk to your instructor or TA about the problem). Back at the computer, double click the Task1 class icon to reveal its code. Minus the comments it should look like this:

public class Task1 {
    public static void main(String args[]) {

 
        Turtle.forward(2000);
        Turtle.turn(1000);
        Turtle.forward(2000);
        Turtle.turn(1000);
        Turtle.forward(2000);
 
        Turtle.stop();
    }
}

Let's examine the Turtle specific code in Task1.java.

        Turtle.forward(2000);
        Turtle.turn(1000);
        Turtle.forward(2000);
        Turtle.turn(1000);
        Turtle.forward(2000);
 
        Turtle.stop();

The forward method tells the Turtle to move forward, the turn method to turn, and the stop method to stop. forward needs to know how long to go forward, and the number is time in milliseconds. Here, we say 2000 milliseconds so that the turtle will go forward for about two seconds. Similarly, the turn method needs to know how many milliseconds to turn. The stop method doesn't need any additional information.

Task Two

Create a new class called Task2 (using the Task1 code as a model) that commands the turtle to move in a approximately a "box" (a square) either clockwise or counter-clockwise. Notice how hard it is to get the robot to turn exactly 90 degrees!

Task Three

Now create a class ("Task3") that moves the turtle in an spiral like the one shown.

Some Optional Problems

Conclusion

In this lab, you learned the following:

• How to write code for the Turtle, download it, and run it.
• How to invoke an object's methods.
• The following Turtle methods: forward, backward, turn and stop.
• How to use sequential control to control the actions of the Turtle.