Robot Soccer
Compass Sensor, IR Emitting Ball, IR Detector, Ultrasonic Sensor, Light Sensor
In Robot Soccer it is useful for your robot to be able to tell the direction to both your goal and your opponent’s goal. A compass can help your robot do this.
There are two kinds of compass available for your NXT robot, Wiltronics compasses and HiTechnic compasses. We will use the latter, as Lego has approved them for use with the NXT robots.
What does the Compass Sensor Do?
The HiTechnic
Compass Sensor contains a digital magnetic compass that measures the
earth’s magnetic field and calculates a compass bearing to the nearest 1°
. It can report a compass bearing to an NXT robot 100 times per second.
Building a Compass Sensor on Your Robot
The
HiTechnic
compass sensor will only operate correctly in a horizontal position, so
you must keep the compass level for it to read correctly.
This is very important.
Please remember this when you building the compass sensor into your
robot.
It is also highly desirable to mount the compass at least 15 cm. (6
inches) away from the motors and 10 cm. (4 inches) away from the NXT
computer brick itself. This is necessary because both the NXT computer
brick and NXT motors produce magnetic fields that can change the compass
sensor readings.
In the picture to the left, you can see the compass sensor is mounted
high, well above both the motors and computer brick.
Construct the
following program. The program
lets
you see the compass reading on
your NXT’s screen.
Set up the compass control block (left of the three blocks above) to look
like this:
The “Number to Text” block (middle of the three blocks above) should look
like this:
The Display block (the rightmost of the three blocks in the NXT-G diagram on
the previous page) should be adjusted to look like this:
Now when you download this program to your NXT and run it, you will see a
number displayed on your NXT’s small computer screen; this number (between
zero and 360) is the compass heading.
Move your robot around, and notice how the reading
changes. Experiment and see if the reading changes when the compass sensor
is tilted one way or another. Do you have to be quite careful to keep the
sensor horizontal, or does this not matter?
Introduction to Part 2
How does the compass Block Work?
Suppose you used the
above
program, and found out that
the compass directions of the two edges of your opponent’s goal were 90 and
100 degrees. Now also suppose that the compass control block in your program
was set up like the one below.
If your robot was in a game and saw the compass
readings of either 90 degrees, or 100 degrees, the compass block would
behave as in the following table:
|
A |
B |
Range |
Absolute |
Compass Block True or False? |
|
85 |
95 |
Inside Range |
90 |
True |
|
85 |
95 |
Inside Range |
100 |
False |
|
85 |
95 |
Outside Range |
90 |
False |
|
85 |
95 |
Outside Range |
100 |
True |
Using the Compass block in a Soccer Program.
Use the
above
program
to find compass headings
for the two edges of your opponent’s goal. Write them down.
Now if you used the “Inside Range” option in your program to
see if your robot is pointed at your opponent’s goal, you could then program
your robot similarly to the program
below.
Note that, any of the sensor
slots 1, 2, 3 or 4 can be used.
A compass sensor block like the one below would tell your robot to obey the
top line of your program if the compass reading is inside the range 90 to
180.
In the example program above, the top line of the
program would tell the robot to go forward if it is facing in the direction
of your opponent’s goal (that is, if the compass reading is inside the range
90 to 180 degrees). If the robot is not pointing at the opponent’s goal,
(that is, the compass reading is outside the range of 90 to 180 degrees) it
will spin around looking for your opponent’s goal.
The program above is one way to program your soccer robot. But is this the only way to arrange your soccer program? Is there a better way to use the compass sensor in your soccer program? What should you do if the ball is between your robot and your opponent’s goal – charge at it? What should you do if the ball is between your robot and your own goal – should you charge at it or do something different? Hints.
· Check if you are out of bounds (marked with black tape) – if you are out of bounds try to return to the soccer field (turn around, go forward, and repeat this until
you are inside the soccer field.
· Find the soccer ball (spin around
until you see the soccer ball, but be careful, too fast a spin and you may
miss it).
· Go to a good place to kick the ball towards your opponent’s goal (if the soccer ball is between you and your own goal, travel to the other side of the soccer ball
so that the soccer ball is between you and your
opponent’s goal).
· Go to the soccer ball (go fast
towards the soccer ball, trying to reach it before your opponent’s robot)