WORKSHOP 3D Lego RCX Challenge: RoboLab Programming

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WORKSHOP 3D Lego RCX Challenge: RoboLab Programming
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. WORKSHOP 3D Lego RCX Challenge
RoboLab Programming
. RoboLab is the product line offered by LEGO dacta built around the Mindstorms technology and the RCX.
The software for the RoboLab products is built over the top of LabView, the same software used by NASA
to program and control the Mars Rover. There are two modes available in the RoboLab software, Pilot and
Inventor. Of these, only Inventor is suitable for our sample program, because there is no way to create a
repeating loop in Pilot. Within Inventor, you can access the programming interface at any on of four 'levels',
which provide progressively more control icons. The sample program is developed in level 4.
Sample Line Tracker in ROBOLAB:Inventor
Let's look at the whole program, then at how it was written.
is the program as it appears in the ROBOLAB software as it is being developed. From here, it can be
transferred directly to the RCX (with the proverbial 'one click' - really) and tested on your real robot.
A direct translation of this program is as follows. See if you can follow along in the diagram as you read the
text. I have capitalized the words that are represented by a symbol above:
When the RUN button is pressed, REPEAT the following 1000 times: REPEAT the following 1000
times: If the value of the LIGHT SENSOR at INPUT 2 is GREATER THAN 50, turn on the MOTOR
at OUTPUT A at POWER LEVEL 3. WAIT 0.02 second, then TURN off OUTPUT A. On the other
hand, if the value of the LIGHT SENSOR at INPUT 2 is LESS THAN OR EQUAL TO 50, turn on the
MOTOR at OUTPUT C at POWER LEVEL 3. WAIT 0.02 second, then TURN off OUTPUT C. GO
BACK to the repeat point. GO BACK to the other repeat point. STOP the program.
Developing the program:
http://www.workshop3d.com/rcx/robolab.htm (1 of 3)3/31/2006 11:43:52 PM
You don't have to know all of the detail to start writing your program. It is written by dragging the various
icons onto the screen and then hooking them together with "string" (the pink lines). You can always move
the icons around later, and revise the way that they are strung together. We'll look at the program at each
stage in it's development.
This is the way a new RoboLab program looks before your write it. The icon on the left means "the program
starts here", or "start doing this when the RUN button is pressed". The one on the left means "the program
ends here". Their relative location implies that operation will proceed from left to right, which will make the
This

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WORKSHOP 3D Lego RCX Challenge: RoboLab Programming
program easy to read.
http://www.workshop3d.com/rcx/robolab.htm (2 of 3)3/31/2006 11:43:52 PM
This first icon added is probably the most important. It represents the light sensor, and indicates that we have
reached a fork in the road. If the light sensor reads greater than some number (note the '>' symbol on the
icon), we will do one thing, and if it reads less than or equal to that number, it will do something else. This
icon is then connected by string to the start icon, indicating that it is the first thing the robot should do: check
the light sensor.
The next two icons are identical. Each represents one of the motors. This is because if the light sensor sees
black, we will run one of the motors, and if it sees white, we will run the other one. If we attach a wheel to
each motor and mount these on the left and right sides of our robot, this will cause the robot to turn left or
right depending upon whether it sees black or white. Note that each of these is connected by string to the
light sensor, one to the "greater than" sign and the other to the "less than or equal to" sign.
Of course, after turning on the selected motor, we must turn it back off again, or the robot will just keep
turning in the selected direction, so these stop signs are added after the motor icons. Again note that the
program flows from left to right, following the pink string.
By adding a merge icon and connecting the whole thing to the stop sign, we have a complete program. It
starts, it checks the sensor, then turns one of the motors on and then off, then stops the program. The actions
are right, but we want it to do this over and over again, not stop.
By adding these 'repeat' brackets (they must be used in pairs), we can make the robot repeat any portion of
the program. These have been inserted before the light sensor is checked, to make sure that it checks it every

WORKSHOP 3D Lego RCX Challenge: RoboLab Programming
time, and after the merge.
http://www.workshop3d.com/rcx/robolab.htm (3 of 3)3/31/2006 11:43:52 PM
This looks pretty good, but some information is missing. The motors for instance, are turned off immediately
after they are turned on. We need to insert a pause icon between turning the motor on and turning it off:
Here, the pauses have been added, as well as a second set of repeat brackets. We need these because through
experimentation I found at that you could only repeat something a limited number of times (1,000 works,
10,000 doesn't - I don't know what the actual limit is). By doubling up the brackets, we can get it to repeat at
least 1,000 x 1,000 = 1,000,000 times. This sounds sufficient, but we'll do the math in a minute to make sure.
Finally, a number of values are added to the icons to give specific direction about the actions. Working from
left to right, we see the 1000 repetitions repeated 1000 times in the blue boxes below the repeat icons. Below
the light sensor is the value 50, which is the value the actual reading will be compared to to decide which
program branch to take. Above the light sensor is the number of the port (2) on the RCX it will be attached
to. Similarly, the port for each motor is indicated (A and C), as well as a "Power Level" value for each, and
the pause icons have each received a value of 0.02 seconds, which is how long each motor will run for before
it stops. That's it. We're done.
Now back to the math. The light sensor is sampled and a motor is pulsed 1,000,000 times. Since each pulse
lasts for 0.02 seconds, the robot will theoretically run for 0.02 seconds times 1,000,000 repetitions = 20,000
seconds, or over five and a half hours. The batteries would probably run out before the program stopped.