M. Pathfinder (Modified Pathfinder)
Before You Begin
What’s the point?
To help students understand that the behavior of a motor (e.g., which way it turns) can be controlled by
switching wire connections or via a program. Instead of rewiring a device in order to change its behavior,
we can use a set of instructions—a program—to do it.
To help students analyze simple motion patterns for a two wheeled vehicle such as moving forward and
backward or turning and to relate these patterns to motor operations and to programming commands. For
example, students will observe a turning vehicle, realizing that one motor is turning. They will then represent
this observation in a computer program.
What you will need:
For Part 2:
For Part 1:
For each group:
• M. Pathfinder for demonstration
• 2 wires, about 40 inches long, with ends
stripped
• 2 alligator clips
• 1 C or 9 Volt battery
• 2 intact long connector wires
•
•
•
•
The Robotics Inventions System™ kit
A disk
A copy of the Technical Guide
A copy of M. Pathfinder building
instructions
• A building bin (optional)
• Student activity sheet
For the class:
• M. Pathfinder transparency # 1
Programs used: Roundtrip, TurnTurn, Long, Short, and Different
© 2002 The LEGO Group. Used here with permission.
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Prepare Ahead:
1.
Make enough copies of the M. Pathfinder building instructions for each working group to have
one.
2.
Make enough copies of the Building and Programming M. Pathfinder student activity sheets for
each student to have one.
3.
Build M. Pathfinder following building directions provided with this manual. Do not attach the RCX
to the base.
4.
Prepare the stripped wires as described in (See Note on Materials box below).
5.
Download programs Roundtrip and TurnTurn onto the RCX.
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Note on Materials
There are several ways to connect Lego™ motors to a battery without connecting to the RCX.
We will describe two: 1) using a stripped connector wire, and 2) using wires with alligator clips.
Stripped connector wire: Since kits don’t come with extra connector wires, you will have to purchase an
additional wire separately. (The Pitsco Lego Dacta online store—www.pldstore.com—offers small packages
of connector wires (sometimes bundled with a touch sensor). To prepare the wire, cut the brick from one
end of the wire, separate the wires along their midsection for a length of about 2 inches, and then strip
about ½ inch of each wire (see first illustration below).
Wires with alligator clips: Another way would be to obtain general purpose electrical wires and 2
alligator clips (Can be found in hobby or electronics stores.) You will need 2 wires, each about 1 ft. in
length. Strip both ends of each of the wires. Connect an alligator clip to one end of each wire. Use the clips
to connect to an intact Lego™ connector wire as illustrated below. Be sure to connect the clips on the
bottom of the brick so that the clips touch the metal part of the brick and the clips don’t touch each other.
It’s best to attach the clips on non-adjacent corners (on the diagonal).
Running Lego motors
without the RCX
New commands
introduced in this activity:
Activity Overview
Pathfinder is a two-wheeled, two-motor vehicle without any sensors
attached. It is an easy-to-build, easy-to-fall-apart vehicle. We have
included building instructions since it is no longer included in the
Constructopedia. We suggest using it as a vehicle (no pun intended) for
introducing RCX Code.
This activity has two parts. Part 1 is designed as a transition between the
Wall Hugging Mouse and programmable robots. In this part, you will
demonstrate running M. Pathfinder’s base without the RCX, showing that
the unit can run on batteries alone and does not have to be programmed
to run. You will show students that M. Pathfinder can run with or without the
RCX. When connected to batteries, it can run forward or backwards, turn
right or left, all depending on wiring. You will then demonstrate how M.
Pathfinder’s behavior can be controlled through a program rather than
through wire switching.
P-1
© 2002 The LEGO Group. Used here with permission.
R/W-1
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In Part 2, students build the modified Pathfinder, following the M. Pathfinder
building instructions. They then run it using the program in slot #1. (This
program is downloaded onto the RCX when firmware is installed.) Students
will notice that M. Pathfinder goes forward, but doesn’t stop by itself. In
order to stop it, they must catch it and turn it off.
Next, students will follow the activity sheet to help them figure out how to
make M. Pathfinder move and stop on its own, using the On, Wait for, Off,
and On For command blocks.
At this point students will be ready to use the newly acquired commands to
write their own programs, instructing M. Pathfinder to move in a variety of
ways (e.g., zigzag, go around a box). Finally, they will program M. Pathfinder
to go towards a wall, hit it, and go backwards. It is important that all
students solve this problem before they move on to work with touch sensors.
Discussion Points
Part 1: Running Lego Motors with and without the RCX
Whole Class Activity
Before students build M. Pathfinder: Show students the base of a pre-built M. Pathfinder without the
RCX. Connect one end of a long connector wire to each motor and then connect the other ends to each
other, stacked on top of one another.
If you are using a stripped connector wire,
connect it to the long connector wires on one end,
with the stripped end free.
If you are using wires with alligator clips,
connect the clips to the long connector wires (see
illustration on the previous page).
Now, take one stripped wire and hold it to one end of a battery. Hold the second stripped wire to the other
end of the battery. Point out that M. Pathfinder is running in one direction now. Then switch which wire is
connected to which end of the battery. Have students observe that M. Pathfinder is running in the opposite
direction.
Ask students what they think you should do to make the base turn in place. If needed, remind them of how
the Wall Hugging Mouse moved (one motor turned on at a time).
Adding the RCX: Show students the RCX. Explain that it is a small computer that runs on batteries and can
hold up to five programs. Remove the stripped wires and connect one end of a long connector wire to
Port A and the other to Port C. (The other ends should remain attached to the motors.)
Hold the RCX in your hand and show students the red on/off button, the gray program button, and the
green run button. Point out that when a program (1-5) is selected, a number is displayed in the display
window, indicating which program is running. In addition, an icon of a running person helps you tell
whether or not a program is running.
Now run the Roundtrip program and have students describe what M. Pathfinder is doing. Point out that this
time you haven’t switched wiring. Then move to the TurnTurn program and run it.
Emphasize that M. Pathfinder is displaying different behavior—moving forward, backward, turning one way
and then the other way—but you haven’t changed the connector wires. The motors are turned on or off
by commands that are coming from a program.
Finally, attach the RCX to the base, using short connector wires, and let it run on its own. Tell students that
they will build M. Pathfinder and program it in the next section (It may or may not be in the same lesson,
depending on the time allocated for the activity.)
Part 2: Building and Programming M. Pathfinder
Small Group Activity
Building M. Pathfinder: Have students work in small groups. Give each group a copy of M. Pathfinder
building instructions and each student a copy of the activity sheet. As they build, talk to students in their
groups, showing them the different buttons on the RCX and encouraging them to look up their function in
the Technical Guide or to search the Help menu for RCX buttons and RCX display window. Make them
aware of the running indicator on the display window and of how to move from one program to another.
When they have finished building, if at all possible, allow students to run their vehicles in a hallway.
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B-5
Beyond the Basics
How can we explain. . .
It sometimes happens that M. Pathfinder runs backwards or spins in place
when using the program in slot #1(even though the program instruction is to
turn on both motors). Just as in the Wall Hugging Mouse, the answer lies in
wiring rather than in the program. If this happens, remind students of what
happened when they switched the wire on the Wall Hugging Mouse and
have them reposition the wires either on the RCX or to the motors. To
reposition the wires students will have to rotate the bricks attached to the
connector wires 90º or 180º.
About running a program
There are two ways to run a program—by pressing the Run (green) button on
the RCX or by clicking the Run button on the screen.
Pressing the Run button on the RCX runs the selected program. Pressing it
once starts the program as indicated by the running person icon in the
display window. Pressing the Run button again, stops the running program.
Clicking the Run button on the screen starts the selected program. The RCX
must be within communication range with the IR tower. This allows you to
track the running program and to get information about the state of sensors
and motors. (See the RCX Sensor Link Wizard section in the Help menu.)
Students are now ready to program their RCX. Decide whether to have students follow the worksheet and
refer to the Technical Guide or the Help menu on their own, or whether to introduce the IR (Infrared) tower
and the download procedure to the class as a whole.
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3-4
Show the class (or small groups) the IR (infrared) tower. Demonstrate how to use it to download a program
onto the RCX. Point out the part of the RCX that must face the IR tower. Explain that the IR tower and the
RCX must be within communication distance, just over a hand-width apart. Programs transmitted from the
computer to the RCX via IR can then be run by the RCX.
Have students work on their own to solve the problem of making M. Pathfinder do things on its own—
recreating a program, downloading, and running it. As they work, talk to them in small groups about the
difference between downloading and saving (one goes to the RCX the other onto a disk).
Programming Highlights
The following section, Programming Highlights, discusses important learning points about programming that
should be explored by the students. You may choose to discuss them with the whole class or group by
group as they arise during students’ work.
Downloading a program and setting the program
slot For a program to run, it has to be downloaded to
H-9
the RCX. The RCX can hold one program in each of its
five program slots. Each time a program is
downloaded into a slot, it overwrites the old program.
Therefore, it is important to know which one is the
active program slot. Use the Program Setting menu on
the screen to check or change the program slot.
Be on the lookout Many times beginners do
not pay attention to which program slot a
program is downloaded. By default, a
program is sent to slot #5. If that is not
changed, the next program is going to land in
the same slot, erasing the older program.
What you can do: Remind students to check
the program slot before they download and to
change it if needed. With practice, this will
become a habit.
The Wizard. Each programming block has a small tab
on its right side. This is the wizard. When clicked, the
wizard opens a dialog box, allowing you to observe,
add or change information related to the block. For
example, clicking on the tab on the On For block,
opens the wizard that allows you to designate motors
to be turned on and to specify the amount of time
they should stay on.
Tab
© 2002 The LEGO Group. Used here with permission.
Click Wizard
In this dialog box, motors
A and C are set to turn
on for 3 seconds.
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How is ON AC different from On AC for 1?
The On AC block turns on the motors connected to
output Ports A and C for an unspecified length of time.
To turn them off, the block Off AC must be used.
However, if the Off block is used immediately after the
On block, the motors will turn on for a fraction of a
second hardly noticeable and then immediately turn
off. To have the motors run for a while, you must delay
the onset of the Off command. This is done by inserting
a Wait for block, which specifies the number of
seconds before the program carries out the Off
command.
The On AC for 1 block actually combines three
commands into one—On AC; Wait for 1; Off AC. That’s
why there’s no need to turn the motors off after using
this command.
Be on the lookout
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What you can do: An analogy to plain
R/W-1
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then the program executes the Beep block. Here the
observable behavior of motors turning and block
execution time are of the same length—3 seconds. A
simple way to put it: it takes “no time” to tell the motors
to turn on, but the program has to stay there for 3
seconds in order to tell motors to turn off, only then it
is free to focus its attention on the Beep block.
3-6
students what’s the difference between
person to “walk for 2 minutes.” Point out
that even though the word “stop” wasn’t
used in the second request, a person will
know to stop after 2 minutes, but will
continue walking when a time is not
specified.
Note: The off block has two modes, the
default mode Coast and an alternate
mode Brake. In coast mode, motors
come to a stop gradually. In brake mode
they come to an abrupt stop. For
demonstration purposes, Brake mode is
more useful than coast mode.
© 2002 The LEGO Group. Used here with permission.
It takes 3 seconds to execute the first block. Only
English commands might be helpful. Ask
asking a person to “walk” and asking a
Executing Blocks. Observing behavior. Consider a
simple stack of blocks containing power commands
such as On, On for, or Off. The blocks execute in a topdown fashion. When the first block finished executing,
the program drops to the next block, and so on.
Different blocks take different amounts of time to
execute. For example, the On block, executes in a
flash; the On AC for 3 block takes 3 seconds to
execute. It is very important to make a distinction
between the time it takes a block to execute and how
long the resulting behavior lasts. In the above example,
it takes a flash for the On block to execute, but the
motors turned on by the block will stay on until another
block turns them off.
Here are some examples to illustrate this point:
Because the command to turn off the
motors is hidden in the On For block,
beginners sometime do not realize that
the motors must be turned ON again if
they are to run.
It takes “no time” to execute the
On AC block because the
program tells motors to turn on
and has no further instructions in
the block. Therefore, it is
immediately free to move on to
the next block, Beep.
Note: You can find additional examples illustrating this
point in Transparency #1.
Be on the lookout
What does the command Reverse Direction do?
The Reverse Direction command is not a movement
command (It does not turn on motors). It is an
orientation command, pointing the motors to move in
the opposite direction when they are turned on.
Motors do not have to be turned on in order to reverse
direction.
If a motor is already turned on, it will run in the opposite
direction as soon as the program carries out the
reverse direction block.
If a motor is not turned on, it is going to be prepared to
move in the opposite direction, but it’s not going to
actually move until it’s turned on.
Here’s an illustration of this distinction: The programs
Needed and NotNeeded relate to whether an On AC
block is needed or is not needed for the motors to
continue to run.
Many students interpret the block Reverse
Direction as a command to start moving in the
opposite direction. Therefore, they expect a
robot to continue to move when the Reverse
Direction block is executed in a program such
as in the program Needed.
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What you can do: Tell students that the
Reverse Direction command is independent of
turning on/off motors. It is not a movement
command, but rather an orientation command
(prepare to move in the opposite direction).
Emphasize that motors will continue to do what
they are commanded to.
In the program NotNeeded, the motors
will turn in one direction for 2 seconds,
then reverse direction and continue
In the program Needed, the motors will turn in
one direction for 2 seconds, stop, and
prepare to reverse direction. They won’t
move in the opposite direction because the
motors were turned off in the On For block.
Therefore, for the motors to run, they must
be turned on again.
© 2002 The LEGO Group. Used here with permission.
turning in the opposite direction.
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How is Reverse Direction different from Set
Direction? Reverse Direction changes the direction in
which a motor is currently running to its opposite. Set
Direction sets the direction a motor should turn in
regardless of the direction it was turning prior to
carrying out the Set Direction command.
In other words, Reverse Direction is relative to a prior
state, while Set Direction is absolute and does not
depend on what was happening previously.
Be on the lookout
Here, too, some students may expect
the motors to turn on when the Set
Direction block is carried out.
In addition, some may not distinguish
between the Reverse Direction and
Set Direction commands, using them
interchangeably.
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What you can do: Have a student
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turn right then ask him or her to reverse
direction and then reverse direction
again. Ask students how it is that the
same command—reverse direction—
resulted in different behaviors, once a
left turn, and once a right turn.
For a helpful illustration, see Set Direction
and Reverse Direction in the Help menu.
Be on the lookout
Wait for! The Wait for block delays the execution of
the command block that follows it. It has no effect on
any command block above it in the stack. On the flip
side of the Wait for block, you can specify the number
of seconds the onset of the next block should be
delayed. (You can use whole numbers or decimals.)
R/W-1
Here are two examples.
on for an unlimited length of time. They’ll stay
on while the program is waiting for 3 seconds
and beeping sound #3 and then they’ll turn off.
So the wait command delayed the execution of
the rest of the stack. But things were
happening during that delay (i.e., The motors
were turning).
© 2002 The LEGO Group. Used here with permission.
In the program, Delay1, motors AC are turned
When using the Wait command,
students expect a pause in the program.
Indeed, sometimes the resulting
behavior looks like a pause—when the
previous command ended and the
execution of the command that follows
is delayed. But other times, it looks like
the program continues carrying out the
previous command without pause.
Note: It is important not to confuse the
Wait for block with the Wait Until block.
R/W-1
The Wait Until block is a Stack Decision
Maker, working with sensor input. The
Wait For block is not a decision maker
and does not depend on sensor input.
In the program, Delay2, motors A and C are turned
on for 2 seconds. Then the program pauses for 3
seconds before it carries out the beep block. While
this program is waiting the motors do not turn
because they were turned off in the first block.
If needed, have students evaluate short stacks to help them understand how
commands relate to each other. Use Transparency #1 to help you guide the discussion.
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SDM-1
Transparency # 1
Motors turn on and a beep sounds.
Motors continue to run while
beeping and continue after the beep
Motors run for 2 seconds and stop,
then beeps sounds.
ends. There is no command that tells
the motors to stop.
Motors turn on and continue to run.
While the Wait for command runs,
there is no change in behavior. Note
that there is no command that tells
the motors to stop.
Motors turn on and a beep sounds
after 2 seconds. This is an example
where the Wait For delays the beep.
Note that there is no command that
tells the motors to stop.
Motors run for 2 seconds and stop.
The program waits for 3 seconds,
which has no effect on behavior.
Note that the motors turn off in the
first block.
Motors run for 3 seconds and stop.
The program waits 2 seconds
before it beeps. This is an example
of Wait for resulting in a pause.
© 2002 The LEGO Group. Used here with permission.
Going Independent
This is a good time to have students use the newly acquired programming commands and concepts. We
recommend allowing students to come up with their own problems to solve or to work on the challenges
suggested in the student activity sheets.
It is important for all the students to write a program, instructing M. Pathfinder to go toward a wall, hit it and
go backwards (not necessarily back to the start line). This is going to help them understand the function
and advantage of using a touch sensor in the following session.
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RCXamples: Here are some sample solutions to some of the problems from the student activity sheets.
Spin (Zero radius turn)
Zig-Zag
See the
Repeat For
section 8-1 for
details.
Turn in a small circle (Fixed radius turn)
© 2002 The LEGO Group. Used here with permission.
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Here are some sample solution programs instructing M. Pathfinder to move forward toward a wall, hit the
wall and then move backward. The amount of time it has to move forward before it hits the wall has to be
determined empirically.
© 2002 The LEGO Group. Used here with permission.
After this set of activities, students will build a new robot. Be sure to have at least one
M. Pathfinder assembled for a demonstration in the “Working with Touch Sensors” activity.
After the demonstration, you can take it apart.
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