Live From Mars was a precursor to Mars Team Online.
Activity 2.3: Robots from
| Teacher Background
The Pathfinder rover, Sojourner, was once called the "Microrover
Flight Experiment." It was designed to test the design and performance
of rovers, as well as to do some interesting science and imaging.
It will be the first autonomous vehicle to explore the surface of
another planet. (The former Soviet Union successfully operated robot
rovers on the Moon, which is a satellite of Earth, not a planet.)
Sojourner has a mobile mass of 11.5 kilograms. On its top is a flat
solar panel 1/4 of a square meter in size which will provide 16
watt-hours of peak power. The rover also has a primary battery that
will provide 150 watt-hours of power. The rover has a height of
280 millimeters with a ground clearance of 130 millimeters. It is
630 millimeters long and 480 millimeters wide. Its six wheels are
on a rocker-bogie suspension system that permits the rover to crawl
over small rocks. Sojourner will be able to climb a 30-degree slope
in dry sand.
Robots and robotic rovers are fascinating to most students and
provide enough material to consume many hours of class time! The
Activity suggested here uses simple items and takes just a few class
periods. For those who are bitten by the robot bug, however, there
are activities that introduce students to sophisticated devices
that more closely mimic robots used in space exploration and demonstrate
other important scientific and engineering principles. (See "Red
Rover, Red Rover," p. 56. The LFM Web Site also provides additional
resources and contacts.)
This Activity will center around wind (balloon) and rubber band-powered
rovers. They are simple, inexpensive and easy to make, but are not
as practical for teaching about motion as rovers powered by electric
motors. Small, battery-powered motors cost a few dollars and solar
cells can be added to investigate rovers powered by solar energy.
Students will construct robots from simple materials and use
them to investigate physical concepts including mass, center of
mass, torque, and friction.
Students will explain (infer) how problems they encounter in
robot construction relates to the design of planetary rovers.
Students will research, plan and construct a rover test-bed that
simulates the martian environment and the challenge faced by the
NASA engineers who built the Mars rover.
center of mass
MPF Project Educator Meredith Olson reports students have had
great success using round pizza trays and a crutch! Emphasizing
the value of learning from experiment, she also had students use
a toilet paper tube for a chassis, and push-up yogurt containers
for wheels. She writes, "We want students to recognize that ingenious
activity can be done everywhere. They do not need to wait to have
spiffy equipment to be clever in the way they solve everyday problems...'Right
answers come from making the materials perform better, not from
doing it the way a teacher may say it should be." Push the engineering
envelope and your students' imaginations!
| Materials: For each Rover Development Team:
eight 12-inch wooden or plastic dowels
two 3-inch wooden or plastic dowels
two 18-inch wooden orplastic dowels
a couple of square feet of stiff cardboard
rubber bands of different strengths and lengths
several plastic drinking straws
several bamboo skewers (from grocery store)
a piece of flexible mesh gutter guard (for house gutters)
3/8 inch plastic tubing
a pair of strong scissors
several pieces of modeling clay the size of golf balls
large rectangular sponge
large button with holes
wooden dowel about 6 inches long
| Materials: For the rover test bed (Mars landscape):
several plywood boards or very stiff pieces of
cardboard each at least 1 foot x 2 feet in size
several pieces of coarse and fine grain sand paper
several pieces of aluminum foil
a couple of piles of books
several rocks or other objects, each an inch or two
high and several inches long (to serve as obstacles)
Any other materials students can find at school or
home, suggested by them or thought of during an in-
class brainstorming session.
Ask students to demonstrate how big they think the Pathfinder rover
is. Then show them a box that is roughly the same size as the rover (height:
28 cm, length: 63 cm, and width: 48 cm; about the size of a laser printer,
but much lighter). Explain that this is the size of the rover body without
its wheels. Discuss.
In this Activity students are going to problem solve and simulate the
work of a Rover Development Team, creating and testing their own mechanical
robotic-rovers. (This Activity can be as open or closed ended as you wish.
Some educators may prefer to allow free-form experimentation, relying
on student trial and error to arrive at final designs. Consistent with
the other Activities in this and previous PTK Guides, the following offers
step-by-step instructions and hints. These can be passed on to the students
from the beginning or used to offer guidance only when they run into difficulty.)
1. Distribute the 12 dowels or plastic rods, a piece of stiff
cardboard that is 3 x 18 inches, some duct tape, and several pieces
of clay each about the size of a golf ball. (Note: commercially
available plastic building set materials may also be used if they
are sturdy.) Instruct each team to use the dowels/rods, the cardboard
and the tape to construct as sturdy a structure as possible. Have
them discuss, construct, non-destructively test, and share designs
with the class. List key design elements of the most sturdy constructions.
Caution students to try to use equal amounts of tape at each of
2. When they are finished, explain that this structure may be
thought of as the framework for an experimental robot rover (Fig.
1). Ultimately, wheels will need to be placed on the frame so it
can move, but first they need to experiment with the structure of
the frame and develop ideas about where instruments might be placed
within. Tell them that in doing this, they must keep in mind the
center of mass of the system because that will affect whether the
rover might tip over when encountering a large rock.
Center of Mass (C.M.) demonstration:
Explain that all objects have a center of mass--a point at which the
object balances. Hold up a meter stick and ask students where you would
have to put your finger to balance it. Demonstrate that their likely guess
at the 50 cm mark was correct. Next, tape a coin on one end of the stick
and repeat the question. Repeat with two coins taped to one end, each
time demonstrating the new center of mass. Next move to a 3-dimensional
object, like a ball. Hold it in different ways. Lead students to the correct
notion that the C.M. is in the center of the sphere.
Produce a second ball inside which you have inserted a fairly large
piece of modeling clay which is securely attached to interior side of
the ball. Ask students where the center of mass is. (They will likely
answer in the center). Hang this ball by a piece of string from various
points. Ask students to infer what is happening. Help them to determine
the C.M. of the second ball, and to realize that an object's C.M. is determined
by how mass is distributed within that object. Discuss why this concept
of center of mass is important to rover design.
5. Students should repeat the above experiments, this time placing the
piece of clay near the bottom of the sides but before they do, challenge
them to make hypotheses as to what effect this will have on the center
of mass and tip-over angles. Record the results, discuss and re-examine
their hypotheses. Discuss. Next, have them place the clay in the center
of the bottom of the frame, i.e., in the middle of the piece of cardboard.
Again make measurements and discuss. Ask students to conclude where they
would place the heaviest instruments within the frame to maximize the
stability of the robot when climbing over rocks or other rough terrain.
Challenge them to redesign the shape of the frame to increase the overall
stability of the rover. (Older students could calculate the volume of
the frame and design a new, more stable frame in a different shape but
with the same total volume).
| 3. Explain that the pieces of clay represent instruments to be
put in the rover. Have students experiment with attaching a piece
of clay near the top of one of the long sides of the frame. Have
them determine the new center of mass. Next, have them slowly and
carefully begin to tip the frame over so that the clay hangs over
the edge of the structure.
4. Using their protractors, have students determine at what
angle the structure becomes unstable, i.e., tips over. Record
the results. Next have students do the same by placing the same
piece of clay near the top of the short side of the frame. Repeat
the center of mass determination and the tipping experiment and
record the results. Discuss the difference. Challenge students
to draw conclusions.
6. Discuss wheels. Ask students to draw conclusions as to the best
size wheels to use on the original frame and/or their redesigned frame.
What advantage do large wheels have? Is there a limit to the size of
wheels that can be used for a particular sized frame? Why? If a total
of 4 wheels on two axles are to be used, where is the best place to
put the axles. Are two axles the best? Why, or why not? Should they
be close together or far apart? Should they be right at the front and
way in the back? Does the answer depend on the weight distribution of
the instruments? Remind them how their decisions will likely affect
the C.M. and overall stability of the rover.
7. Distribute more cardboard, scissors, dowels and straws to each
team and have them cut out and add the wheels and axles to their frames.
Once complete, have them experiment again with the C.M. and determine
the tip over angles of their wheeled rovers. What effect did the wheels
and axles have on the C.M.? Did they help or hurt the overall stability?
Have each team determine how big a rock their rovers can negotiate,
under two different conditions: (1) if the rock passes directly under
the rover and, (2) if the rock passes under one or more wheels.
Powering the Rovers
Challenge students in a class discussion or as part of individual
design projects to come up with realistic ways of propelling their
rovers over rough terrain. Blow up a balloon and let it go, or
remind students of their Activity using balloon rockets. Give
each team a long balloon and challenge them to figure out a propulsion
system that can be attached to their frames (Fig. 3).
Ask them to think about where the force of the balloon will
be directed and challenge them to apply this knowledge to where,
relative to the C.M. of the frame, they should place their balloon
for maximum stability. When complete, have each team propel their
rovers across the classroom. How could the system be improved?
Redesign and test if necessary.
Rubber Band Power:
Give each team a button, a large, strong rubber band and a dowel
about as long as the diameter of one of their rover's wheels.
Have them disassemble the rear wheels and axle and attach the
rubber band as shown in Fig. 3, p. 38 (or challenge them to figure
out how to use these materials to power their rovers).
Have students wind up their rubber bands using the dowel attached
to one of the wheels and, placing the rover on the floor, have
each team test theirs in turn. Redesign, if necessary, for improvements.
Note that the tighter the rubber band is wound, the more powerfully
and faster energy is transferred to the rear wheels. Is there
such a thing as having too much power transferred too quickly?
What happens if this occurs? Challenge students to consider and
investigate the effects of using different sized wheels, the materials
and design of the wheels themselves (see the image of Sojourner
on the LFM poster, and on the accompanying NASA publication) and
the nature of the surface on which the rover moves. Make changes
if possible including covering the rims of the wheels with coarse
rubber or thin strips from a rectangular sponge. This can lead
to an important discussion of friction and even torque among older
8. After appropriate rover redesigns, clear an area in the hall,
gym or play ground and have an "Ares Vallis 500". Award prizes
for the teams whose rovers went the farthest and/or the fastest.
Discuss with the class the differences in design which led to
the winners. Ask them if speed is necessarily a good thing for
a planetary rover, especially if it's maneuvering in unknown terrain.
9. Next, have the class design a course for the rovers to navigate.
Use appropriate pieces of stiff cardboard, books, tape, different
kinds of sand paper, loose sand and rocks. An example is shown
Have each team run their rover over the course one at a time.
Note which rovers succeeded, which failed, and why. Challenge
each team to make adjustments in their rovers (or make overall
adjustments to the course if it seems too challenging for most)
and run the trials again. Discuss all that was learned.
| Expand/Adapt/ Connect
Challenge students to take what they have learned from this
Activity and use it to design a more advanced robot rover. Tell
them that, in this hypothetical case, they might have a budget
of a few hundred dollars. Ask them what such a rover could do
that their simple rovers could not. Discuss this in light of the
fact that a planetary rover is a long distance from Earth where
two-way communication can take a long time and the terrain can
be very unfamiliar.
Students may also want to investigate and build a Bogie rover
with a separate hinged set of wheels. Such designs have advantages
in planetary investigations because they add greater capability
in helping rovers maneuver over rocks and other uneven terrain.
Have them take such a rover by hand over their course, feeling
the forces encountered as the rover confronts obstacles. Discuss
advantages of the rocker bogie design over the fixed axle designs
they built before.
Go on-line and research Sojourner's actual design in greater
Discuss how their own rocker bogie design is similar or different.
When running the rover over their Mars terrains, students might
want to add a time-delay handicap simulating the time involved
in sending messages between Earth and Mars.
Schools might want to collaborate with other schools via e-mail
and teleconferencing (CU-SeeMe), exchanging ideas and actually
directing rovers at remote locations.