Engineers, Everyday Heroes

Background

When Canadarm was launched in 1981 Canada became a world leader in space robotics. Canadarm is like a human arm with nerves of copper wiring, bones of graphite fibre, motors in place of muscles and a thin, protective "skin". It can move like your arm as its joints rotate: two at the shoulder, one at the elbow and three at the wrist. In space, Canadarm can lift up to 266,000 kg, the mass of a school bus, using less electricity than a tea kettle. Its brain is a computer that helps astronauts operate it from a distance.

Teacher Information

Canadarm's end effector is an elegantly simple snare: three wires on rotating cylinders that come together as rotary motion occurs.The wires form a triangle that shrinks to a single point as the parts turn.The wires grab a post mounted on a satellite (grapple fixture in space terminology) to secure it.This is how astronauts can capture the Hubble Space Telescope for servicing or assemble the many parts of the International Space Station. In this challenge students make a simple "hand" using a design patterned after Canadarm's end effector. It illustrates how basic concepts, such as properties of materials and rotary motion are used in the development of space hardware.

Glossary

End effector The "hand" mechanism at the end of a robotic arm used to grab space objects.	Pneumatic A closed system operated by the pressure of a gas (usually air).
Snare A mechanism designed to grab something.	Hydraulic A closed system operated by the pressure of a liquid (usually oil or water).
Grapple fixture A post mounted on a satellite or other space object for grabbing and moving it with a robotic arm.	Piston A cylinder moving within a tube and exerting pressure on,or receiving pressure from, a gas or liquid within the tube.

Engage

Invite students to share what they know about Canadarm, the space shuttle and the contributions of Canadian astronauts and engineers. Use Canadian websites to gather information on Canadarm (www.mdrobotics.ca) and the Mobile Servicing System (www.space.gc.ca). Show a movie or simulation demonstrating how Canadarm's hand (end effector) works. Canadian videotapes on Canadarm, Mobile Servicing System and Space Missions cost $10 each and are available from the Canadian Space Resource Centre at 1-877-449-2772.

Preparing for the Activity

Materials for each group:

two plastic or Styrofoam containers (cups, soup bowls);
string (coloured is nice), thread or fishing line;
tape (duct, fibre); and
scissors, metric ruler, sand paper.

Suggested Grouping

Two students

Making an End Effector

Nest the two containers together and cut through them along the dotted line where indicated in the diagram. Smooth the cut edges by scraping them over sandpaper that is laid on a tabletop.
Cut three pieces of string 8 cm. long (for small cups), 12 cm. long for bowls.Adjust the length depending on the container diameter.
Tape the end of the first string to the inside of the inner container near the cut end.Tape the other end of the string to the outside of the outer container but leave it a little loose.
Repeat step three two more times, placing each piece of string about one-third further (120 degrees) around the container from the first string.
While holding the inner container, rotate the outer one until the three strings cross each other. The strings will have some slack. Pull the ends of the string on the outside until they are straight and intersect exactly in the middle of the cylinder opening. Press down on the tape to secure the string.

Open position

Rotate outer cup

Continue rotating to close snares
Use the end effector to pick up an object such as a pencil. Open the end effector so that the strings are not crossing each other. Move the end effector over the end of the pencil. Rotate the outer cylinder to close the strings around the pencil and grasp it.
The strings may slide off the smooth sides of the pencil.What could be added the pencil to make certain it can be caught by the end effector?

Results

Students test their end effector and make improvements (such as shortening or lengthening the three strings or using different-sized containers). Securing the strings more tightly improves the operation and performance. Students share their ideas and write about their experiments in their journals.

Extending the Activity

Students can design and build a grapple fixture that can be mounted on other objects allowing them to be snared by the end effector. Students can mount an end effector at the end of a one metre-long pole and use controls to make it grab a grapple fixture. They can even mechanize the operation by adding a pneumatic or hydraulic piston so that it can be operated remotely (see photo below).

Building an End Effector for a Space Arm