Gravity-Powered Robot Walker

Walking robots have been a favorite project of robotics labs for a while. And if the robots are two-legged -- or walk as if they were two-legged -- scientists naturally look to human movement as a guide. Why reinvent the wheel (or the foot)?

One branch of this biomimetic research into bipedal walking is in the area of non-motorized movement. These models are known as passive-dynamic walkers. Look it up on YouTube and you'll find dozens of examples. Some, like the original Tinker Toy model from Cornell University, are just amusing playthings. More recent versions have reached such a life-like state that they fall into the Uncanny Valley. But gravity-powered robot walkers don't have to be creepy.

This Gravity Powered Robot Walker is as simple and friendly as can be. It comes from my book BOTS! Robotics Engineering with Makerspace Activities for Kids, and it works great in workshops with kids ages 7 and up. All it takes is some stiff cardstock, a few basic crafts items, and a bamboo barbecue skewer.

Most kids can put together a working model in about an hour. Getting it to walk the way they want can take a little longer -- and provides an excellent opportunity to teach kids about the process of troubleshooting.

To make your robot walker, you will need:

• stiff cardstock (such as recycled advertising postcards) -- see template PDF
• scissors
• sharp pencil
• foam plate or piece of scrap corrugated cardboard
• peel-and-stick craft foam, 2-4 small pieces
• mini craft sticks about 2 ½ inches (6 ½ centimeters) long, or regular craft sticks or coffee stirrers cut to size
• glue stick
• bamboo barbecue skewer or other thin straight stick or rod, about 10 inches (25 centimeters) long
• wooden or plastic beads
• masking tape, zip ties, or rubber bands

Cut out the two legs from the cardstock, using the PDF template or following the design shown.

Lay each leg on a foam plate or piece of corrugated cardboard. Use the sharp point of the pencil to poke a hole through the circle at the top of each leg as shown. Fold up on the dotted line to make the feet.

Cut out two pieces of peel-and-stick craft foam. Attach the foam to the bottom of each foot for padding and traction.

Glue a mini-craft stick on top of each foot, right next to the leg. Position the craft stick in the same spot on each foot so the legs will be balanced. The added weight helps the legs to swing.

Slide one big bead onto the skewer and center it. This will be the "head." The bead(s) should cover about ½ inch (15 millimeters) of the skewer.

Slide one of the legs onto the skewer through the hole with the foot on the outside.The leg and the bead should be almost touching. Slide on the second leg on the same way.

Take two beads that fit a little tightly on the skewer. These will be the "shoulders." Slip them onto the skewer to hold the legs in place – making sure there is just enough space for the legs to swing back and forth easily. If the outside beads are not staying in place, wrap a rubber band, zip-tie, or little piece of masking tape around the skewer to keep them from sliding around.

Finally, stick a bead on each end of the skewer. It should be tight enough to stay on. If not, attach with tape. (You can glue the beads on, but first make sure the legs are in the center of the skewer and your mini-walker is balanced. Avoid getting any glue on the legs!)

Make a test ramp with a long flat surface that you can tilt slightly, like a large sheet of cardboard or foamcore. For added traction and to help keep the walker moving straight, put some strips of masking tape down the length of your ramp.

To test the walker, set it at the top of the ramp and gently tap one end of skewer. Because it doesn't have knees, in order to lift one foot and then the other off the ground, the walker tips side to side as it makes its way downhill.

• If the legs aren't swinging, check that the holes are big enough to avoid rubbing on the skewer. You can stretch the holes a little by pulling gently.

• If the legs are buckling or sliding out sideways, try adding a "shin guard" of craft foam to the outside of each leg. Make sure it doesn't rub against the skewer.

• If the walker veers to one side, try readjusting the beads to shift the weight a tiny bit. Also check that the beads are not sliding around loosely. If they are, a little masking tape will keep them in place.

Remember: If it walks at all, it is working! It just needs a small adjustment somewhere. Keep fiddling with it until you get it.

When it works, you will amaze your friends and family with your cardboard robot that walks by itself without any motor or batteries!