Introduction: Steerable Vibrating Tensegrity Robot
A tensegrity structure is made out of stretchy cords and stiff struts. It can flex and compress when dropped or squeezed, and then spring back into shape. It also has a high degree of compliance, which means it won’t harm people or equipment around it. That, together with its resilience, makes a tensegrity a useful framework for robots that need to withstand jolts or twist and turn themselves through irregular spaces.
This ultra-simple robot is based on work by professor John Rieffel and students at Union College in Schenectady, NY. They create tensegrity robots that have programmable bodies. With the right design, you can steer them with just a vibrating motor. The robots turn left or right depending on the frequency of the vibrations.
The body of this version is made out of drinking straws and rubber bands. It's based on a tensegrity icosahedron holiday ornament project by Bre Pettis for the Make magazine website in 2007.
Once you’ve built your tensegrity structure, you can quickly put together a circuit to make your robot move using:
- a tiny vibrating motor
- a dimmer switch to make it run faster or slower, and
- a bar graph indicator that shows how much power you’re supplying to the motor.
When you attach the motor to the tensegrity, the structure will shake and move across the table.
I built my circuit quickly and easily using littleBits, electronic modules that snap together magnetically. If you have all the parts, you can build your own Tensegrity Robot in less than an hour. When you’re done, you can reuse the littleBits in other projects.
This project originally appeared in my book Making Simple Robots: Exploring Cutting-Edge Robotics with Everyday Stuff. You can find more great robotics projects for kids and beginners in my latest book, BOTS!, from Nomad Press.
- 6 drinking straws (TIP: Keep some spare straws on hand while you’re working. If a straw bends, you’re better off replacing it than trying to fix it.)
- 6 long rubber bands, roughly 5 inches long
- 6 short rubber bands -- an inch or two long
- removable adhesive, such as glue dots
- littleBits modules:
- wire, and
- vibration motor
Step 1: Cut Straws
Cut 6 pieces of straw to no more than about 5" long.
On each straw, cut a slit on either end, making sure that the slits are aligned (i.e., both vertical). The slits should be ¼" deep — enough to hold the rubber band in place, but not so much that the straw begins to weaken and bend.
Step 2: Connect Straws
Line up 2 straws and wrap a small rubber band loosely around each end of the pair.
Do the same to a second pair of straws and slide them perpendicularly between the first 2 straws to form an "X" shape.
Step 3: Add More Straws
Take the last 2 straws and wrap a small rubber band around one end.
Slide them through the intersection of the other straws so that they’re perpendicular to the first 2 pairs, and then wrap a small rubber band around the other end.
Step 4: Stretch Rubber Bands
Twist one pair of straws so their slits are horizontal and facing you, with one above the other. Stretch a long rubber band from the horizontal slit of the upper straw, up and over a pair of perpendicular straws, and to the other end of the straw, passing it through all four slits.
Repeat with the remaining straws.
Adjust the rubber bands so they’re even.
Step 5: Cut Rubber Bands and Adjust
Cut away the small rubber bands so that the tensegrity structure springs open.
Adjust the pairs of straws so they’re parallel and not touching.
Step 6: Assemble the LittleBits
Now assemble the littleBits electronic circuit that will make your tensegrity bot go:
- Plug the power module (or “Bit”) into the battery.
- Attach the dimmer switch module for turning the voltage up or down.
- Connect the bar graph module to the dimmer switch. This is a Bit with five rows of miniature LEDs; as more power goes through it, more LEDs light up.
- Attach one or more wires. The wire modules are short, so use 2 or 3 to make sure your robot has room to move.
- Finally, add the vibrating motor. This is a small disc, about the size of a pill, with 2 thin wires attaching it to a magnetic base.
Step 7: Make It Move
To try out your tensegrity robot, attach your electronic circuit to your straw model. Situate the vibrating motor so that none of the electronics get in the way of the tensegrity structure’s motion. Decide where you’d like to attach the disc end of your motor. Use tape or another adhesive to hold it onto one of the straws. Stretch the motor wire along the straw and attach the motor base and wire base to it. Turn on the motor and slowly increase the power with the dimmer switch. You’ll start to see the rubber bands vibrate in sympathy, and your tensegrity robot should start to shimmy along the table. See if you can steer it to the right and left by adjusting the power. If your robot doesn’t move, try attaching the motor higher or lower on the structure. Moving the robot’s center of gravity a little off-center can help overcome its inertia. Now that the robot works, experiment with placing the motor in different locations on the tensegrity structure — in the center, off on one corner — to see which position produces the most reliable and interesting movements. Varying the speed and placement of the motor will produce different kinds of motion, giving the robot a kind of physical intelligence. GOING FURTHER While this simple tensegrity robot moves through vibration, advanced tensegrity robots move by contracting their cables and changing shape so they can roll. For an even greater challenge, think about how you could design your robot to do the same. Or break out of the prototyping stage and build a new version of this circuit without using littleBits. Starting here, you’ll be well on your way to making your own advanced tensegrity robots.
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