Motion System Design

Re-inventing the wheel

Circular wheels have been moving people and machines for thousands of years. Now, two engineers are testing non-circular wheels for use in MEMS applications, robotics, micro machines, and toys. Stephen Derby of Distributed Robotics LLC and Steven Winckler of Global Composites Inc., both located in Troy, N.Y., built and are operating a model car that gets around on four square wheels.

The prototype is designed like a typical car: Four wheels connect together mechanically and turn clockwise or counterclockwise. To minimize wobbliness, the square wheels are each offset by a 22.5° angle (¼ a 90° turn). This also allows all four wheels to rotate in unison and negotiate turns. Rubber tips on the wheel vertices provide greater traction on rough surfaces.

A drive mechanism consisting of a motor-driven shaft, lateral arm, and offset weight (a mass on the arm's end) generates force to turn the wheels and move the car. The shaft extends vertically from the car's center, holding the lateral arm above the wheels. As the shaft rotates, forces created by the offset weight move from wheel to wheel. Peak forces coincide when the weight lines up with a wheel, causing one of its four sides to flatten parallel to the surface and the remaining three wheels to rotate.

Derby and Winckler say that, theoretically, forces besides gravity may be employed to rotate the wheels. One such scenario involves placing four electromagnets in each corner of the car's body and energizing them in succession. Without a drive, lateral arm, and weight, the car can potentially run faster and smoother.

To see more square and elliptical wheel cars in action, visit

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