The Fascination of Technology Mini-Robot Hops Over Stair Steps like a Frog

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In our “Fascination with Technology” section, we showcase impressive research and development projects for design engineers every week. Today: a robot weighing just one gram that hops up and down stair steps with centimeter-level precision thanks to a bionic direct-drive system that requires no springs.

When it comes to exploring hard-to-reach environments, many people look to the sky at flying drones. But flying is energy-intensive. In the world of insects and amphibians, hopping is a far more popular—and nearly two orders of magnitude more energy-efficient—mode of locomotion. While a mosquito must constantly expend energy while flying, a flea uses energy only at the moment of takeoff.For developers of microrobots, the flea has therefore long served as the biological model. The principle: A spring mechanism is tensioned across parts of the exoskeleton (or robot housing) and then suddenly released. However, this design has significant drawbacks on the microscale. 

The Problem with the "Flea" Mechanism

While robots that rely on springs and pawls can cover considerable distances, controlling the jump distance is extremely difficult. “Uncoiling a spring is an all-or-nothing action,” explains Sawyer Fuller, associate professor of mechanical engineering at the University of Washington (UW). Furthermore, the necessary mechanical components—such as springs and latches—are highly complex, prone to failure, and difficult to manufacture on a miniature scale. The solution developed by the research team led by Fuller and Ph.D. student Hanquan Wang for their new robot “DirectHop” is therefore to eliminate the spring entirely from the design.

Direct Drive: From a Flea to a Frog

Instead, the engineers drew inspiration from a frog’s leg muscles, which directly power a jump. “We discovered that a very small electric motor can accelerate quickly enough to execute the jump directly,” said Wang. The major advantage of this direct actuation system is that the jump height can be calibrated with centimeter precision simply by adjusting the current supplied to the motor.

The kinematics of the robot, which weighs only about 1 gram, are as simple as they are effective: The tiny electric motor is attached to a tower-like structure via a piece of sturdy fishing line. When the motor accelerates, it winds up the line in a fraction of a second and pulls itself up the tower. This catapults the entire robot into the air. As the motor ascends, three folding legs extend to stabilize the system. At full power, DirectHop can easily clear a standard stair step.

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Shift in Focus Toward Self-Correction

Another challenge with hopping microrobots is landing. Since the system inevitably starts to wobble in the air, it usually lands on its side. Here, too, bionics—specifically, the box turtle—proved helpful. The engineers designed a roll cage that protects the robot in the event of a side impact. When the DirectHop is on the ground, the motor on the tower structure moves back down. Through this targeted shift in the center of gravity, the robot automatically rolls back onto its feet in 90 percent of cases—and is immediately ready for the next jump.

Affordable Swarm Robotics for Industry

DirectHop is currently still in the prototype stage and is powered and controlled via a cable. However, the researchers are already working on the next generation: Integrated solar cells, microbatteries, vibration motors for orientation, and a miniaturized camera are expected to enable fully autonomous missions in the future, such as climbing stairs on its own. Since the components used are extremely small and inexpensive, the designers estimate production costs of about $10 per unit. In the future, swarms of hundreds of such semi-disposable robots could be used for inspections—for example, to detect gas leaks in refineries or to monitor agricultural land. If a single robot fails, the rest of the swarm simply takes over the task.