DirectHop Shows Tiny Robots Can Make Big Jumps

Jumping can be a very effective way for tiny robots to get around. A robot that weighs just a gram can easily hop over obstacles that would be impossible to climb with wheels or legs. But most jumping robots at this scale rely on springs that slowly store energy before releasing it all at once. That produces impressive jumps, but it also adds mechanical complexity and makes controlling exactly how high the robot jumps difficult.

Researchers have developed a different approach with DirectHop, a 1-gram jumping robot that skips the spring entirely. Instead, a tiny DC motor directly provides the burst of power needed to launch the robot into the air. This provides a simpler mechanism that can vary its jump height and get itself upright again after landing.

DirectHop measures about 50 mm and is built around an 820 mg coreless DC motor that moves up and down relative to the robot's foot. The motor is connected to the foot through a 40-mm-travel Sarrus linkage made from carbon fiber composites, which constrains that movement to a straight line. Two pieces of lightweight fishing line are attached to the motor shaft to form a tiny spooling mechanism. For a jump, the motor rapidly winds one line around its shaft, pulling against the linkage and driving the foot downward as the rest of the robot accelerates upward. Once the foot leaves the ground, the robot continues upward under its own momentum. The second line lets the motor reverse this process after landing, pulling the motor back toward the foot.

Unlike a spring-powered jumper, the force produced during that launch can be controlled directly. Motor torque is proportional to the current flowing through its windings, and the linkage translates that torque into force against the ground. Increasing the commanded current therefore produces a stronger push and a higher jump. Testing showed a strong linear relationship between motor current and jump height, with an R² value of about 0.98. At the upper end, DirectHop reached a center-of-mass jump height of 28.6 cm. At the opposite extreme, adding small feet to tilt the robot allowed it to make forward hops of less than 5 mm.

DirectHop can come down from a jump in unpredictable orientations, so the researchers surrounded it with a carbon fiber roll cage inspired by a tortoise shell. After landing, the motor reverses and reels itself back toward the foot. Moving the motor, which accounts for more than 80% of the robot's mass, shifts the center of mass while the rapid retraction gives the robot a kick out of otherwise stable positions. Gravity then finishes rolling it upright.

That system successfully self-righted DirectHop in nine out of ten trials, with recovery from a worst-case inverted position taking an average of four seconds. The team also demonstrated four consecutive jumps, stopping only after the external power tether began interfering with the robot.

DirectHop is still tethered, but the researchers believe an autonomous version could carry its own battery and control electronics. They also estimate that solar cells could eventually supply enough energy for periodic jumps. Future versions are expected to add forward locomotion, directional steering, and onboard control, turning the tiny jumper from a laboratory prototype into a robot capable of navigating on its own.