Robots Can Save Energy with Hopping

Jumping is an important means of locomotion for animals, and this form of movement could be important to future generations of robots too.  Jumping has been extensively studied in biological organisms which use stretched tendons to store energy.

In research that could extend the range of future rescue and exploration robots, scientists have found that hopping robots could significantly reduce the amount of energy they use by adopting a unique two-part "stutter jump."

The study shows how a short hop before a big jump could allow spring-based "pogo stick" robots to reduce their power consumption tenfold.

After analyzing nearly 20,000 jumps made by a simple laboratory robot under a wide range of conditions,  the researchers devised a formula for the two-part jump.

"If we time things right, the robot can jump with a tenth of the power required to jump to the same height under other conditions," said Daniel Goldman, an assistant professor, School of Physics, Georgia Institute of Technology. "In the stutter jumps, we can move the mass at a lower frequency to get off the ground. We achieve the same takeoff velocity as a conventional jump, but it is developed over a longer period of time with much less power."

The Georgia Tech research into robot jumping began with a goal of learning how hopping robots would interact with complicated surfaces be it sand, granular material  or debris from disaster. It was then Goldman realized that he'd need to know more about the physics of jumping to separate the surface issues from the factors controlled by the dynamics of jumping.

Goldman asked Jeffrey Aguilar, a graduate student at the George W. Woodruff School of Mechanical Engineering, to construct the simplest jumping robot.

Aguilar constructed a one-kilogram robot composed of a spring beneath a mass capable of moving up and down on a thrust rod.  With the help of computer controls he tried to vary the starting position of the mass on the rod, the amplitude of the motion, the pattern of movement and the frequency of movement applied by an actuator built into the robot's mass. A high-speed camera and a contact sensor measured and recorded the height of each jump.

"The preparatory hop allows the robot to time things such that it can use a lower energy to get to the same jump height," Goldman explained. "You really don't have to move the mass rapidly to get a good jump."

The amount of energy that can be stored in batteries can limit the range and duration of robotic missions, so the stutter jump could be helpful for small robots that have limited power. 

"If you're a small robot and you want to jump over an obstacle, you could save energy by using the stutter jump even though that would take longer," said Goldman. "But if a hazard is threatening, you may need to expend the additional energy to make a quick jump to get out of the way."

The simple jumping robot turned out to be a useful system to study, not only because of the interesting behaviors that turned up, but also because the results were contradictory to what the researchers had expected. 

"In physics, we often study the steady-state solution," Goldman noted. "If we wait enough time for the transient phenomena to die off, then we can study what's left. It turns out that in this system, we really care about the transients."

The research was reported October 26 in the journal Physical Review Letters.