MIT scientists have developed a cheetah robot that can run and jump, untethered, across grass - and the approach it uses is similar to the way world-class sprinters, such as Usain Bolt, race.
The cheetah is the fastest land animal on Earth, able to accelerate to 96 kph in just a few seconds. As it ramps up to top speed, a cheetah pumps its legs in tandem, bounding until it reaches a full gallop.
Now Massachusetts Institute of Technology researchers have developed an algorithm for bounding that they have successfully implemented in a robotic cheetah - a four-legged assemblage of gears, batteries, and electric motors that weighs about as much as its feline counterpart.
The team took the robot for a test run on MIT's Killian Court, where it bounded across the grass at a steady clip.
In experiments on an indoor track, the robot sprinted up to 16 kph, even continuing to run after clearing a hurdle. The researchers estimate that the current version of the robot may eventually reach speeds of up to 48 kph.
The key to the bounding algorithm is in programming each of the robot's legs to exert a certain amount of force in the split second during which it hits the ground, in order to maintain a given speed: In general, the faster the desired speed, the more force must be applied to propel the robot forward.
Sangbae Kim, an associate professor of mechanical engineering at MIT, hypothesises that this force-control approach to robotic running is similar, in principle, to the way world-class sprinters race.
"Many sprinters, like Usain Bolt, don't cycle their legs really fast. They actually increase their stride length by pushing downward harder and increasing their ground force, so they can fly more while keeping the same frequency," Kim said.
Kim said that by adapting a force-based approach, the cheetah-bot is able to handle rougher terrain, such as bounding across a grassy field.
In treadmill experiments, the team found that the robot handled slight bumps in its path, maintaining its speed even as it ran over a foam obstacle.
As an animal bounds, its legs touch the ground for a fraction of a second before cycling through the air again.
The percentage of time a leg spends on the ground rather than in the air is referred to in biomechanics as a "duty cycle"; the faster an animal runs, the shorter its duty cycle.
Kim and his colleagues developed an algorithm that determines the amount of force a leg should exert in the short period of each cycle that it spends on the ground.
That force, they reasoned, should be enough for the robot to push up against the downward force of gravity, in order to maintain forward momentum.
In experiments, the team ran the robot at progressively smaller duty cycles, finding that, following the algorithm's force prescriptions, the robot was able to run at higher speeds without falling.
Kim said the team's algorithm enables precise control over the forces a robot can exert while running.
