“Cheetah” Robot’s Electric Motors Provide a Streamlined Stride

A 70-pound “cheetah” robot designed by MIT researchers may soon outpace its animal counterparts in running efficiency: In treadmill tests, the researchers have found that the robot — about the size and weight of an actual cheetah — wastes very little energy as it trots continuously for up to an hour and a half at 5 mph. The key to the robot’s streamlined stride: lightweight electric motors, set into its shoulders, that produce high torque with very little heat wasted.

The motors can be programmed to quickly adjust the robot’s leg stiffness and damping ratio — or cushioning — in response to outside forces such as a push, or a change in terrain. Part of the challenge in powering running machines with electric motors is that such robots require a flexible response upon impact, and high power, torque and efficiency — characteristics that have historically been difficult to achieve with electric motors.

To combat heat loss from motors, the group proposed a high-torque-density motor — a motor that produces a significant amount of torque at a given weight and heat production. The team analyzed the relationship between motor size and torque, and designed custom motors that exceed the torque performance of commercially available electric motors. The team found that such high-torque motors require fewer gears — a characteristic that would improve efficiency even more, as there would be less machinery through which energy could dissipate.

In addition to heat given off by a motor, the group found that another major source of energy loss comes from the force of impact as a robot’s leg hits the ground. The cheetah-bot’s electric motors capture this energy, feeding it back to the system to further power the robot. Mounting motors and gears at the hip joint would also reduce energy loss by minimizing leg inertia.

The researchers also attached strips of Kevlar to connect sections of the robot’s legs, simulating the structure of tendons along a bone. The Kevlar strengthens the leg with little additional weight, and further reduces the leg’s inertia. The group also constructed a flexible spine out of rings of polyurethane rubber, sandwiched between vertebra-like segments. The spine moves along with the rear legs, and can store elastic energy while galloping.