March 8, 2013
MIT Robotic Cheetah Rivals Real Animals In Energy Efficiency
Lee Rannals for redOrbit.com - Your Universe Online
Thanks to a group of researchers, one high-tech robotic cheetah is proving itself to be dominant above its flesh-and-blood counterparts in terms of running efficiency.A team of MIT mechanical engineers, led by Sangbae Kim, has developed a 70-pound robotic cheetah that wastes very little energy as it runs for an hour and a half at 5-miles-per-hour.
Design principles to minimize energy waste led to the team developing a robot with lightweight electric motors that produce high torque with very little energy wasted in the form of heat. Kim explained part of the challenge in powering running machines with electric motors like those used in these types of robots need a flexible response upon impact, high power, torque and efficiency.
According to the team, most wasted energy comes from heat given off by the motor, energy dissipated through mechanical transmission, and inefficient control. The group proposed a high-torque-density motor to help produce a significant amount of torque at a given weight and heat production. After analyzing the relationship between motor size and torque, they designed custom motors that exceed the torque performance of commercially available electric motors.
Some researchers use springs and dampers in series with motors to help protect the robot from impacts during locomotion. However, they have found it is difficult to control a spring's stiffness and damping ratio.
“With our system, we can make our robotic leg behave like a spring or damper without having physical springs, dampers or force sensors,” Kim says.
Another source of energy loss comes from the force of impact as a robot's legs hit the ground, which can jar the machine and cause damage. Engineers must use dampers to minimize shaking and to stabilize these systems.
“The majority of impact energy goes back to the battery because the damping is created by custom-designed electric control of the motor,” Kim says. “[The motor] regenerates energy that would have been lost.”
Kim says mount motors and gears at the hip joint help to reduce energy loss by minimizing leg inertia. With the team's design, 85 percent of the weight of the leg is concentrated at the hip joint, thus keeping the rest of the leg relatively lightweight.
The team attached strips of Kevlar to connect sections of the robot's legs, simulating the structure of tendons along a bone. This technique helps to strengthen the leg with little additional weight, further reducing the leg's inertia. They also used polyurethane rubber to construct a flexible spine. Kim believes when the spine and rear legs move together, it helps to store elastic energy while galloping.
To test the robot cheetah, they put the 70-pound machine on a treadmill at a steady 5-miles-per-hour pace. They then measured the voltage and current of the battery, as well as that from each motor, then calculated the robot's efficiency of locomotion.
The engineers found their robot cheetah was more efficient than other robotic competitors like Boston Dynamic's BigDog and Honda's two-legged robot, ASIMO. They also found the cheetah robot falls into roughly the same energy-efficiency range as humans, cheetahs and hunting dogs.
The researchers are now working on assembling a set of new motors to help improve the cheetah's galloping speeds to up to 35-miles-per-hour.
“The cheetah robot has really pushed the technology in efficient motor design, low-loss transmissions, and low-inertia legs,” says Ron Fearing, a professor of electrical engineering and computer science at the University of California at Berkeley, who was not involved with the MIT project.
“By combining these with the regenerative motor drive system, so that mechanical energy from the leg can recharge the battery, that in my opinion has made a huge difference in efficiency, [and] an important step forward in making efficient, electrically driven running robots.”