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MIT “cheetah” robot rivals running animals in efficiency

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  • kevinr
    MIT “cheetah” robot rivals running animals in efficiency Mon, 03/11/2013 - 10:28am A 70-pound “cheetah” robot designed by MIT researchers may soon
    Message 1 of 1 , Mar 11, 2013
      MIT “cheetah” robot rivals running animals in efficiency

      Mon, 03/11/2013 - 10:28am

      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. The researchers will present the
      efficiency results and design principles for their electric motor at the
      International Conference on Robotics and Automation in May.

      Sangbae Kim, the Esther and Harold E. Edgerton Assistant Professor in
      MIT’s Department of Mechanical Engineering, says achieving
      energy-efficiency in legged robots has proven extremely difficult.
      Robots such as Boston Dynamic’s “Big Dog” carry heavy gasoline engines
      and hydraulic transmissions, while other electrically powered robots
      require large battery packs, gears, force sensors and springs to
      coordinate the joints in a robot’s leg. All this weighty machinery can
      add up to significant wasted energy, particularly when a robot’s legs
      need to make frequent contact with the ground in order to trot or gallop.

      “In order to send a robot to find people or perform emergency tasks,
      like in the Fukushima disaster, you want it to be autonomous,” Kim says.
      “If it could run for more than two hours and search a large field, that
      would be useful. But one of the reasons why people think it’s impossible
      to make an electric robot that does this is because efficiencies have
      been pretty bad.”

      Kim adds that 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 understand how an electrically powered system might waste little
      energy while running, the researchers first looked at general sources of
      energy loss in running robots. They found that most wasted energy comes
      from three sources: heat given off by a motor; energy dissipated through
      mechanical transmission, such as losses to friction through multiple
      gear trains; and inefficient control, such as energy lost through a
      heavy-footed step, as opposed to a smoother and more gentle gait.

      The group then came up with design principles to minimize such energy
      waste. 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

      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. Many researchers
      have used springs and dampers in series with motors to protect the robot
      from forceful impacts during locomotion, but it’s difficult to control a
      spring’s stiffness and damping ratio—which can be a problem if a robot
      has to traverse disparate surfaces, such as asphalt and sand.

      “With our system, we can make our robotic leg behave like a spring or
      damper without having physical springs, dampers or force sensors,” Kim

      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. Such forces can be strong enough to shake a machine
      and potentially cause damage. Engineers need to use dampers, or shock
      absorbers, to minimize shaking and stabilize such systems. But Kim says
      such dampers act to dissipate energy each time a leg meets the ground.

      In contrast, the cheetah-bot’s electric motors capture this energy,
      feeding it back to the system to further power the robot.

      “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 adds that mounting motors and gears at the hip joint would also
      reduce energy loss by minimizing leg inertia: Some legged robots are
      designed with motors and gearboxes at each joint along a leg, which can
      be cumbersome and can lose more energy at every impact. With Kim’s
      design, 85% of the weight of the leg is concentrated at the hip joint,
      keeping the rest of the leg relatively lightweight.

      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. Kim hypothesizes that the spine moves along with the rear
      legs, and can store elastic energy while galloping.

      To test the efficiency of the robot, the researchers ran it on a
      treadmill at a steady 5-mph clip. They measured the voltage and current
      of the battery, as well as that from each motor. They calculated the
      robot’s efficiency of locomotion—also known as cost of transport—and
      found that it was more efficient than robotic competitors such as Big
      Dog and Honda’s two-legged robot, ASIMO.

      After digging through the literature on animal locomotion, the
      researchers plotted the cost of transport of various running, flying and
      swimming animals. They found that, not surprisingly, fliers were more
      efficient than runners, although swimmers were the most efficient
      movers. The cheetah robot, according to Kim’s calculations, falls around
      the efficiency range of humans, cheetahs and hunting dogs.

      Currently the team is assembling a set of new motors, designed by
      Jeffrey Lang, a professor of electrical engineering at MIT. Kim expects
      that once the group outfits the robot with improved motors, the cheetah
      robot will be able to gallop at speeds of up to 35 mph, with an
      efficiency that rivals even fliers. The researchers are convinced that
      this approach can exceed biological muscle in many aspects, including
      power, torque and responsiveness.

      “There are so many ways to design, and each legged robot has a different
      system,” Kim says. “If you design the motor properly, it’s more
      powerful, simpler robotics.”

      Ron Fearing, a professor of electrical engineering and computer science
      at the University of California at Berkeley, says that simple springs
      can work well in small robots running on smooth terrain. But for
      rougher, more unpredictable terrain, he says the energy-recovery system
      of the MIT cheetah has big advantages.

      “The cheetah robot has really pushed the technology in efficient motor
      design, low-loss transmissions, and low-inertia legs,” says Fearing, who
      did not contribute to the research. “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.”

      In addition to Kim and Lang, the paper’s co-authors include Sangok Seok,
      Albert Wang, Meng Yee Chuah and David Otten, all of MIT.

      This research was funded by the Defense Advanced Research Projects
      Agency’s Maximum Mobility and Manipulation (M3) program.

      Source: Massachusetts Institute of Technology
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