Making Robots More Like Us

The research was not initially focused on solving the problem of human interaction, but the scientists soon realized the implications, recalled Dr. Pratt, who is now the project manager for the Defense Advanced Research Projects Agency's Robotics Challenge, an upcoming contest that is intended to advance robotics technology to be used in natural disasters and other emergencies."It actually started with numerically controlled machine tools," he said — using computer-controlled robots to perform milling tasks.For those manufacturing uses, what mattered was the precise positioning of the robot limb. However, Dr. Pratt was focused on developing walking robots that could move in the natural world, and force was more significant than precision to meet that challenge: "There the position of the limb didn't matter so much, but what mattered was how hard was the robot pressing on the world, and how hard the world was pressing back on the robot," he said.

The solution was to put something elastic, like a spring, between the motor and the joint. These are now described as series elastic actuators, and the technique of installing them is now widely used as a low-cost solution for robots that are both nonthreatening to humans and able to move more agilely in the natural world.  "In the Darpa Robotics Challenge, almost all of the robots that are being used there have series elastic actuation or other types of compliant control," he said. "The reason is both because it makes the locomotion task easier and the manipulation task easier, and it also makes it possible for the robot to be gentle when it does things and not make things worse."

Dr. Pratt recalled an incident when the researchers first realized that series elastic actuation was the key to freeing robots from their cages. While working on an early humanoid robot named COG, in a project led by Rodney Brooks, the founder of Rethink Robotics who was then director of the M.I.T. artificial intelligence lab, they were demonstrating how the robot could do tasks like writing with a pencil and paper. However, there was a bug in the software, causing the robot's arm to repeatedly bang the table.Dr. Brooks decided it was an opportunity to demonstrate the safety of the technology. He placed himself between the table and the arm, which began spanking him.