Bipedal humanoid robots can step over obstacles and negotiate stairs where their wheeled counterparts cannot, but this comes with the risk of falling down. Naturally, humanoid robots will never be accepted in society if they break when they fall down. The bigger the robot, the more likely it is that it will damage itself during a fall and be unable to get up.
In 2003 the HRP-2P was the first full-scale humanoid that could fall over safely and get back up, and so far remains alone; not even Honda’s ASIMO can do this. As soon as it detected that it was falling, the HRP-2P would bend its knees and back, which helped to reduce the ground impact. This motion, called “UKEMI”, is quite similar to how the SONY QRIO would react when falling over to reduce the risk of damaging its components.
Originally AIST used a modified version of the HRP-2L to test the falling procedure, but this wasn’t ideal as it lacked an upper body. Later they tested falls on the real HRP-2P by having a karate master kick the robot.
As one can imagine, the risk of damaging such an expensive robot meant that such experiments had to be kept to a minimum. AIST’s researchers also realized that such a complex robot wasn’t necessary for doing initial falling tests, and that a simpler robot could be used as a stunt double. In 2007 they built the low cost HRP-2FX, which had the minimum number of actuators and sensors required for the experiments. Basically they cut the HRP-2P down the middle to reduce the number of actuators, but it had the same measurements, actuators, and electronics. Additionally, they kept its main computer and power source apart from its body. It was able to replicate how the real robot would fall forward and backward, and if it got damaged it would be much cheaper to replace.
They were able to reliably reduce the ground impact after implementing an optimized falling motion. However, this falling motion was not calculated in real-time, but in advance. Having the robot calculate a safe landing in real-time is a goal for the future.
Images by AIST from “An Optimal Planning of Falling Motions of a Humanoid Robot” and “The First Human-size Humanoid that can Fall Over Safely and Stand-up Again” (Humanoid Robotics Group, Intelligent systems institute, AIST).