Researchers at KAIST have published details of RAIBO2, the four-legged robot that completed a full marathon on a single battery charge nearly two years ago. The 42.195 km run took place at the Sangju Dried Persimmon Marathon, with RAIBO2 finishing in 4 hours, 19 minutes, and 52 seconds. In a paper appearing this week in Nature, the team, led by associate professor Jemin Hwangbo, explains how the robot achieved more than three times the range per charge typical of recent quadrupeds.
Most legged robots get around 20 km per charge, largely because their joints must constantly expend energy supporting the machine's weight and absorbing repeated foot-ground impacts. RAIBO2 improved on this by combining lightweight mechanical design, efficient motor-driving circuitry, and a locomotion policy tuned through reinforcement learning. The robot covered the course at an average speed of 2.64 m/s while climbing 286 m of elevation change on slopes up to 18.4 degrees, using 1,280 Wh from a 2,016 Wh battery. Its cost of transport, a measure of energy per unit weight and distance, was 0.25, beating the researchers' human benchmark of 0.37.
The efficiency record does not make the robot autonomous. The authors note that stairs or mountainous terrain could require different energy-loss models and postures, and that autonomous navigation would depend on planners that can identify energy-optimal paths from a global map. They present their loss models as a foundation for tackling those challenges, potentially extending the range of future legged robots in disaster response, industrial inspection, or other settings where prolonged battery life is critical.