Robot dog RAIBO2 ran a hilly marathon on one charge after engineers cut its energy losses

RAIBO2 had already covered a marathon distance before Sangju ^[7]. On a flat athletic field, following a GPS-guided route, it ran 43 km in 4 hours 40 minutes on one charge ^[7]. In an earlier race over uneven ground against other competitors, though, its battery ran out around the 37 km mark ^[8]. The developers blamed that partly on frequent pace changes as the robot adjusted to the runners near it ^[8].

For field work, that failure is the most informative result in the record. The robot had shown it could cover the distance, so the developers' explanation points to how a crowd made it move. The researchers spell out why legs pay for that. "Unlike wheeled robots, quadrupeds continuously expend energy at their joints to support body weight and incur kinetic energy losses during intermittent foot-ground contact," they wrote in the Nature paper, published Sept. 23 ^[6][3].

The team then enlarged the battery by 33% ^[9], to a stated 2,016 watt-hours ^[15], so the earlier pack held roughly 1,500 Wh ^[19]. Scaling the 37 km failure by battery alone gives about 49 km ^[20]. The researchers now estimate a maximum of 67 km per charge, 25 km beyond the marathon ^[11]. That estimate is for straight paths, so the gap between 49 and 67 km blends the control and hardware changes with an easier course. The team credits lightweight transmission parts, a low-loss motor-driving circuit and a locomotion policy tuned to limit energy use, trained in its RaiSim simulator on slopes, stairs and icy roads ^[13].

Dividing the pack by that straight-path range gives roughly 30 Wh per kilometre ^[21]. According to the researchers, the range is more than three times what existing quadrupeds manage per charge ^[15], and the robot's cost of transport sits about a third below the human benchmark ^[22]. On descents it recovers some energy through regeneration. That offsets part of the climbing cost, not all of it ^[14].

At Sangju the average pace was about 9.8 km/h ^[17], against about 9.2 km/h on the flat field ^[18]. The report does not say whether the flat run used the smaller pack, what RAIBO2 carried, or how it was steered during the race.

Legged robots can already cross stairs and rugged trails that stop wheeled machines ^[16]. Their limit has been endurance, because powering several joints to lift and stabilise a body uses a great deal of energy ^[16]. On a road course, this work pushes that limit past a full marathon with range left over ^[11]. I think it supports a narrower conclusion than that the battery is now the constraint. In the one failure on record, the battery ran out partly because of how the robot had to move, and the remedy changed the movement as well as the pack ^[8][10]. I'd expect inspection and patrol jobs to look more like that crowded race than like a steady road marathon.