Survey: gaits of dogs and quadruped robots

Dogs walk, trot, pace and gallop, and none has been recorded bounding or pronking. Pupper's policies all trot, at commanded speeds where a 3 kg mammal would still walk, and the gait command cannot express a walk because it fixes each foot's time on the ground at half a stride.

• Animals change gait at nearly fixed Froude numbers, Fr = v²/(g·h): nine horses of 90–720 kg switched from walk to trot near Fr 0.35. A 3.17 kg mammal is predicted to start trotting at 0.79 m/s; Pupper's commands of 0.3–0.75 m/s are Fr 0.05–0.33 at its 0.175 m hip height
• In MuJoCo and MJX rollouts the walking, multi-terrain, command and Stanford policies all trot: diagonal feet land within 0.06–0.12 of a stride of each other, each foot is down 52–60 % of the stride, and there is no flight below 0.75 m/s
• The command policy's gait input never left the trot: after 902 M steps with pace, bound and pronk commanded it still trotted, and quadrupling the gait penalty made it stand still instead. The deployed command policy stands with all four feet down when given those gaits
• Evidence that another gait helps on terrain is narrow: in Walk These Ways' simulated block test the trot and the pace lasted longest (0.88 and 0.87 of the episode, against 0.82 for pronk and bound), and Spot supports stairs only in its trot
• Position: keep the trot as the terrain gait. For broader commands, add a duty input and a walk first (offsets 0.5, 0.34, 0 with a duty near 0.65), and retry pace, bound and pronk once from a fresh start with timing scaled to Pupper's size
• Unexplained: the same policies step at 1.90–2.32 Hz in MuJoCo and at 1.47–1.86 Hz in the project's MJX evaluation rollouts