[joreilly]

I'm not sure any of the big legged robots are using direct drive actuation yet; MIT Cheetah and Cheetah mini use quasi-direct drives with a sub-10:1 gear ratio for super high proprioceptive sensing. ANYmal actually uses series-elastic actuators, since they allow the actuator to operate at a higher speed and lower output torque, thus making them more efficient than a quasi-direct drive setup found in other quadrupeds [1]. They also allow for some energy reuse, since they can release energy stored by compressing the spring. The downside is, like you said, reduced control bandwidth, since the spring element will behave optimally under a given set of conditions and, going outside that operating range, will either be more stiff or less than is ideal.

That said, for the kind of work ANYmal is currently being deployed on (site inspection mainly, I believe), they're hardly doing hyper-dynamic movements, so I think selecting SEAs over quasi-direct drive was a fairly judicious choice. Just like it makes more sense to have quasi-direct drives in a highly dynamic robot like the MIT Cheetah; it depends on the use case.

[1]: https://www.researchgate.net/figure/Comparison-of-EV-electri...