  Animals locomote robustly and agile, albeit significant sensorimotor delays
of their nervous system. The sensorimotor control of legged robots is
implemented with much higher frequencies-often in the kilohertz range-and
sensor and actuator delays in the low millisecond range. But especially at
harsh impacts with unknown touch-down timing, legged robots show unstable
controller behaviors, while animals are seemingly not impacted. Here we examine
this discrepancy and suggest a hybrid robotic leg and controller design. We
implemented a physical, parallel joint compliance dimensioned in combination
with an active, virtual leg length controller. We present an extensive set of
systematic experiments both in computer simulation and hardware. Our hybrid leg
and controller design shows previously unseen robustness, in the presence of
sensorimotor delays up to 60 ms, or control frequencies as low as 20 Hz, for a
drop landing task from 1.3 leg lengths high and with a passive compliance ratio
of 0.7. In computer simulations, we report successful drop-landings of the
hybrid compliant leg from 3.8 leg lengths (1.2 m) for a 2 kg quadruped robot
with 100 Hz control frequency and a sensorimotor delay of 35 ms. The results of
our presented hybrid leg design and control provide a further explanation for
the performance robustness of animals, and the resulting discrepancy between
animals and legged robots.
