  Robotic systems are increasingly relying on distributed feedback controllers
to tackle complex and latency-prone sensing and decision problems. These
demands come at the cost of a growing computational burden and, as a result,
larger controller latencies. To maximize robustness to mechanical disturbances
and achieve high control performance, we emphasize the necessity for executing
damping feedback in close proximity to the control plant while allocating
stiffness feedback in a latency-prone centralized control process.
Additionally, series elastic actuators (SEAs) are becoming prevalent in
torque-controlled robots during recent years to achieve compliant interactions
with environments and humans. However, designing optimal impedance controllers
and characterizing impedance performance for SEAs with time delays and
filtering are still under-explored problems. The presented study addresses the
optimal controller design problem by devising a critically-damped gain design
method for a class of SEA cascaded control architectures, which is composed of
outer-impedance and inner-torque feedback loops. Via the proposed controller
design criterion, we adopt frequency-domain methods to thoroughly analyze the
effects of time delays, filtering and load inertia on SEA impedance
performance. These results are further validated through the analysis,
simulation, and experimental testing on high-performance actuators and on an
omnidirectional mobile base.
