  This paper proposes a unified robust motion controller for the position and
force control problems of compliant robot manipulators driven by Series Elastic
Actuators (SEAs). It is shown that the dynamic model of the compliant robot
includes not only matched but also mismatched disturbances that act on the
system through a different channel from the control input. To tackle this
complex robust control problem, the unified robust motion controller is
synthesised by employing a second-order Disturbance Observer (DOb), which
allows us to estimate not only disturbances but also their first and second
order derivatives, and a novel controller design approach in state space. By
using the Brunovsky canonical form transformation and the estimations of
disturbances and their first and second order derivatives, the dynamic model of
the robot is reconstructed so that a new system model that includes only
matched disturbances is obtained for compliant robots driven by SEAs. The
robust position and force controllers are simply designed by eliminating the
matched disturbances of the reconstructed system model via the conventional
DOb-based robust control method. The stability and performance of the proposed
robust motion controllers are verified by simulations.
