  This paper deals with the robust force and position control problems of
Series Elastic Actuators. It is shown that a Series Elastic Actuator's force
control problem can be described by a second-order dynamic model which suffers
from only matched disturbances. However, the position control dynamics of a
Series Elastic Actuator is of fourth-order and includes matched and mismatched
disturbances. In other words, a Series Elastic Actuator's position control is
more complicated than its force control, particularly when disturbances are
considered. A novel robust motion controller is proposed for Series Elastic
Actuators by using Disturbance Observer and Sliding Mode Control. When the
proposed robust motion controller is implemented, a Series Elastic Actuator can
precisely track desired trajectories and safely contact with an unknown and
dynamic environment. The proposed motion controller does not require precise
dynamic models of the actuator and environment. Therefore, it can be applied to
many different advanced robotic systems such as compliant humanoids and
exoskeletons. The validity of the motion controller is experimentally verified.
