  An important performance metric for series-elastic actuators is the range of
impedance which they can safely render. Advanced torque control, using
techniques such as the disturbance observer, improve torque tracking bandwidth
and accuracy, but their impact on safe impedance range is not established.
However, to define a safe impedance range requires a practical coupled
stability condition. Here, passivity-based conditions are proposed for two
variants of DOB torque control, and validated experimentally in a
high-stiffness environment. While high-gain PD torque control has been shown to
reduce Z-width, it is here shown that a DOB reduces the need for high-gain PD
feedback and allows a higher rendered impedance. A dynamic feedforward
compensator is proposed which increases the maximum safe impedance of the DOB,
validated in experimentally in collision with high-stiffness environments and
manual excitation.
