  Nonlinear stiffness SEAs (NSEAs) inspired by biological muscles offer promise
in achieving adaptable stiffness for assistive robots. While assistive robots
are often designed and compared based on torque capability and control
bandwidth, NSEAs have not been systematically designed in the frequency domain
due to their nonlinearity. The describing function, an analytical concept for
nonlinear systems, offers a means to understand their behavior in the frequency
domain. This paper introduces a frequency domain analysis of nonlinear series
elastic actuators using the describing function method. This framework aims to
equip researchers and engineers with tools for improved design and control in
assistive robotics.

Robotics