Compact nonlinear springs with user defined torque-deflection profiles for series elastic actuators

Abstract

Series elastic actuators often use linear metal
springs in their drivetrains, which requires design compromises
between torque resolution and actuation bandwidth. Nonlinear
springs (NLSs), with variable stiffness, overcome this limitation,
enabling both high torque resolution and high bandwidth.
Current NLS designs combine variable cam structures with
off-the-shelf linear springs, which increases the overall size of
these torque transmitting elements. NLS size could be reduced
by using other materials as an elastic element. We present an
optimization-based synthesis method for NLSs that are compact
and encode a user-defined torque-deflection profile using elastic
elements with an arbitrary stiffness profile. We experimentally
validate the proposed method by creating a NLS prototype
and testing it on an actuator testbed. The prototype uses
rubber as the elastic element, resulting in a compact design
that generates the desired torque profile, although hysteresis of
the rubber material partially compromises performance. The
results suggest that the proposed method successfully generates
compact NLS designs, but that rubber elements need to be
carefully chosen to mitigate hysteresis.