Control and evaluation of series elastic actuators with nonlinear rubber springs

Abstract

Series elastic actuators primarily use linear
springs in their drivetrains, which introduces a design tradeoff:
soft springs provide higher torque resolution at the cost of
system bandwidth, whereas stiff springs provide a fast response
but lower torque resolution. Nonlinear springs (NLSs) poten-
tially incorporate the benefits of both soft and stiff springs,
but such springs are often large. An NLS design was recently
proposed that combines a variable radius cam with a rubber
elastic element, enabling a compact spring design. However,
the rubber introduces hysteresis, which can lead to poor
torque tracking if not accounted for in the controller. To
overcome this limitation, we here propose a state observer that
captures hysteretic effects exhibited by the rubber to provide
an accurate estimate of actuator torque. We perform torque-
control experiments with this observer on an actuator testbed
and compare the performance of the NLS to both soft and stiff
linear metal springs. Experiments show that the NLS exhibits
improved output impedance compared to both linear springs,
and comparable bandwidth to the stiff linear spring up to 1.5
Hz. However, the hysteresis in the urethane rubber introduces
instability in higher-frequency conditions, suggesting that future
NLS designs can be improved by use of a different rubber as
the elastic element.