Effect of compliance location in series elastic actuators

Jonathon W. Sensingera1a2 c1, Lawrence E. Burkarta3, Gill A. Pratta4a5 and Richard F. ff. Weira6a7

a1 Rehabilitation Institute of Chicago, Center for Bionic Medicine, Chicago, Illinois, USA

a2 Physical Medicine and Rehabilitation/Mechanical Engineering, Northwestern University, Chicago, Illinois, USA

a3 Kyocera, San Diego, California, USA

a4 Franklin W. Olin College of Engineering, Needham, Massachusetts, USA

a5 Defense Advanced Research Projects Agency, Arlington, Virginia, USA

a6 Department of Bioengineering, University of Colorado, Denver, Colorado, USA

a7 VA Eastern Colorado Health Care System, Aurora, Colorado, USA


Series elastic actuators have beneficial properties for some robot applications. Several recent implementations contain alternative placements of the compliant element to improve instrumentation design. We use a class 1 versus class 2 lever model and energy-port methods to demonstrate in this paper that these alternative placements should still be classified as series elastic actuators. We also note that the compliance of proximal series elastic actuators is reflected by an augmented gear ratio dependent on the nominal gear ratio, which is significant for small gear ratios and approaches unity for large gear ratios. This reflected compliance is shown to differ depending on the sign of the gear ratio. We demonstrate that although the reflected compliance is only marginally influenced by the magnitude of the gear ratio, there are several notable differences, particularly for small gear ratios.

(Accepted May 01 2013)

(Online publication June 07 2013)


  • Biomimetic robots;
  • Design;
  • Mechatronic systems;
  • Humanoid robots;
  • Surgical robots


c1 Corresponding author. E-mail: