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Actuator With Angle-Dependent Elasticity for Biomimetic Transfemoral Prostheses

  • Authors: Pfeifer, Serge; Pagel, Anna; Riener, Robert; Vallery, Heike

Despite tremendous improvements in recent years, lower-limb prostheses are still inferior to their biological counterparts. Most powered knee joints use impedance control, but it is unknown which impedance profiles are needed to replicate physiological behavior. Recently, we have developed a method to quantify such profiles from conventional gait data. Based on this method, we derive stiffness requirements for knee prostheses, and we propose an actuation concept where physical actuator stiffness changes in function of joint angle. The idea is to express stiffness and moment requirements as functions of angle, and then to combine a series elastic actuator (SEA) with an optimized nonlinear transmission and parallel springs to reproduce the profiles. By considering the angle-dependent stiffness requirement, the upper bound for the impedance in zero-force control could be reduced by a factor of two. We realize this ANGle-dependent ELAstic Actuator (ANGELAA) in a leg, with rubber cords as series elastic elements. Hysteresis in the rubber is accounted for, and knee moment is estimated with a mean error of 0.7 Nm. The nonlinear parallel elasticity creates equilibria near 0◦ as well as 90◦ knee flexion, frequent postures in daily life. Experimental evaluation in a test setup shows force control bandwidth around 5–9 Hz, and a pilot experiment with an amputee subject shows the feasibility of the approach. While weight and power consumption are not optimized in this prototype, the incorporated mechatronic principles may pave the way for cheaper and lighter actuators in artificial legs and in other applications where stiffness requirements depend on kinematic configuration.

Posted on: October 22, 2014

Minimalist Design of a 3-Axis Passive Compliant Foot for Sprawling Posture Robots

Authors: Melo, K. ; Horvat, T. ; Ijspeert, A. J.


A sprawled posture amphibious biorobot resembling a salamander had helped us in conquering scientific questions of the locomotion of these animals and offer us with technological possibilities for applications in disaster response. However, so far the foot structure of these robots is simplified in a ball foot that occludes many interesting aspects of the rich dynamics of interaction with the ground that these animals/robots have. In this paper, we present a minimalist design of a three degree of freedom foot that uses passive mechanics, soft materials and simple fabrication techniques to achieve features observed in sprawling posture animals walking gaits. The parameters of our design can be adjusted for specific visco-elastic properties (stiffness, damping) and easily adapted to different sizes and forms. We presented the fabrication technique used to achieve several configurations of the foot structure. Stress analysis of the design helped us to verify the right selection of materials and configurations to achieve desired behaviors. We expect to use these feet for better understanding the limb-ground interaction in sprawled animals as well as improving the locomotion capabilities of biorobots in general.


  • Published in: 2019 2nd IEEE International Conference on Soft Robotics (Robosoft 2019), 788-794
  • DOI: 10.1109/ROBOSOFT.2019.8722792
  • Read paper
  • Date: 2019
Posted on: May 10, 2020