Vrije Universiteit Brussel

Series-Parallel Elastic Actuation (SPEA)


For applications with many degrees of freedom, lightweight and compact designs with high torque requirements (e.g. exoskeletons, humanoids, agile locomotion, prostheses, manipulators) the current actuation technology is insufficient. Variable Impedance Actuators (VIAs) are being developed to bring bio-inspiration into mechanical systems to extend the possibilities of traditional stiff actuation and improve the requirements regarding safety and energy efficiency. Most of the developed actuators consist or can be modeled as a series arrangement of a motor controlling the equilibrium and a spring of which the stiffness can be mechanically controlled. Disadvantage of this series arrangement is that all the torque generated by the joint, also needs to be generated by the (electric) motor, as shown in Fig.1. Parallel springs are already deployed to reduce the torque requirements of the motor, but in these designs the parallel stiffness is fixed in the design. In this paper a novel actuator is presented containing series and parallel elastic elements where the engagement of the parallel springs can be actively controlled. Therefore we call this novel category of actuators Series-Parallel Elastic Actuators (SPEA). Main inspiration source here is a biological muscle that consists of a large set of parallel and series muscle fibers. The difference is that not all parallel springs in the SPEA need to have their own force converter (mostly electrical motor), but are tensioned by an intermittent mechanism actuated by a single motor.


The Series-Parallel Elastic Actuator (SPEA) consists of a bundle of parallel compliant elements, for which every compliant element can be contracted one after the other. A single spring can be in three phases as shown in the lower part of figure 1.

  1. In unpretensioned phase the spring is at its rest length connected to both sides of the link.
  2. In pretensioned phase the spring is extended with its sides connected to both links. All forces that are exerted will not pass the motor because it is not in the force path present.
  3. In the pretension phase the motor controls the length of the spring and brings it from unpretensioned phase to pretensioned phase or back.
The forces that are exerted on the spring in unpretension phase will go through the motor. Since most of the springs are in unpretensioned or pretensioned phase and only one or a few are in pretension phase, only a portion of the total torque exerted on the link will be felt by the motor as can be seen in figure underneath.

The problem is then brought back to developing an intermittent mechanism that with a single motor can achieve the springs to be in one of the three phases. This novel idea drastically reduces the torque requirements of the motor and increases the energy efficiency since it features variable load cancellation and selective spring recruitment and locking.



C: articles in scientific journals with an international referee system

  1. Concept of a Series-Parallel Elastic Actuator for a Powered Transtibial Prosthesis
    Mathijssen Glenn, Cherelle Pierre, Lefeber Dirk, Vanderborght Bram
    MDPI Actuators, vol.2, n. 3, pp.59 - 73, 2013

I1: communications at international congresses / symposia integrally published in proceedings

  1. Series-Parallel Elastic Actuation (SPEA) with intermittent mechanism for reduced motor torque and increased efficiency
    Mathijssen Glenn, Brackx Branko, Van Damme Michael, Lefeber Dirk, Vanderborght Bram
    from Proceedigs International Conference on Intelligent Robots and Systems November 2013 (IROS 2013), pp.5841 - 5846, 2013.

I2: communications at international congresses / symposia not published or only available as an abstract

  1. Novel Design of a Series-Parallel Elastic Actuator (SPEA)
    Mathijssen Glenn, Brackx Branko, Van Damme Michael, Van Ham Ronald, Lefeber Dirk, Vanderborght Bram
    from ICRA 2013 Workshop: Human Robot Interaction (HRI) for Assistance and Industrial Robots. Scientific Knowledge, Standards and Regulatory Framework. How do I design for the real world?, 2013
  2. Series-Parallel Elastic Actuation (SPEA) for Reduced Torque Requirements
    Mathijssen Glenn, Brackx Branko, Van Damme Michael, Van Ham Ronald, Lefeber Dirk, Vanderborght Bram
    HFR2012 ? Brussels: 5th International Workshop on Human-Friendly Robotics, 2012

More info

More info about this actuator and its applications can be obtained from Glenn Mathijssen (email), RaphaŽl Furnemont (email) and Bram Vanderborght (email). Up

©2012 • Vrije Universiteit Brussel • Dept. MECH • Pleinlaan 2 • 1050 Elsene
• Tel.: +32-2-629.28.06 • Fax: +32-2-629.28.65 • webmaster