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LabsHumanoids & Human Centred MechatronicsProjects ■ Variable Stiffness Actuators

Variable Stiffness Actuators (VSAs)

This research explores:

  • Design of VSAs based on
    • Adjustable lever arm mechanisms (AwAS-I, AwAS-II, CompAct-VSA)
    • Antagonistic pneumatic muscles
  • Control strategies
    • Stiffness and position control
  • Estimators
    • Mechanical stiffness (in collaboration with UNIROMA1)
    • Mechanical impedance (in collaboration with UNIPI)

Team members:



vsa a
Fig.1 - AwAS-I: conceptual schematics

The Actuator with Adjustable Stiffness (AwAS-I) is based on a compliant lever which rotates around a fixed pivot point. Its compliance is due to two springs attached antagonistically on the lever at a variable distance (lever arm) as shown in Fig.1. The stiffness adjustment is done through moving the springs along the lever and changing the lever arm. The longer the lever arm is the stiffer the mechanism.

Motor M1 (position motor) is connected to the intermediate link. Motor M2 (stiffness motor) is assembled on the intermediate link and is attached to the ballscrew which converts rotary motion of M2 into linear motion of the ballscrew’s nut. Springs are connected between the ball screw and output link and move together with the ballscrew’s nut.

vsa b vsa c
Fig.2 - AwAS-I: CAD cross section view Fig.3 - AwAS-I: fully assembled prototype

AwAS-II   [Video]

AwAS-I: conceptual schematic
Fig.4 - AwAS-II: conceptual schematics

The Actuator with Adjustable Stiffness (AwAS-II) is based on a compliant lever. However in this version springs are kept fixed but the pivot point is variable. The stiffness of the lever depends on the ratio L1/L2 and can be adjusted from zero (pivot reaches springs position) to completely rigid.

The lever is connected to the output link through a rotational joint. An intermediate link is connected to motor (M1) for the link positioning. A motor (M2) is assembled on the intermediate link and it adjusts the stiffness by changing the position of the pivot through rotation of the ballscrew. Two torsion springs are placed between the lever and the output link.

vsa b2 AwAS-II: fully assembled prototype
Fig.5 - AwAS-II: CAD cross section view Fig.6 - AwAS-II: fully assembled prototype


The CompAct Variable Stiffness Actuator is based on a compliant lever as in Fig. 4. However this version makes use of a cam shaped lever arm with a variable pivot axis actuated by a rack and pinion transmission system. The proposed concept permits the realization of an actuation unit with a wide range of stiffness and fast regulation. This realization results in a highly integrated and modular assembly.

Compact CAD AwAS-II: fully assembled prototype
Fig.7 - Variable stiffness module. A) Joint connecting the link and the cam. B) Joint Axis. C) Cam shaped lever arm. P) Pivot. E) Cam roller. F) Rack and pinion transmission. G) Motor. H) Springs Fig.8 - AwAS-II: fully assembled prototype

Selected publications:


  • A. Jafari, N. G. Tsagarakis, D. G. Caldwell: “A Novel Intrinsically Energy Efficient Development of a Novel Actuator with Adjustable Stiffness (AwAS)”, IEEE Transactions on Mechatronics, Vol. 18, No. 1, 2013.



  • N. G. Tsagarakis, I. Sardellitti, D. G. Caldwell: A new variable stiffness actuator (CompAct-VSA): Design and Modelling, IROS 2011.
  • F. Flacco, A. De Luca, I. Sardellitti, N. G. Tsagarakis: “Robust estimation of Variable Stiffness in Flexible Joints”, IROS 2011.
  • A. Serio, G. Grioli, I. Sardellitti, N. G. Tsagarakis, A. Bicchi: “A decoupled impedance observer for a variable stiffness robot”, ICRA 2011.
  • M. Catalano, G. Grioli, M. Garabini, F. Bonomo, N. G. Tsagarakis, A. Bicchi, “VSA-CubeBot: a modular variable stiffness platform for multiple degrees of freedom robots”, IEEE International Conference on Robotics and Automation, (ICRA 2011),Shanghai.
  • A. Jafari, N. G. Tsagarakis, and D. G. Caldwell, “Exploiting natural dynamics for energy minimization using an actuator with adjustable stiffness (awas),” Int. Conf. on Rob. and Aut., 2011.
  • A. Jafari, N. G. Tsagarakis, and D. G. Caldwell, “AwAS-II: A New Actuator with Adjustable Stiffness Based on the Novel Principle of Adaptable Pivot Point and Variable Lever Ratio”, IEEE International Conference on Robotics and Automation , ICRA, 2011.


Last Updated on Monday, 18 February 2013 11:04


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