irisVSA: Infinite-rotation infinite-stiffness Variable Stiffness Actuator towards physical human–robot-interaction

Mohammed A. Ramadan, Mohammad Ismail Awad, Mohamed Nassim Boushaki, Zhenwei Niu, Kinda A. Khalaf, Irfan Hussain

Research output: Contribution to journalArticlepeer-review


In Human–Robot-Collaboration applications, variable stiffness actuators provide an attractive actuation solution due to their well-established capabilities in ensuring accuracy and safety. Although, several existing VSAs have achieved an infinite-range of stiffness, those are generally relatively bulky, incapable of attaining an infinite-range of rotation, and their stiffness adjustment rate is rather slow. In this paper, we introduce a new infinite-rotation infinite-stiffness Variable Stiffness Actuator (irisVSA). The novelty of this actuator lies in its design topology, where two long-armed torsional springs are mounted on the output link and connected to the input link via a Force Contact Point (FCP). The variation in stiffness is realized by altering the relative distance from the FCP to the joint rotation center (effective arm). This unique topology offers a wide range of stiffness ranging from transparency (0 stiffness) to physically rigid. Moreover, the rate of stiffness adjustment is considerably fast (0.7 sec) through the utilization of a hypocycloid gear train. The working principle, design, prototyping, and characterization process for the irisVSA are detailed here. A Nonlinear Proportional–Integral–Derivative Controller is proposed to ensure the system's accuracy. The safety of the actuator was evaluated by subjecting it to both impact force criterion experiments as well as Head Injury Criterion. © 2023
Original languageAmerican English
StatePublished - 2023


  • Actuators
  • Human robot interaction
  • Rotation
  • Topology
  • Collaboration applications
  • Compliant manipulation
  • Contact points
  • Design topologies
  • Human-robot collaboration
  • Physical humanrobot interaction (phri)
  • Stiffness actuators
  • Stiffness adjustments
  • Variable stiffness
  • Variable stiffness actuator
  • Stiffness


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