The Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS): Towards Adaptive and Safe Physical Human-Robot Interaction

  • Hamed Assem Toubar

Student thesis: Master's Thesis


The recent application of the principle of discrete adjustment of stiffness has substantially enhanced the performance of Clutched Elastic Actuators (CEA), especially in terms of their capability for rapid-stiffness-switching. This key improvement plays a vital role in safety considerations, which are of particular relevance to physical Human-Robot-Interaction (pHRI) applications. However, currently the only proposed CEA for this purpose suffers from bulkiness, which limits its potential application in multi-degrees-of-freedom manipulators. In this thesis, the Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS) is introduced as a novel CEA, in which the stiffness can be modulated by adding/subtracting the appropriate number of elastic elements via electromagnetic tooth clutches. The novelty lies in the unique design topology, where three concentric shafts are connected in series through clutches. Each shaft is further attached to a pair of elastic elements. The number of involved pairs of springs, which depends on the engagement/disengagement of the clutches, allows for the attainment of three different levels of stiffness. The realized design is significantly compact relative to the previously proposed CEA that was based on this principle, while keeping the same advantages over Series Elastic Actuators (SEAs) in terms of adaptive stiffness, and over Variable Stiffness Actuators (VSAs) in terms of the superior safety switching time. This thesis details the working principle, the design of the actuator, the prototype development, the stiffness and dynamic modeling, the dynamic parameters estimation, the joint stiffness characterization, an adaptive gain-scheduled PID controller used for accurate joint position control, and the evaluation of the actuator's safety in pHRI via the Head-Injury Criterion (HIC).
Date of AwardJun 2022
Original languageAmerican English


  • Series Elastic Actuators
  • Clutched Elastic Actuators
  • physical Human-Robot Interaction
  • Compliant Manipulation.

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