Design, Modelling and Control of a Compliant Robot Joint System for Safe Human-Robot Collaboration in Manufacturing

  • Mohammed Ahmed Ramadan

Student thesis: Master's Thesis


In Human-Robot-Collaboration applications, variable stiffness actuators provide an attractive actuation solution due to their capabilities in ensuring accuracy and safety. Infinite range of stiffness has been achieved by following the principle of adjusting the transmission between the elastic element and the output link. However, most of the VSA with infinite stiffness range are relatively bulky, incapable of achieving infinite range of rotation and the stiffness adjustment rate is relatively slow. In this paper, the infinite rotation infinite stiffness Variable Stiffness Actuator (irisVSA) is presented. The novelty of this actuator lies in its design topology, where two long-armed torsional springs mounted on the output link and connected with input link via Force Contact Point (FCP). The stiffness variation is achieved by altering the distance from the FCP to the joint rotation center (effective arm). When the effective arm is null, the output link is disengaged from the input side creating a zero stiffness. When the effective arm reaches its maximum value, the energy will flow directly from the input to the output link reflecting a rigid actuator. The FCP's position is altered via a hypocycloidal gear set which enables rapid coverage of stiffness range (within 0.7 seconds). Moreover, as the stiffness adjustment mechanism is located on the input side, this enabled the unlimited range of rotation. The principle, design and prototyping of irisVSA is illustrated. The characterization process aimed to identify the dynamic parameters of irisVSA, and evaluate the capabilities of irisVSA in terms of accuracy and safety. To ensure accuracy, a Nonlinear Proportional-Integral-Derivative Controller was designed to evaluate the accuracy for both no load and load conditions. To evaluate the safety of the proposed actuator, irisVSA was subjected to impact force criterion experiments (static collision and Dynamic Collision) along with the simulation of Head Injury Criterion (HIC). The results show the significance of the proposed actuator in terms of accuracy and safety.
Date of AwardDec 2021
Original languageAmerican English


  • Variable Stiffness Actuator
  • Variable Lever Mechanism
  • Non-Linear PID Control
  • Human-Robot-Interaction.

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