Politecnico di Torino - Corso Duca degli Abruzzi, 24 - 10129 Torino, ITALY

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Variable stiffness rotational joint

Collaborative roboticsdecoupled controldifferential transmissionRoboticsvariable stiffness joint


Rotary joint able to modify its torsional stiffness, decoupling the control of the stiffness from the control of the equilibrium configuration. The stiffness control allows to limit the energy transferred if a collision occurs, efficient management of storage and release of energy in impulsive / explosive movements and precise control of the exchanged torque.

Technical features

The core mechanism of the joint is able to decouple the control of the torsional stiffness of a rotating shaft from the control of its equilibrium configuration, acting independently on two rotary joints configured in parallel. It exploits a circular bending spring and a differential mechanism to decouple the regulation of position and stiffness. This allows to realize a joint that can be installed in series between a motor and a load, keeping both motors in parallel without requiring the rotation of electrical components. Furthermore, the particular configuration of the elastic transmission allows to modify the stiffness with a limited energy consumption, contributing to reduce the total power installed on the device.

Possible Applications

  • Robotic applications characterized by force-based interaction;
  • Collaborative robotics;
  • Rehabilitation robotics;
  • Entertainment robotics.


  • Unlimited rotation range, able to allow continuous motions;
  • Stiffness adjustment that occurs with reduced energy consumption;
  • Decoupling of equilibrium position and stiffness adjustment;
  • Can be integrated into existing mechanical transmissions.