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Electromagnetic actuator with variable compliance

haptic interfaceInformatica Tsd EnJoint with variable complianceprothesisRoboticsSpherical Joint


The proposed system is an electromagnetic actuator with variable compliance, a simple and compact device able to simultaneously produce an active and a passive reaction force/torque with variable compliance.  The presence of a single actuator capable of active and passive operation offers significant advantages in the design of robotic, biomedical and industrial systems, both in terms of reducing complexity and increasing the performance and functionality of the systems in which the actuator is integrated. Current solutions in the field of electromagnetic and mechanical actuators are based on separate and complex devices that are difficult to integrate in small volumes.

Technical features

The system is designed to provide an active electromagnetic torque and a passive internal reaction torque. The passive reaction torque is obtained thanks to a magnetorheological fluid (MRF) contained in the gap between stator and rotor. This fluid is composed of micro-particles of iron dispersed in a base of oil or water. At rest it behaves like a common low viscosity lubricant fluid. If the fluid is excited by an external magnetic field, however, it changes its rheological properties becoming semi-solid. In this way, by appropriately controlling the magnetic field of excitation of the MRF, it is possible to modulate the passive reaction torque obtaining a device with variable compliance.
The system operates with 3 degrees of freedom (DoF) being able to move in the z-x and z-y plane with an angle amplitude of about ± (60° – 70°) and being able to rotate 360° around the z axis.

Possible Applications

  • Robotics, as a spherical “joint” for connections between moving parts;
  • Biomedical, for the design of special prostheses or haptic interfaces;
  • Design of Joysticks (i.e. in the cloche of flight simulators or in video games);
  • Construction of wind generators as a controlled starting / braking system.


  • Simple and compact;
  • Increased performance and integration into various systems;
  • Production of an active force/torque and a passive reaction force with variable compliance;
  • Ability to simulate human body joints or recreate force feedback reproducing a constrained movement.