Design of a variable impedance differential actuator for wearable robotics applications

In the design of wearable robots, the possibility of dynamically regulating the mechanical output impedance is crucial to achieve an efficient and safe human-robot interaction and to produce useful emergent dynamical behaviors. In this paper we propose a Variable Impedance Differential Actuator (VIDA) for wearable robotics applications. The system comprises two actuators (one being an impedance-controlled rotary Series Elastic Actuator) connected through a Harmonic Drive in a differential configuration used to separately control output position and mechanical impedance. Design choices regarding the overall architecture and the single components are presented and discussed. The mechanical structure also comprises a custom-made torsion spring designed after a CAD/FEM optimization. An electromechanical model of the system has been developed and a control strategy, based on the equilibrium point approach, is simulated to validate the performances of the system against system requirements. The actuation architecture allows to implement a control strategy where an equilibrium position and impedance field are simultaneously and independently regulated. This is possible still adopting very simple control laws: two controls for position and impedance regulation of the two input shafts.