pHRI in Assistive and Rehabilitation Robots: Neural Constraints and Compliant Joints/Actuators Design

The development of robotic systems for physical Human-Robot Interaction (pHRI) in rehabilitation and assistive applications demands advanced solutions in terms of mechanical structure, actuation system and control strategies to face several issues such as safety, dynamical adaptability and biomechanical compatibility. To cope with the complex features of the human body and of the neural adaptation mechanisms, one should ideally be capable of designing the interaction, i.e. of introducing external artifacts effectively providing the desired assistance while optimally interacting with the user. Successful designs can be pursued if neural mechanisms underlying human motor control are understood and if proper components are embedded in the robotic artifacts to enhance intrinsic dynamics: on the one hand the features of physiological motion and neuromotor strategies adopted to perform motor tasks have not to be hindered; on the other hand, body dynamics can be supplemented and enriched by external agents making desired complex behaviors emerge as a form of constructive perturbation. This thesis contributes to the improvement of pHRI in the design of assistive and rehabilitation robots with respect to these two research fields. The topic of human intrinsic motor strategies and possible influence of robots perturbations is addressed presenting a work on human wrist movements during kinematically redundant pointing tasks in free motion and during interaction with a state-of-the-art robot for post-stroke neurorehabilitation. Starting from the demonstration that human natural motion is perturbed by the robot during assessment, the effects of system mechanical impedance reduction are analyzed, using a direct force control scheme to minimize human-robot interaction forces. The work demonstrates the possibility of using a simple control approach to cope with neural strategies in performing motor tasks, which are not properly taken into account in current design of rehabilitation robots. The topic of robots designs leveraging potentialities of intrinsic structural properties is addressed starting from the analysis of double actuation compliant systems used to vary impedance in different application fields. A classification and a critical comparison of possible architectures are carried out to provide guidelines for future designs of actuators. On this basis, a new actuation system, Variable Impedance Differential Actuator (VIDA), is proposed: an electromechanical model of the actuator is developed and performances are evaluated in simulations. A compact rotary Series Elastic Actuator (SEA), suitable to be used in wearable robotics applications, is also presented. Components are described and experimental characterization of prototypes, in terms of torque-angle characteristic of the elastic element and of torque/impedance control schemes, are reported. Moreover, a purely mechanical rotary passive ViscoElastic Joint (pVEJ) is presented. It includes two functionally distinct submodules, a torsion spring and a torsion damper connected in parallel. The modularity of the system allows to have a generic stiffness-angle characteristics by substituting a single component. Damping coefficient can be regulated through a valve. Characterization of the developed prototype using a custom dynamometric test-bed is reported. The active and passive joints developed and tested within this work are suitable to be included in assistive robots to exploit properties of passive low-impedance components.