Design, realization and experimental characterisation of a controllable-compliance joint of a robotic exoskeleton for assist-as-needed rehabilitation

Nowadays rehabilitation robotics has increasingly widespread applications into the biomedical field. One of the most promising strategies for the control of rehabilitation robots is the "assist-as-needed" approach, which is conceived to adapt the level of assistance of the robot on the basis of the physical characteristics and ability of the patient undergoing the robotic therapy. Within the framework of "assist-as-needed" control, the optimal adaptation of the robot performance can be achieved through some strategies that exploit the intrinsic compliance of soft actuators to regulate the interaction between the robot and the patient.This paper presents the current steps of the mechatronic design of a biorobotic joint with controllable compliance, towards the direction of the realization of a wearable and soft exoskeleton for "assist-as-needed" rehabilitation. The biorobotic joint is actuated by pneumatic artificial muscles (PAMs) that are soft actuators with variable stiffness.As result of the process of design and validation of the mechanism of regulation of the joint compliance, some experimental tests, which involve the measurements of pressure and force of the used PAMs, have been performed in order to characterize the performance of the mechanism of regulation influenced by the nonlinear and hysteretic response of the PAMs.The regulation mechanism enables the implementation of a novel and effective control strategy guaranteeing safe and biomimetic performance for the optimal regulation of the mechanical compliance of biorobotic joints over the full ranges of pressure and motion of the joint. The proposed control law overcomes some limiting assumptions taken in existing strategies of control of the pressure of the PAMs involved into the regulation of the mechanical compliance of rehabilitation robots actuated by pneumatic muscles.