Design, Computational Modelling and Experimental Characterization of Bistable Hybrid Soft Actuators for a Controllable-Compliance Joint of an Exoskeleton Rehabilitation Robot

This paper presents the mechatronic design of a biorobotic joint with controllable compliance, for innovative applications of “assist-as-needed” robotic rehabilitation mediated by a wearable and soft exoskeleton. The soft actuation of robotic exoskeletons can provide some relevant advantages in terms of controllable compliance, adaptivity and intrinsic safety of the control performance of the robot during the interaction with the patient. Pneumatic Artificial Muscles (PAMs), which belong to the class of soft actuators, can be arranged in antagonistic configuration in order to exploit the variability of their mechanical compliance for the optimal adaptation of the robot performance during therapy. The coupling of an antagonistic configuration of PAMs with a regulation mechanism can achieve, under a customized control strategy, the optimal tuning of the mechanical compliance of the exoskeleton joint over full ranges of actuation pressure and joint rotation. This work presents a novel mechanism, for the optimal regulation of the compliance of the biorobotic joint, which is characterized by a soft and hybrid actuation exploiting the storage/release of the elastic energy by bistable Von Mises elastic trusses. The contribution from elastic Von Mises structure can improve both the mechanical response of the soft pneumatic bellows actuating the regulation mechanism and the intrinsic safety of the whole mechanism. A comprehensive set of design steps is presented here, including the optimization of the geometry of the pneumatic bellows, the fabrication process through 3D printing of the mechanism and some experimental tests devoted to the characterization of the hybrid soft actuation. The experimental tests replicated the main operating conditions of the regulation mechanism; the advantages arising from the bistable hybrid soft actuation were evaluated in terms of static and dynamic performance, e.g., pressure and force transition thresholds of the bistable mechanism, linearity and hysteresis of the actuator response

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

Abstract—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

Modelling and identification of the asymmetric hysteresis in the viscoelastic response of the fingertip under indentation: A multistate friction model with switching parameters

This paper presents a dynamic model for the identification of asymmetric hysteresis in the force response of the fingertip under cyclic local deformation of the fingertip soft tissues. Viscoelastic effects, marked anisotropy and nonlinearity contribute to the hysteretic behaviour of the force of the fingertip undergoing indentation. The fingertip force in response to an indentation stimulus varies along the loading and unloading cycles; the resulting hysteresis loops are asymmetric. The asymmetry arises from the simultaneous convexity of the branches of the hysteresis loop. The proposed model belongs to the class of multistate friction models that can effectively describe the hysteresis in the presliding regime of motion of general mechanisms with friction, by exploiting a mechanical analogy obtained through the concatenation of multiple elasto-plastic elements. The multistate model, which provides the mechanical representation of the fingertip response through a set of newly-conceived switching elements and viscoelastic blocks, can reproduce the convex and asymmetric hysteresis loops of the fingertip mechanical response, including the viscoelastic effects of stress relaxation and the influence of time interval between consecutive cycles of indentation. The model has been validated through the experimental identification of fingertip indentation tests performed on an optomechanical test-rig. This model potentially opens the way for efficient model-based control strategies of servomechanisms involved in tactile and haptic displays and interfaces