Variable stiffness ankle actuator for use in robotic-assisted walking: Control strategy and experimental characterization

Abstract

Despite the increasing interest in using powered ankle-foot orthoses for assistive purposes, their development and benchmarking still present core challenges. Powered orthoses have to be safe and provide adequate torque while keeping limited size and weight. The discordance of these requirements is a challenge for the development of these devices. This paper describes the control strategy and characterization of a compact variable stiffness actuator, to be used in an assistive ankle-foot orthosis for impaired subjects. The results of the characterization experiments show the advantageous behavior of the actuator and its performance in providing different relevant assistive torque profiles, with different actuator stiffnesses, during emulated walking experiments. However, some divergences in the results obtained in different testing conditions highlight the need for more general benchmarking techniques. Towards this objective, the paper also proposes a novel performance indicator that can be used to better evaluate the performance of robotic actuators both in quasi-static and in dynamic conditions. The article concludes with a call for research on new benchmarking techniques, to understand more in-depth series elastic's actuators behavior under dynamic conditions.

Walking with a powered ankle-foot orthosis: the effects of actuation timing and stiffness level on healthy users

“…The influence of this parameter on the actuator's performance has been already shown in several test-bench experiments [37,40]. In [37] it was shown that no single stiffness level of the ankle actuator used in the MAPO could be defined as optimal based on the actuator's torque tracking performance, due to the dependence of the benefits of a specific actuator's stiffness level on both the assistive torque reference and the user's ankle kinematics. However, the effect of the actuator's stiffness level on the users has not been yet studied.…”

“…1. The design and characterization of the MACCEPA-based ankle actuator implemented in the MAPO have been already presented in detail in [37,38]. Briefly, the ankle VSA is used to actuate the MAPO in both directions of the sagittal plane (Additional file 1).…”

“…The value of v * was set to 0.268, in order to keep the walking speed v limited. This choice was taken to ensure the actuator's capability in tracking the desired torque profile during the walking trials [37]. The walking trials performed in the first session were meant to collect baseline walking data during normal walking (NW) and during the unassisted walking trial called zero torque (ZT).…”

“…Despite the wellrecognized benefits of VSAs, their employment in wearable robots is still limited due to their design complexity, the increased weight, and the challenging control strategies needed [30]. Among the developed VSAs, the MAC-CEPA has been already implemented in wearable robots such as lower limbs exoskeletons [31][32][33] and powered knee [34][35][36] and ankle orthoses [37][38][39].…”

Robust Tracking Control of Variable Stiffness Joint Based on Feedback Linearization and Disturbance Observer With Estimation Error Compensation

“…In the current research field of the flexible robot, variable stiffness joint (VSJ) is a kind of flexible robot actuation joint with independent and controllable characteristics of position and stiffness [1]. The inherent flexibility (affected by the internal elastic elements of the joint) and the adjustability of the joint output stiffness make the VSJ suitable for physical human-robot interaction applications such as the roboticassistive devices [2,3], exoskeleton robot [4], prosthetic joint [5], wearable devices [6,7], etc. When the VSJ is used as the actuated joint of the rehabilitation training robot, the therapist can adjust the joint stiffness of the robot according to the patients' rehabilitation training process.…”