A cable-based series elastic actuator with conduit sensor for wearable exoskeletons

Abstract

Abstract-There is currently a scarcity of wearable robotic devices that can practically provide physical assistance in a range of real world activities. Soft wearable exoskeletons, or exosuits, have the potential to be more portable and less restrictive than their rigid counterparts. In this paper, we present the design of an actuation system that has been optimized for use in a soft exosuit for the human arm. The selected design comprises a DC motor and gearbox, a flexible cable conduit transmission, and a custom series elastic force sensor. Placed in series with the transmission conduit, the custom compliant force sensor consists of a translational steel compression spring with a pair of Hall effect sensors for measuring deflection. The custom sensor is validated as an accurate means of measuring cable tension, and it is shown that it can be used in feedback to control the cable tension with high bandwidth. The dynamic effect of the cable-conduit transmission on the force felt at the user interface is characterized by backdriving the system as it renders a range of virtual impedances to the user. We conclude with recommendations for the integration of such an actuation system into a full wearable exosuit.

Remote Actuation Systems for Fully Wearable Assistive Devices: Requirements, Selection, and Optimization for Out-of-the-Lab Application of a Hand Exoskeleton

“…In cable-based RAS, a rope or a cable mechanically connects the actuation unit to the output. Most of the reviewed RAS use Bowden cables consisting of a cable guided in a sheath (Letier et al, 2006 ; Veneman et al, 2007 ; Kong et al, 2009 ; Wang et al, 2009 ; Vitiello et al, 2013 ; Agarwal et al, 2015 ; Asbeck et al, 2015 ; Bartenbach et al, 2015 ; Cempini et al, 2015 ; In et al, 2015 ; Nycz et al, 2015 , 2016 ; Cappello et al, 2016 ; Choi et al, 2016 ; Choi et al, 2019 ; Dinh et al, 2016 ; Kalantari and Ghaffari, 2016 ; Norman et al, 2016 ; Xiloyannis et al, 2016 , 2019 ; Blumenschein et al, 2017 ; Jeong and Cho, 2017 ; Popov et al, 2017 ; Yun S.-S. et al, 2017 ; Randazzo et al, 2018 ; Yi et al, 2018 ; Burns et al, 2019 ; Dwivedi et al, 2019 ; Tran et al, 2020 ). Electrical motors are most commonly used to actuate the cable transmission.…”

“…Alternatively, in Hofmann et al ( 2018 ) and Jeong and Cho ( 2017 ), bending angle sensors combined with dynamic feed-forward friction compensation were presented, such that no sensors were needed at the output. Often, series elastic elements (SEE) have been used to measure the force transmitted by Bowden cables at the output (Agarwal et al, 2015 ; Blumenschein et al, 2017 ; Marconi et al, 2019 ). SEE have also been used to pretension the wires and make the transmission more compliant (Veneman et al, 2007 ).…”

“…Actuation units of cable-based transmission systems are mostly composed of rotational electromagnetic motors in combination with gears (Veneman et al, 2007 ; Asbeck et al, 2015 ; Nycz et al, 2015 ; Xiloyannis et al, 2016 , 2019 ; Blumenschein et al, 2017 ; Park et al, 2019 ; Rose and O'Malley, 2019 ), pulleys (Vitiello et al, 2013 ; Choi et al, 2019 ), lead screws (Cempini et al, 2015 ), or winches (Letier et al, 2006 ; Agarwal et al, 2015 ; In et al, 2015 ; Cappello et al, 2016 ; Dinh et al, 2016 ; Kalantari and Ghaffari, 2016 ; Xiloyannis et al, 2016 ; Jeong and Cho, 2017 ; Thielbar et al, 2017 ; Tran et al, 2020 ). Alternatively, linear motors and servo motors, directly attached to the cable, have been used (Choi et al, 2016 ; Norman et al, 2016 ; Nycz et al, 2016 ; Yun S.-S. et al, 2017 ; Kim and Park, 2018 ; Lemerle et al, 2018 ; Burns et al, 2019 ).…”

“…Bidirectional rotational motion has often been achieved by connecting a pair of Bowden cables to a pulley (pull-pull torque transmission system). When linear motion is required at the output, the end of the cable most often serves as output and is attached directly to strategically chosen anchor points, e.g., on a glove or an exosuit either in unidirectional (Asbeck et al, 2015 ; Bartenbach et al, 2015 ; In et al, 2015 ; Nycz et al, 2015 ; Norman et al, 2016 ; Xiloyannis et al, 2016 , 2019 ; Blumenschein et al, 2017 ; Kang et al, 2018 ; Kim and Park, 2018 ; Choi et al, 2019 ; Dwivedi et al, 2019 ; Rose and O'Malley, 2019 ; Tran et al, 2020 ) or in a push–pull configuration (Nycz et al, 2016 ; Randazzo et al, 2018 ).…”

A Bowden Cable-Based Series Elastic Actuation Module for Assessing the Human Wrist

“…With reference to seminal wrist assessment studies [7,8,12] and rehabilitation devices [11,15,16], we identified the following requirements as paramount to develop a 2-DOF wrist stiffness and range of motion assessment device: 1) increase range of motion, 2) increase torque output, and 3) incorporate direct torque measurement. A Bowden cable-based actuation scheme, which locates the motors off-board as used in [17,18], was adopted for increased device range of motion and torque output. However, the benefits of using Bowden cables comes at the cost of reduced accuracy in torque measurement and control.…”