skip to main content

Title: Design of a Hybrid SMA-Pneumatic based Wearable Upper Limb Exoskeleton
Upper limb mobility impairments affect individuals at all life stages. Exoskeletons can assist in rehabilitation as well as performing Activities of Daily Living (ADL). Most commercial assistive devices still rely on rigid robotics with constrained biomechanical degrees of freedom that may even increase user exertion. Therefore, this paper discusses the iterative design and development of a novel hybrid pneumatic actuation and Shape Memory Alloy (SMA) based wearable soft exoskeleton to assist in shoulder abduction and horizontal flexion/extension movements, with integrated soft strain sensing to track shoulder joint motion. The garment development was done in two stages which involved creating (1) SMA actuators integrated with soft sensing, and (2) integrating pneumatic actuation. The final soft exoskeleton design was developed based on the insights gained from two prior prototypes in terms of wearability, usability, comfort, and functional specifications (i.e., placement and number) of the sensors and actuators. The final exoskeleton is a modular, multilayer garment which uses a hybrid and customizable actuation strategy (SMA and inflatable pneumatic bladder).
Award ID(s):
Publication Date:
Journal Name:
International Symposium on Wearable Computers Design Exhibition
Sponsoring Org:
National Science Foundation
More Like this
  1. This paper investigates the tradeoffs between design variables important for the development of a mobility support soft exoskeleton for horizontal shoulder adduction. The soft exoskeleton utilizes discreet shape memory alloy (SMA) spring actuators to generate the required torque to move the arm segment, while preserving the qualities of a soft, wearable garment solution. A pilot benchtop test involving varying power input, actuator anchor position, actuator orientation, and added weight, was investigated to evaluate their effects against the degree of motion the soft exoskeleton allows. The results show that the power input, actuator anchor position, and simulated limb weight each affect the ultimate horizontal adduction angle the exoskeleton is able to induce. Further, the project highlights a crucial point in regard to the tradeoffs between functionality and wearability: when actuator orientation was investigated, we found a decrement in functionality (as measured by maximum achievable horizontal adduction angle) when the actuators were constrained close to the body. This shows that when aiming to improve the hypothetical system’s wearability/usability, the effective torque that can be generated is reduced. Together these findings demonstrate important design considerations while developing a wearable, soft exoskeleton system that is capable of effectively supporting movement of the body whilemore »maintaining the comfort and discreetness of a regular garment.

    « less
  2. Stimuli-responsive hydrogels are candidate building blocks for soft robotic applications due to many of their unique properties, including tunable mechanical properties and biocompatibility. Over the past decade, there has been significant progress in developing soft and biohybrid actuators using naturally occurring and synthetic hydrogels to address the increasing demands for machines capable of interacting with fragile biological systems. Recent advancements in three-dimensional (3D) printing technology, either as a standalone manufacturing process or integrated with traditional fabrication techniques, have enabled the development of hydrogel-based actuators with on-demand geometry and actuation modalities. This mini-review surveys existing research efforts to inspire the development of novel fabrication techniques using hydrogel building blocks and identify potential future directions. In this article, existing 3D fabrication techniques for hydrogel actuators are first examined. Next, existing actuation mechanisms, including pneumatic, hydraulic, ionic, dehydration-rehydration, and cell-powered actuation, are reviewed with their benefits and limitations discussed. Subsequently, the applications of hydrogel-based actuators, including compliant handling of fragile items, micro-swimmers, wearable devices, and origami structures, are described. Finally, challenges in fabricating functional actuators using existing techniques are discussed.
  3. High-performance actuators are crucial to enable mechanical versatility of wearable robots, which are required to be lightweight, highly backdrivable, and with high bandwidth. State-of-the-art actuators, e.g., series elastic actuators (SEAs), have to compromise bandwidth to improve compliance (i.e., backdrivability). We describe the design and human-robot interaction modeling of a portable hip exoskeleton based on our custom quasi-direct drive (QDD) actuation (i.e., a high torque density motor with low ratio gear). We also present a model-based performance benchmark comparison of representative actuators in terms of torque capability, control bandwidth, backdrivability, and force tracking accuracy. This paper aims to corroborate the underlying philosophy of “design for control“, namely meticulous robot design can simplify control algorithms while ensuring high performance. Following this idea, we create a lightweight bilateral hip exoskeleton to reduce joint loadings during normal activities, including walking and squatting. Experiments indicate that the exoskeleton is able to produce high nominal torque (17.5 Nm), high backdrivability (0.4 Nm backdrive torque), high bandwidth (62.4 Hz), and high control accuracy (1.09 Nm root mean square tracking error, 5.4% of the desired peak torque). Its controller is versatile to assist walking at different speeds and squatting. This work demonstrates performance improvement compared with state-of-the-art exoskeletons.
  4. Recent work in fluid-driven soft robots has demonstrated the potential to achieve high power-to-weight ratios, low fabrication costs, and improved safety, making them well suited for interactive tasks. However, the low speed of pneumatic actuation prevents use of these robots in more dynamic tasks. This paper aims to design, characterize, and model a bistable elastomeric actuator for swift tasks (BEAST). This actuator enables both fast actuation and mechanical compliance, and is designed by integrating silicone and polyethylene terephthalate (PET) in a bendy straw structure. The BEAST contains three states - compressed, natural, and stretched states. Two operation modes - compressed and stretched modes, are defined to model the continuous elongation dynamics before and after the quickly switching around the natural state. A set of design rules and a novel fabrication method are presented to develop the BEAST. The actuator characterization shows that the maximum extension ratio, snapping speed, and output force of the BEAST to be 0.58, 1.5m/s, and 48N, respectively. A hybrid linear parameter varying (HLPV) model is developed to describe the pressure-dependent dynamics of the actuator. The actuators are evaluated in an object sorting task where both fast and gentle behaviors are demonstrated.
  5. In the world of soft-robotic medical devices, there is a growing need for low profile, non-rigid, and lower power actuators for soft exoskeletons and dynamic compression garments. Advanced compression garments with integrated shape memory materials have been developed recently to alleviate the functional and usability limitations associated with traditional compression garments. These advanced garments use contractile shape memory alloy (SMA) coil actuators to produce dynamic compression on the body through selective heating of the SMA material. While these garments can create spatially- and temporally-controllable compression, typical SMA materials (e.g., 70°C Flexinol) consume considerable power and require considerable thermal insulation to protect the wearer during the heating phase of the SMA actuation. Alternative SMA materials (e.g., NiTi #8 by Fort Wayne Metals, Inc.) transform below room temperature and do so using no applied electrical power and generate no waste heat. However, these materials are challenging to dynamically control and require active refrigeration to reset to material. In theory, low-temperature SMA actuators made from materials like NiTi #8 may maintain additional dynamic actuation capacity once equilibrated to room temperature (i.e., the material may not fully transform), as the SMA phase transformation temperature window expands when the material experiences applied stress. This papermore »investigates this possibility: we manufactured and tested low-temperature NiTi coil actuators to determine the magnitude of the additional force that can be generated via Joule heating once the material has equilibrated to room temperature. SMA spring actuators made from NiTi #8 consumed 84% less power and stabilized at significantly lower temperatures (26.0°C vs. 41.2°C) than SMA springs made from 70°C Flexinol, when actuated at identically fixed displacements (100% nominal strain) and when driven to produce equal forces (∼3.35N). This demonstration of low-power, minimal-heat exposure SMA actuation holds promise for many future wearable actuation applications, including dynamic compression garments.

    « less