Design of Shape Memory Alloy-Based Soft Wearable Robot for Assisting Wrist Motion

Abstract

In this paper, we propose a shape memory alloy (SMA)-based wearable robot that assists the wrist motion for patients who have difficulties in manipulating the lower arm. Since SMA shows high contraction strain when it is designed as a form of coil spring shape, the proposed muscle-like actuator was designed after optimizing the spring parameters. The fabricated actuator shows a maximum force of 10 N and a maximum contraction ratio of 40%. The SMA-based wearable robot, named soft wrist assist (SWA), assists 2 degrees of freedom (DOF) wrist motions. In addition, the robot is totally flexible and weighs 151g for the wearable parts. A maximum torque of 1.32 Nm was measured for wrist flexion, and a torque of larger than 0.5 Nm was measured for the other motions. The robot showed the average range of motion (ROM) with 33.8, 30.4, 15.4, and 21.4 degrees for flexion, extension, ulnar, and radial deviation, respectively. Thanks to the soft feature of the SWA, time cost for wearing the device is shorter than 2 min as was also the case for patients when putting it on by themselves. From the experimental results, the SWA is expected to support wrist motion for diverse activities of daily living (ADL) routinely for patients.

Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges

“…Maximum Energy Efficiency [14] Dielectric elastomer (CF19-2186) 100, 0 63 (Linear area strain) 0.2 - [14] Dielectric elastomer (HS3) 280, 0 117 (Linear area strain) 0.16 - [3,18] Shape memory alloy wire (NiTi) -6-8 100 3~10 [19,20] Shape memory alloy coil spring (NiTi) -300 >100 3~10 [3] Shape memory polymer (polyurethane) -100-400 2 10 [3] Piezoelectric ceramic (PZT) -0.2 0.1 90 [3] Piezoelectric polymer (PVDF) -0.1 0.0024 - [3] Conductive polymer (PANI) -10 23 <5 [3] Ionic gel (polyelectrolyte) -40 0.06 30 [21] PVC gel actuator -76 -- [22] Coiled actuator (Nylon) -50 0.84 - [15,18] Natural human muscle -40 0.008~0.04 35-40…”

“…Artificial muscles are designed to mimic human muscle characteristics. A typical human muscle displays a maximum of 40% strain in the axial direction [13,19]. However, such large deformation is not attained with just long linear dielectric elastomer actuators.…”

Robotic micromanipulation: a) actuators and their application

“…The SMA based actuators can be used in many applications, including robotics, soft robotics, medicine, aerospace, etc. [126]. The advantages of SMA drives are high power-to-weight ratio, biocompatibility, small size, simple mechanical design and quiet operation.…”

An Assistive Soft Wrist Exosuit for Flexion Movements With an Ergonomic Reinforced Glove

“…Still, soft exosuits can offer the possibility of reducing weight and enhancing ergonomics due to their mechanical features. The SWA wrist exosuit (Jeong et al, 2019 ) (in Figure 1E ) is a shape memory alloy (SMA)-based wearable robot that assists 2 DoF of the wrist motion and it is based on coil spring-shaped SMA that acts like a muscle. It is a light solution (weight 151 g) and it can exert a maximum force of 10 N. Despite this solution is completely flexible, the SMA actuators offers a limited bandwidth for industrial exosuits due to the need of heat dissipation that underlie its physical working principle.…”