Development of a Polymer-Based Tendon-Driven Wearable Roboti

发布时间：2018-04-22 16:57

Abstract— This paper presents the development of a polymer-based tendon-driven wearable robotic hand, Exo-Glove Poly. Unlike the previously developed Exo-Glove, a fabric-based tendon-driven wearable robotic hand, Exo-Glove Poly was developed using silicone to allow for sanitization between users in multiple-user environments such as hospitals. Exo-Glove Poly was developed to use two motors, one for the thumb and the other for the index/middle finger, and an under-actuation mechanism to grasp various objects. In order to realize Exo-Glove Poly, design features and fabrication processes were developed to permit adjustment to different hand sizes, to protect users from injury, to enable ventilation, and to embed Teflon tubes for the wire paths. The mechanical properties of Exo-Glove Poly were verified with a healthy subject through a wrap grasp experiment using a mat-type pressure sensor and an under-actuation performance experiment with a specialized test set-up. Finally, performance of the Exo-Glove Poly for grasping various shapes of object was verified, including objects needing under-actuation.
 

1. INTRODUCTION

Recently, owing to the rapidly increasing number of spinal cord injury [1] and stroke [2] patients, many wearable devices have been developed to assist with activities of daily living (ADLs) or to help rehabilitation. Since the hand is the most important body part with respect to ADLs, many researchers have been developing wearable hand robots for patients with hand motor function impairment. Conventionally, these robots were designed in three ways: a link-based rigid exoskeleton [2-6], a polymer-based soft exoskeleton using pneumatic actuation [7-8], and a fabric-based soft exoskeleton using a tendon drive [9-11].
A link-based rigid exoskeleton has the advantages of easy force transmission and easy control, but the wearable part is bulky because of the need to align robotic joints to the human finger joints. To improve the compactness of the hand-wearable part of the robot, soft exoskeletons made of polymer and fabric have been proposed. The compliant and soft characteristics of soft robots make them easy to customize, to adjust to the user’s hand, and to don and doff. On the other hand, the robot’s compliance increases the nonlinearity of the system, making such robots complicated and difficult to control.


 
The soft exoskeleton hand robot also has its pros and cons, according to the actuation method that it uses. A pneumatic-actuated soft exoskeleton is basically manufactured with polymers. The key design issue is how to design and fabricate the air chamber in the pneumatic actuator. Pneumatic actuators are usually placed on the dorsal side of the hand to transmit evenly distributed force on the finger. However, problems remain to be solved for pneumatic-actuated soft exoskeleton robots: They can only perform flexion, and they are not portable owing to the requirement for an air compressor system. A tendon-driven soft exoskeleton based on fabric is portable, but force transmission to the finger is weak. The wearable part is more compact than for pneumatic soft exoskeleton, and portability is ensured because small linear actuators and rotary actuators are adequate for the robot. However, unlike the pneumatic actuator, it is difficult to provide the right amount of force on each finger joint, high wire tension on the tendon is not evenly distributed on the hand. Additionally, the extreme compliance of the fabric used raises extra barriers to overcome in manufacturing, force distribution, and control.
In a previous study, we developed a fabric-based tendon-driven wearable hand robot, Exo-Glove, to assist people with loss of hand mobility to perform ADLs, especially those with spinal cord injury (SCI) at C5 to C7 [11]. Exo-Glove enables the wearer to grasp using only the thumb and the index and middle fingers. The system uses under-actuation to permit the wearer to grasp various objects adaptively and to reduce the number of actuators needed. The robot uses two actuators, one for thumb flexion/extension and the other for index/middle finger flexion/extension. However,

Exo-Glove has limitations when used in a multiple-user environment such as a hospital because of the difficulty of sanitizing it between uses. The fabric portion of the robot absorbs sweat, but the fabric is not suitable for frequent cleaning. Sanitization is not a problem for solo users, who are using Exo-Glove in daily living as an assistive device.
In this paper, we present a version of the Exo-Glove that uses polymer instead of fabric. This new polymer-based tendon-driven wearable robotic hand, Exo-Glove Poly (Fig. 1), has new design features and a new fabrication process owing to the change of materials. We verified the mechanical performance of Exo-Glove Poly via pressure sensors and a distinctive test set-up using load cells. Successful grasping performance was verified through experiments in which volunteers wearing the robot grasped various objects.

1. EXO-GLOVE POLY

1. Design Criteria

As mentioned above, owing to the sanitation problem hospitals prefer wearable robots to be made of polymers rather than fabric. Impurities such as sweat on a polymer-based wearable robot can be cleaned easily by wiping it with an alcohol swab. Thus, Exo-Glove Poly was developed to be a fabric-free soft wearable robotic hand.
Exo-Glove and Exo-Glove Poly differ not only in their base materials but also in terms of design perspective and fabrication process. In Exo-Glove, the compliant fabric is comfortable to wear and adapts easily to different hand sizes. Even though fabric is so compliant that it cannot maintain its structural shape by itself, nevertheless the fabric glove can support all parts of the wire path. Polymer is also compliant, but it has very different material properties.
By choosing the proper compliant polymer as the base material, the hand wearable part of the robot can match the performance of a fabric glove in terms of comfort and ability to support its structural shape. Adaptability to different hand sizes will be obtained through novel design features.
When using fabrics, other components can be easily added by sewing or adhesives. In contrast, because polymer is manufactured by a molding process, adding additional components is not possible. Therefore, all necessary components must be seriously planned for in the design stage.
Moreover, considering the fact that silicon is not capable of ventilation, Exo-Glove Poly should aspire to cover the least amount of area on the hand, consistent with maintaining functionality.

2. Parts and Materials

As shown in Fig. 2, the Exo-Glove Poly wearable part consists of a main body, a thumb body, three thimbles, and five straps. These parts are fabricated with silicone (KE-1300 or KE-1300T, Shinetsu). Silicone was also used to build RAPAEL [12], a rehabilitation system with a wearable hand robot that has an embedded bending sensor, from Neofect. The material properties of the silicone are shown in Table 1.

The wearable part is connected to the actuator unit through a tendon anchoring support (TAS) and sheath. Velcro is used to attach the wearable part to the TAS. The TAS was fabricated with a three-dimensional (3D) printer (Objet Connex 260, Stratasys) using VeroWhite. The TAS was designed to fit the distal part of the wrist joint to enable force transmission of the wire, determine the wire path, and anchor one end of the sheath [13]. The other end of the sheath is anchored to the actuator.
To reduce friction in the wire path, a Teflon tube is embedded into the silicone. A tension spring with a diameter of 2.6mm was used for the sheath. The Teflon tube is also inserted in the sheath to decrease the friction of the wire along the sheath.

1. Main Body and Thumb Body

The wearable part was manufactured as two separate bodies, a thumb body for thumb actuation and a main body for index/middle finger actuation. When designing the body design, three features were considered.
The first design feature is the diamond shapes between the straps and the thimble (Fig. 3, red box), which have two functions. One is to create an extendable design that permits adjustment of the distance between the straps and the thimble during actuation and that accommodates different finger lengths. The other function is to prevent skin abrasion during actuation caused by contact between the wire and skin. These functions can be seen in Fig. 4 (a, b).
The second design feature is embedded Teflon tubes that determine the wire path on the palm of the hand (Fig 3, green box). The wire paths were designed in two layers, one for thumb flexion (Fig. 3,blue line) and the other for index/middle finger flexion with under-actuation (Fig. 3, purple lines). Also, to avoid disturbing thumb movement, the index/middle finger flexion wire paths were curved. Owing to the curved two-layer wire paths, the Teflon tubes could not be embedded during the molding process. As a counterplan, the tubes were embedded in concave paths on the main body. Four concave paths for under-actuation of index/middle finger flexion was designed on the top side and another single concave path was designed for thumb flexion on the bottom side of the main body. After the main body was molded-in, Teflon tubes were placed on the concave paths and additional silicone was plastered on.

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