IEEE Robotics & Automation Magazine - March 2015 - 99

Insertion
Extensor
of Muscle
Origin
O
g
Muscles Digitorum
of Muscle

Tendons Flexor
Digitorum
Profundus

Pulleys
(Annular
Ligament)

Figure 3. The human finger works via tendons, muscles, and
pulleys acting on the skeleton. The muscles that connect the
finger are located in the forearm and hand.

experimentally with both a healthy and a disabled subject.
The results demonstrated that the Exo-Glove restored grasping function to the disabled subject.
A Bioinspired Soft Tendon Routing System
The Exo-Glove transmits the tension of the tendon to the body
in a manner inspired by the human hand, for which tendons
transmit muscular forces to induce flexion and extension of
the fingers.
Three components are required for human tendons to
function properly: 1) an insertion, 2) the origin of the muscles, and 3) a pulley (see Figure 3). Muscles are connected to
tendons, and tendons are connected to their insertion points
and to the origin point of the muscles. The pulley is the annular ligament that determines the path of the tendons.
Based on the structure of the human hand, the Exo-Glove
has two tendons for each finger and the thumb, several types
of straps that together form the pulley, and supporting structures that form the origin of the muscles. These components
are designed to transmit normal forces to the body. To generate an appropriate finger trajectory for different users, the tendon path of the routing system can be adjusted by changing
the length and position of the fabric straps.

Fabric Straps
as Pulleys

Thimbles for
Muscle Insertion

Teflon
Tubes
T
b
(b)

(a)

Figure 4. A comparison of the interfaces for tendon attachment.
The green line on the dorsal side represents the extensor
tendon, and the red line on the volar side represents the flexor
tendon. (a) A case in which the tendon is directly attached to
the glove. (b) A case in which the tendon is attached through
a thimble-like strap and is fixed near the finger by straps. The
dotted lines on the fabric straps are the stitch lines that form the
hollow tubes in which Teflon tubes are installed.

shown in Figure 3. Without the pulleys, the tendons would
separate from the finger, a phenomenon referred to as bowstringing [14].
The Exo-Glove pulleys are made from fabric straps and a
Teflon tube and are indicated in gray, as shown in Figure 4(b).
The straps are attached to the dorsal side, with their two ends
located on the left and right sides of the phalanges. Both ends
of the straps are rolled up and stitched to hold the Teflon
tubes that minimize friction when the flexor tendons move.
The straps are made inextensible to maintain the moment
arms of the tendons. The positions of the straps determine the

Fabric
Straps

Extensor
Tendons
TA Support
(Brace)

Actuation
Units

Design Details of the Exo-Glove
Muscle Insertion Point: A Fingertip Thimble
In soft wearable hand robots, special interfaces attach the tendons to the digits. The Exo-Glove uses a glove interface. One
of the problems of using a glove interface is that it stretches.
As can be seen from Figure 4(a), the part of the glove near the
attachment point is stretched in the proximal direction when
the flexor tendon is pulled.
To solve this problem, the tendons are attached to the finger by a thimble-like strap made from inextensible fabric, as
shown in Figure 4(b). When the flexor tendon is pulled, the
strap pushes the back and tip of the phalange, minimizing
deformation of the attachment points.
Pulleys: Inextensible Fabric Straps and Teflon Tubes
In the hand, the annular ligaments attached to the bone act
as pulleys, and they determine the path of the tendons, as

Flexor
Outer Sheath of
Tendons Wrist Bowden Cable
Joint
(a)
TExtension

Releasing Extensor
Ta, Extension

TFlexion

Ta, Flexion
Winding Flexor
(b)
Figure 5. The dotted lines indicate the sections of tendons and
Bowden cables that lie on the far side of the hand. (a) The
black arrows indicate the force exerted on the TA support by the
Bowden cables, and the red arrows indicate the distributed force
exerted on the TA support by the hand. (b) While the tendon
moves toward the joint, the tension applied to the joint is greater
than the tension at the actuator.

march 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2015