Multi-fingered Robotic Hand based on Hybrid Mechanism of
Tendon-Driven and Jamming Transition
Kaori Mizushima1, Student Member, IEEE, Takumi Oku2, Yosuke Suzuki3, Member, IEEE,
Tokuo Tsuji3, Member, IEEE, and Tetsuyou Watanabe3, Member, IEEE

Abstract— This study presents a novel four-fingered robotic
hand to attain a soft contact and high stability under
disturbances while holding an object. Each finger is constructedusing a tendon-driven skeleton, granular materials
corresponding to finger pulp, and a deformable rubber skin.
This structure provides soft contact with an object, as well as
high adaptation to its shape. Even if the object is deformableand fragile, a grasping posture can be formed without
deforming the object. If the air around the granular materials in
the rubber skin and jamming transition is vacuumed, thegrasping posture can be fixed and the object can be grasped
firmly and stably. A high grasping stability under disturbances
can be attained. Additionally, the fingertips can work as a small
jamming gripper to grasp an object smaller than a fingertip. An
experimental investigation indicated that the proposed
structure provides a high grasping force with a jamming
transition with high adaptability to the object’s shape.

I. INTRODUCTION
Robots have recently been required to work in humanenvironments. Such robots should provide a sense of security.
When focusing on the capability of handling objects, therobots should have softness to prevent injury to human users
and periphery objects, as well as rigidity to firmly and stably
grasp an object. Moreover, the robots should have a variety ofgrasping styles for adoption to a wide variety of shapes andproperties (e.g., stiffness and fragility) of the objects. This
study presents a robotic hand satisfying different andconflicting demands (see Fig. 1). The key solution is a hybridmechanism of a tendon-driven and jamming transition. The
hybrid structure enables forming a grasping posture when incontact with an object through small forces and impacts, andto fix the grasping posture using a jamming transition to
firmly and stably grasp the object. The main contributions of
this mechanism are as follows.
High adaptability: Each finger link is constructed using arubber bag filled with granular materials. Therefore, thesurface is soft during a grasping motion, and the contact
surfaces of the fingers deform according to the shape of theobject. Compared to conventional tendon-driven robotic
hands with relatively hard surfaces [1]–[10], a larger contact
area can be attained, which enhances a stable grasp.
*1K. Mizushima is with the Graduated school of Natural science andTechnology, Kanazawa University, Kakuma-machi, Kanazawa, 9201192
Japan (e-mail: k_mizushima@stu.kanazawa-u.ac.jp).
2
T. Oku is with the YKK corporation.
3
Y. Suzuki, T. Tsuji, and T. Watanabe are with the Faculty of MechanicalEngineering, Institute of Science and Engineering, Kanazawa University,
Kakuma-machi, Kanazawa, 9201192 Japan (e-mail: te-watanabe@ieee.org).

Figure 1. Multi-fingered robotic hand based on hybrid jamming transition
and tendon-driven mechanism.

Conventional jamming grippers [11][12][13]–[19] have tostrongly press the rubber bag against the object in one
direction for grasping. The presented robotic hand has joints
and can envelope an object through a closing motion. Owing
to this enveloping motion, the object can be pressed fromdifferent directions, and the object can be grasped using small
forces adapting to the shape of the object. Deformable andfragile objects can then be grasped without changing theshapes of the objects.
High stability under disturbances: If the jamming
transition is conducted after the grasping motion, the grasping
posture is fixed. The rigidity of both the surfaces and joints isincreased. Additionally, in the grasping posture, the surfacesof the fingers are deformed according to the shape of theobject. Thus, even if a disturbance force is applied to theobject, the object can be grasped stably without increasing thegrasping forces.
Segmented chamber mechanism: The jamming transitionof the elongated rubber bag does not work if the bending or
tensile force is applied to the bag. Such forces extend in space,
and the density of the granular materials cannot decrease, andthe jamming therefore fails. This paper presents a novel
structure to resolve this issue. The chambers of the granularmaterials are separated for each finger link, and the chambers
move with the rotation of the finger link, allowing thevolumes of the chambers to remain constant.
Small jamming gripper style utilizing fingertip: If thetarget object is smaller than the diameter of the fingertip, thefingertip can be utilized as a small jamming gripper, which isan additional grasping style of the presented robotic hand.
II. RELATED WORKS
A. Multi-fingered hand with tendon-driven mechanism
The human hand has a very high DOF, which facilitates thegrasping of various objects within a living space. In thiscontext, a number of multi-fingered robotic hands imitating ahuman hand have been developed [1]–[10]. Tendon-driven

B. Jamming transition
A jamming transition is a phenomenon in which the state of
the granular materials changes from soft to rigid, or vice versa,
by changing the density of the granular materials [11][12].
There have been many previous studies on robotic hands
utilizing a jamming transition [13]–[19]. One representative
study is a jamming gripper developed by Brown et al. [15].
The gripper consists of a rubber bag filled with coffee beans,
and its stiffness was changed by controlling the pressure of
the air inside the bag. The grasping process is as follows. 1) A
rubber bag with granular materials is pressed against theobject, and then deforms along the shape of the object. 2) The
deformed gripper is changed to a solid-like state by sucking
the air around the granular materials. The state is then fixed,
and the object can be grasped. Some other examples include a
jammable manipulator developed by Cheng et al. [16], atwo-fingered hand having a jammable fingertip by Hou et al.

[17], and a two-layered jamming gripper by Fujita et al. [18].
Using this structure, the rigidity can be simply controlled
through an air pressure control. One drawback is that thegripper has to strongly press against the object for the
purposes of grasping, and is difficult to adopt for use withfragile or deformable objects. The presented study avoids thisissue by using a jamming transition not for the grasping
directly but for fixing and maintaining the grasping posture.
III. DESIGN AND FUNCTION
A. Hand design
Fig. 2 shows the developed robotic hand constructed withfour fingers. The actuator (ROBOTIS, DYNAMIXELXM430-W350-R) is located at the backside of the palm for
driving all four fingers. The actuator is connected to anindependent-motion-driving unit (described later). The sizeof the palm is 90 mm × 82 mm, and the length of each finger
is 100 mm. The fingers were arranged with angles of α and β(Fig. 2), allowing the enveloping, cylindrical, and precision
grasps to be realized using a single actuator. An angle α of 20°
provides an intermediate grasping style between theenveloping grasp and the parallel-jaw-gripper style. It should
be noted that the parallel-jaw-gripper style is effective forgrasping a cylindrical object. As shown in Fig. 3(a), each pairof the two antipodal fingers can be in opposition. An angle βof 30° provides a surface contact between the two antipodal
fingertips, when pulling the driving wire without jamming.
These settings enable a stable precision grasp. The available
grasping styles are shown in Fig. 3 with the maximum andminimum sizes of graspable objects.
Independent-motiondriven unit
(30°)

)
( 20°

100mm

82mm

systems have been popularly utilized to construct such
multi-fingered robotic hands. Multiple joints can be driven
using a single wire, i.e., one actuator. Actuators can beembedded in the palm or wrist. The fingers can then be slim.
Thus, a small sized, lightweight, and low-cost robotic handwith a high DOF can be constructed. Examples of robotic
hands based on a tendon-driven system including the i-HY
hand by Odhner et al. [2], Pisa/IIT SoftHand by Catalano et al.
[3], SDM Hand by Dollar et al. [4], a three-fingered gripper
with passive rotational joints by Backus et al. [5], and athree-fingered robotic hand with active and passive tendons[6]. A popular structure for a robotic hand utilizing atendon-driven system is as follows. A wire is attached to afinger link. Its corresponding joint is rotated through a pullingof the wire, which leads to a closing motion of the robotic
hand. If a torsion spring is embedded in each joint, the robotic
hand is opened through a release of the pulling force. To
achieve a lightweight, low-cost, and easily maintained system,
the utilization of a small number of actuators is preferable. It
is possible to pull all of the wires to rotate the joints using
only one actuator. However, if the motion of a certain finger
is restricted through contact with an object, the motions of allother fingers are also restricted, and the grasping could fail.
This issue can be resolved by embedding an elastic
component such as a spring into the wire[3], [6]. The
embedded spring for a finger restricted by an object is
extended, and a closing motion of all fingers that are not in
contact with the object is available. The robotic hand can thengrasp the object by adapting to its shape. However, there is adrawback to this particular structure. The magnitude of thegenerable grasping force is determined not by the power of
the actuator but by the stiffness of the embedded spring. The
stiffness should slight to enhance the adaptation to the object
shape, but sufficient to achieve a large grasping force. The
installation of a brake between the wire and spring could be asolution. However, the components used in the brake could
increase the finger size, which should be avoided for use in awide range of fields. In addition, to attain a high adaptability
to the object’s shape, a soft surface is more preferable.

90mm(a) Inside

45mm

110mm

(b) Front(c) SideFigure 2. Outlook of the developed robotic hand

Minimum
graspable size(a) Enveloping grasp

Maximum
graspable size

45mm
100mm

(b) Precision grasp

Surfacecontact

Maximum
graspable size
110mm

Figure 3. The available grasping styles with the maximum and minimum
sizes of graspable objects; (a) enveloping grasp, (b) Precision grasp.

Skelton

Outer rubber skin

Vinyl tube

Wire
Fingerbase

Torsion spring

Particles

Suction tube

Figure 4. Schematic view of the finger structure.

Wire ( Vinyl tube )Vinyl tube issealed(a)

Air flows through theclearance between the Joint block
links and the joint blocks
Suction tube
Screw hole forfixing the joint blockHole
for a shaft

Joint block

Path for the particles
and air(b)(c)

Figure 5. Skeleton of the finger: (a) photograph, (b) CAD model without ajoint bock, and (c) CAD model with a joint block.
(2) The flexed finger(1) Initial stateForce(3) Load is applied after (4) The lower area
jamming transition
was inflated(a)Expand(b)

Figure 6. (a) Schematic view and (b) photograph of a case in which an
external force is exerted on the flexed finger without a joint block.

Force
Joint block

The joint blocks prevents
the increase of the volumeof the chamber.
Figure 7. Schematic view of a case in which an external force is exerted on
the flexed finger with a joint block.

Bending direction

Applicable force [N]Load

Automatic
positioning stage

Load

Force gauge

B. Finger structure
Fig. 4 shows a schematic view of the finger structure
constructed using a skeleton, particles, and an outer rubberskin (Ecoflex 00-30). Fig. 5 shows the skeleton of a finger
made using a 3D printer (Stratasys uPrintSE). The skeleton iscomposed of four links and three rotational joints. The
protruded part at each joint is called the “joint block,” and isconnected with the distal-side link. The role of the joint blockis to provide separated spaces inside the outer portion of therubber. Air flows through the clearance between the links andjoint blocks, whereas the particles do not. Thus, all segmented
chambers can be jammed by sucking the air around theparticles through a suction tube embedded at the root of thefinger. One wire is attached to the distal joint block, and goes
through the holes in the other joint blocks and finger base. All
joints of the finger are driven through a pulling of the wire. To
prevent air from leaking from the holes, the wire is covered
with a vinyl tube, which is tightly sealed at the finger base.
The joint bock has a role in separating the chamber into
three chambers for the four links, and to enhance the increasein rigidity owing to the jamming transition. The increase inrigidity is caused by an increase in the particle density. Thereis one weakness, however. The rigidity decreases when
applying a load, which contributes to an increase in thechamber volume, blocking the operation of a jamming
transition. A typical example is a case in which an external
force is exerted on the flexed finger, as shown in Fig. 6. In Fig.
6(b), it can be seen that the lower part of the proximal joint is
infracted. To resolve this issue, the joint block has a shape inwhich the inside part is a flat plate and the outside part isquarter-spherical. This prevents an undesirable movement of
the particles, which increases the volume of the chamber, as
shown in Fig. 7. It should be noted that a finger stiffened by ajamming transition can return to its original soft state throughan unjamming process, as shown in the attached video clip.
To verify the proposed structure of the skeleton, we
experimentally investigated how much force can be applied to
the finger, which is closely related with the magnitude ofrigidity (Fig. 8). The experimental method applied is asfollows. The finger base was fixed such that the finger was
horizontally arranged with its inside facing downward. Afterthe finger was expanded straightly, a jamming transition wasapplied. A vertical load was then applied to the fingertip. Fourkinds of fingers were prepared: (a) no skeleton, (b) a skeleton
without joint blocks, and (c) a skeleton with the joint blocks.
We compared the reactive force when the fingers were
pushed with the same displacement (7 mm). A jamming
transition was applied using a vacuum pump (G-10DA,
UKVAC KIKO, Inc., with an ultimate pressure of 1.3 Pa).
Five trials were applied. Fig. 8 shows the results. It can beseen that the proposed structure, namely, (c) provides an
outstanding performance, which validates the structure. It
should be remarked that a low applicable force occurs when
there is no skeleton, which indicates the importance of askeleton for enhancing the rigidity.

1.4
1.2

Load

1
0.8
0.6

Load

0.4
0.2
0

(a)

(b)

(c)

Figure 8. Applicable forces for different skeleton structures. The left figure
illustrates the experimental set up while the right figure displays the results;(a) no skeleton, (b) three joints and links, and (c) three joints and links withjoint blocks.

D. Independent-motion-driven unit
To utilize the high adaptability of the proposed finger
mechanism, the wires of the four fingers need to be pulledindividually. As mentioned in section I, if simply pulling thefour wires of the four fingers using a single actuator, aproblem will occur (see the left side of Fig. 12), namely, themotions of all fingers will be restricted by the restriction
caused by contact between one finger and the object. Herein,
we applied a method in which we embedded a spring in thewire, similar to a series of elastic actuators (see the right sideof Fig. 12). The independent-motion-driving unit provides acompact structure, allowing a solution to be realized. Fig. 13
shows a schematic structure of the unit. The unit wasconstructed using four smaller units for the four fingers. Eachsmall unit was constructed using an inner ring, a torsion
spring, and an outer ring. The four inner rings are serially

4.98 g(a)30.0 g(b)21.0 g(c)37.1 g(d)

PulleyBending
direction

Pull

Automatic]positioning stage

Move

Force gauge

Figure 9. Examined granular materials: (a) coffee powder, (b) beads, (c)rice, and (d) gravel.

Figure 10. Schematic view of the experimental setup for selecting granularmaterials.
5
4.5

Pulling force [N]C. Particle selection
It is preferable for the particles to not disturb the rotation of
the finger joints before a jamming transition, whereas thejoints can be strongly fixed after a jamming transition. From
this viewpoint, we experimentally investigated which type of
granular material is preferable. The target materials areshown in Fig. 9. Coffee powder, which has a small particle
size, is a popular material used for jamming grippers. A beadis an industrial product, and the variances in its size and shapeare small. Rice is long and thin, and its variances in size andshape are large. Gravel has larger variances in size and shapethan rice. The average size of a particle is largest for gravel,
followed in order by rice, beads, and coffee powder.
Fig. 10 shows the experimental setup applied. The
procedure is as follows. 1) The base of the expanded finger
was fixed such that the inside of the finger faced upward. 2)
The wire for closing the finger was attached to a force gauge(DS2-50N, IMADA) mounted on an automatic poisoning
stage (MX2-500N, IMADA) through a pulley. 3) We pulled
the wire using the automatic poisoning stage, and the pulling
force was measured with and without a jamming transition. A
jamming transition was conducted using a vacuum pump
(G-10DA, UKVAC KIKO, Inc., with an ultimate pressure of
1.3 Pa). To evaluate the stiffness of the structures, we
compared the increased pulling forces when pulling the same
distance (7 mm) starting from the same initial pulling force(0.5 N). We repeated the same experiment five times.
Fig. 11 shows the results; the left bar indicates a case inwhich a jamming transition was not applied, whereas the rightbar shows a case in which a jamming transition was applied.
Without a jamming transition, coffee powder provided theleast stiffness, and the smoothest joint motion was therefore
expected. The second best was obtained using rice. With ajamming transition, rice provided the largest amount ofstiffness. Because a soft surface constructed using granularmaterials normally provides a soft contact, a high stiffness
owing to a jamming transition is more important for firm andstable grasping. Therefore, we selected rice as the granularmaterial for the developed robotic hand.

4
3.5
3
2.5
2
1.5
1
0.5
0

Coffee powder

Bead

Rice

GravelFigure 11. Pulling force when utilizing different granular materials: without(left bar) or with (right bar) a jamming transition.

ObjectObjectSpringPull

Pull
Expandby pulling

Pull

Pull
Motor shaft

Motor shaft

Figure 12. Role of independent-motion-driving unit.
Motor

Inner ring
Torsion spring

Side view
Outer ring

Figure 13. Schematic view of independent-motion-driving unit.

Press strongly

Jamming

Pick up

Small object

Figure 14. Function of jamming gripper. This is useful to grasp a small orthin object that is difficult to grasp with another grasping style.

connected and driven by a motor. The outer ring is attached toa wire to pull each finger. The driven torque is transmitted tothe outer rings through the torsion springs connected to theinner rings individually. When the motor is driven, the four
smaller units are rotated and the four wires are initially pulled.
If the motion of one finger is restricted through contact withan object, the torsion spring is extended instead of pulling thewire in the corresponding unit. Thus, the motions of the otherfingers are not hindered.
E. Small jamming gripper utilizing fingertip
The fingertip of the developed robotic hand can be utilized
as a jamming gripper, as shown in Fig. 14. The size of agraspable object is smaller than the fingertip. This function isuseful when grasping an object that is too small or thin, and isdifficult to grasp using other grasping styles.
IV. EXPERIMENTAL EVALUATION
To evaluate the functions of the developed robotic hand,several experiments were conducted, the results of which are
shown herein.
A. Grasping test
Figs. 3 and 15 shows the results of the grasping tests. A
baseball, mug, and cord were grasped using an enveloping
grasping style. A plastic bottle (500 ml) and a tomato canwere grasped using a cylindrical grasping style. A black ball,a box, a small ornament, and a bunch of keys were grasped
using a precision grasping style. The results indicate that thedeveloped robotic hand can grasp a wide variety of objects byadapting to the shapes of the objects and utilizing different
grasping styles. Off course, there are the limitations. The handfailed to grasp a card, large box and bottle because of too thin,
too large, and too heavy for precision grasp, respectively.
Focusing on the grasping of a small mug, one finger was
extended owing to a contact made with the handle, whereas
the other fingers were flexed. This validates the efficacy of
the independent-motion-driving unit. We tried to grasp alarge mug using a style in which one finger was inserted into

the handle, and another finger supported the weight of the
mug while making contact with the lower part of the handle.
A load was exerted on the lower finger such that a flection
occurred and the wire was relaxed; however, the finger
posture was maintained as a constant, thereby validating theefficacy of the jamming transition.
B. Grasping stability and disturbance
Herein, we investigated the grasping stability under adisturbance through two types of operations (see the videoclip). Fig. 16 shows an operation in which a deformableplastic tube including two tennis balls was grasped andshaken. It can be seen that the tube was not deformed, and thegrasping posture was maintained. For the operation shown in
Fig. 17, we grasped a deformable paper box including water,
poured the water into a deformable plastic cup, grasped thecup, and poured the water into a vase. A deformation of thegrasped objects, which are fragile and deformable, was notobserved. The grasping posture was maintained even whenpouring the water while tilting the objects. From the two
operations, the high grasping stability under a disturbancewas observed.
C. Functionality as a small jamming gripper
Herein, we investigated the functionality of the fingertip asa small jamming gripper. The target objects were M2 to M8
sized bolts, as shown in Fig. 18. Each bolt was placed on a flat
surface, and pressed by the fingertip. Subsequently, ajamming transition was applied, and the fingertip was picked
up. We considered the operation to be successful if the bolt
was also picked up or absorbed. The operation for each bolt
was applied four times. Table I shows the success rate. A low
success rate was obtained when grasping the M2 and M8
bolts. An M2 bolt was too small compared to the thickness of
the rubber skin, and an M8 bolt was too big compared to thesizes of the fingertips and the granular materials. The
available range within which fingertip can work as a jamming
gripper was close to the size of the M3 to M6 bolts.

Plastic bottle

Precision
grasp

Enveloping/Cylindricalgrasp
Tomato can

Mug (small)Thin pen

Cord

Ball (small) A bunch of keys

Mug (large)

The load was exerted on the lower
finger in the bending direction

Card

Box (large)

Exampleof failure
Too thin

Too large

Plastic bottle

Too heavy for
precision grasp

Figure 15. Results of grapsing test

Box (small)

Small ornament

(1)

(2)

(3)

(4)

REFERENCES[1]

Center of gravity moves

Figure 16. Operation in which a deformable plastic tube including two tennis
balls was grasped and shaken.

Paper box(a)

Plastic cup

(b)

(c)

Figure 17. Operation in which a deformable paper box including water was
grasped, (a) the water was poured into a deformable plastic cup, (b) the cup
was grasped, and (c) the water was poured into a vase.
M6×10

M8×15

Bolt

Figure 18. Target objects and overview of the grasping test utilizing thefingertip as a jamming gripper.
TABLE I.
Type andlengthSuccess
rate

SUCCESS RATE OF GRAPSING TEST UTILIZING THE
FINGERTIP AS A JAMMING GRIPPER

M2×4

M3×4

M4×5

M5×5

M6×10

M8×15

1/4

4/4

4/4

4/4

2/4

0/4

V. CONCLUSION
This study presented a novel multi-fingered hand based on
the hybrid mechanism of a tendon-driven and jamming
transition. Each finger was constructed using a tendon-driven
rigid skeleton, granular materials (rice) corresponding tofinger pulp, and deformable rubber skin. With this structure,
the robotic hand can make a grasping posture while adapting
to the shape of the object and without its deformation, andhold the object by fixing the grasping posture using a
jamming transition. The experimental results indicate that thehand can grasp a wide variety of objects with different
grasping styles according to the object shape or operation. A
fixed grasping posture using a jamming transition wasmaintained even when several external disturbances wereexerted. This indicates that a high grasping stability under
disturbances can be achieved. Additionally, the fingertips canwork as small jamming grippers, and the experimental results
show that this function is available for objects whose size is
close to that of M3 to M6 sized bolts. Future work will
involve a realization of autonomous operations utilizing thedeveloped robotic hand.

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